P. Peylin, J. Polcher, G. Bonan, D. L. Williamson, and K. Laval. Comparison of two complex land surface schemes coupled to the National Center for Atmospheric Research general circulation model. Journal of Geophysical Research, 102:19, August 1997. [ bib | DOI | ADS link ]
Two climate simulations with the National Center for Atmospheric Research general circulation model (version CCM2) coupled either to the Biosphere Atmosphere Transfer Scheme (BATS) or to Sechiba land surface scheme are compared. Both parameterizations of surface-atmosphere exchanges may be considered as complex but represent the soil hydrology and the role of vegetation in very different ways. The global impact of the change in land surface scheme on the simulated climate appears to be small. Changes are smaller than those obtained when comparing either one of these schemes to the fixed hydrology used in the standard CCM2. Nevertheless, at the regional scale, changing the land-surface scheme can have a large impact on the local climate. As one example, wre detail how circulation patterns are modified above the Tibetan plateau during the monsoon season. Elsewhere, mainly over land, changes can also be important. In the tropics, during the dry season, Sechiba produces warmer surface temperatures than does BATS. This warming arises from differences in the soil hydrology, both storage capacity and the dynamics of soil water transport. Over the Tundra biotype, the formulation of the transpiration induces significant differences in the energy balance.
S. Bony, K.-M. Lau, and Y. C. Sud. Sea Surface Temperature and Large-Scale Circulation Influences on Tropical Greenhouse Effect and Cloud Radiative Forcing. Journal of Climate, 10:2055-2077, August 1997. [ bib | DOI | ADS link ]
Two independent sets of meteorological reanalyses are used to investigate relationships between the tropical sea surface temperature (SST) and the large-scale vertical motion of the atmosphere for spatial and seasonal variations, as well as for El Niño/La Niña episodes of 1987-88. Supergreenhouse effect (SGE) situations are found to be linked to the occurrence of enhanced large-scale rising motion associated with increasing SST. In regions where the large-scale atmospheric motion is largely decoupled from the local SST due to internal or remote forcings, the SGE occurrence is weak. On seasonal and interannual timescales, such regions are found mainly over equatorial regions of the Indian Ocean and western Pacific, especially for SSTs exceeding 29.5degC. In these regions, the activation of feedback processes that regulate the ocean temperature is thus likely to be more related to the large-scale remote processes, such as those that govern the monsoon circulations and the low-frequency variability of the atmosphere, than to the local SST change.The relationships among SST, clouds, and cloud radiative forcing inferred from satellite observations are also investigated. In large-scale subsidence regimes, regardless of the SST range, the cloudiness, the cloud optical thickness, and the shortwave cloud forcing decrease with increasing SST. In convective regions maintained by the large-scale circulation, the strong dependence of both the longwave (LW) and shortwave (SW) cloud forcing on SST mainly results from changes in the large-scale vertical motion accompanying the SST changes. Indeed, for a given large-scale rising motion, the cloud optical thickness decreases with SST, and the SW cloud forcing remains essentially unaffected by SST changes. However, the LW cloud forcing still increases with SST because the detrainment height of deep convection, and thus the cloud-top altitude, tend to increase with SST. The dependence of the net cloud radiative forcing on SST may thus provide a larger positive climate feedback when the ocean warming is associated with weak large-scale circulation changes than during seasonal or El Niño variations.
