ATMOSPHERIC MOISTURE MEASUREMENTS - A MICROWAVE RADIOMETER - RADIOSONDE COMPARISON

The NASA/Goddard Space Flight Center Crustal Dynamics Project microwave Water Vapor Radiometer (WVR-J03) is used to measure the thermal emission of the sky at three frequencies (20.7, 22.2, and 31.4 GHz). Measurements were taken during the Atmospheric Moisture Intercomparison Study (ATMIS) held at Wallops Island, VA during April 1989. These measurements were compared with brightness temperatures inferred from measurements from VAISALA radiosonde packages launched every 3 hours during the experiment period. An error analysis for the radiosonde-inferred brightness temperatures was performed assuming reasonable random uncertainties for the pressure, temperature, and humidity measurements and propagating these uncertainties through the analysis algorithm. For the assumed uncertainties (sigma(P) = sigma(T) = 0.84 K, and sigma(RH) = 5% RH) the dominant contribution to the total uncertainty comes from the temperature measurement (66% - 88%) whereas the relative humidity measurement contributes only 2% - 8%, except in the vicinity of the water vapor line, where the contribution is 10% - 20%. Sky brightness temperature random errors range from 0.03 K to 0.6 K, with systematic errors between 0.4 K and 1.8 K. Two different water vapor emission models were used for the derivation of the brightness temperatures from the radiosonde measurements. The Liebe model and VAISALA radiosonde data give better agreement with the WVR for the 20.7 and 22.2 GHz frequencies (mean differences [WVR - VAI] of -0.32 +/- 0.56 K and -0.22 +/- 0.77 K, respectively) than does the Waters model and VAISALA radiosonde data. Agreement is best at 31.4 GHz using the Waters model (mean difference [WVR - VAI] -0.28 +/- 0.5 K), although in all cases the differences are less than the estimated uncertainties on the radiosonde inferred brightness temperatures. Differences between the two models increase as the moisture content increases and vary as a function of frequency.