The wet tropospheric range correction is designed to correct the observed altimetric sea-surface elevations for the propagation delay caused by atmospheric water vapor and cloud liquid water. Although it is smaller than the corresponding `dry tropospheric range correction,' the wet correction is more complex, with possibly rapid variations in both time and space. The correction can vary from just a few millimeters in dry, cold air to more than 40 cm in hot, wet air.
On TOPEX/POSEIDON, the wet troposphere correction is based on the measurements of the T/P on-board microwave radiometer (TMR). This is a 3-channel (operating at 18, 21, and 37 Gigahertz), nadir-viewing instrument, which provides nearly direct measurements of the wet-troposphere correction by monitoring the strong water-vapor absorption line centered at 22.235 GHz. The radiometer and its corresponding data-processing algorithms are described in a series a three papers published in the IEEE TGRS [see Ruf et al., 1995; Janssen et al., 1995; Keihm et al., 1995]. The radiometer was calibrated in flight during the first few months of the T/P mission; this calibration work is described by Ruf et al. [1994]. Further work concerning the validation of this instrument includes Stum [1994] and Keihm & Ruf [1995].
The stability of the on-board microwave radiometer has been closely monitored. Comparisons of the TMR path delay against SSMI, VLBI, and GPS derived path delays have identified drifting in the 18 GHz TMR channel resulting in an ~1.2 mm/yr drift described in Keihm et al.[2000]. During recent comparisons against the Jason-1 microwave radiometer (JMR) a bias was discovered in the TMR brightness temperatures and path delay between satellite yaw states. Correction algorithms have been developed for the path delay drift (Ruf and Brown, 2002) and the yaw state bias (Brown, Ruf, and Keihm, 2002) that are essential for monitoring long term mean sea level rates.
REFERENCES:
Brown S, Ruf, C S, and S Keihm, Brightness Temperature and Path Delay Correction for Topex Microwave Radiometer Yaw State Bias, 8 August 2002 Technical Memo.
Ruf, C S, and S Brown, Topex Microwave Radiometer 18 GHz Drift - Revisited, 29 May 2002 Technical Memo.
Keihm S, Zlotnicki, V, and C S Ruf, TOPEX Microwave Radiometer Performance Evaluation, 1992-1998, IEEE Transactions on Geoscience & Remote Sensing, 38, 1379-1386, May 2000.
Janssen, M A, C S Ruf, and S J Keihm, TOPEX/Poseidon microwave radiometer (TMR): 2. Antenna pattern correction and brightness temperature algorithm, IEEE Transactions on Geoscience & Remote Sensing, 33, 138-146, 1995.
Keihm, S J, and C S Ruf, Role of water vapor radiometers for in-flight calibration of the TOPEX microwave radiometer, Marine Geodesy, 18, 139-156, 1995.
Keihm, S J, M A Janssen, and C S Ruf, TOPEX/Poseidon microwave radiometer (TMR): 3. Wet troposphere range correction algorithm and pre-launch error budget, IEEE Transactions on Geoscience & Remote Sensing, 33, 147-161, 1995.
Ruf, C S, S J Keihm, B Subramanya, and M A Janssen, TOPEX/Poseidon microwave radiometer performance and in-flight calibration, Journal of Geophysical Research, 99, 24915-24926, 1994.
Ruf, C S, S J Keihm, and M A Janssen, TOPEX/Poseidon microwave radiometer (TMR): 1. Instrument description and antenna temperature calibration, IEEE Transactions on Geoscience & Remote Sensing, 33, 125-137, 1995.
Stum, J., A comparison between TOPEX microwave radiometer, ERS-1 microwave radiometer, and ECMWF derived wet tropospheric corrections, Journal of Geophysical Research, 99, 24927-24939, 1994.
EXAMPLE:
This plot shows the mean wet troposphere correction during
cycle 30 of the Topex/Poseidon mission, i.e. during the 10-day
interval starting 7 July 1993.
The mean correction during this time period was 156 mm, the
maximum 454 mm, the minimum 0 mm.