R. D. Cess, M. H. Zhang, G. L. Potter, V. Alekseev, H. W. Barker, S. Bony, R. A. Colman, D. A. Dazlich, A. D. Del Genio, M. DéQué, M. R. Dix, V. Dymnikov, M. Esch, L. D. Fowler, J. R. Fraser, V. Galin, W. L. Gates, J. J. Hack, W. J. Ingram, J. T. Kiehl, Y. Kim, H. Le Treut, X.-Z. Liang, B. J. McAvaney, V. P. Meleshko, J. J. Morcrette, D. A. Randall, E. Roeckner, M. E. Schlesinger, P. V. Sporyshev, K. E. Taylor, B. Timbal, E. M. Volodin, W. Wang, W. C. Wang, and R. T. Wetherald. Comparison of the seasonal change in cloud-radiative forcing from atmospheric general circulation models and satellite observations. Journal of Geophysical Research, 102:16, July 1997. [ bib | DOI | ADS link ]
We compare seasonal changes in cloud-radiative forcing (CRF) at the top of the atmosphere from 18 atmospheric general circulation models, and observations from the Earth Radiation Budget Experiment (ERBE). To enhance the CRF signal and suppress interannual variability, we consider only zonal mean quantities for which the extreme months (January and July), as well as the northern and southern hemispheres, have been differenced. Since seasonal variations of the shortwave component of CRF are caused by seasonal changes in both cloudiness and solar irradiance, the latter was removed. In the ERBE data, seasonal changes in CRF are driven primarily by changes in cloud amount. The same conclusion applies to the models. The shortwave component of seasonal CRF is a measure of changes in cloud amount at all altitudes, while the longwave component is more a measure of upper level clouds. Thus important insights into seasonal cloud amount variations of the models have been obtained by comparing both components, as generated by the models, with the satellite data. For example, in 10 of the 18 models the seasonal oscillations of zonal cloud patterns extend too far poleward by one latitudinal grid.
G. Krinner, C. Genthon, Z.-X. Li, and P. Le van. Studies of the Antarctic climate with a stretched-grid general circulation model. Journal of Geophysical Research, 102:13, June 1997. [ bib | DOI | ADS link ]
A stretched-grid general circulation model (GCM), derived from the Laboratoire de Météorologie Dynamique (LMD) GCM is used for a multiyear high-resolution simulation of the Antarctic climate. The resolution in the Antarctic region reaches 100 km. In order to correctly represent the polar climate, it is necessary to implement several modifications in the model physics. These modifications mostly concern the parameterizations of the atmospheric boundary layer. The simulated Antarctic climate is significantly better in the stretched-grid simulation than in the regular-grid control run. The katabatic wind regime is well captured, although the winds may be somewhat too weak. The annual snow accumulation is generally close to the observed values, although local discrepancies between the simulated annual accumulation and observations remain. The simulated continental mean annual accumulation is 16.2 cm y-1. Features like the surface temperature and the temperature inversion over large parts of the continent are correctly represented. The model correctly simulates the atmospheric dynamics of the rest of the globe.
S. Bony, Y. Sud, K. M. Lau, J. Susskind, and S. Saha. Comparison and Satellite Assessment of NASA/DAO and NCEP-NCAR Reanalyses over Tropical Ocean: Atmospheric Hydrology and Radiation. Journal of Climate, 10:1441-1462, June 1997. [ bib | DOI | ADS link ]
This study compares the atmospheric reanalyses that have been produced independently at the Data Assimilation Office (DAO) of Goddard Laboratory for Atmospheres and at the National Centers for Environmental Prediction (NCEP). These reanalyses were produced by using a frozen state-of-the-art version of the global data assimilation system developed at these two centers. For the period 1987-88 and for the tropical oceanic regions of 30degS-30degN, surface and atmospheric fields related to atmospheric hydrology and radiation are compared and assessed, wherever possible, with satellite data. Some common biases as well as discrepancies between the two independent reassimilation products are highlighted.Considering both annual averages and interannual variability (1987-88), discrepancies between DAO and NCEP reanalysis in water vapor, precipitation, and clear-sky longwave radiation at the top of the atmosphere are generally smaller than discrepancies that exist between corresponding satellite estimates. Among common biases identified in the reanalyses, the authors note an underestimation of the total precipitable water and an overestimation of the shortwave cloud radiative forcing in warm convective regions. Both lead to an underestimation of the surface radiation budget. The authors also note an overestimaton of the clear-sky outgoing longwave radiation in most tropical ocean regions, as well as an overestimation of the longwave radiative cooling at the ocean surface.Surface latent and sensible heat fluxes differ by about 20 and 3 W m2, respectively, in the two reanalyses. Differences in the surface radiation budget are larger than the uncertainties of satellite-based estimates. Biases in the surface radiation fluxes derived from the reanalyses are primarily due to incorrect shortwave cloud radiative forcing and, to a lesser degree, due to a deficit in the total precipitable water and a cold bias at lower-tropospheric temperatures.This study suggests that individual features and biases of each set of reanalyses should be carefully studied, especially when using analyzed surface fluxes to force other physical or geophysical models such as ocean circulation models. Over large regions of the tropical oceans, DAO and NCEP reanalyses produce surface net heat fluxes that can differ by up to 50 W m2 in the average and by a factor of 2 when considering interannual anomalies. This may lead to vastly different thermal forcings for driving ocean circulations.
