Abstract
An analysis of the atmospheric impact on ground brightness temperature (Tg) is performed for numerous land surface types at commonly-used frequencies (i.e., 1.4 GHz, 6.93 GHz, 10.65 GHz, 18.7 GHz, 23.8 GHz, 36.5 GHz and 89.0 GHz). The results indicate that the atmosphere has a negligible impact on Tg at 1.4 GHz for land surfaces with emissivities greater than 0.7, at 6.93 GHz for land surfaces with emissivities greater than 0.8, and at 10.65 GHz for land surfaces with emissivities greater than 0.9 if a root mean square error (RMSE) less than 1 K is desired. To remove the atmospheric effect on Tg, a generalized atmospheric correction method is proposed by parameterizing the atmospheric transmittance τ and upwelling atmospheric brightness temperature Tba↑. Better accuracies with Tg RMSEs less than 1 K are achieved at 1.4 GHz, 6.93 GHz, 10.65 GHz, 18.7 GHz and 36.5 GHz, and worse accuracies with RMSEs of 1.34 K and 4.35 K are obtained at 23.8 GHz and 89.0 GHz, respectively. Additionally, a simplified atmospheric correction method is developed when lacking sufficient input data to perform the generalized atmospheric correction method, and an emissivity-based atmospheric correction method is presented when the emissivity is known. Consequently, an appropriate atmospheric correction method can be selected based on the available data, frequency and required accuracy. Furthermore, this study provides a method to estimate τ and Tba↑ of different frequencies using the atmospheric parameters (total water vapor content in observation direction Lwv, total cloud liquid water content Lclw and mean temperature of cloud Tclw), which is important for simultaneously determining the land surface parameters using multi-frequency passive microwave satellite data.
Highlights
Instantaneous satellite observations have been used in a variety of applications to estimate ocean and land surface temperatures, land surface emissivity, and regional evapotranspiration [1,2,3,4,5,6]
The minimum root mean square error (RMSE) of ΔT, which are equal to the standard deviations of ΔT, are obtained when the emissivities are 0.9066, 0.9430, 0.9569, 0.9599, 0.9381, 0.9518, and 0.9146 at frequencies of 1.4 GHz, 6.93 GHz, 10.65 GHz, 18.7 GHz, 23.8 GHz, 36.5 GHz and 89.0 GHz, respectively
Analyses of the atmospheric impacts were performed for many types of land surfaces, and an appropriate atmospheric correction method was developed for each frequency (i.e., 1.4 GHz, 6.93 GHz, 10.65 GHz, 18.7 GHz, 23.8 GHz, 36.5 GHz and 89.0 GHz)
Summary
Instantaneous satellite observations have been used in a variety of applications to estimate ocean and land surface temperatures, land surface emissivity, and regional evapotranspiration [1,2,3,4,5,6]. Previous studies have shown that atmosphere has significant effects on surface parameter retrieval, especially from optical and infrared remotely-sensed data, that must be corrected [7,8,9,10,11,12]. Microwave indices (e.g., microwave polarization difference index and microwave vegetation index) are often used to retrieve land surface parameters due to their simplicity and practicability, but most studies ignored the effect of the atmosphere [24, 25]. Several studies of physically-based algorithms for land surface parameter retrieval ignored the effect of the atmosphere [22, 26]. Ignoring atmospheric effects leads to larger errors in the retrieval of land surface parameters and limits the application of passive microwave data, especially at high frequencies. Little research has focused on the atmospheric impacts and corrections for all-weather conditions, all underlying surfaces and all commonly-used frequencies due to the complex microwave emissions and interactions between the atmosphere and the underlying
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