A multiyear intercomparison of wet troposphere corrections from TOPEX/Poseidon, ERS-1, and ERS-2 Microwave Radiometers and the European Centre for Medium-Range Weather Forecasts model

Wet troposphere corrections to altimeter measurements calculated from the T OPEX/Poseidon (T/P) Microwave Radiometer (TMR) and the ERS-1 and ERS-2 Micr owave Radiometers (EMR I and EMR2) are compared to each other and to Europe an Centre for Medium-Range Weather Forecasts (ECMWF) model data. The most r ecently published correction algorithm for the EMR1 data [Stum et al., 1998 ] is applied. The suggested drift correction for TMR data [Keihm et al., 19 98, 2000] is also evaluated. The corrected EMR1 data (1991-1996) produce a global (to +/- 66 degrees the T/P latitude range) long-term mean wet tropos phere correction 6 and 13 mm lower than TMR and ECMWF, respectively. The EM R2 data (1995-1999) yield a mean wet troposphere correction 2 and 9 mm lowe r than TMR and ECMWF, respectively. After removing mean differences all thr ee microwave radiometers reproduce similar long-term zonal wet troposphere corrections compared to the ECMWF model (10-14 mm rms) and to each other (5 -9 nim rms) with some zonally periodic differences, most < 10 mm. The ECMWF model shows variations compared to the radiometers of over 30 mm before 19 95, about 20-30 mm from 1995 to 1997, and up to 20 mm from 1998 to 1999. Th e intersatellite differences include a latitudinally dependent annual signa l, reaching 10 nun in amplitude. Before correcting for the TMR drift there exists a global relative TMR-EMR1 drift of -1.6 +/- 0.4 mm y(-1), from 1992 to 1996. After correcting for the TMR drift the TMR-EMR I trend is reduced to -0.4 +/- 0.2 mm. y(-1), supporting the TMR drift correction. The TMR-EM R2 trend changes sign (direction) after an anomaly that occurred in one of the EMR2 brightness temperatures during June 1996. Before the anomaly, with (without) the TMR correction the relative TMR-EMR2 trend is -3.0 +/- 1.9 ( 4.0 +/- 2.1) nun y(-1), again supporting the TMR drift correction. After th e anomaly, through 1997 the TMR-EMR trend is 3.7 +/- 1.2 mm y(-1), and from 1998 to 1999 it is 0.8 +/- 0.6 mm y(-1).