Estimation of Attenuation at TRMM Precipitation Radar Channel

The Tropical Rainfall Measuring Mission (TRMM) satellite is a novel mission to observe rainfall from space. The precipitation radar onboard the TRMM operates at 13.8 GHz. It measures several parameters which help to understand the cloud microphysics, viz. the characteristics of the melting layer, rainfall intensity, etc. However, at such a high frequency, the raindrops cause absorption and scattering of the signal leading to attenuation. Thus, the signal strength reduces, causing degradation of the signal. The knowledge of attenuation is of immense importance to the link designers. Besides, it has its applications in defense. The knowledge of attenuation is also necessary for climate study as it gives an insight into drop-size distribution. In this paper, the authors have attempted to estimate attenuation from the ITU-R model by using the TRMM-retrieved rainfall as an input. They have validated the attenuation measured by the TRMM against that estimated using the ITU-R model. The study includes a few locations in the Indian subcontinent, namely, Bangalore (12.97°N, 77.59°E), Guwahati (26.14°N, 91.74°E), Chennai (13.08°N, 80.27°E), Delhi (28.70°N, 77.10°E), Kakdwip (21.87°N, 88.18°E), Kolkata (22.57°N, 88.36°E), Karaikal (10.92°N, 79.83°E), Mumbai (19.07°N, 72.87°E), Panjim (15.49°N, 73.82°E), Puri (19.81°N, 85.83°E), Machilipatnam (16.19°N, 81.13°E), Vishakhapatnam (17.68°N, 83.21°E), Salem (11.66°N, 78.14°E), Trivandrum (8.52°N, 76.93°E), and Mangalore (12.91°N, 74.85°E). Some of the locations so chosen lie along the East and West coasts of India. The period of study includes 1998-2002. The study shows that over Trivandrum and Kakdwip, the two matches very well. However, over other locations, sometimes the TRMM-estimated attenuation values overestimate. At times, these underestimate. The RMS error over these locations varies between 0.177 at Trivandrum to 22.004 at Bangalore. Besides, the investigation shows that the rainfall versus attenuation relationship is not always a power relation as accepted globally. Instead, the relationship between the two varies from one location to another.