T. H. Chen, A. Henderson-Sellers, P. C. D. Milly, A. J. Pitman, A. C. M. Beljaars, J. Polcher, F. Abramopoulos, A. Boone, S. Chang, F. Chen, Y. Dai, C. E. Desborough, R. E. Dickinson, L. Dümenil, M. Ek, J. R. Garratt, N. Gedney, Y. M. Gusev, J. Kim, R. Koster, E. A. Kowalczyk, K. Laval, J. Lean, D. Lettenmaier, X. Liang, J.-F. Mahfouf, H.-T. Mengelkamp, K. Mitchell, O. N. Nasonova, J. Noilhan, A. Robock, C. Rosenzweig, J. Schaake, C. A. Schlosser, J.-P. Schulz, Y. Shao, A. B. Shmakin, D. L. Verseghy, P. Wetzel, E. F. Wood, Y. Xue, Z.-L. Yang, and Q. Zeng. Cabauw Experimental Results from the Project for Intercomparison of Land-Surface Parameterization Schemes. Journal of Climate, 10:1194-1215, June 1997. [ bib | DOI | ADS link ]
In the Project for Intercomparison of Land-Surface Parameterization Schemes phase 2a experiment, meteorological data for the year 1987 from Cabauw, the Netherlands, were used as inputs to 23 land-surface flux schemes designed for use in climate and weather models. Schemes were evaluated by comparing their outputs with long-term measurements of surface sensible heat fluxes into the atmosphere and the ground, and of upward longwave radiation and total net radiative fluxes, and also comparing them with latent heat fluxes derived from a surface energy balance. Tuning of schemes by use of the observed flux data was not permitted. On an annual basis, the predicted surface radiative temperature exhibits a range of 2 K across schemes, consistent with the range of about 10 W m2 in predicted surface net radiation. Most modeled values of monthly net radiation differ from the observations by less than the estimated maximum monthly observational error (10 W m2). However, modeled radiative surface temperature appears to have a systematic positive bias in most schemes; this might be explained by an error in assumed emissivity and by models' neglect of canopy thermal heterogeneity. Annual means of sensible and latent heat fluxes, into which net radiation is partitioned, have ranges across schemes of30 W m2 and 25 W m2, respectively. Annual totals of evapotranspiration and runoff, into which the precipitation is partitioned, both have ranges of 315 mm. These ranges in annual heat and water fluxes were approximately halved upon exclusion of the three schemes that have no stomatal resistance under non-water-stressed conditions. Many schemes tend to underestimate latent heat flux and overestimate sensible heat flux in summer, with a reverse tendency in winter. For six schemes, root-mean-square deviations of predictions from monthly observations are less than the estimated upper bounds on observation errors (5 W m2 for sensible heat flux and 10 W m2 for latent heat flux). Actual runoff at the site is believed to be dominated by vertical drainage to groundwater, but several schemes produced significant amounts of runoff as overland flow or interflow. There is a range across schemes of 184 mm (40% of total pore volume) in the simulated annual mean root-zone soil moisture. Unfortunately, no measurements of soil moisture were available for model evaluation. A theoretical analysis suggested that differences in boundary conditions used in various schemes are not sufficient to explain the large variance in soil moisture. However, many of the extreme values of soil moisture could be explained in terms of the particulars of experimental setup or excessive evapotranspiration.
