The utilization of the altimetric satellite’s on-board radiometer for atmospheric observation is demonstrated. This study uses the Wet Tropospheric Correction (WTC) data from the Topex/Jason altimetry mission series (Topex/Poseidon, Jason-1, Jason-2/OSTM, and Jason-3). The data spans nearly 30 years, making them sufficient for climate study. Precipitable Water Vapor (PWV) is derived from the WTC and used to study the water vapor variability over the Tropical Indian Ocean (TIO). Standard EOF (Empirical Orthogonal Function) analysis on the derived PWV anomaly (PWVA) in the TIO generates two leading modes. The first mode has a dipole spatial structure that explains 18.3% of the total variance, and the second one has a basin-wide homogeneous structure that explains 12.3% of the total variance. Correlation analysis with IOD and ENSO monitoring indices has shown that these two modes are associated with the two interannual variabilities. Further analyses using composite techniques are done to distinguish the different of mechanism and impact between the two. A new monitoring index is proposed here which uses the altimetry-derived PWV anomaly data. The index is computed from the difference of PWV anomaly between the dipole regions in TIO, which effectively describes the activity of the Indian Ocean Walker Circulation (IOWC). This research showcased the feasibility of using the data measured by the radiometer of altimetric satellites for atmospheric studies, a potential continuation of this research would be the use of the synchronous altimeter-radiometer measurement for more advanced analysis such as the ocean-atmosphere coupling.

Altimetry satellites have an on-board microwave radiometer whose original function is to measure the Tropospheric Wet Delay (TWD) for their main sea level measurement using radar altimeter. The TWD, as it reflects the amount of water vapor in the atmosphere, could be used to derive the Precipitable Water Vapor (PWV) which would be a good parameter to study the atmosphere variability, especially in Indonesia where rainfall is the main variable of its climate. Studies on altimetry-derived PWV are conducted, which include: analyzing their correlation with rainfall observation dataset, and assessing how well they could capture some key atmospheric phenomena in Indonesia. The results show that there are fairly high correlation between the altimetry-derived PWV and the rainfall data in monthly-scale with 0.63 Pearson correlation coefficient. Moreover, they could very well capture some key phenomena in Indonesia such as the monsoon and El Niño Southern Oscillation (ENSO) variation. Thus, a new way of utilizing altimetry satellite data is introduced in this paper: to observe the atmosphere using its water vapor data derived from the microwave radiometer. This open the possibility of utilizing altimetry satellites for both oceanographic and meteorological studies as it could measures both oceanic and atmospheric parameter, with its radar altimeter and microwave radiometer, respectively.

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