2018 .

(41 publications)

T. B. Richardson, P. M. Forster, T. Andrews, O. Boucher, G. Faluvegi, D. Fläschner, Ø. Hodnebrog, M. Kasoar, A. Kirkevåg, J.-F. Lamarque, G. Myhre, D. Olivié, B. H. Samset, D. Shawki, D. Shindell, T. Takemura, and A. Voulgarakis. Drivers of Precipitation Change: An Energetic Understanding. Journal of Climate, 31:9641-9657, December 2018. [ bib | DOI | ADS link ]

A. Dommo, N. Philippon, D. A. Vondou, G. Sèze, and R. Eastman. The June-September Low Cloud Cover in Western Central Africa: Mean Spatial Distribution and Diurnal Evolution, and Associated Atmospheric Dynamics. Journal of Climate, 31:9585-9603, December 2018. [ bib | DOI | ADS link ]

D. Fläschner, T. Mauritsen, B. Stevens, and S. Bony. The Signature of Shallow Circulations, Not Cloud Radiative Effects, in the Spatial Distribution of Tropical Precipitation. Journal of Climate, 31:9489-9505, December 2018. [ bib | DOI | ADS link ]

D. Coppin and S. Bony. On the Interplay Between Convective Aggregation, Surface Temperature Gradients, and Climate Sensitivity. Journal of Advances in Modeling Earth Systems, 10:3123-3138, December 2018. [ bib | DOI | ADS link ]

This study explores the extent to which convective aggregation interacts with sea surface temperature (SST) and affects climate sensitivity. For this purpose, radiative-convective equilibrium simulations are run with a general circulation model coupled to an ocean mixed layer, and several types of perturbations are imposed to the ocean-atmosphere system. Convective aggregation turns out to be much more sensitive to temperature in coupled experiments than in prescribed SST experiments. But changes in convective aggregation induced by a doubling of the CO2 concentration are always smaller than changes associated with the transition from a non-aggregated to an aggregated state. If aggregation changes were acting alone, they would exert a strong negative feedback on global mean surface temperature. However, in a coupled framework, aggregation changes interact with the SST and generate SST gradients that strengthen the positive low-cloud feedback associated with changes in SST pattern. This overcompensates the negative feedback due to aggregation changes and leads to a larger equilibrium climate sensitivity than in the absence of SST gradients. Although this effect might be model specific, interactions between convective aggregation and the spatial distribution of SST appear crucial to assess the impact of convective aggregation on climate sensitivity.

X. Li, Y. Balkanski, Z. Wu, T. Gasser, P. Ciais, F. Zhou, L. Li, S. Tao, S. Peng, S. Piao, R. Wang, T. Wang, and B. Li. Analysis of slight precipitation in China during the past decades and its relationship with advanced very high radiometric resolution normalized difference vegetation index. International Journal of Climatology, 38:5563-5575, December 2018. [ bib | DOI | ADS link ]

G. Krinner, C. Derksen, R. Essery, M. Flanner, S. Hagemann, M. Clark, A. Hall, H. Rott, C. Brutel-Vuilmet, H. Kim, C. B. Ménard, L. Mudryk, C. Thackeray, L. Wang, G. Arduini, G. Balsamo, P. Bartlett, J. Boike, A. Boone, F. Chéruy, J. Colin, M. Cuntz, Y. Dai, B. Decharme, J. Derry, A. Ducharne, E. Dutra, X. Fang, C. Fierz, J. Ghattas, Y. Gusev, V. Haverd, A. Kontu, M. Lafaysse, R. Law, D. Lawrence, W. Li, T. Marke, D. Marks, M. Ménégoz, O. Nasonova, T. Nitta, M. Niwano, J. Pomeroy, M. S. Raleigh, G. Schaedler, V. Semenov, T. G. Smirnova, T. Stacke, U. Strasser, S. Svenson, D. Turkov, T. Wang, N. Wever, H. Yuan, W. Zhou, and D. Zhu. ESM-SnowMIP: assessing snow models and quantifying snow-related climate feedbacks. Geoscientific Model Development, 11:5027-5049, December 2018. [ bib | DOI | ADS link ]

This paper describes ESM-SnowMIP, an international coordinated modelling effort to evaluate current snow schemes, including snow schemes that are included in Earth system models, in a wide variety of settings against local and global observations. The project aims to identify crucial processes and characteristics that need to be improved in snow models in the context of local- and global-scale modelling. A further objective of ESM-SnowMIP is to better quantify snow-related feedbacks in the Earth system. Although it is not part of the sixth phase of the Coupled Model Intercomparison Project (CMIP6), ESM-SnowMIP is tightly linked to the CMIP6-endorsed Land Surface, Snow and Soil Moisture Model Intercomparison (LS3MIP).

D. Bharath Kumar, S. Verma, O. Boucher, and R. Wang. Constrained simulation of aerosol species and sources during pre-monsoon season over the Indian subcontinent. Atmospheric Research, 214:91-108, December 2018. [ bib | DOI | ADS link ]

This study was designed to deliver a better concurrence between model estimates and observations, of atmospheric aerosol species, and predict their spatial distribution as consistently as possible. A free running aerosol simulation (freesimu) in a general circulation model (GCM) was performed, and further the simulated aerosol optical depth (AOD) was constrained with the observed AOD. The present study was carried out during the pre-monsoon season and for the Tigerz experiment which was conducted at stations over the Indo-Gangetic plain (IGP) and the Himalayan foot-hills in northern India. Our formulation of the constrained aerosol simulation (constrsimu) was based upon an identification of the freesimu with the most consistent estimates of aerosol characteristic among the three freesimu. The three freesimu (differing in source of emissions and model horizontal resolution) were carried out with the general circulation model (GCM) of Laboratoire de Météorologie Dynamique (LMD-ZT GCM). Black carbon (BC), organic carbon (OC), and sulfate-other water soluble (Sul-ows) estimated from constrsimu amounted to 70%-100% compared to that from freesimu being 20%-50% of their measured counterparts. Among the aerosol species, the pre-monsoon mean concentration of dust was considerably high over most part of the Indian subcontinent; the anthropogenic aerosol species were, however, specifically predominant over the IGP (mostly 8-12 μg m-3 for Sul-ows, OC). The constrsimu estimated total submicron aerosol mass concentration revealed its alarmingly high value over the northern and north-western India ( 100 μg m-3 and as high as 300 μg m-3). While the high value of observed AOD was found being mainly due to dust (AOD due to dust greater than 0.3) over the northern-northwestern IGP, it was due to Sul-ows (AOD due to Sul-ows as high as 0.4) over the eastern IGP, eastern coastline, and the Bay of Bengal. Temporal trend of fine (FM) and coarse mode (CM) AOD from constrsimu estimates and that derived from Tigerz experiment were in phase with each other for most of the days and exhibited a strong positive correlation coefficient. Source of Tigerz aerosols was mainly due to a predominant influence of dust from Africa/west Asia followed by that from northwest India, and of anthropogenic emissions originating in the IGP. A 200% increase was inferred for potential black carbon emissions (using India emission inventory implemented in a GCM) to obtain a concurrence between observed and freesimu BC concentration.