V. Giraud, J. C. Buriez, Y. Fouquart, F. Parol, and G. Seze. Large-Scale Analysis of Cirrus Clouds from AVHRR Data: Assessment of Both a Microphysical Index and the Cloud-Top Temperature. Journal of Applied Meteorology, 36:664-675, June 1997. [ bib | DOI | ADS link ]
An algorithm that allows an automatic analysis of cirrus properties from Advanced Very High Resolution Radiometer (AVHRR) observations is presented. Further investigations of the information content and physical meaning of the brightness temperature differences (BTD) between channels 4 (11 m) and 5 (12 m) of the radiometer have led to the development of an automatic procedure to provide global estimates both of the cirrus cloud temperature and of the ratio of the equivalent absorption coefficients in the two channels, accounting for scattering effects. The ratio is useful since its variations are related to differences in microphysical properties. Assuming that cirrus clouds are composed of ice spheres, the effective diameter of the particle size distribution can be deduced from this microphysical index.The automatic procedure includes first, a cloud classification and a selection of the pixels corresponding to the envelope of the BTD diagram observed at a scale of typically 100 × 100 pixels. The classification, which uses dynamic cluster analysis, takes into account spectral and spatial properties of the AVHRR pixels. The selection is made through a series of tests, which also guarantees that the BTD diagram contains the necessary information, such as the presence of both cirrus-free pixels and pixels totally covered by opaque cirrus in the same area. Finally, the cloud temperature and the equivalent absorption coefficient ratio are found by fitting the envelope of the BTD diagram with a theoretical curve. Note that the method leads to the retrieval of the maximum value of the equivalent absorption coefficient ratio in the scene under consideration. This, in turn, corresponds to the minimum value of the effective diameter of the size distribution of equivalent Mie particles.The automatic analysis has been applied to a series of 21 AVHRR images acquired during the International Cirrus Experiment (ICE'89). Although the dataset is obviously much too limited to draw any conclusion at the global scale, it is large enough to permit derivation of cirrus properties that are statistically representative of the cirrus systems contained therein. The authors found that on average, the maximum equivalent absorption coefficient ratio increases with the cloud-top temperature with a jump between 235 and 240 K. More precisely, for cloud temperatures warmer than 235 K, the retrieved equivalent absorption coefficient ratio sometimes corresponds to very small equivalent spheres (diameter smaller than 20 m). This is never observed for lower cloud temperatures. This change in cirrus microphysical properties points out that ice crystal habits may vary from one temperature regime toanother. It may be attributed to a modification of the size and/or shape of the particles.
P. L. Read, M. Collins, F. Forget, R. Fournier, F. Hourdin, S. R. Lewis, O. Talagrand, F. W. Taylor, and N. P. J. Thomas. A GCM climate database for mars: for mission planning and for scientific studies. Advances in Space Research, 19:1213-1222, May 1997. [ bib | DOI | ADS link ]
The construction of a new database of statistics on the climate and environment of the Martian atmosphere is currently under way, with the support of the European Space Agency. The primary objectives of this database are to provide information for mission design specialists on the mean state and variability of the Martian environment in unprecedented detail, through the execution of a set of carefully validated simulations of the Martian atmospheric circulation using comprehensive numerical general circulation models. The formulation of the models used are outlined herein, noting especially new improvements in various schemes to parametrize important physical processes, and the scope of the database to be constructed is described. A novel approach towards the representation of large-scale variability in the output of the database using empirical eigenfunctions derived from statistical analyses of the numerical simulations, is also discussed. It is hoped that the resulting database will be of value for both scientific and engineering studies of Mars' atmosphere and near-surface environment.
