Abstract
Retrieval of ice cloud properties using passive terahertz wave radiometer from space has gained increasing attention currently. A multi-channel regression inversion method for passive remote sensing of ice water path (IWP) in the terahertz band is presented. The characteristics of the upward terahertz radiation in the clear-sky and cloudy-sky are first analyzed using the Atmospheric Radiative Transfer Simulator (ARTS). Nine representative center frequencies with different offsets are selected to study the changes of terahertz radiation caused by microphysical parameters of ice clouds. Then, multiple linear regression method is applied to the inversion of IWP. Combinations of different channels are selected for regression to eliminate the influence of other factors (i.e., particle size and cloud height). The optimal fitting equation are obtained by the stepwise regression method using two oxygen absorption channels (118.75 ± 1.1 GHz, 118.75 ± 3.0 GHz), two water vapor absorption channels (183.31 ± 1.0 GHz, 183.31 ± 7.0 GHz), and two window channels (243.20 ± 2.5 GHz, 874.4 ± 6.0 GHz). Finally, the errors of the proposed inversion method are evaluated. The simulation results show that the absolute errors of this method for the low IWP cases are below 7 g/m2, and the relative errors for the high IWP cases are generally ranging from 10 to 30%, indicating that the multi-channel regression inversion method can achieve satisfactory accuracy.
Highlights
Ice clouds, often occurring in the upper troposphere and the lower stratosphere, play a significant role in the balance of the energy budget since they can both reflect the incoming solar radiation and trap thermal infrared emission from the lower atmosphere and the surface
The simulation results show that the absolute errors of this method for the low ice water path (IWP) cases are below 7 g/m2, and the relative errors for the high IWP cases are generally ranging from 10 to 30%, indicating that the multi-channel regression inversion method can achieve satisfactory accuracy
The upward thermal radiation generated by the surface and the lower layer of the warmer atmosphere is reduced by the scattering effect of the ice cloud particles
Summary
Often occurring in the upper troposphere and the lower stratosphere, play a significant role in the balance of the energy budget since they can both reflect the incoming solar radiation and trap thermal infrared emission from the lower atmosphere and the surface. For passive remote sensing in the infrared and optical band, because of the high absorbing properties of ice clouds in this band, the penetrability of the electromagnetic wave is very low when cloud is thick. It should be noted that the wavelengths applied by both the optical and microwave instruments to remote sense the ice clouds are much smaller or larger than the actual size of the ice crystals, so the signals received are not very sensitive to the cloud particles. To solve this problem, many researchers are resort to use the terahertz wave, which is located between infrared and microwave band. The method of multi-channel linear regression is adopted to eliminate the influence of other correlative parameters (i.e., particle size and cloud height), so as to realize the inversion of IWP directly by passive remote sensing in the terahertz band
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