J. Jouhaud, J.-L. Dufresne, J.-B. Madeleine, F. Hourdin, F. Couvreux, N. Villefranque, and A. Jam. Accounting for Vertical Subgrid-Scale Heterogeneity in Low-Level Cloud Fraction Parameterizations. Journal of Advances in Modeling Earth Systems, 10:2686-2705, November 2018. [ bib | DOI | ADS link ]

Many general circulation models (GCMs) assume some heterogeneity of water amounts in their grid boxes and use probability density functions to parameterize cloud fractions CF and amounts of condensed water qc. Most GCM cloud schemes calculate the CF as the volume of the grid box that contains clouds (CFvol), whereas radiative fluxes primarily depend on the CF by surface (CFsurf), that is, the surface of the grid box covered by clouds when looking from above. This discrepancy matters as previous findings suggest that CFsurf is typically greater than CFvol by about 30%. In this paper we modify the single column model version of the LMDz GCM cloud scheme by introducing the vertical subgrid-scale heterogeneity of water content. This allows to distinctly compute the two fractions, CFvol and CFsurf, as well as the amount of condensed water qc. This study is one of the first to take into account such vertical subgrid-scale heterogeneity in a GCM cloud scheme. Three large eddy simulation cases of cumuliform boundary layer clouds are used to test and calibrate two different parameterizations. These new developments increase cloud cover by about 10% for the oceanic cases RICO and Barbados Oceanographic Meteorological Experiment and by up to 50% for the continental case ARM. The change in condensed water reduces the liquid water path by 10-20% and therefore the cloud opacity by 5-50%. These results show the potential of the new scheme to reduce the too few, too bright bias by increasing low-level CF and decreasing cloud reflectance.

C. J. Smith, R. J. Kramer, G. Myhre, P. M. Forster, B. J. Soden, T. Andrews, O. Boucher, G. Faluvegi, D. Fläschner, Ø. Hodnebrog, M. Kasoar, V. Kharin, A. Kirkevâg, J.-F. Lamarque, J. Mülmenstädt, D. Olivié, T. Richardson, B. H. Samset, D. Shindell, P. Stier, T. Takemura, A. Voulgarakis, and D. Watson-Parris. Understanding Rapid Adjustments to Diverse Forcing Agents. Geophysical Research Letters, 45:12, November 2018. [ bib | DOI | ADS link ]

Rapid adjustments are responses to forcing agents that cause a perturbation to the top of atmosphere energy budget but are uncoupled to changes in surface warming. Different mechanisms are responsible for these adjustments for a variety of climate drivers. These remain to be quantified in detail. It is shown that rapid adjustments reduce the effective radiative forcing (ERF) of black carbon by half of the instantaneous forcing, but for CO2 forcing, rapid adjustments increase ERF. Competing tropospheric adjustments for CO2 forcing are individually significant but sum to zero, such that the ERF equals the stratospherically adjusted radiative forcing, but this is not true for other forcing agents. Additional experiments of increase in the solar constant and increase in CH4 are used to show that a key factor of the rapid adjustment for an individual climate driver is changes in temperature in the upper troposphere and lower stratosphere.

G. Myhre, R. J. Kramer, C. J. Smith, Ø. Hodnebrog, P. Forster, B. J. Soden, B. H. Samset, C. W. Stjern, T. Andrews, O. Boucher, G. Faluvegi, D. Fläschner, M. Kasoar, A. Kirkevâg, J.-F. Lamarque, D. Olivié, T. Richardson, D. Shindell, P. Stier, T. Takemura, A. Voulgarakis, and D. Watson-Parris. Quantifying the Importance of Rapid Adjustments for Global Precipitation Changes. Geophysical Research Letters, 45:11, October 2018. [ bib | DOI | ADS link ]

Different climate drivers influence precipitation in different ways. Here we use radiative kernels to understand the influence of rapid adjustment processes on precipitation in climate models. Rapid adjustments are generally triggered by the initial heating or cooling of the atmosphere from an external climate driver. For precipitation changes, rapid adjustments due to changes in temperature, water vapor, and clouds are most important. In this study we have investigated five climate drivers (CO2, CH4, solar irradiance, black carbon, and sulfate aerosols). The fast precipitation responses to a doubling of CO2 and a 10-fold increase in black carbon are found to be similar, despite very different instantaneous changes in the radiative cooling, individual rapid adjustments, and sensible heating. The model diversity in rapid adjustments is smaller for the experiment involving an increase in the solar irradiance compared to the other climate driver perturbations, and this is also seen in the precipitation changes.

A. Gāinusā-Bogdan, F. Hourdin, A. K. Traore, and P. Braconnot. Omens of coupled model biases in the CMIP5 AMIP simulations. Climate Dynamics, 51:2927-2941, October 2018. [ bib | DOI | ADS link ]

Despite decades of efforts and improvements in the representation of processes as well as in model resolution, current global climate models still suffer from a set of important, systematic biases in sea surface temperature (SST), not much different from the previous generation of climate models. Many studies have looked at errors in the wind field, cloud representation or oceanic upwelling in coupled models to explain the SST errors. In this paper we highlight the relationship between latent heat flux (LH) biases in forced atmospheric simulations and the SST biases models develop in coupled mode, at the scale of the entire intertropical domain. By analyzing 22 pairs of forced atmospheric and coupled ocean-atmosphere simulations from the CMIP5 database, we show a systematic, negative correlation between the spatial patterns of these two biases. This link between forced and coupled bias patterns is also confirmed by two sets of dedicated sensitivity experiments with the IPSL-CM5A-LR model. The analysis of the sources of the atmospheric LH bias pattern reveals that the near-surface wind speed bias dominates the zonal structure of the LH bias and that the near-surface relative humidity dominates the east-west contrasts.

O. Richet, J.-M. Chomaz, and C. Muller. Internal Tide Dissipation at Topography: Triadic Resonant Instability Equatorward and Evanescent Waves Poleward of the Critical Latitude. Journal of Geophysical Research (Oceans), 123:6136-6155, September 2018. [ bib | DOI | ADS link ]

Several studies have shown the existence of a critical latitude where the dissipation of internal tides is strongly enhanced. Internal tides are internal waves generated by barotropic tidal currents impinging rough topography at the seafloor. Their dissipation and concomitant diapycnal mixing are believed to be important for water masses and the large-scale ocean circulation. The purpose of this study is to clarify the physical processes at the origin of this strong latitudinal dependence of tidal energy dissipation. We find that different mechanisms are involved equatorward and poleward of the critical latitude. Triadic resonant instabilities are responsible for the dissipation of internal tides equatorward of the critical latitude. In particular, a dominant triad involving the primary internal tide and near-inertial waves is key. At the critical latitude, the peak of energy dissipation is explained by both increased instability growth rates, and smaller scales of secondary waves thus more prone to break and dissipate their energy. Surprisingly, poleward of the critical latitude, the generation of evanescent waves appears to be crucial. Triadic instabilities have been widely studied, but the transfer of energy to evanescent waves has received comparatively little attention. Our work suggests that the nonlinear transfer of energy from the internal tide to evanescent waves (corresponding to the 2f-pump mechanism described by Young et al., 2008, <A href=“https://doi.org/10.1017/S0022112008001742”>https://doi.org/10.1017/S0022112008001742</A>) is an efficient mechanism to dissipate internal tide energy near and poleward of the critical latitude. The theoretical results are confirmed in idealized high-resolution numerical simulations of a barotropic M2 tide impinging sinusoidal topography in a linearly stratified fluid.

