Usage of Differential Absorption Method in the Thermal IR: a Case Study of Quick Estimate of Clear-sky Column Water Vapor

The concept of differential absorption has been widely used in UV and shortwave remote sensing. This study explores how to extend such concept to the thermal-IR for fast estimation of the total column water vapor (CWV) from clear-sky IR radiances. Using AIRS (Atmospheric Infrared Sounder) radiances as a case study, double difference of radiances at two pairs of pre-selected AIRS channels (812.531 cm^-1, 814.029 cm^-1, 827.747 cm^-1and 829.299 cm^-1) can be used to suppress the influence of continuum absorption and to highlight contrasts due to weak water vapor line absorptions. To take emission into account, brightness temperatures at another two AIRS channels (963.836 cm^-1 and 748.6 cm^-1) are used as surrogates of surface temperature (BT _963.8) and lapse rate (Δ BT _963.8-748.6) in the lower troposphere. As a result, a three-dimensional look-up table (LUT) can be constructed based on training data sets. Given the dependence on surface type, 3 LUTs are constructed for ocean, land excluding desert and desert only. Such LUTs enable us a fast estimate of CWV directly from the spectral radiances without any a prior information or formal retrieval. The performance of the method is tested using synthetic AIRS radiances based on ECMWF-ERA reanalysis (Figure 1) as well as the Thermodynamic Initial Guess Retrieval (TIGR2000 v1.2) profiles. It is also tested using AIRS L2 cloud-cleared radiances and against AIRS version6 CWV retrievals. The comparisons show that the near-global mean bias of this method is within ± 0.07 cm and the root-mean-square fractional error is about 33%. This method has applications in quick estimate of CWV when accuracy requirement is not high, such as a first guess for other more sophisticated retrieval algorithms for CWV. Key words: total column water vapor; retrieval; differential absorption method; thermal-IR; AIRS