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

A new regional relationship is derived for wet tropospheric range correction h for radar altimeter and precipitable water (W) over the Indian Ocean using ship observations for the period 1982-91. The W varied over the range of 20-80 mm, thus providing total variability expected over 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 relation. 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 the Indian Ocean. Comparisons with earlier models show a factor of 2 improvement in the accuracy of the correction over this region. The W values have also been derived using the NOAA High-Resolution Infrared Sounder data for the years 1980, 1981, and 1984. These monthly mean W(3-yr 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 about the values of corrections required over this region. Here h obtained using W from a sample pass of the Seasat Scanning Multichannel Microwave Radiometer (SMMR) and h provided by the Fleet Numeric Oceanographic Center (FNOC) have been compared to point out deviations of FNOC model-derived h values from h obtained from SMMR-derived W. The Special Sensor Microwave/Imager-derived W values for a sample pass over the Arabian Sea and the Bay of Bengal have been used to estimate h values from our derived relation between h and W. The Bay of Bengal exhibits high h values compared to those over the Arabian Sea. This study demonstrates 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 provides instantaneous W measurements for studying various oceanographic phenomena.