W. Li, Z. Jiang, X. Zhang, and L. Li. On the Emergence of Anthropogenic Signal in Extreme Precipitation Change Over China. Geophysical Research Letters, 45:9179-9185, September 2018. [ bib | DOI | ADS link ]

The detection of anthropogenic influences on climate extremes at regional scale is important for the development of national climate change policy. Global climate simulations from phase 5 of the Coupled Model Intercomparison Project under the Representative Concentration Pathway 8.5 scenario are used to examine the time at which an anthropogenic influence becomes detectable in extreme precipitation over China and the change in probability of extreme precipitation with certain magnitudes when the changes are detectable. Anthropogenic influence is not significantly detected over China in the observational record or simulations from 1961 to 2012 based on the test of field significance. Simulations indicate that such change would become detectable in the future by around 2035. Large changes would already manifest by the time of signal detection; for example, extreme precipitation events that occur on average once every 20, 50, and 100 years in current (1986-2005) climate would reduce to about 15, 34, and 63 years on average by the time of detection around 2035.

X. Zeng, D. Klocke, B. J. Shipway, M. S. Singh, I. Sandu, W. Hannah, P. Bogenschutz, Y. Zhang, H. Morrison, M. Pritchard, and C. Rio. Future Community Efforts in Understanding and Modeling Atmospheric Processes. Bulletin of the American Meteorological Society, 99:ES159-ES162, September 2018. [ bib | DOI | ADS link ]

B. Kravitz, P. J. Rasch, H. Wang, A. Robock, C. Gabriel, O. Boucher, J. N. S. Cole, J. Haywood, D. Ji, A. Jones, A. Lenton, J. C. Moore, H. Muri, U. Niemeier, S. Phipps, H. Schmidt, S. Watanabe, S. Yang, and J.-H. Yoon. The climate effects of increasing ocean albedo: an idealized representation of solar geoengineering. Atmospheric Chemistry & Physics, 18:13097-13113, September 2018. [ bib | DOI | ADS link ]

Geoengineering, or climate intervention, describes methods of deliberately altering the climate system to offset anthropogenic climate change. As an idealized representation of near-surface solar geoengineering over the ocean, such as marine cloud brightening, this paper discusses experiment G1ocean-albedo of the Geoengineering Model Intercomparison Project (GeoMIP), involving an abrupt quadrupling of the CO2 concentration and an instantaneous increase in ocean albedo to maintain approximate net top-of-atmosphere radiative flux balance. A total of 11 Earth system models are relatively consistent in their temperature, radiative flux, and hydrological cycle responses to this experiment. Due to the imposed forcing, air over the land surface warms by a model average of 1.14 K, while air over most of the ocean cools. Some parts of the near-surface air temperature over ocean warm due to heat transport from land to ocean. These changes generally resolve within a few years, indicating that changes in ocean heat content play at most a small role in the warming over the oceans. The hydrological cycle response is a general slowing down, with high heterogeneity in the response, particularly in the tropics. While idealized, these results have important implications for marine cloud brightening, or other methods of geoengineering involving spatially heterogeneous forcing, or other general forcings with a strong land-ocean contrast. It also reinforces previous findings that keeping top-of-atmosphere net radiative flux constant is not sufficient for preventing changes in global mean temperature.

J. Vial, C. Cassou, F. Codron, S. Bony, and Y. Ruprich-Robert. Influence of the Atlantic Meridional Overturning Circulation on the Tropical Climate Response to CO2 Forcing. Geophysical Research Letters, 45:8519-8528, August 2018. [ bib | DOI | ADS link ]

The increase of atmospheric greenhouse gases is expected to affect the hydrological cycle and large-scale precipitation patterns. In parallel, unforced natural variability on decadal-to-multidecadal timescales can also modulate forced changes at the regional scales. Based on multimember ensembles from a coupled General Circulation Model, we investigate the sensitivity of CO2-forced changes in tropical precipitation and atmospheric circulation to fluctuations of the Atlantic Multidecadal Overturning Circulation (AMOC). We show that contrasted AMOC states yield considerable differences in equatorial Pacific precipitation forced changes, by impacting the direct (within a year) CO2-induced weakening of the Walker circulation. We use global atmospheric energetics, as a theoretical backdrop, to explain the relationship between the tropical atmospheric circulation and the AMOC state. A physical mechanism is then proposed, relating the direct CO2-forced weakening of the atmospheric tropical circulation to its climatological strength in unperturbed climate and indirectly to the AMOC state.

P. Drobinski, N. D. Silva, G. Panthou, S. Bastin, C. Muller, B. Ahrens, M. Borga, D. Conte, G. Fosser, F. Giorgi, I. Güttler, V. Kotroni, L. Li, E. Morin, B. Önol, P. Quintana-Segui, R. Romera, and C. Z. Torma. Scaling precipitation extremes with temperature in the Mediterranean: past climate assessment and projection in anthropogenic scenarios. Climate Dynamics, 51:1237-1257, August 2018. [ bib | DOI | ADS link ]

In this study we investigate the scaling of precipitation extremes with temperature in the Mediterranean region by assessing against observations the present day and future regional climate simulations performed in the frame of the HyMeX and MED-CORDEX programs. Over the 1979-2008 period, despite differences in quantitative precipitation simulation across the various models, the change in precipitation extremes with respect to temperature is robust and consistent. The spatial variability of the temperature-precipitation extremes relationship displays a hook shape across the Mediterranean, with negative slope at high temperatures and a slope following Clausius-Clapeyron (CC)-scaling at low temperatures. The temperature at which the slope of the temperature-precipitation extreme relation sharply changes (or temperature break), ranges from about 20 degC in the western Mediterranean to 10 degC in Greece. In addition, this slope is always negative in the arid regions of the Mediterranean. The scaling of the simulated precipitation extremes is insensitive to ocean-atmosphere coupling, while it depends very weakly on the resolution at high temperatures for short precipitation accumulation times. In future climate scenario simulations covering the 2070-2100 period, the temperature break shifts to higher temperatures by a value which is on average the mean regional temperature change due to global warming. The slope of the simulated future temperature-precipitation extremes relationship is close to CC-scaling at temperatures below the temperature break, while at high temperatures, the negative slope is close, but somewhat flatter or steeper, than in the current climate depending on the model. Overall, models predict more intense precipitation extremes in the future. Adjusting the temperature-precipitation extremes relationship in the present climate using the CC law and the temperature shift in the future allows the recovery of the temperature-precipitation extremes relationship in the future climate. This implies negligible regional changes of relative humidity in the future despite the large warming and drying over the Mediterranean. This suggests that the Mediterranean Sea is the primary source of moisture which counteracts the drying and warming impacts on relative humidity in parts of the Mediterranean region.

