Microwave Radiometric Calibration Of Airborne Weather Radars Under Rain-free Conditions Over The Ocean

Abstract

Because o f the grouing i n te res t in the e f f e c t o f the global d i s t r i b u t i o n o f r a i n f a l l on climate, the space agencies of Japan arid the United States have i n i t i a t e d the developnent o f a Tropical Rain Measuring Mission (TRMM) s a t e l l i t e . The s a t e l l i t e u i l l carry a 14 GHz radar along u i t h 10, 19, 22, 37 and 86 GHz radiometers. I n order t o s imulate those measurements, a major e f f o r t has been i n i t i a t e d t o measure p r e c i p i t a t i o n from airborne radars and microuave radiometers (1). I n the present study, an a i r c r a f t uas f i t t e d u i t h nadi r paint i n g radars and radiometers operat ing a t near ly the same frequencies as those that u i l l be used on the TRMM s a t e l l i t e . Deploying 10, 19, 34 and 92 GHz microuave radiometers permit ted the measured br ightness temperatures t o be used t o cMnpute the ocean surface u ind speed and the t o t a l path at tenuat ion o f a r a i n f r e e a t mosphere. The normalized surface radar backscatter ing cross section, a t 10 and 35 GHz, was then computed using the surface u ind speed. The e f f e c t i v e surface radar backscatter ing cross sect ion per u n i t area, a t these frequencies, uas ca lcu lated by apply ing the knouledge of the transmission of the ra in f ree atmosphere. Adding measurements of the returned surface radar pouer, allowed the determinat ion of the radar constants.