Tropospheric dryness and clouds over tropical Indian Ocean

Dry layers in the free troposphere over Tropical Oceans have been studied for their role in convective activity and for their effects in the radiation budget. Previous studies concentrated, mostly, on the Western Pacific region because of the abundance of observations during the TOGA-COARE. This study aims to document the occurrence of dry layers over the Indian Ocean and to investigate the cloudiness observed, but poorly documented, during such events. One month (March 1999, during INDOEX) of combined Visible/Infrared and Microwave data from the TRMM radiometers had been processed to classify observations in terms of total precipitable water vapour (TPWV), cloud occurrence and type. Soundings (1978-2004) from the Seychelles station have been analysed to validate the capability, through the analysis of TPWV, to detect dry layers. The study area (40 degrees E-80 degrees E, 30 degrees S-30 degrees N) had been portioned into 2.5 degrees x 2.5 degrees boxes to investigate the spatial distribution of occurrence of dry events with associated cloudiness. South of the ITCZ cloudiness associated with low TPWV is due to low-level clouds: probably generated by shallow convection trapped by the trade inversion. North of the ITCZ cirrus are mostly observed during relatively dry events. Further analyses, concentrating on possible links between occurrence of cirrus and TPWV, suggest, in addition to the obvious mechanism (i.e. the higher the moisture, the higher the convective activity and as a consequence the higher the occurrence of cirrus), a second one that would be responsible of relatively high occurrence of cirrus for low TPWV. A case (14th-15th March 1999) is studied in detail with TRMM observations, sounding data and METEOSAT imagery. The observed cirrus, generated by convection, migrate over relatively dry air of extra-tropical origin. Cirrus extend as filaments for more than 1000 km in length and about 50 to 100 km in width, and they last for 2-3 days. A retrieval method is designed and applied to the data giving as cirrus top pressure approximately 250 hPa while the retrieved effective radius value is consistent with what expected, from literature, for dissipating cirrus at that pressure. The vertical structure of the atmosphere, observed during such event suggests the hypothesis that a combination of presented radiative and dynamical mechanisms could be responsible, through the supply of moisture from lower levels, of increasing the cirrus lifetime and, as a consequence, increasing the occurrence of cirrus over relatively dry air columns. A larger data set is investigated to confirm the results based on the March 1999 data set analysis. The average vertical structure of the atmosphere, during such events, as obtained from the Seychelles sounding data set (> 7000 soundings) analysis confirms the occurrence of the features observed in the study case. Similarly, the analysis of 105 days of TRMM orbits over the box containing the Seychelles station, confirms the statistical features that indicate a possible relationship between the occurrence of dry air and associated cirrus. However, possible interaction between dry layer and cirrus occurrence should be investigated in a detailed modelling framework (beyond the scope of this study) such as the one offered by the cloud resolving models.