C. Genthon, R. Forbes, E. Vignon, A. Gettelman, and J.-B. Madeleine. Comment on “Surface Air Relative Humidities Spuriously Exceeding 100% in CMIP5 Model Output and Their Impact on Future Projections” by K. Ruosteenoja et al. (2017). Journal of Geophysical Research (Atmospheres), 123:8724-8727, August 2018. [ bib | DOI | ADS link ]

F. Adloff, G. Jordà, S. Somot, F. Sevault, T. Arsouze, B. Meyssignac, L. Li, and S. Planton. Improving sea level simulation in Mediterranean regional climate models. Climate Dynamics, 51:1167-1178, August 2018. [ bib | DOI | ADS link ]

For now, the question about future sea level change in the Mediterranean remains a challenge. Previous climate modelling attempts to estimate future sea level change in the Mediterranean did not meet a consensus. The low resolution of CMIP-type models prevents an accurate representation of important small scales processes acting over the Mediterranean region. For this reason among others, the use of high resolution regional ocean modelling has been recommended in literature to address the question of ongoing and future Mediterranean sea level change in response to climate change or greenhouse gases emissions. Also, it has been shown that east Atlantic sea level variability is the dominant driver of the Mediterranean variability at interannual and interdecadal scales. However, up to now, long-term regional simulations of the Mediterranean Sea do not integrate the full sea level information from the Atlantic, which is a substantial shortcoming when analysing Mediterranean sea level response. In the present study we analyse different approaches followed by state-of-the-art regional climate models to simulate Mediterranean sea level variability. Additionally we present a new simulation which incorporates improved information of Atlantic sea level forcing at the lateral boundary. We evaluate the skills of the different simulations in the frame of long-term hindcast simulations spanning from 1980 to 2012 analysing sea level variability from seasonal to multidecadal scales. Results from the new simulation show a substantial improvement in the modelled Mediterranean sea level signal. This confirms that Mediterranean mean sea level is strongly influenced by the Atlantic conditions, and thus suggests that the quality of the information in the lateral boundary conditions (LBCs) is crucial for the good modelling of Mediterranean sea level. We also found that the regional differences inside the basin, that are induced by circulation changes, are model-dependent and thus not affected by the LBCs. Finally, we argue that a correct configuration of LBCs in the Atlantic should be used for future Mediterranean simulations, which cover hindcast period, but also for scenarios.

A. Harzallah, G. Jordà, C. Dubois, G. Sannino, A. Carillo, L. Li, T. Arsouze, L. Cavicchia, J. Beuvier, and N. Akhtar. Long term evolution of heat budget in the Mediterranean Sea from Med-CORDEX forced and coupled simulations. Climate Dynamics, 51:1145-1165, August 2018. [ bib | DOI | ADS link ]

This study evaluates the Mediterranean Sea heat budget components from a set of forced and coupled simulations performed in the frame of the Med-CORDEX project. The simulations use regional climate system models (RCSMs) dedicated to the Mediterranean area and driven by the ERA40/ERA-Interim reanalyses. The study focuses on the period 1980-2010. Interannual variations of the average net heat flux at the sea surface are consistent among models but the spread in the mean values is large (from -4.8 to +2.2 Wm-2) with the coupled models showing the lowest heat loss from the sea. For the heat flux at the Strait of Gibraltar both interannual variations and mean values show a large intermodel spread. The basin average temperature shows positive trends with highest values in the coupled models; it also shows interannual variations that are in good agreement with observations. The heat content rate is calculated based on the derivative of the average temperature and is found to be significantly correlated for most models with the net heat flux at the sea surface (average correlation 0.5) but not with the net heat flux through the Strait of Gibraltar (average correlation 0.2), suggesting that in the considered RCSMs the interannual variability of the heat content rate is mainly driven by the surface heat fluxes. The resemblance between the simulated and observed heat content rates is stronger in the forced models than in the coupled ones. This is explained by the stronger constraint applied to the forced models by the use of the surface temperature relaxation to observations. The temperature of the outflowing water through the Strait of Gibraltar shows positive and significant trends, also higher in the coupled models. It is suggested that the Mediterranean Sea warming found in most models and in particular in the coupled ones, induces a change of the hydrographic conditions that affects the Strait of Gibraltar.

A. Obermann, S. Bastin, S. Belamari, D. Conte, M. A. Gaertner, L. Li, and B. Ahrens. Mistral and Tramontane wind speed and wind direction patterns in regional climate simulations. Climate Dynamics, 51:1059-1076, August 2018. [ bib | DOI | ADS link ]

The Mistral and Tramontane are important wind phenomena that occur over southern France and the northwestern Mediterranean Sea. Both winds travel through constricting valleys before flowing out towards the Mediterranean Sea. The Mistral and Tramontane are thus interesting phenomena, and represent an opportunity to study channeling effects, as well as the interactions between the atmosphere and land/ocean surfaces. This study investigates Mistral and Tramontane simulations using five regional climate models with grid spacing of about 50 km and smaller. All simulations are driven by ERA-Interim reanalysis data. Spatial patterns of surface wind, as well as wind development and error propagation along the wind tracks from inland France to offshore during Mistral and Tramontane events, are presented and discussed. To disentangle the results from large-scale error sources in Mistral and Tramontane simulations, only days with well simulated large-scale sea level pressure field patterns are evaluated. Comparisons with the observations show that the large-scale pressure patterns are well simulated by the considered models, but the orographic modifications to the wind systems are not well simulated by the coarse-grid simulations (with a grid spacing of about 50 km), and are reproduced slightly better by the higher resolution simulations. On days with Mistral and/or Tramontane events, most simulations underestimate (by 13 % on average) the wind speed over the Mediterranean Sea. This effect is strongest at the lateral borders of the main flowthe flow width is underestimated. All simulations of this study show a clockwise wind direction bias over the sea during Mistral and Tramontane events. Simulations with smaller grid spacing show smaller biases than their coarse-grid counterparts.

L. Cavicchia, E. Scoccimarro, S. Gualdi, P. Marson, B. Ahrens, S. Berthou, D. Conte, A. Dell'Aquila, P. Drobinski, V. Djurdjevic, C. Dubois, C. Gallardo, L. Li, P. Oddo, A. Sanna, and C. Torma. Mediterranean extreme precipitation: a multi-model assessment. Climate Dynamics, 51:901-913, August 2018. [ bib | DOI | ADS link ]

Exploiting the added value of the ensemble of high-resolution model simulations provided by the Med-CORDEX coordinated initiative, an updated assessment of Mediterranean extreme precipitation events as represented in different observational, reanalysis and modelling datasets is presented. A spatiotemporal characterisation of the long-term statistics of extreme precipitation is performed, using a number of different diagnostic indices. Employing a novel approach based on the timing of extreme precipitation events a number of physically consistent subregions are defined. The comparison of different diagnostics over the Mediterranean domain and physically homogeneous sub-domains is presented and discussed, focussing on the relative impact of several model configuration features (resolution, coupling, physical parameterisations) on the performance in reproducing extreme precipitation events. It is found that the agreement between the observed and modelled long-term statistics of extreme precipitation is more sensitive to the model physics, in particular convective parameterisation, than to other model configurations such as resolution and coupling.

A. Benedetti, J. S. Reid, P. Knippertz, J. H. Marsham, F. Di Giuseppe, S. Rémy, S. Basart, O. Boucher, I. M. Brooks, L. Menut, L. Mona, P. Laj, G. Pappalardo, A. Wiedensohler, A. Baklanov, M. Brooks, P. R. Colarco, E. Cuevas, A. da Silva, J. Escribano, J. Flemming, N. Huneeus, O. Jorba, S. Kazadzis, S. Kinne, T. Popp, P. K. Quinn, T. T. Sekiyama, T. Tanaka, and E. Terradellas. Status and future of numerical atmospheric aerosol prediction with a focus on data requirements. Atmospheric Chemistry & Physics, 18:10615-10643, July 2018. [ bib | DOI | ADS link ]

