A method for assessing the quality of model‐based estimates of ground temperature and atmospheric moisture using satellite data

Abstract

A method is developed for validating model‐based estimates of atmospheric moisture and ground temperature using satellite data. The approach relates errors in estimates of clear‐sky longwave fluxes at the top of the Earth‐atmosphere system to errors in geophysical parameters. The fluxes include clear‐sky outgoing longwave radiation (CLR) and radiative flux in the window region between 8 and 12 μm (RadWn). The approach capitalizes on the availability of satellite estimates of CLR and RadWn and other auxiliary satellite data, and multiple global four‐dimensional data assimilation (4‐DDA) products. The basic methodology employs off‐line forward radiative transfer calculations to generate synthetic clear‐sky longwave fluxes from two different 4‐DDA data sets. Simple linear regression is used to relate the clear‐sky longwave flux discrepancies to discrepancies in ground temperature (δTg) and broad‐layer integrated atmospheric precipitable water (δpw). The slopes of the regression lines define sensitivity parameters which can be exploited to help interpret mismatches between satellite observations and model‐based estimates of clear‐sky longwave fluxes. For illustration we analyze the discrepancies in the clear‐sky longwave fluxes between an early implementation of the Goddard Earth Observing System Data Assimilation System (GEOS2) and a recent operational version of the European Centre for Medium‐Range Weather Forecasts data assimilation system. The analysis of the synthetic clear‐sky flux data shows that simple linear regression employing δTg and broad layer δpw provides a good approximation to the full radiative transfer calculations, typically explaining more than 90% of the 6 hourly variance in the flux differences. These simple regression relations can be inverted to “retrieve” the errors in the geophysical parameters. Uncertainties (normalized by standard deviation) in the monthly mean retrieved parameters range from 7% for δTg to ∼20% for the lower tropospheric moisture between 500 hPa and surface. The regression relationships developed from the synthetic flux data, together with CLR and RadWn observed with the Clouds and Earth Radiant Energy System instrument, are used to assess the quality of the GEOS2 Tg and pw. Results showed that the GEOS2 Tg is too cold over land, and pw in upper layers is too high over the tropical oceans and too low in the lower atmosphere.