| Millimeter-wave is close to the size of cloud particles, so millimeter-wave radar has now been constructed and operated for cloud observations. The ground-based 37-GHz and 94-GHz meteorological Doppler radars have been the main implements to detect clouds. In the CloudSat Mission there will be a 3-millimeter cloud profile radar (CPR). It is very interesting to investigate the combination of CloudSat/CPR with TRMM/PR which has been in space to get the information of the vertical structure of clouds from thin to thick clouds. Thereinto, the correction of attenuation is a big problem in millimeter-wave cloud radar detection. A study of the characteristics of atmospheric attenuation at 37 and 94 GHz is presented in this paper. The primary aim of this study is to provide some basic information to the atmospheric correction for 94 GHz cloud radar. The atmospheric attenuation at 37 and 94 GHz have been calculated and analyzed in different cloudy atmospheres. The feasibility of performing the correction is discussed on the basis of dual- or triple- wavelength transmission and backscattering measurements.This paper calculates the 94GHz and 37GHz attenuation in various model atmosphere by using Liebe's algorithms, and also the microwave attenuation of cloud by using Rayleigh's approximation . As for a space-based cloud radar, the change of the radar reflectivity n and the change of attenuation rate along with the cloud liquid water content have been compared between 37 GHz and 94GHz. The transmittance T of single level cloud for different heights and liquid water content has also beencalculated at 2 frequencies. By defining A=n tR, which combines both effects of radarreflectivity and transmittance, we have examined the values of A at 37GHz and 94GHz for high, middle and low cloud liquid water contents.Main conclusions drawn from the analyses of calculating results are as follows: (1) The microwave absorption of atmosphere gas have obvious attenuation at the wavelengths for cloud detection, thereinto, the water attenuation effect changes greatly, so the 94GHz cloud detecting radar should have water vapor attenuation correction scheme. (2) When 37GHz and 94GHz radars detect clouds, the difference between the atmosphere and cloud attenuation and the large variety of radar reflectivity conduce the intension of the radar backscatter signals change. (3) For the thin cloud layer and low water content of cloud, 37-GHz radar backscatter signals are not as good as 94-GHz radar, that is to say that the 94GHz radar has better capability in thin clouds detection. For thick layer and high water content of cloud, due to strong attenuation effect, the 94-GHz radar backscatter signals are less then the 37-GHz radar. (4) Considering thedisadvantage factors in atmospheric attenuation, the space-based 94-GHz radar had best to detect high-level thin clouds. When it uses to detect low thin clouds, the atmospheric gas attenuation should be corrected. For the strong attenuation due to thick clouds, the ability to detect the middle and bottom part of clouds will be reduced greatly, retrieval of vertical structure of the whole cloud requires attenuation correction with odier information. (5) In order to have the ability to detect the vertical structure of clouds form thin to thick clouds, the radar systems of cloud observations in future had better use dual- or triple- wavelength (eg., 94, 37 and 13 GHz). |
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