Assessment of the Jason Microwave Radiometer's Measurement of Wet Tropospheric Path Delay Using Comparisons to SSM/I and TMI

Abstract

The Jason microwave radiometer (JMR) provides a crucial correction due to water vapor in the troposphere, and a much smaller correction due to liquid water, to the travel time of the Jason-1 altimeter radar pulse. An error of any size in the radiometer's measurement of wet path delay translates as an error of equal size in the measurement of sea surface height, the ultimate quantity that the altimetric system should yield. The estimate of globally-averaged sea surface height change associated with climate change, requires that uncertainties in the trends in such a global average be accurate to much better than the signal of 1–2 mm/yr. We first compare the JMR observations to those from the TOPEX/Poseidon radiometer (TMR) over approximately six months, since the intent of Jason is to continue the 10-year time series of precision ocean surface topography initiated by T/P. We then assess the stability of the JMR measurement by comparing its wet path delay to those of other orbiting radiometers over 22 months, specifically the Special Sensor Microwave Imager aboard the Defense Meteorological Satellite Program (DMSP-SSM/I) series of satellites, and the Tropical Rainfall Mapping Mission's Microwave Imager (TMI), as well as the European Center for Medium Range Weather Forecasting's (ECMWF) atmospheric numerical model estimate of water vapor. From the combined set, we obtain a robust assessment of the stability of JMR measurements. We find, that JMR is in remarkable agreement with TMR, only 2.5 mm longer, and 6–7 mm standard deviation on their difference in 0.5 degree averages; that JMR has experienced a globally-averaged step-function change, yielding an apparent shortening in wet path delay estimates of 4–5 mm around October 2002 (Jason cycles 28–32); that this step-function is visible only in the 23.8 GHz channel; and that the 34 GHz channel appears to drift at a rate of −0.4K/year. In addition, we find that, while in 2002 there was no evidence of sensitivity to the Jason satellite's attitude (a correlation of the wet path delay with yaw state), in 2003 there are strong (2–3 mm, up to 7 mm globally averaged) changes associated with such yaw state. These JMR issues were all found in the first 22 months of Jason's geophysical data records (GDR) data, and thus they apply to any investigations that use such data without further corrections.