Abstract
This article presents a new method for retrieving the Ice Water Path (IWP), the median volume equivalent sphere diameter (Dme) of thin ice clouds (IWP < 100 g/m2, Dme < 80 μm) in the Terahertz band. The upwelling brightness temperature depressions caused by the ice clouds at 325.15, 448.0, 664.0 and 874.0 GHz channels are simulated by the Atmospheric Radiative Transfer Simulator (ARTS). The simulated forward radiative transfer models are taken as historical data for the M5 model tree algorithm to construct a set of piecewise functions which represent the relation of simulated brightness temperature depressions and IWP. The inversion results are optimized by an empirical relation of the IWP and the Dme for thin ice clouds which is summarized by previous studies. We inverse IWP and Dme with the simulated brightness temperature and analyze the inversion performance of selected channels. The 874.4 ± 6.0 GHz channel provides the most accurate results, because of the strong brightness temperature response to the change of IWP in the forward radiative transfer model. In order to improve the thin ice clouds IWP and Dme retrieval accuracy at the middle-high frequency channels in Terahertz band, a dual-channel inversion method was proposed that combines the 448.0± 3.0 GHz and 664.0 ± 4.2 GHz channel. The error analysis shows that the results of the 874.4 ± 6.0 GHz channel and the dual-channel inversion are reliable, and the IWP inversion results meet the error requirement range proposed by previous studies.
Highlights
Several researchers have studied the distribution characteristics and radiative impacts of thin ice clouds [1,2,3]
Improving the microphysical-property inversion accuracy of the thin ice clouds is conducive to studying the radiation effects of ice clouds and reducing the uncertainty in General Circulation Models (GCMs) attributed to ice clouds
To evaluate the inversion results, part of the forward radiative transfer model where Ice Water Path (IWP) and Dme conform to Formula 1 is noted as testing data, and the IWP and Dme in the test set are taken as the true value for inversion
Summary
Several researchers have studied the distribution characteristics and radiative impacts of thin ice clouds [1,2,3]. These researchers demonstrate that the existence of thin ice clouds is ubiquitous, and it plays an important role in future upper troposphere analysis. IWP, ice crystal shape and particle size are the critical parameters that describe and modify the net radiative effect of ice cloud [4]. Improving the microphysical-property inversion accuracy of the thin ice clouds is conducive to studying the radiation effects of ice clouds and reducing the uncertainty in GCMs attributed to ice clouds. The wavelength of THz radiation is in the middle of visible and microwave bands, comparable to the size of typical particles in ice clouds [8]
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