K.-M. Lau, H.-T. Wu, and S. Bony. The Role of Large-Scale Atmospheric Circulation in the Relationship between Tropical Convection and Sea Surface Temperature. Journal of Climate, 10:381-392, March 1997. [ bib | DOI | ADS link ]
In this paper, the authors study the influence of the large-scale atmospheric circulation on the relationship between sea surface temperature (SST) and tropical convection inferred from outgoing longwave radiation (OLR). They find that under subsidence and clear sky conditions there is an increase in OLR with respect to SST at a rate of 1.8-2.5 Wm2 (degC)1. In regions of large-scale ascending motions, which is correlated to, but not always collocated with, regions of warm water, there is a large reduction of OLR with respect to SST associated with increase in deep convection. The rate of OLR reduction is found to be a strong function of the large-scale motion field. The authors find an intrinsic OLR sensitivity to SST of approximately 4 to 5 Wm2 (degC)1 in the SST range of 27deg-28degC, under conditions of weak large-scale circulation. Under the influence of strong ascending motion, the rate can be increased to 15 to 20 Wm2 (degC)1 for the same SST range. The above OLR-SST relationships are strongly dependent on geographic locations. On the other hand, deep convection and large-scale circulation exhibit a nearly linear relationship that is less dependent on SST and geographic locations.The above results are supported by regression analyses. In addition, they find that on interannual timescales, the relationship between OLR and SST is dominated by the large-scale circulation and SST changes associated with the El Niño-Southern Oscillation. The relationship between anomalous convection and local SST is generally weak everywhere except in the equatorial central Pacific, where large-scale circulation and local SST appear to work together to produce the observed OLR-SST sensitivity. Over the equatorial central Pacific, approximately 45%-55% of the OLR variance can be explained by the large-scale circulation and 15%-20% by the local SST.Their results also show that there is no fundamental microphysical or thermodynamical significance to the so-called SST threshold at approximately 27degC, except that it represents a transitional SST between clear-sky/subsiding and convective/ascending atmospheric conditions. Depending on the ambient large-scale motion associated with basin-scale SST distribution, this transitional SST can occur in a range from 25.5deg to 28degC. Similarly, there is no magic to the 29.5degC SST, beyond which convection appears to decrease with SST. The authors find that under the influence of strong large-scale rising motion, convection does not decrease but increases monotonically with SST even at SST higher than 29.5degC. The reduction in convection is likely to be influenced by large-scale subsidence forced by nearby or remotely generated deep convection.
R. Roca, L. Picon, M. Desbois, H. Le Treut, and J.-J. Morcrette. Direct comparison of meteosat water vapor channel data and general circulation model results. Geophysical Research Letters, 24:147-150, 1997. [ bib | DOI | ADS link ]
Following a model to satellite approach, this study points out the ability of the general circulation model (GCM) of the Laboratoire de Météorologie Dynamique to reproduce the observed relationship between tropical convection and subtropical moisture in the upper troposphere. Those parameters are characterized from Meteosat water vapor equivalent brightness temperatures (WVEBT) over a monthly scale. The simulated WVEBT field closely resembles to the observed distribution. The pure water vapor features and the convective areas are well located and their seasonal variations are captured by the model. A dry (moist) bias is found over convective (subsiding) areas, whereas the model globally best acts over Atlantic ocean than over Africa. The observed and simulated seasonal variations show that an extension of the ITCZ is correlated to a moistening of the upper troposphere in subtropical areas. Those results imply a positive large scale relationship between convective and subsiding areas in both observation and simulation, and suggest the relevance of our approach for further climatic studies.
Z.-X. Li, K. Ide, H. L. Treut, and M. Ghil. Atmospheric radiative equilibria in a simple column model. Climate Dynamics, 13:429-440, 1997. [ bib | DOI | ADS link ]
An analytic radiative-equilibrium model is formulated where both short- and longwave radiation are treated as two-stream (down- and upward) fluxes. An equilibrium state is defined in the model by the vertical temperature profile. The sensitivity of any such state to the model atmosphere's optical properties is formulated analytically. As an example, this general formulation is applied to a single-column 11-layer model, and the model's optical parameters are obtained from a detailed radiative parametrization of a general circulation model. The resulting simple column model is then used to study changes in the Earth-atmosphere system's radiative equilibrium and, in particular, to infer the role of greenhouse trace gases, water vapor and aerosols in modifying the vertical temperature profile. Multiple equilibria appear when a positive surface-albedo feedback is introduced, and their stability is studied. The vertical structure of the radiative fluxes (both short- and longwave) is substantially modified as the temperature profile changes from one equilibrium to another. These equilibria and their stability are compared to those that appear in energy-balance models, which heretofore have ignored the details of the vertical temperature and radiation profiles.