Numerical prediction of aerosol particle properties has become an important activity at many research and operational weather centers. This development is due to growing interest from a diverse set of stakeholders, such as air quality regulatory bodies, aviation and military authorities, solar energy plant managers, climate services providers, and health professionals. Owing to the complexity of atmospheric aerosol processes and their sensitivity to the underlying meteorological conditions, the prediction of aerosol particle concentrations and properties in the numerical weather prediction (NWP) framework faces a number of challenges. The modeling of numerous aerosol-related parameters increases computational expense. Errors in aerosol prediction concern all processes involved in the aerosol life cycle including (a) errors on the source terms (for both anthropogenic and natural emissions), (b) errors directly dependent on the meteorology (e.g., mixing, transport, scavenging by precipitation), and (c) errors related to aerosol chemistry (e.g., nucleation, gas-aerosol partitioning, chemical transformation and growth, hygroscopicity). Finally, there are fundamental uncertainties and significant processing overhead in the diverse observations used for verification and assimilation within these systems. Indeed, a significant component of aerosol forecast development consists in streamlining aerosol-related observations and reducing the most important errors through model development and data assimilation. Aerosol particle observations from satellite- and ground-based platforms have been crucial to guide model development of the recent years and have been made more readily available for model evaluation and assimilation. However, for the sustainability of the aerosol particle prediction activities around the globe, it is crucial that quality aerosol observations continue to be made available from different platforms (space, near surface, and aircraft) and freely shared. This paper reviews current requirements for aerosol observations in the context of the operational activities carried out at various global and regional centers. While some of the requirements are equally applicable to aerosol-climate, the focus here is on global operational prediction of aerosol properties such as mass concentrations and optical parameters. It is also recognized that the term “requirements” is loosely used here given the diversity in global aerosol observing systems and that utilized data are typically not from operational sources. Most operational models are based on bulk schemes that do not predict the size distribution of the aerosol particles. Others are based on a mix of “bin” and bulk schemes with limited capability of simulating the size information. However the next generation of aerosol operational models will output both mass and number density concentration to provide a more complete description of the aerosol population. A brief overview of the state of the art is provided with an introduction on the importance of aerosol prediction activities. The criteria on which the requirements for aerosol observations are based are also outlined. Assimilation and evaluation aspects are discussed from the perspective of the user requirements.

L. Liu, D. Shawki, A. Voulgarakis, M. Kasoar, B. H. Samset, G. Myhre, P. M. Forster, Ø. Hodnebrog, J. Sillmann, S. G. Aalbergsjø, O. Boucher, G. Faluvegi, T. Iversen, A. Kirkevåg, J.-F. Lamarque, D. Olivié, T. Richardson, D. Shindell, and T. Takemura. A PDRMIP Multimodel Study on the Impacts of Regional Aerosol Forcings on Global and Regional Precipitation. Journal of Climate, 31:4429-4447, June 2018. [ bib | DOI | ADS link ]

T. Tang, D. Shindell, B. H. Samset, O. Boucher, P. M. Forster, Ø. Hodnebrog, G. Myhre, J. Sillmann, A. Voulgarakis, T. Andrews, G. Faluvegi, D. Fläschner, T. Iversen, M. Kasoar, V. Kharin, A. Kirkevåg, J.-F. Lamarque, D. Olivié, T. Richardson, C. W. Stjern, and T. Takemura. Dynamical response of Mediterranean precipitation to greenhouse gases and aerosols. Atmospheric Chemistry & Physics, 18:8439-8452, June 2018. [ bib | DOI | ADS link ]

Atmospheric aerosols and greenhouse gases affect cloud properties, radiative balance and, thus, the hydrological cycle. Observations show that precipitation has decreased in the Mediterranean since the beginning of the 20th century, and many studies have investigated possible mechanisms. So far, however, the effects of aerosol forcing on Mediterranean precipitation remain largely unknown. Here we compare the modeled dynamical response of Mediterranean precipitation to individual forcing agents in a set of global climate models (GCMs). Our analyses show that both greenhouse gases and aerosols can cause drying in the Mediterranean and that precipitation is more sensitive to black carbon (BC) forcing than to well-mixed greenhouse gases (WMGHGs) or sulfate aerosol. In addition to local heating, BC appears to reduce precipitation by causing an enhanced positive sea level pressure (SLP) pattern similar to the North Atlantic Oscillation-Arctic Oscillation, characterized by higher SLP at midlatitudes and lower SLP at high latitudes. WMGHGs cause a similar SLP change, and both are associated with a northward diversion of the jet stream and storm tracks, reducing precipitation in the Mediterranean while increasing precipitation in northern Europe. Though the applied forcings were much larger, if forcings are scaled to those of the historical period of 1901-2010, roughly one-third (3117 %) of the precipitation decrease would be attributable to global BC forcing with the remainder largely attributable to WMGHGs, whereas global scattering sulfate aerosols would have negligible impacts. Aerosol-cloud interactions appear to have minimal impacts on Mediterranean precipitation in these models, at least in part because many simulations did not fully include such processes; these merit further study. The findings from this study suggest that future BC and WMGHG emissions may significantly affect regional water resources, agricultural practices, ecosystems and the economy in the Mediterranean region.

J. Fuglestvedt, J. Rogelj, R. J. Millar, M. Allen, O. Boucher, M. Cain, P. M. Forster, E. Kriegler, and D. Shindell. Implications of possible interpretations of `greenhouse gas balance' in the Paris Agreement. Philosophical Transactions of the Royal Society of London Series A, 376:20160445, May 2018. [ bib | DOI | ADS link ]

The main goal of the Paris Agreement as stated in Article 2 is `holding the increase in the global average temperature to well below 2degC above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5degC'. Article 4 points to this long-term goal and the need to achieve `balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases'. This statement on `greenhouse gas balance' is subject to interpretation, and clarifications are needed to make it operational for national and international climate policies. We study possible interpretations from a scientific perspective and analyse their climatic implications. We clarify how the implications for individual gases depend on the metrics used to relate them. We show that the way in which balance is interpreted, achieved and maintained influences temperature outcomes. Achieving and maintaining net-zero CO2-equivalent emissions conventionally calculated using GWP100 (100-year global warming potential) and including substantial positive contributions from short-lived climate-forcing agents such as methane would result in a sustained decline in global temperature. A modified approach to the use of GWP100 (that equates constant emissions of short-lived climate forcers with zero sustained emission of CO2) results in global temperatures remaining approximately constant once net-zero CO2-equivalent emissions are achieved and maintained. Our paper provides policymakers with an overview of issues and choices that are important to determine which approach is most appropriate in the context of the Paris Agreement.

This article is part of the theme issue `The Paris Agreement: understanding the physical and social challenges for a warming world of 1.5degC above pre-industrial levels'.

K. Van Weverberg, C. J. Morcrette, J. Petch, S. A. Klein, H.-Y. Ma, C. Zhang, S. Xie, Q. Tang, W. I. Gustafson, Y. Qian, L. K. Berg, Y. Liu, M. Huang, M. Ahlgrimm, R. Forbes, E. Bazile, R. Roehrig, J. Cole, W. Merryfield, W.-S. Lee, F. Cheruy, L. Mellul, Y.-C. Wang, K. Johnson, and M. M. Thieman. CAUSES: Attribution of Surface Radiation Biases in NWP and Climate Models near the U.S. Southern Great Plains. Journal of Geophysical Research (Atmospheres), 123:3612-3644, April 2018. [ bib | DOI | ADS link ]

Many Numerical Weather Prediction (NWP) and climate models exhibit too warm lower tropospheres near the midlatitude continents. The warm bias has been shown to coincide with important surface radiation biases that likely play a critical role in the inception or the growth of the warm bias. This paper presents an attribution study on the net radiation biases in nine model simulations, performed in the framework of the CAUSES project (Clouds Above the United States and Errors at the Surface). Contributions from deficiencies in the surface properties, clouds, water vapor, and aerosols are quantified, using an array of radiation measurement stations near the Atmospheric Radiation Measurement Southern Great Plains site. Furthermore, an in-depth analysis is shown to attribute the radiation errors to specific cloud regimes. The net surface shortwave radiation is overestimated in all models throughout most of the simulation period. Cloud errors are shown to contribute most to this overestimation, although nonnegligible contributions from the surface albedo exist in most models. Missing deep cloud events and/or simulating deep clouds with too weak cloud radiative effects dominate in the cloud-related radiation errors. Some models have compensating errors between excessive occurrence of deep cloud but largely underestimating their radiative effect, while other models miss deep cloud events altogether. Surprisingly, even the latter models tend to produce too much and too frequent afternoon surface precipitation. This suggests that rather than issues with the triggering of deep convection, cloud radiative deficiencies are related to too weak convective cloud detrainment and too large precipitation efficiencies.

