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G. Szejwach and M. Desbois

Abstract

Cumulative histograms taken from eight consecutive infrared pictures of GOES 1 are used to study the spatio-temporal evolution of mesoscale cloud patterns. A model is proposed to approximate the volume of cloud lying in a particular area. Then, the cloud volume variation between successive pictures in the same area is deduced. This volume variation is used to define a cloud development index. Development or dissipation of cloud formations can be distinguished using this index.

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D. Ramond, H. Corbin, M. Des̄bois, G. Szejwach, and P. Waldteufel

Abstract

A polar jet stream flowing over northern Europe is investigated using both the radiosounding network and the METEOSAT water vapor (WV) channel imagery. A maximum radiance band associated with the polar jet stream is shown to accurately delineate the tropopause break; a major difference in the subtropical jet stream radiative signature is noted, however. An analytic approximation of the radiative transfer equation is established for the METEOSAT WV channel and leads to the conclusion that the radiance field, away from the polar frontal zones, is representative of the temperature field on a water vapor isosteric surface.

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L. A. Toledo Machado, M. Desbois, and J-Ph Duvel

Abstract

The structural properties of convective cloud clusters of tropical Africa and the Atlantic Ocean are studied using six summers of Meteosat satellite data in the atmospheric infrared window. A cluster at a given brightness temperature threshold is defined as the area covered by adjacent cloud cells with brightness temperature lower than the threshold. The clusters are classified according to the area they cover and the position of their center of mass. Results show that the convective cluster number can be approximated by a power law of the radius with an exponent around −2. This gives a nearly equal contribution of each cluster size to the mean high cloud cover for a given brightness temperature threshold. Using the visible channel (0.4–1.1 μm) of Meteosat, we show that the part of the clusters with reflectance larger than 0.7 also follows a power law.

The cluster-size distributions remain similar for different subregions and seasons, even if they are subject to large variations in the mean cloudiness. We further inspect the relatively large diurnal and interdiurnal variations of the cluster-size distribution. We also look at the variations of the cluster-size distribution as a function of a vertical extension, defined as the lowest brightness temperature reached by the cluster. We find preferential sizes that increase as the vertical extension increases. We also show that the distance between clusters, defined as the minimum distance between clouds of the same size, also follows a well-defined distribution.

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L. A. Toledo Machado, J-Ph Duvel, and M. Desbois

Abstract

Using Metecosat satellite data in the atmospheric infrared window, the authors study short time-scale fluctuations of the size distribution of tropical convective cloud clusters for July to September 1989. A cluster at a given brightness-temperature threshold (T IR) is defined as the area covered by adjacent cloudy pixels with brightness temperature lower than the threshold. The clusters are classified according to the area they cover and the position of their center of mass.

Over land regions of West Africa, the size distribution undergoes a very coherent diurnal behavior with development of small cells between noon and 1500 LST that later grow or merge into larger clusters. Over the Atlantic Ocean, the highest cloudiness has a weak maximum extent in early morning, while cloudiness at lower levels (but with infrared brightness temperature T IR < 253 K) is more extended in the afternoon. This diurnal behavior is primarily due to large cloud clusters (r > 100 km at T IR = 218 K), suggesting that the diurnal variation over the ocean results mainly from internal variations of large convective systems and not from the initiation of convection at a given hour of the day. This is confirmed by the analysis of 15 large convective systems propagating over the ocean.

In agreement with previous studies, we find that the high cloud cover is maximum within the trough of easterly waves. At midlevel (T IR = 253 K), these waves modulate mostly the number of clusters with radii larger than 200 km. At colder levels (T IR = 218 K), while the wave modulates the number of clusters at all sizes, the clusters are organized at larger scale within the trough. The cluster size also depends on the wave amplitude with larger mean cluster size when the amplitude is larger. These results show that over the ITCZ, the trough phase of the wave more promotes the development of large clusters than it favors the initial stage of the convection.

