The quality of satellite precipitable water content data and their impact on analyzed moisture fields

Abstract

The quality of satellite Precipitable Water Content (PWC) moisture data is assessed by comparing them with radiosondes and the first-guess humidityfield from the ECMWF operational model. Collocation studies show that PWC data from satellites are of comparable quality to radiosonde data and that the error in the inferred relative humidity varies from 15% in the Tropics to 35% in the extra-tropics, similar to the typical error levels of radiosonde humidity data. The model first-guess provides moisture information of comparable accuracy. Using a large sample of radiosonde and first-guess moisture information over North America, the typical forecast error correlation scale for the humidity analysis variable has been found to be 250 km at the surface rising to 300 km at upper levels. Finally, impact studies have shown that the inclusion of satellite PWC data in the ECMWF humidity analysis has a small but beneficial impact on the forecast, especially of the forecast rain in the Tropics.