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1998 .

(4 publications)

M. Doutriaux-Boucher, J. Pelon, V. Trouillet, G. SèZe, H. Le Treut, P. Flamant, and M. Desbois. Simulation of satellite lidar and radiometer retrievals of a general circulation model three-dimensional cloud data set. Journal of Geophysical Research, 103:26025, October 1998. [ bib | DOI | ADS link ]

The inclusion of a backscatter lidar on a space platform for a radiation mission, as proposed by various space agencies, aims to bring new information on three-dimensional cloud distribution, with a special emphasis on optically thin cirrus clouds, which are presently poorly detected by passive sensors. Key issues for such cloud observational studies are the detection of multilayered cloud systems, thin cirrus, and fractional cloud cover, knowledge that would improve our understanding of the global radiation budget. To assess the impact of such lidar measurements on cloud climatology, a 1 month cloud data set has been simulated with a general circulation model (GCM). The cloud detection capability of a spaceborne scanning backscatter lidar is assessed with the use of two detection schemes, one based on limitations in the detected cloud optical depth and the other based on lidar signal-to-noise ratio. The cloud information retrieved from passive radiometric measurements using a procedure like that used in the International Satellite Cloud Climatology Project is also simulated from the same GCM cloud data set. It is shown that a spaceborne backscatter lidar can improve significantly the retrieval of thin cirrus clouds as well as underlying cloud layers. High-level cloud retrieval from a spaceborne lidar therefore appears as a powerful complement to radiometric measurements for improving our knowledge of actual cloud climatology.

O. P. Sharma, H. Le Treut, G. Sèze, L. Fairhead, and R. Sadourny. Interannual Variations of Summer Monsoons: Sensitivity to Cloud Radiative Forcing. Journal of Climate, 11:1883-1905, August 1998. [ bib | DOI | ADS link ]

The sensitivity of the interannual variations of the summer monsoons to imposed cloudiness has been studied with a general circulation model using the initial conditions prepared from the European Centre for Medium-Range Forecasts analyses of 1 May 1987 and 1988. The cloud optical properties in this global model are calculated from prognostically computed cloud liquid water. The model successfully simulates the contrasting behavior of these two successive monsoons. However, when the optical properties of the observed clouds are specified in the model runs, the simulations show some degradation over India and its vicinity. The main cause of this degradation is the reduced land-sea temperature contrast resulting from the radiative effects of the observed clouds imposed in such simulations. It is argued that the high concentration of condensed water content of clouds over the Indian land areas will serve to limit heating of the land, thereby reducing the thermal contrast that gives rise to a weak Somali jet. A countermonsoon circulation is, therefore, simulated in the vector difference field of 850-hPa winds from the model runs with externally specified clouds. This countermonsoon circulation is associated with an equatorial heat source that is the response of the model to the radiative effects of the imposed clouds. Indeed, there are at least two clear points that can be made: 1) the cloud-SST patterns, together, affect the interannual variability; and 2) with both clouds and SST imposed, the model simulation is less sensitive to initial conditions. Additionally, the study emphasizes the importance of dynamically consistent clouds developing in response to the dynamical, thermal, and moist state of the atmosphere during model integrations.

M. Doutriaux-Boucher and G. Sèze. Significant changes between the ISCCP C and D cloud climatologies. Geophysical Research Letters, 25:4193-4196, 1998. [ bib | DOI | ADS link ]

We analyse one year of cloud data from the ISCCP C and D datasets. The two datasets differ by their retrieval algorithms and their definitions of the cloud types defined from the cloud top pressure and cloud optical depth. The differences between the two datasets are first described in terms of the total cloud cover, as well as its repartition in low, middle, and high level cloudiness. We also project the ISCCP C cloud classes into the ISCCP D cloud types to circumvent the problem of different cloud type definitions in the two datasets. The differences between the two datasets are then also investigated in terms of the most frequent cloud type.

C. Vanbauce, J. C. Buriez, F. Parol, B. Bonnel, G. Sèze, and P. Couvert. Apparent pressure derived from ADEOS-POLDER observations in the oxygen A-band over ocean. Geophysical Research Letters, 25:3159-3162, 1998. [ bib | DOI | ADS link ]

The POLDER radiometer was on board the ADEOS satellite from August 1996 to June 1997. This instrument measures radiances in eight narrow spectral bands of the visible and near infrared spectrum. Two of them are centered on the O2 A-band in order to infer cloud pressure. By assuming the atmosphere behaves as a pure absorbing medium overlying a perfect reflector, an apparent pressure Papp is derived from POLDER data. For validation purposes, Papp is first compared to the sea-surface pressure Ps for clear-sky conditions; Papp is found to be close to Ps (within ~30 hPa) for measurements in the sunglint region. For overcast conditions, Papp differs from the cloud-top pressure mainly because of multiple scattering inside the cloud. When Papp is compared to the cloud pressure determined from brightness temperature measurements, large differences are observed (typically 180 hPa).

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