A Method for Correcting Nonlinear Effects of Temperature and Water Vapor in Radiative Transfer in Thermal Infrared Regions-An Application to the Brightness Temperature of Hypothetical Channels-

Abstract

It has been shown by Aoki (2004, 2005) that the differential of multi-channel radiance data can be compressed into about two order smaller channels of hypothetical radiances, with loosing negligible information content. In these methods the weighting functions of the original channels are expanded with empirical orthogonal functions (EOFs), and a set of hypothetical radiances, whose weighting functions are the EOFs, are obtained by a linear combination of original channels. It was noted in these papers that to incorporate this method into the assimilation scheme of the numerical prediction, a massive look-up table is required to be applicable for various atmospheric profiles. In this table, the atmospheric profiles of temperature and water vapor, the coefficients for converting the original brightness temperatures to the hypothetical brightness temperatures, and the empirical orthogonal functions are included. A sufficiently large number of model atmospheres should be prepared for the table, so that the differential brightness temperature for an arbitrary atmospheric profile can be given by a linear correction to the brightness temperature for one of the model atmospheres in the table. The data size of such a look-up table could become as large as it impacts to the computer system.In the present paper, a preliminary study is carried out on the effect of the nonlinearity of the radiative transfer in the brightness temperature of hypothetical channels in thermal infrared regions 640-760 cm−1 and 1300-1600 cm−1. A simple method for the nonlinearity correction is proposed. It is shown that by applying the correction method proposed, the distances of the neighboring atmospheric profiles in the look-up table can be extended to more than 6 K for the temperature, and about 0.3 g/cm2 for the precipitable water.