ATMOSPHERIC WATER VAPOR PROFILES DERIVED FROM REMOTE-SENSING RADIOMETER MEASUREMENTS

Abstract

The feasibility and preliminary testing of a low cost, remote-sensing air-borne, double bolometer technique for inferring atmospheric water vapor is illustrated. To deduce the water vapor profile with commercially available equipment, the radiative transfer equation is solved for the water vapor transmissivity employing an input data remote radiometer-measured upward irradiances obtained at aircraft holding levels. Radiometers sensitive in two separate spectral bands are used. The primary radiometer covers the 4.39 to 20.83µ, broad atmospheric radiation band, and the second, for surface temperature deduction, covers the atmospheric window region, 7.35 to 13.16µ. The transfer solution results are acquired from computer programs developed specifically for this purpose. Results indicate an accuracy for inferred total tropospheric water vapor and mixing ratio profiles close to that of the standard sounding electrical hygrometer. The absolute accuracy of the radiosonde hygrometer, considering surface calibration procedures, and for a single ascent, is not better than ±12 percent. The absolute accuracy is greatest for ?dry? soundings where the largest changes in irradiance occur for given changes in moisture. Specifically, tests for a vertical profile averaging 6.00 gm./kg. of water vapor produce an average error of 0.70 gm./kg. in the inferred mixing ratio. The average error in mixing ratio obtained by this technique for profiles averaging 2.3 gm./kg. is 0.05 gm./kg. The implications for use on high-flying aircraft or on rockets with highly sensitive radiometers are obvious. The primary purpose in reporting this research is to suggest a technique and illustrate its use. It is clear that with more sensitive bolometer radiometers with selective band pass filters a considerable increase in accuracy can be achieved.