INTERNATIONAL SYMPOSIUM ON INFORMATION TECHNOLOGY IN OCEANOGRAPHY (ITO-98) GOA, INDIA 12-16 OCTOBER 1998

The presence of rain in the sub-satellite track can significantly degrade altimeter measurements by introducing an attenuation of the backscattered signal, a change in its path length through the atmosphere and change in the mean square slope of the sea surface. This can cause errors, not only in the measurement of the precise satellite altitude and hence in sea surface topography, but also in the determination of wind speed and wave height. A sensitivity of dual frequency microwave observations by TOPEX/Poseidon (T/P) altimetry at Ku and C band (13.6 GHz and 5.3 GHz respectively) has been attempted to utilise for their rainfall estimation potential. First the sensitivity of both the channels have been studied for the varying sea state conditions. Under the rain-free conditions, the effects of winds and roughness on the backscattered signal has been simulated assuming specular point model at nadir viewing geometry of T/P radar altimeter. The radar reflectivity at two widely separated frequencies at Ku and C band linearly decreases upto 25 m/sec while there is a marked difference for decreasing trend for L and X band which has been introduced for comparison. The simultaneous observations of radar scattering at same frequency (Ku and C band) of T/P during the raining spells is just contrary to the non-raining cases and thus shows an explicit possibility of retrievability of rain. Thus the potential use of differential backscattering (d s ° ) measurements at nadir for measuring precipitation by Ku and C band observations from T/P has been estimated and analysed. The results shows that the oceanic background is common for Ku and C band channels from calm to even high sea state conditions. Tile results thus illustrate that d s ° can have important application in the quantitative measurements of rainfall for its satellite altimetric correction on instantaneous basis for measurements of sea surface elevations and associated dynamic topography. On the monthly scales also, the rain characterizing capability of T/P have been compared with the microwave radiometric data from the Special Sensor Microwave/lmgaer (SSM/1). The dual frequency altimetric observation of rain resembles with all the mean monthly rain features on the global scale.

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Copyright© National Institute Of Oceanography, Goa,India,1998
updated on 7th July'98