H. Masunaga and S. Bony. Radiative Invigoration of Tropical Convection by Preceding Cirrus Clouds. Journal of Atmospheric Sciences, 75:1327-1342, April 2018. [ bib | DOI | ADS link ]

C. J. Morcrette, K. Van Weverberg, H.-Y. Ma, M. Ahlgrimm, E. Bazile, L. K. Berg, A. Cheng, F. Cheruy, J. Cole, R. Forbes, W. I. Gustafson, M. Huang, W.-S. Lee, Y. Liu, L. Mellul, W. J. Merryfield, Y. Qian, R. Roehrig, Y.-C. Wang, S. Xie, K.-M. Xu, C. Zhang, S. Klein, and J. Petch. Introduction to CAUSES: Description of Weather and Climate Models and Their Near-Surface Temperature Errors in 5 day Hindcasts Near the Southern Great Plains. Journal of Geophysical Research (Atmospheres), 123:2655-2683, March 2018. [ bib | DOI | ADS link ]

We introduce the Clouds Above the United States and Errors at the Surface (CAUSES) project with its aim of better understanding the physical processes leading to warm screen temperature biases over the American Midwest in many numerical models. In this first of four companion papers, 11 different models, from nine institutes, perform a series of 5 day hindcasts, each initialized from reanalyses. After describing the common experimental protocol and detailing each model configuration, a gridded temperature data set is derived from observations and used to show that all the models have a warm bias over parts of the Midwest. Additionally, a strong diurnal cycle in the screen temperature bias is found in most models. In some models the bias is largest around midday, while in others it is largest during the night. At the Department of Energy Atmospheric Radiation Measurement Southern Great Plains (SGP) site, the model biases are shown to extend several kilometers into the atmosphere. Finally, to provide context for the companion papers, in which observations from the SGP site are used to evaluate the different processes contributing to errors there, it is shown that there are numerous locations across the Midwest where the diurnal cycle of the error is highly correlated with the diurnal cycle of the error at SGP. This suggests that conclusions drawn from detailed evaluation of models using instruments located at SGP will be representative of errors that are prevalent over a larger spatial scale.

H.-Y. Ma, S. A. Klein, S. Xie, C. Zhang, S. Tang, Q. Tang, C. J. Morcrette, K. Van Weverberg, J. Petch, M. Ahlgrimm, L. K. Berg, F. Cheruy, J. Cole, R. Forbes, W. I. Gustafson, M. Huang, Y. Liu, W. Merryfield, Y. Qian, R. Roehrig, and Y.-C. Wang. CAUSES: On the Role of Surface Energy Budget Errors to the Warm Surface Air Temperature Error Over the Central United States. Journal of Geophysical Research (Atmospheres), 123:2888-2909, March 2018. [ bib | DOI | ADS link ]

Many weather forecast and climate models simulate warm surface air temperature (T2m) biases over midlatitude continents during the summertime, especially over the Great Plains. We present here one of a series of papers from a multimodel intercomparison project (CAUSES: Cloud Above the United States and Errors at the Surface), which aims to evaluate the role of cloud, radiation, and precipitation biases in contributing to the T2m bias using a short-term hindcast approach during the spring and summer of 2011. Observations are mainly from the Atmospheric Radiation Measurement Southern Great Plains sites. The present study examines the contributions of surface energy budget errors. All participating models simulate too much net shortwave and longwave fluxes at the surface but with no consistent mean bias sign in turbulent fluxes over the Central United States and Southern Great Plains. Nevertheless, biases in the net shortwave and downward longwave fluxes as well as surface evaporative fraction (EF) are contributors to T2m bias. Radiation biases are largely affected by cloud simulations, while EF bias is largely affected by soil moisture modulated by seasonal accumulated precipitation and evaporation. An approximate equation based upon the surface energy budget is derived to further quantify the magnitudes of radiation and EF contributions to T2m bias. Our analysis ascribes that a large EF underestimate is the dominant source of error in all models with a large positive temperature bias, whereas an EF overestimate compensates for an excess of absorbed shortwave radiation in nearly all the models with the smallest temperature bias.

A. K. Behera, E. D. Rivière, V. Marécal, J.-F. Rysman, C. Chantal, G. Sèze, N. Amarouche, M. Ghysels, S. M. Khaykin, J.-P. Pommereau, G. Held, J. Burgalat, and G. Durry. Modeling the TTL at Continental Scale for a Wet Season: An Evaluation of the BRAMS Mesoscale Model Using TRO-Pico Campaign, and Measurements From Airborne and Spaceborne Sensors. Journal of Geophysical Research (Atmospheres), 123:2491-2508, March 2018. [ bib | DOI | ADS link ]

In order to better understand the water vapor (WV) intrusion into the tropical stratosphere, a mesoscale simulation of the tropical tropopause layer using the BRAMS (Brazilian version of Regional Atmospheric Modeling System (RAMS)) model is evaluated for a wet season. This simulation with a horizontal grid point resolution of 20 km × 20 km cannot resolve the stratospheric overshooting convection (SOC). Its ability to reproduce other key parameters playing a role in the stratospheric WV abundance is investigated using the balloon-borne TRO-Pico campaign measurements, the upper-air soundings over Brazil, and the satellite observations by Aura Microwave Limb Sounder, Microwave Humidity Sounder, and Geostationary Operational Environmental Satellite 12. The BRAMS exhibits a good ability in simulating temperature, cold-point, WV variability around the tropopause. However, the simulation is typically observed to be warmer by 2.0degC and wetter by 0.4 ppmv at the hygropause, which can be partly affiliated with the grid boundary nudging of the model by European Centre for Medium-Range Weather Forecasts operational analyses. The modeled cloud tops show a good correlation (maximum cross-correlation of 0.7) with Geostationary Operational Environmental Satellite 12. Furthermore, the overshooting cells detected by Microwave Humidity Sounder are observed at the locations, where 75% of the modeled cloud tops are higher than 11 km. Finally, the modeled inertia-gravity wave periodicity and wavelength are comparable with those deduced from the radio sounding measurements during TRO-Pico campaign. The good behavior of BRAMS confirms the SOC contribution in the WV abundance, and variability is of lesser importance than the large-scale processes. This simulation can be used as a reference run for upscaling the impact of SOC at a continental scale for future studies.

