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Michel Desbois

Abstract

A data set of 56,000 winds determined from the horizontal displacements of EOLE balloons at the 200 mb level in the Southern Hemisphere during the period October 1971–February 1972 is utilized for the computation of planetary- and synoptic-scale kinetic energy space spectra. However, the random distribution of measurements in space and time presents some problems for the spectral analysis. Two different approaches are used, i.e., a harmonic analysis of daily .wind values at equi-distant points obtained by space-time interpolation of the data, and a correlation method using the direct measurements. Both methods give similar results for small wavenumbers, but the second is more accurate for higher wavenumbers (k≥10). The spectra show a maximum at wavenumbers 5 and 6 due to baroclinic instability and then decrease for high wavenumbers up to wavenumber 35 (which is the limit of the analysis), according to a k p law with p close to 3.

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Daniel Cadet
and
Michel Desbois

Abstract

Low-level wind fields over the western Indian Ocean west of 65°E have been determined from the analysis of METEOSAT images from 10 to 22 May 1978 (one field every two days). The data show the abrupt change in the low-level airflow circulation which takes place over the western Indian Ocean during the burst of the monsoon. The burst of the monsoon along the African coast is characterized by the sudden establishment of the Somali low-level jet and associated strong cross-equatorial flow. This modification which augurs the beginning of the monsoon season over India is closely related to the first rainfalls over the southern part of the western coast of India which occur with a lag of 3–4 days.

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Daniel Cadet
and
Michel Desbois

Abstract

A fluctuation in the intensity of the Somali low-level jet flowing along the East African coast during the Indian summer monsoon is studied. It is shown that this variation of the intensity of cross-equatorial flow in western Indian Ocean results from the interaction between extratropical perturbations moving eastward at midlatitudes of the Southern Hemisphere and southeast trade winds of the monsoon circulation over the southern equatorial Indian Ocean.

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Genevieve Seze
and
Michel Desbois

Abstract

New developments of a cloud classification scheme based on histogram clustering by a statistical method are presented. Use of time series of geostationary satellite pictures as well as for construction of composite images representative of the surface properties and then for the identification of significative cloud classes is discussed. Spatial variances are introduced as additional parameters of the classification, with the aim to better separate clouds from the surface and the different kinds of more or less homogeneous cloud classes.

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Maud Roulleau
and
Michel Desbois

Abstract

The wind correction to the evaporation rate of electrically suspended water droplets has been measured for Reynolds numbers between 1 and 6. The results agree well with those of Kinzer and Gunn. During evaporation, Rayleigh instabilities have also been observed. At instability, the charge variation ΔQ is of the same order as that observed by other workers, but under the present experimental conditions there is no detectable change of mass.

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Michel Desbois
and
Françoise Désalmand

A summary of the North Atlantic Treaty Organization's Advanced Research Workshop Global Precipitations and Climate Change that reviewed the current understanding of global precipitation distribution, a s related to climate evolution, on various timescales, is presented. The workshop, which took place at L'Agelonde, France, 27 September–1 October 1993, was held to assess the availability and quality of data that could be used to improve the understanding of the processes determining precipitation changes through initialization and validation of existing climate models. Furthermore, several examples of the use of precipitation data were presented. A summary of the recommendations for future studies, which arose from the exchanges between the modeling and observational communities, is included.

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Laurence Picon
and
Michel Desbois

Abstract

Mean monthly images from the water vapor channel of METEOSAT characteristically contain large-scale spatial structures, especially in tropical regions. The aim of this paper is to establish connections between these structures and large-scale circulation features. For this purpose, statistical relationships between radiances and some meteorological parameters provided by ECMWF analyses are studied.

Temporal correlations are computed for two sizes of regions, in order to compare temporal changes associated with both large-scale circulations and smaller scale systems. The correlations obtained are poor, suggesting that the chosen parameters are not well related at short time scales.

Temporal averages appear more suitable for these comparisons. As expected, the mean relative humidity yields the best correlation with the mean water vapor radiances. A (weaker) relationship exists also with mean dynamic fields: large water vapor radiances are almost always related to subsidence in the middle troposphere, divergence near the surface, and convergence in the upper troposphere. However, there is regional variability in the results., one explanation may be different contributions from horizontal advecion and vertical motions to the humidity of the middle troposphere.

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Michel Legrand
,
Michel Desbois
, and
Kwami Vovor

Abstract

The data of the ECLATS experiment (at Niamey, 1980) are used to study the daily variations of the sensitivity of ground temperature to atmospheric dustiness. In the presence of dust, ground temperature is lower during daytime and higher than normal during nighttime. During night, the influence of dust on ground temperature prevails over other factors. The sensitivity of temperature to dustiness is evaluated and used jointly with ECLATS aircraft soundings to assess the impact of atmospheric dust on the radiance measured by the METEOSAT-I thermal infrared sensor. The results obtained demonstrate the ability of METEOSAT to detect dust clouds during nighttime. A method of optimized imaging is described and applied to a case study.

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Yves Arnaud
,
Michel Desbois
, and
Joel Maizi

Abstract

With the objective of budding climatological statistics on propagating African convective systems and extracting pertinent parameters for the evaluation of precipitation an automatic method of tracking clouds with cold tops on Meteosat infrared (IR) images is established. This method takes into account eventual separation or merging of clouds. Results obtained in several cases not only demonstrate the capability of the method to perform correct tracking in different situations, but also show that objective determination of parameters, such as propagation speeds, system area, and volume index, is possible. The analysis of the time evolution of these last parameters allows a clear characterization of the cloud life cycle with its growing and decreasing stages, which may be useful for improving precipitation-estimation methods based only on cold-cloud occurrences or cloud-top temperatures.

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Michel Desbois
,
Genevieve Seze
, and
Gerard Szejwach

Abstract

A statistical classification method based on clustering on three-dimensional histograms is applied to the three channels of the METEOSAT imagery [Visible (VIS)–Infrared Window (IR)–Infrared Water Vapor (WV)]. The results of this classification are studied for different cloud cover cases over tropical regions. For high-level cloud classes, it is shown that the bidimensional histogram IR-WV allows one to deduce the cloud top temperature even for semi-transparent clouds.

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