Analysis of tropical convection using satellite data

Abstract

Organised convection in the tropics occurs primarily in the Intertropical Convergence Zone (ITCZ). Many of the important phenomena in the tropics such as El Niño or the monsoons involve large displacements of the ITCZ. Understanding the nature of the space-time variations of the ITCZ is therefore an important problem in tropical meteorology. The work reported in this thesis is aimed at increasing our knowledge of the nature of the intraseasonal and interannual variations of the ITCZ using digitised satellite data.

Major features of the intraseasonal variations of the ITCZ over the Indian longitudes during the summer monsoon have been brought out by earlier studies of satellite imagery. These are:

Oscillations in the intensity of the continental ITCZ between active spells and weak spells or breaks.

A bimodal distribution of convection with latitude due to the presence of an ITCZ over the equatorial Indian Ocean as well as one over the continent.

Poleward propagations of the ITCZ from the equatorial Indian Ocean onto the continent at intervals of 2–6 weeks.

In order to study whether these features are observed over other parts of the tropics, an objective method to delineate the ITCZ from the 2.5° daily data on outgoing longwave radiation (OLR) and albedo has been developed (Chapter 3). The method involves identification of the grid points with a large fraction of deep convective clouds by the imposition of a bispectral threshold (185 W/m² for daytime OLR and 0.5 for albedo). Such selected grid points are then filtered to retain only the large-scale convection. The variations of the ITCZ deduced by this method compare well with that obtained by the subjective analysis of satellite imagery (Sikka and Gadgil, 1980). The region of convection delineated is very similar to that obtained by the analysis of the equivalent blackbody temperature data on pixel-scale (Murakami, 1983).

The intraseasonal variation of the ITCZ over different parts of the tropics is analysed in Chapter 4. It is found that oscillation between active and weak spells is a basic feature of the ITCZ. Bimodality is found only over the Asian monsoon zone. Poleward propagations are seen over the Asian winter monsoon regions only in some years.

The interannual variation of the ITCZ has been studied in Chapter 5. Since rainfall is known to be well correlated with the presence of highly reflective clouds in the tropics, we expected the variation in the number of convective days in a month (counted as the number of days on which the threshold criteria for OLR and albedo are satisfied) to be related to the variation in the rainfall over the Indian region. However, the interannual variation of number of convective days during the period 1974–1985 was found to be not completely consistent with that of rainfall. Therefore, the variations of a more basic variable, viz., monthly mean OLR were analysed. This analysis of the variation of the OLR and rainfall over the Indian region suggests the possibility of a systematic bias in the OLR (both daytime as well as daily-averaged) with consistently lower values during 1982–85 as compared with 1974–78. It is also shown that the bias can be removed by an empirical formula akin to the one used in earlier corrections.