Abstract. This study assesses a common analytic expression used to compute upper tropospheric humidity (UTH) from satellite brightness temperatures at 6.7/•m. Vertical averaging methods for UTH were found to have significant impact on the algorithm. The temperature lapse parameter • had more latitudinal variance than previously found; however, large vertical gradients in midlatitude profiles degraded UTH retrievals. The scaled reference pressure parameter P0 did sufficiently improve the radiance to humidity relationship with exception to midlatitude profiles where P0 exceeds 1.7. A new method for computing the reference pressure parameter is presented using HIRS channel 6 observations. Error analysis of radiance to humidity relationship indicates that absolute UTH errors increase with increasing humidity with largest errors of 15% per 1 K bias between simulated and observed brightness temperatures.
IntroductionThe term upper tropospheric humidit), (UTH) refers to a layer-averaged t•aa_qaa hpq• r•lqt•w• humidity computed from polar orbiting or geostationary satellite observations at water vapor channels near 6.7 /•m. Because UTH is derived from satellite radiance (or brightness temperature, the terms are used interchangeably in this paper), differing assumptions about the radiance to humidity relationship can produce significant biases between methods. In this study we examine the strengths and weaknesses of the various assumptions used in the relationship between UTH and satellite radiance. to differing assumptions about what parameters need to be accounted for in the radiance to humidity relationship. In section 3 we examined the characteristics of the vertical temperature and moisture profiles for tropical and midlatitude climate zones. We assess the impact that various assumptions and climate zones have on the UTH algorithm in section 4. We introduce a UTH algorithm for HIRS observations by using an optimum satellite temperature channel to predict the reference pressure in section 5. We then summarize the results of our study in section 6.
UTH MethodologySoden and Bretherton [1993] derived a relation between UTH and 6.7/•m brightness temperature using ({RH) Po/ a + bT6. 7 = in /3 cos 0/' where T6. 7 is the observed 6.7 /•m brightness temperature (HIRS channel 12 is used in this paper, but water vapor channels found on other satellites could apply), (RH) is the vertically integrated relative humidity (or UTH), 7o (-P[T : T6.7]/300 hPa) is the scaled reference pressure, /3 (-(P/T d T/dp)) is the vertically integrated dimensionless lapse rate parameter, 0 is the satellite view angle, and coefficients a and b are fitted parameters. Moisture variations contribute most of the variance in T6.7; however, variations due to temperature become significant in the midlatitudes. Two temperaturedependent parameters found in (1) are Po and/3. Po represents the pressure where the temperature is equal to T6. 7 and is approximated by assuming the mean T6. 7 = 240 K. Po accounts for latitudinal changes in upper tropospheric temp...