Physical properties of maritime low clouds as retrieved by combined use of Tropical Rainfall Measurement Mission Microwave Imager and Visible/Infrared Scanner : Algorithm

摘要

Satellite remote sensing studies on the microphysical and optical properties of clouds have constructed an active research field in the last decades. Clouds are observed over a wide spectral range from the visible/infrared to the microwave, and either shortwave or microwave measurement is used to evaluate the liquid water path (LWP). On the other hand, to date, there have been few cloud studies based on combined measurement by a visible/infrared imager and a microwave radiometer aboard the same platform. In this paper a physical inversion algorithm for the combined use of visible/infrared and microwave sensors is proposed to retrieve the cloud physical quantities such as LWP and the effective droplet radius, each of which is determined in two different ways. The current version of the algorithm has been developed for application to the Tropical Rainfall Measurement Mission (TRMM) sensors, i.e., Visible and Infrared Scanner (VIRS) and TRMM Microwave Imager (TMI). The cloud top temperature obtained from the VIRS analysis is used as an input to the TMI analysis to reduce uncertainties in estimation of LWP. Total errors in LWP are estimated to range from 11 to 30 g/m2. In the algorithm the beam-filling efficiency of clouds for TMI footprints is corrected by the cloud fraction evaluated from the VIRS measurements. For application, global analysis is performed with 3-monthly data from January to March 2000. The scatter diagram of the shortwave-retrieved LWP (LWPshrt) versus the microwave-retrieved LWP (LWPmicr) shows characteristic trends for both precipitating and nonprecipitating clouds. Vertical inhomogeneity of the cloud droplet size accounts for small excess of LWPshrt over LWPmicr for nonprecipitating clouds, while precipitating clouds produce LWPmicr larger than LWPshrt, owing to the presence of raindrops. These tendencies are reinforced by examination of the global distributions of the shortwave-retrieved droplet radius Re(NV) and the microwave counterpart defined by LWP divided by the cloud optical thickness Re(MV). The result implies that difference in those effective radii reflects a microphysical mechanism to expedite or suppress the conversion of the cloud water into rainfall.