Sea State and Rain: A Second Take on Dual-Frequency Altimetry

摘要

TOPEX and Jason were the first two dual-frequency altimeters in space, with both operating at K_u- and C-band. Thus, each gives two measurements of the normalized backscatter, σ~0, (from which wind speed is calculated) and two estimates of wave height. Departures from a well-defined relationship between the K_u - and C-band σ~0 values give an indication of rain. This study investigates differences between the two instruments using data from Jason's verification phase. Jason's K_u -band estimates of wave height are ～1.8% less than TOPEX's, whereas its σ~0 values are higher. When these effects have been removed the root mean square (rms) mismatch between TOPEX and Jason's K_u-band observations is close to that for TOPEX's observations at its two frequencies, and the changes in σ~0 with varying wave height conditions are the same for the two altimeters. Rain flagging and quantitative estimates of rain rate are both based on the atmospheric attenuation derived from the σ~0 measurements at the two frequencies. The attenuation estimates of TOPEX and Jason agree very well, and a threshold of -0.5 dB is effective at removing the majority of spurious data records from the Jason GDRs. In the high σ~0 regime, anomalous data can be caused by processes other than rain. Consequently, for these low wind conditions, neither can reliable rain detection be based on altimetry alone, nor can a generic rain flag be expected to remove all suspect data.