F. DéSalmand, A. Szantai, L. Picon, and M. Desbois. Systematic observation of westward propagating cloud bands over the Arabian Sea during Indian Ocean Experiment (INDOEX) from Meteosat-5 data. Journal of Geophysical Research (Atmospheres), 108:8004, September 2003. [ bib | DOI | ADS link ]
During the field experiment phase of the Indian Ocean Experiment (INDOEX), linear cloud formations parallel to the West Indian coast and propagating westward have been observed. Meteosat-5 images are used for the description of the life cycle of these events. Single cloud bands, or main cloud bands followed by (up to 10) secondary parallel cloud lines with a wavelike pattern, have been observed daily during four periods in the dry season of 1999 (15 January, 16-19 February, 27 February to 7 March, and 1-3 April 1999). During these four periods, one single cloud band or a packet of cloud bands is generated every day at nighttime. Their length reaches several hundreds of kilometers, their width lies below 40 km, and their duration in some cases exceeds 24 hours. The smaller cloud lines observed behind the leading cloud line are narrower and are separated by a distance of 2-10 km. Cloud tops are about 3-8degC colder than the Arabian Sea, corresponding to an altitude between 500 and 1200 m during the night and below 2500 m during daytime. Cloud bands travel westward over the Arabian Sea at a speed around 13 m/s, greater than the wind speed measured in the surrounding area. The motion of the cloud band presents similarities with the wavelike propagation of atmospheric phenomena such as the Australian morning glories. Common elements to the different cases are the following: weak low-level winds close to the southwestern Indian coast when the cloud bands appear, winds with a northerly component in the vicinity of the northwestern Indian coast, the presence of a sea-land breeze circulation along the West Indian coast, and the presence of high concentrations of pollutants over the northeastern Arabian Sea. However, existing physical links between these elements (if any) still have to be investigated.
L. Picon, R. Roca, S. Serrar, J. L. Monge, and M. Desbois. A new METEOSAT “water vapor” archive for climate studies. Journal of Geophysical Research (Atmospheres), 108:4301, May 2003. [ bib | DOI | ADS link ]
Water vapor plays an important role in the climate system through a number of mechanisms spanning a wide range of space and timescale. Since 1977, the METEOSAT satellites are equipped with a radiometer dedicated to the measurements of upper tropospheric humidity (UTH) which covers a relevant range of scales for a better understanding of the water vapor role in the climate. Due to the changes of the satellites and the calibration techniques over the last 20 years, this water vapor METEOSAT archive is not homogeneous and cannot be directly used for climatic studies. Hence the authors present in this paper a newly homogenized METEOSAT water vapor channel archive. Two main types of anomalies entail the original METEOSAT archive. The first one corresponds to the successive improvements of the calibration procedure. In this case, a statistical correction technique based on comparisons between ECMWF-simulated brightness temperature (BT) and water vapor METEOSAT-observed BT is developed. The second type of anomaly concerns the METEOSAT radiometer changes over the time. While still measuring the UTH, the details of the filter function, indeed, evolved over the last 20 years. In this second case, the correction is based on a physical method implying simulations of the same scene by different radiometer filter functions. Two major cases are documented in detail for September 1987 and for February 1994. Sensitivity analysis of the techniques is conducted and the methods are shown to be robust with respect to the details of their implementations. The efficiency of the two methods is then evaluated. The resulting archive reveals water vapor seasonal cycle features in better agreement with climatological estimates. The new homogenized METEOSAT archive consists of 3-hourly total sky radiance at the 0.625deg × 0.625deg resolution over the July 1983 to February 1994 period, offering the opportunity to investigate the variability of the regional UTH from synoptic scales to interannual and interdecadal scales.