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M. Legrand, J. J. Bertrand, M. Desbois, L. Menenger, and Y. Fouquart

Abstract

Optical depth of Saharan dust derived from photometric measurements made during the dry season at a Sahelian site (Niamey, Republic of Niger) is compared with METEOSAT-2 radiance in the 10.5–12.5 μm channel for different times of the daily cycle. The ability of retrieving dual optical depth using the outgoing radiance of infrared atmospheric window is clearly demonstrated for the middle of the day. Results obtained with nighttime data through a relation between dust optical depth and visibility are also discussed. The major causes of error are identified and quantitatively estimated.

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C. J. Stubenrauch, G. Seze, N. A. Scott, A. Chedin, M. Desbois, and R. S. Kandel

Abstract

Gaining a better understanding of the influence of clouds on the earth's energy budget requires a cloud classification that takes into account cloud height, thickness, and cloud cover. The radiometer ScaRaB (scanner for radiation balance), which was launched in January 1994, has two narrowband channels (0.5–0.7 and 10.5–12.5 µm) in addition to the two broadband channels (0.2–4 and 0.2–50 µm) necessary for earth radiation budget (ERB) measurements in order to improve cloud detection. Most automatic cloud classifications were developed with measurements of very good spatial resolution (200 m to 5 km). Earth radiation budget experiments (ERBE), on the hand, work at a spatial resolution of about 50 km (at nadir), and therefore a cloud field classification adapted to this scale must be investigated. For this study, ScaRaB measurements are simulated by collocated Advanced Very High Resolution Radiometer (AVHRR) ERBE data. The best-suited variables for a global cloud classification are chosen using as a reference cloud types determined by an operationally working threshold algorithm applied to AVHRR measurements at a reduced spatial resolution of 4 km over the North Atlantic. Cloud field types are then classified by an algorithm based on the dynamic clustering method. More recently, the authors have carried out a global cloud field identification using cloud parameters extracted by the 3I (improved initialization inversion) algorithm, from High-Resolution Infrared Sounder (HIRS)-Microwave Sounding Unit (MSU) data. This enables the authors first to determine mean values of the variables best suited for cloud field classification and then to use a maximum-likelihood method for the classification. The authors find that a classification of cloud fields is still possible at a spatial resolution of ERB measurements. Roughly, one can distinguish three cloud heights and two effective cloud amounts (combination of cloud emissivity and cloud cover). However, only by combining flux measurements (ERBE) with cloud field classifications from sounding instruments (HIRS/MSU) can differences in radiative behavior of specific cloud fields be evaluated accurately.

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W. B. Rossow, F. Mosher, E. Kinsella, A. Arking, M. Desbois, E. Harrison, P. Minnis, E. Ruprecht, G. Seze, C. Simmer, and E. Smith

Abstract

The International Satellite Cloud Climatology Project (ISCCP) will provide a uniform global climatology of satellite-measured radiances and derive an experimental climatology of cloud radiative properties from these radiances. A pilot study to intercompare cloud analysis algorithms was initiated in 1981 to define a state-of-the-art algorithm for ISCCP. This study compared the results of applying six different algorithms to the same satellite radiance data. The results show that the performance of all current algorithms depends on how accurately the clear sky radiances are specified; much improvement in results is possible with better methods for obtaining these clear-sky radiances. A major difference between the algorithms is caused by their sensitivity to changes in the cloud size distribution and optical properties: all methods, which work well for some cloud types or climate regions, do poorly for other situations. Therefore, the ISCCP algorithm is composed of a series of steps, each of which is designed to detect some of the clouds present in the scene. This progressive analysis is used to retrieve an estimate of the clear sky radiances corresponding to each satellite image. Application of a bispectral threshold is then used as the last step to determine the cloud fraction. Cloudy radiances are interpreted in terms of a simplified model of cloud radiative effects to provide some measure of cloud radiative properties. Application of this experimental algorithm to produce a cloud climatology and field observation programs to validate the results will stimulate further research on cloud analysis techniques as part of ISCCP.

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