A. A. Wing, K. A. Reed, M. Satoh, B. Stevens, S. Bony, and T. Ohno. Radiative-convective equilibrium model intercomparison project. Geoscientific Model Development, 11:793-813, March 2018. [ bib | DOI | ADS link ]

RCEMIP, an intercomparison of multiple types of models configured in radiative-convective equilibrium (RCE), is proposed. RCE is an idealization of the climate system in which there is a balance between radiative cooling of the atmosphere and heating by convection. The scientific objectives of RCEMIP are three-fold. First, clouds and climate sensitivity will be investigated in the RCE setting. This includes determining how cloud fraction changes with warming and the role of self-aggregation of convection in climate sensitivity. Second, RCEMIP will quantify the dependence of the degree of convective aggregation and tropical circulation regimes on temperature. Finally, by providing a common baseline, RCEMIP will allow the robustness of the RCE state across the spectrum of models to be assessed, which is essential for interpreting the results found regarding clouds, climate sensitivity, and aggregation, and more generally, determining which features of tropical climate a RCE framework is useful for. A novel aspect and major advantage of RCEMIP is the accessibility of the RCE framework to a variety of models, including cloud-resolving models, general circulation models, global cloud-resolving models, single-column models, and large-eddy simulation models.

T. B. Richardson, P. M. Forster, T. Andrews, O. Boucher, G. Faluvegi, D. Fläschner, M. Kasoar, A. Kirkevâg, J.-F. Lamarque, G. Myhre, D. Olivié, B. H. Samset, D. Shawki, D. Shindell, T. Takemura, and A. Voulgarakis. Carbon Dioxide Physiological Forcing Dominates Projected Eastern Amazonian Drying. Geophysical Research Letters, 45:2815-2825, March 2018. [ bib | DOI | ADS link ]

Future projections of east Amazonian precipitation indicate drying, but they are uncertain and poorly understood. In this study we analyze the Amazonian precipitation response to individual atmospheric forcings using a number of global climate models. Black carbon is found to drive reduced precipitation over the Amazon due to temperature-driven circulation changes, but the magnitude is uncertain. CO2 drives reductions in precipitation concentrated in the east, mainly due to a robustly negative, but highly variable in magnitude, fast response. We find that the physiological effect of CO2 on plant stomata is the dominant driver of the fast response due to reduced latent heating and also contributes to the large model spread. Using a simple model, we show that CO2 physiological effects dominate future multimodel mean precipitation projections over the Amazon. However, in individual models temperature-driven changes can be large, but due to little agreement, they largely cancel out in the model mean.

M. Benetti, J.-L. Lacour, A. E. Sveinbjörnsdóttir, G. Aloisi, G. Reverdin, C. Risi, A. J. Peters, and H. C. Steen-Larsen. A Framework to Study Mixing Processes in the Marine Boundary Layer Using Water Vapor Isotope Measurements. Geophysical Research Letters, 45:2524-2532, March 2018. [ bib | DOI | ADS link ]

We propose a framework using water vapor isotopes to study mixing processes in the marine boundary layer (MBL) during quiescent conditions, where we expect evaporation to contribute to the moisture budget. This framework complements the existing models, by taking into account the changing isotopic composition of the evaporation flux (δe), both directly in response to the mixing and indirectly in response to mixing and surface conditions through variations in MBL humidity. The robustness of the model is demonstrated using measurements from the North Atlantic Ocean. This shows the importance of considering the δe variability simultaneous to the mixing of the lower free troposphere to the MBL, to simulate the MBL water vapor, whereas a mixing model using a constant δe fails to reproduce the data. The sensitivity of isotope observations to evaporation and shallow mixing further demonstrates how these observations can constrain uncertainties associated with these key processes for climate feedback predictions.

R. Wang, E. Andrews, Y. Balkanski, O. Boucher, G. Myhre, B. H. Samset, M. Schulz, G. L. Schuster, M. Valari, and S. Tao. Spatial Representativeness Error in the Ground-Level Observation Networks for Black Carbon Radiation Absorption. Geophysical Research Letters, 45:2106-2114, February 2018. [ bib | DOI | ADS link ]

There is high uncertainty in the direct radiative forcing of black carbon (BC), an aerosol that strongly absorbs solar radiation. The observation-constrained estimate, which is several times larger than the bottom-up estimate, is influenced by the spatial representativeness error due to the mesoscale inhomogeneity of the aerosol fields and the relatively low resolution of global chemistry-transport models. Here we evaluated the spatial representativeness error for two widely used observational networks (AErosol RObotic NETwork and Global Atmosphere Watch) by downscaling the geospatial grid in a global model of BC aerosol absorption optical depth to 0.1deg × 0.1deg. Comparing the models at a spatial resolution of 2deg × 2deg with BC aerosol absorption at AErosol RObotic NETwork sites (which are commonly located near emission hot spots) tends to cause a global spatial representativeness error of 30%, as a positive bias for the current top-down estimate of global BC direct radiative forcing. By contrast, the global spatial representativeness error will be 7% for the Global Atmosphere Watch network, because the sites are located in such a way that there are almost an equal number of sites with positive or negative representativeness error.

C. Kleinschmitt, O. Boucher, and U. Platt. Sensitivity of the radiative forcing by stratospheric sulfur geoengineering to the amount and strategy of the SO2injection studied with the LMDZ-S3A model. Atmospheric Chemistry & Physics, 18:2769-2786, February 2018. [ bib | DOI | ADS link ]

The enhancement of the stratospheric sulfate aerosol layer has been proposed as a method of geoengineering to abate global warming. Previous modelling studies found that stratospheric aerosol geoengineering (SAG) could effectively compensate for the warming by greenhouse gases on the global scale, but also that the achievable cooling effect per sulfur mass unit, i.e. the forcing efficiency, decreases with increasing injection rate. In this study we use the atmospheric general circulation model LMDZ with the sectional aerosol module S3A to determine how the forcing efficiency depends on the injected amount of SO2, the injection height, and the spatio-temporal pattern of injection. We find that the forcing efficiency may decrease more drastically for larger SO2 injections than previously estimated. As a result, the net instantaneous radiative forcing does not exceed the limit of -2 W m-2 for continuous equatorial SO2 injections and it decreases (in absolute value) for injection rates larger than 20 Tg S yr-1. In contrast to other studies, the net radiative forcing in our experiments is fairly constant with injection height (in a range 17 to 23 km) for a given amount of SO2 injected. Also, spreading the SO2 injections between 30o S and 30o N or injecting only seasonally from varying latitudes does not result in a significantly larger (i.e. more negative) radiative forcing. Other key characteristics of our simulations include a consequent stratospheric heating, caused by the absorption of solar and infrared radiation by the aerosol, and changes in stratospheric dynamics, with a collapse of the quasi-biennial oscillation at larger injection rates, which has impacts on the resulting spatial aerosol distribution, size, and optical properties. But it has to be noted that the complexity and uncertainty of stratospheric processes cause considerable disagreement among different modelling studies of stratospheric aerosol geoengineering. This may be addressed through detailed model intercomparison activities, as observations to constrain the simulations of stratospheric aerosol geoengineering are not available and analogues (such as volcanic eruptions) are imperfect.

