Precipitable water and surface humidity over global oceans from special sensor microwave imager and European Center for Medium Range Weather Forecasts

Abstract

The two most promising methods of estimating precipitable water W over global oceans rely on spaceborne microwave radiometers and atmospheric general circulation models. Global fields of W from the special sensor microwave imager (SSMI) were compared with those from the European Center for Medium Range Weather Forecasts (ECMWF) model. They agree over most ocean areas; both data sets capture the two annual cycles examined and the interannual anomalies during an El Nifio-Southern Oscillation episode. However, they show significant differences in the dry air masses over the eastern tropical-subtropical oceans, particularly in the southern hemisphere. In these regions, comparisons with radiosonde data indicate that overestimation by the ECMWF model accounts for a large part of the differences. As a check on the W differences, surface level specific humidity O derived from W, using a statistical relation, were compared with O from the ECMWF model. The differences in O were found to be consistent with the differences in W, and indirectly validating the Q-W relation. In both W and Q, SSMI was able to discern clearly the equatorial extension of the tongues of dry air in the eastern tropical ocean, while both ECMWF and climatological fields have reduced spatial gradients and weaker intensity. One of the reasons for the ECMWF moisture overestimation is suggested to the poor resolution of Tiros-N operational vertical sounder data which were assimilated into the ECMWF model. 1.

The Mean State of the Troposphere

“…Maximum values of more than 50 kg m -2 reside throughout the year over Indonesia and the western tropical Pacific. Comparisons between the global estimates of precipitable water from ECMWF and the global ocean estimates from the special sensor microwave imager reveal that the two datasets agree over most ocean areas, but that large differences are evident in the dry air masses over the eastern tropical and subtropical oceans, particu-larly in the SH (Liu et al 1992;Trenberth and Guillemot 1994). In these regions, it appears as though the ECMWF data overestimate the total moisture content of the atmosphere.…”

Global trends (1996–2003) of total column precipitable water observed by Global Ozone Monitoring Experiment (GOME) on ERS‐2 and their relation to near‐surface temperature

“…It should be noted that the SSM/I observations sample all atmospheric conditions while for the GOME observations only mainly cloud free scenes during daytime are analyzed (see section 2.5). Detailed information on the SSM/I data set and its validation can be found by Liu et al [1992], Sun [1993], and Wentz [1997].…”

Water vapour total columns from SCIAMACHY spectra in the 2.36 μm window

“…The Moderate Resolution Imaging Spectrometer (MODIS) on the Terra and Aqua satellites, and the Medium Resolution Imaging Spectrometer (MERIS) on Envisat exploit various spectral windows near 900 nm to retrieve total columns of water vapour (Gao and Kaufman, 2003;Li et al, 2003).…”

“…Measurements in microwaves were made by the TOVS (Tiros Operational Vertical Sounder) systems and by the SSM/I (Special Sensor Microwave/Imager) series of satellites (Liu et al, 1992). Ocean topography missions like Topex/Poseidon and Jason feature microwave radiometers as part of their measurement system.…”