1991 .

(9 publications)

F. Cheruy, R. S. Kandel, and J. P. Duvel. Outgoing longwave radiation and its diurnal variations from combined Earth Radiation Budget Experiment and Meteosat observations :2. Using Meteosat data to determine the longwave diurnal cycle. Journal of Geophysical Research, 96:22, December 1991. [ bib | DOI | ADS link ]

For April and July 1985, applying the narrow to broadband conversion of part 1 to Meteosat observations obtained at 3-hour intervals (ISCCP B2 data), we determine the monthly mean radiant exitance as well as the mean diurnal variation, over 2.5deg×2.5deg latitude-longitude regions of tropical Africa and the neighboring Atlantic Ocean. We compare these determinations with those obtained directly from the ERBS and NOAA 9 Earth Radiation Budget Experiment (ERBE) data for this month, for which the time sampling is sparser and not so uniform. Excellent agreement is obtained in most cases, in particular for the overall monthly means. However, for the monthly mean diurnal variation there are situations in which the ERBE time sampling, the nature of the ERBE diurnal modeling scheme and the convolution of weather system changes with the diurnal cycle, combine to produce significant differences between the ERBE determination and the Meteosat result. These differences would mostly have been much smaller had the third ERBE instrument package been in operation (at 0730/1930 LT) as originally planned. We consider possible improvements in diurnal interpolation procedures, but note that there is no general way to remove bias resulting from inadequate time sampling.

F. Cheruy, R. S. Kandel, and J. P. Duvel. Outgoing longwave radiation and its diurnal variation from combined ERBE and Meteosat observations: 1. Estimating OLR from Meteosat data. Journal of Geophysical Research, 96:22, December 1991. [ bib | DOI | ADS link ]

Radiative transfer calculations show that measurements in two relatively narrow spectral domains, corresponding to the atmospheric infrared window and to a band of water vapor absorption, can account for nearly all of the variance of outgoing longwave radiation integrated over the entire thermal spectrum. Statistical analyses of simultaneous colocated Meteosat radiance and Earth Radiation Budget Experiment (ERBE) radiant exitance data, for the months of November 1984 and April and July 1985, yield regression coefficients for estimating longwave radiant exitance MLW from radiances observed in the Meteosat infrared (IR) window and water vapor (WV) channels, with root-mean-square errors of the order of or less than 10 W m-2. The coefficients thus obtained are in good agreement with coefficients relating MLW, IR and WV from analysis of results of radiative transfer calculations. Meteosat data can then be used to estimate MLW at times not sampled by ERBE. Although some biases appear to exist over areas of heterogeneous cloud cover, they should not significantly affect studies of strong diurnal variations.

W. Yu, G. Sèze, H. Le Treut, and M. Desbois. Comparison of radiance fields observed by satellite and simulated by the LMD general circulation model. Dynamics of Atmospheres and Oceans, 16:147-165, October 1991. [ bib | DOI | ADS link ]

A time series of International Satellite Cloud Climatology Project (ISCCP) B2 data has been used to study the spatio-temporal variability of cloud radiance fields and to compare it with similar diagnostics obtained from a numerical simulation with the Laboratoire de Météorologie Dynamique general circulation model (GCM). We first compare zonal means of the observed and simulated fluxes: the largest differences appear mainly above land, probably due to a diurnal cycle effect. A computation of the cloud radiative forcing is made to distinguish model errors in the clear sky or in cloudy areas: it shows that the radiative effect of the simulated clouds is generally smaller than that of observed ones. The influence of the spatial resolution on the variability of the visible and infrared radiance fields is examined. The results show that the variability due to the better spatial resolution of the satellite data cannot be simulated by the GCM; however, the variability of the simulated radiances is comparable to that of the satellite data when they are spatially averaged on the GCM mesh. The temporal variability of the spatially averaged observed and simulated radiances has a similar spatial distribution but the model results are slightly smaller. The memory of the precedent state shown by the autocorrelation function is longer for the GCM than for the observation. However, the periods obtained by a spectral analysis in the mid-latitude regions are approximately the same in the observation and the simulation.

F. Cheruy and R. S. Kandel. Use of meteosat data for validation of the diurnal variation of the outgoing longwave radiation produced by ERBE. Dynamics of Atmospheres and Oceans, 16:73-84, October 1991. [ bib | DOI | ADS link ]

Radiative transfer calculations have been performed to demonstrate the usefulness of the Meteosat observations in the relative narrow-band of the water vapour absorption (WV, 5.7-7.1 μm) in addition to the observations in the atmospheric infrared window (IR, 10.5-12.5 μm) to deduce the integrated thermal outgoing longwave radiation (OLR). A statistical analysis of colocated and nearly simultaneous Meteosat and the Earth Radiation Budget Experiment (ERBE) data has yielded regression coefficients for estimating the OLR with Meteosat data during the months of April and July 1985. These results have been used to study the mean diurnal variation of the outgoing longwave radiation. The results show that in some cases, because of inadequate time sampling, the form (and especially the phase) of the longwave (LW) diurnal cycle is incorrectly determined by ERBE, but that Meteosat data can improve the determination. In nearly all cases, such errors have little or no influence on the determination of monthly mean LW flux fields.

R. S. Foster, L. Fairhead, and D. C. Backer. A spectral study of four millisecond pulsars. Astrophysical Journal, 378:687-695, September 1991. [ bib | DOI | ADS link ]

Flux density measurements of four millisecond pulsars, PSR 1620-26, 1821-24, PSR 1855 + 09, and PSR 1937 + 21, have been made to determine their spectral indices in the frequency range between 425 MHz and 3 GHz. The four objects are shown to have indices that range from -1.3 to -2.6. The luminosities of these four pulsars are spread over nearly three orders of magnitude. An analytic pulse component model is developed for each object. Individual components are allowed to have different spectral indices and hence different component ratios as a function of frequency. Component separations are evaluated as a function of frequency. The analytic models are used to determine dispersion measures with a precision better than 0.01 pc/cu cm for each object.

G. Sèze and W. B. Rossow. Effects of satellite data resolution on measuring the space/time variations of surfaces and clouds. International Journal of Remote Sensing, 12:921-952, May 1991. [ bib | DOI | ADS link ]

G. Sèze and W. B. Rossow. Time-cumulated visible and infrared radiance histograms used as descriptors of surface and cloud variations. International Journal of Remote Sensing, 12:877-920, May 1991. [ bib | DOI | ADS link ]

H. Le Trent and Z.-X. Li. Sensitivity of an atmospheric general circulation model to prescribed SST changes: feedback effects associated with the simulation of cloud optical properties. Climate Dynamics, 5:175-187, March 1991. [ bib | DOI | ADS link ]

L. Fairhead. Astrometric analysis of timing observations of the fast pulsar PSR 1937 + 214. Astronomy Astrophysics, 241:289-296, January 1991. [ bib | ADS link ]

A new analysis of the first two years of timing data acquired on the fast pulsar PSR 1937+214 at Arecibo is presented. Parameters are evaluated with various models based on two ephemerides, two atomic time scales and two TT-TB time transformations and comparison is carried out with previously published results. We provide evidence that systematic errors induced by the model adopted are 5 to 10 times larger than the formal uncertainties calculated by the fitting procedure. One must therefore exercise great care in the reduction method when comparing results from different timing programs for precise astrometric applications.