E. Vignon, F. Hourdin, C. Genthon, B. J. H. Van de Wiel, H. Gallée, J.-B. Madeleine, and J. Beaumet. Modeling the Dynamics of the Atmospheric Boundary Layer Over the Antarctic Plateau With a General Circulation Model. Journal of Advances in Modeling Earth Systems, 10:98-125, January 2018. [ bib | DOI | ADS link ]

Observations evidence extremely stable boundary layers (SBL) over the Antarctic Plateau and sharp regime transitions between weakly and very stable conditions. Representing such features is a challenge for climate models. This study assesses the modeling of the dynamics of the boundary layer over the Antarctic Plateau in the LMDZ general circulation model. It uses 1 year simulations with a stretched-grid over Dome C. The model is nudged with reanalyses outside of the Dome C region such as simulations can be directly compared to in situ observations. We underline the critical role of the downward longwave radiation for modeling the surface temperature. LMDZ reasonably represents the near-surface seasonal profiles of wind and temperature but strong temperature inversions are degraded by enhanced turbulent mixing formulations. Unlike ERA-Interim reanalyses, LMDZ reproduces two SBL regimes and the regime transition, with a sudden increase in the near-surface inversion with decreasing wind speed. The sharpness of the transition depends on the stability function used for calculating the surface drag coefficient. Moreover, using a refined vertical grid leads to a better reversed “S-shaped” relationship between the inversion and the wind. Sudden warming events associated to synoptic advections of warm and moist air are also well reproduced. Near-surface supersaturation with respect to ice is not allowed in LMDZ but the impact on the SBL structure is moderate. Finally, climate simulations with the free model show that the recommended configuration leads to stronger inversions and winds over the ice-sheet. However, the near-surface wind remains underestimated over the slopes of East-Antarctica.

R. Séférian, S. Baek, O. Boucher, J.-L. Dufresne, B. Decharme, D. Saint-Martin, and R. Roehrig. An interactive ocean surface albedo scheme (OSAv1.0): formulation and evaluation in ARPEGE-Climat (V6.1) and LMDZ (V5A). Geoscientific Model Development, 11:321-338, January 2018. [ bib | DOI | ADS link ]

Ocean surface represents roughly 70 % of the Earth's surface, playing a large role in the partitioning of the energy flow within the climate system. The ocean surface albedo (OSA) is an important parameter in this partitioning because it governs the amount of energy penetrating into the ocean or reflected towards space. The old OSA schemes in the ARPEGE-Climat and LMDZ models only resolve the latitudinal dependence in an ad hoc way without an accurate representation of the solar zenith angle dependence. Here, we propose a new interactive OSA scheme suited for Earth system models, which enables coupling between Earth system model components like surface ocean waves and marine biogeochemistry. This scheme resolves spectrally the various contributions of the surface for direct and diffuse solar radiation. The implementation of this scheme in two Earth system models leads to substantial improvements in simulated OSA. At the local scale, models using the interactive OSA scheme better replicate the day-to-day distribution of OSA derived from ground-based observations in contrast to old schemes. At global scale, the improved representation of OSA for diffuse radiation reduces model biases by up to 80 % over the tropical oceans, reducing annual-mean model-data error in surface upwelling shortwave radiation by up to 7 W m-2 over this domain. The spatial correlation coefficient between modeled and observed OSA at monthly resolution has been increased from 0.1 to 0.8. Despite its complexity, this interactive OSA scheme is computationally efficient for enabling precise OSA calculation without penalizing the elapsed model time.

H. Benveniste, O. Boucher, C. Guivarch, H. Le Treut, and P. Criqui. Impacts of nationally determined contributions on 2030 global greenhouse gas emissions: uncertainty analysis and distribution of emissions. Environmental Research Letters, 13(1):014022, January 2018. [ bib | DOI | ADS link ]

Nationally Determined Contributions (NDCs), submitted by Parties to the United Nations Framework Convention on Climate Change before and after the 21st Conference of Parties, summarize domestic objectives for greenhouse gas (GHG) emissions reductions for the 2025-2030 time horizon. In the absence, for now, of detailed guidelines for the format of NDCs, ancillary data are needed to interpret some NDCs and project GHG emissions in 2030. Here, we provide an analysis of uncertainty sources and their impacts on 2030 global GHG emissions based on the sole and full achievement of the NDCs. We estimate that NDCs project into 56.8-66.5GtCO2eqyr-1 emissions in 2030 (90% confidence interval), which is higher than previous estimates, and with a larger uncertainty range. Despite these uncertainties, NDCs robustly shift GHG emissions towards emerging and developing countries and reduce international inequalities in per capita GHG emissions. Finally, we stress that current NDCs imply larger emissions reduction rates after 2030 than during the 2010-2030 period if long-term temperature goals are to be fulfilled. Our results highlight four requirements for the forthcoming climate regime: a clearer framework regarding future NDCs design, an increasing participation of emerging and developing countries in the global mitigation effort, an ambitious update mechanism in order to avoid hardly feasible decarbonization rates after 2030 and an anticipation of steep decreases in global emissions after 2030.

J.-L. Lacour, C. Risi, J. Worden, C. Clerbaux, and P.-F. Coheur. Importance of depth and intensity of convection on the isotopic composition of water vapor as seen from IASI and TES δD observations. Earth and Planetary Science Letters, 481:387-394, January 2018. [ bib | DOI | ADS link ]

We use tropical observations of the water vapor isotopic composition, derived from IASI and TES spaceborne measurements, to show that the isotopic composition of water vapor in the free troposphere is sensitive to both the depth and the intensity of convection. We find that for any given precipitation intensity, vapor associated with deep convection is isotopically depleted relative to vapor associated with shallow convection. The intensity of precipitation also plays a role as for any given depth of convection, the relative enrichment of water vapor decreases as the intensity of precipitation increases. Shallow convection, via the uplifting of enriched boundary layer air into the free troposphere and the convective detrainment, enriches the free troposphere. In contrast, deep convection is associated with processes that deplete the water vapor in the free troposphere, such as rain re-evaporation. The results of this study allow for a better identification of the parameters controlling the isotopic composition of the free troposphere and indicate that the isotopic composition of water vapor can be used to evaluate the relative contributions of shallow and deep convection in global models.

C. W. Stjern, H. Muri, L. Ahlm, O. Boucher, J. N. S. Cole, D. Ji, A. Jones, J. Haywood, B. Kravitz, A. Lenton, J. C. Moore, U. Niemeier, S. J. Phipps, H. Schmidt, S. Watanabe, and J. Egill Kristjánsson. Response to marine cloud brightening in a multi-model ensemble. Atmospheric Chemistry & Physics, 18:621-634, January 2018. [ bib | DOI | ADS link ]

Here we show results from Earth system model simulations from the marine cloud brightening experiment G4cdnc of the Geoengineering Model Intercomparison Project (GeoMIP). The nine contributing models prescribe a 50 % increase in the cloud droplet number concentration (CDNC) of low clouds over the global oceans in an experiment dubbed G4cdnc, with the purpose of counteracting the radiative forcing due to anthropogenic greenhouse gases under the RCP4.5 scenario. The model ensemble median effective radiative forcing (ERF) amounts to -1.9 W m-2, with a substantial inter-model spread of -0.6 to -2.5 W m-2. The large spread is partly related to the considerable differences in clouds and their representation between the models, with an underestimation of low clouds in several of the models. All models predict a statistically significant temperature decrease with a median of (for years 2020-2069) -0.96 [-0.17 to -1.21] K relative to the RCP4.5 scenario, with particularly strong cooling over low-latitude continents. Globally averaged there is a weak but significant precipitation decrease of -2.35 [-0.57 to -2.96] % due to a colder climate, but at low latitudes there is a 1.19 % increase over land. This increase is part of a circulation change where a strong negative top-of-atmosphere (TOA) shortwave forcing over subtropical oceans, caused by increased albedo associated with the increasing CDNC, is compensated for by rising motion and positive TOA longwave signals over adjacent land regions.