A Method for Assessing the Quality of Model-Based Estimates of Ground Temperature and Atmospheric Moisture Using Satellite Data

General Sciences Corporation, NASA Goddard Space Flight Cenlcr. Grcenhc[t, M;u_,l;mdAbstract. A method is developed for validating model-l_;tsed estimates of atmosphericmoisture and ground temperature using satellite data. The approach relates errors inestimates of clear-sky longwave fluxes at the top of the Earth-atmosphere system to errorsin geophysical parameters. The fluxes include cicar-sky outgoing longwavc radiation(CI_P,) and radiative flux in the window region between 8 and 12/.tin (P, adWn). Theapproach capitalizes on the availability of satellite estimates of CI.R and RadWn andother attxiliary satellite data, and multiple global four-dimensional data assimihttio,1(4-DDA) products. The basic methodology employs off-line fo,ward radiative transfercalculations to generate _nthetic clear-sky Iongwave fluxes from two different 4-DDAdata sets. Simple linear regression is used to relate the clear-sky longwave fluxdiscrepancies to discrepancies in ground temperature (87"_) and bro;td-layer integratedatmospheric precipitable water (6pw). The slopes of the regression lines dcline sensitivityparameters which can be exploited to help interpret mismatches between satelliteobservations and model-based estimates of clear-sky longwavc fluxes. For illustration weanalyze the discrepancies in the clear-sky Iongwave fluxes bctwecn an earlyimplementation of the Goddard Earth Observing System Data Assimilatio,t System(GEOS2) and a recent operational vcrsion of the Europc:m Centre for Mcdium-RatlgeWeather Forecasts data assimilation system. The analysis of the synthetic clca,-sky fluxdata shows that simple linear regression ctnploying 87",_ and broad layer 5pw provides :tgood approximation to the full radiative transfer calculations, typically explaining morethan 90% of the 6 hourly variance in the flux differences. These simple regressionrelations can be inverted to "retrieve" the errors in the geophysical parameters.Uncertainties (normalized by standard deviation) in the monthly mcnn retrievedparameters range from 7% for 8Tg to _20% for the lower tropospheric moisture between500 hPa and surface. The regression rchttionships developed from the synthetic flux data,together with CLR and RadWn observed with the Clouds and E;,rth Radiant EnergySystem instrument, are used to assess the quality of the GEOS2 7"0 and pw. Rcsttltsshowed that the GEOS2 T O is too cold over land, and pw in Ul)pcr layers is too high overthe tropical oceans and too low in the lower attar)sphere.