Precursors to Convection over Peninsular India during March–May in AMSU-B Data

Abstract

In the present work the locations of precursors to summer thunderstorms over peninsular India are sought in images from the Advanced Microwave Sounding Unit-B (AMSU-B). At these locations, moisture maxima and brightness temperature (BT) minima may be expected. Prior literature suggests that the 150-GHz channel is useful in detecting the BT minima due to moisture in the lowest 1 km, the 183.3 ± 7- GHz channel is useful for detecting middle-level moist layers and clouds, and the other 183.3-GHz channels are useful for upper-air features such as high-level dry air incursion or upper-air troughs. Cloudy or moist pixels and low-emissivity ground pixels have similar BTs. The extraction of the locations of BT minima is therefore difficult. The scale 1 or 2 à trous wavelet transform (WT) allows unambiguous location of the BT minima due to clouds or moist regions from AMSU-B images. On 3?4 April 2001 there was a middle-level moisture incursion over the peninsula and on 17?18 April 2001 there was an upper-level dry air incursion. The wavelet components of AMSU-B data were extracted during these two events. From the Meteosat-5 infrared images it was verified that convection occurred within 2 h of the AMSU-B pass in the low-BT regions separated by the à trous WT. On the other hand, peninsular discontinuities on 3?4 April 2001 and 7 April 2002 and other days in 2002 showed that useful precursors could not be extracted from the 150-GHz signal. The BTs for the 150-GHz channel in dry air are affected significantly by ground altitude and by changes in surface emissivity. Failure and success in precursor detection are both attributed to the variability in the BTs.