WATER-VAPOR CORRECTIONS FOR RADAR ALTIMETER AND ITS VARIABILITY OVER THE INDIAN-OCEAN

A new regional relationship is derived for wet tropospheric range corr ection h for radar altimeter and precipitable water (W) over the India n Ocean using ship observations for the period 1982-91. The W varied o ver the range of 20-80 mm, thus providing total variability expected o ver tropical oceans. A fifth-order polynomial between h and W gave an rms error of 2.3 mm when compared with h computed using direct relatio n. Model results have also been compared with an earlier relation over the Indian Ocean and with the global relation, and it has been found that the present model yields the lowest rms error in h values over th e Indian Ocean. Comparisons with earlier models show a factor of 2 imp rovement in the accuracy of the correction over this region. The W val ues have also been derived using the NOAA High-Resolution Infrared Sou nder data for the years 1980, 1981, and 1984. These monthly mean W(3-y r averaged) values have been used to study the variabilities in W. The Indian Ocean depicts large variabilities of W even on a monthly scale . The monthly mean map of h has also been given to get a rough idea ab out the values of corrections required over this region. Here h obtain ed using W from a sample pass of the Seasat Scanning Multichannel Micr owave Radiometer (SMMR) and h provided by the Fleet Numeric Oceanograp hic Center (FNOC) have been compared to point out deviations of FNOC m odel-derived h values from h obtained from SMMR-derived W. The Special Sensor Microwave/Imager-derived W values for a sample pass over the A rabian Sea and the Bay of Bengal have been used to estimate h values f rom our derived relation between h and W. The Bay of Bengal exhibits h igh h values compared to those over the Arabian Sea. This study demons trates the usefulness of the proposed regional relation between h and W for application to satellite-borne altimeter data, such as the ERS-1 and Topex/Poseidon missions, where an onboard microwave radiometer pr ovides instantaneous W measurements for studying various oceanographic phenomena.