Estimation of precipitation from space-based platforms

Abstract

Measuring precipitation intensity from spaceborne sensors is a highly difficult problem whose solution is yet to be completely reached. While the problems of physical and space-time representativeness of groundbased measurements are to some extent typical, also of the space-based ones, spaceborne sensing adds a few more issues that need to be considered when trying to make a quantitative use of data. The indirect character of retrievals from ground-based radars, for example, is even more exacerbated from the satellite passive remote sensing perspective, which deals with radiation scattered or emitted from the clouds in the visible (VIS), infrared (IR) and passive microwave (PMW) spectral bands. These retrievals of precipitation characteristics and intensity from space became significantly less indirect when the Tropical Rainfall Measuring Mission (TRMM) was launched in 1997 (Kummerow et al. 1998, 2000) with the first radar for precipitation ever in space. It is not the scope of the present Chapter to go in depth into the field of rainfall measurements from space and the reader is referred to the book edited by Levizzani et al. (2007), which represents the most recent and perhaps complete overview of the state of the art in the field. Other notable reviews were compiled with a general perspective (Levizzani et al. 2001) and with a focus on over land applications (Petty 1995) and climatology (Kidd 2001). An historical perspective of the field is offered by Barrett and Martin (1981). Note that the problem of measuring precipitation from space can be decomposed into more than one step to be necessarily tackled and solved before pretending to obtain any quantitative result: (1) assess the physical content of the radiance measurements with respect to cloud hydrometeor content and precipitation formation mechanisms, (2) identify cloud type in terms of precipitation content, and (3) delineate precipitation areas. A crude simplification could delimit the problem to two major aspects: (a) delimit rain areas, and (b) quantitatively estimate precipitation. In fact, before making any attempt to estimate the amount of rain falling from a particular cloud seen from a satellite sensor, we must first make sure that the cloud is indeed precipitating and this is far from being an easy task. The first part of the Chapter will briefly examine the status of precipitation estimates from space using the available passive and active sensors and give a perspective on possible improvements from advances in sensor technology and better physical understanding of cloud vertical structure. The second part will discuss the potential of multispectral observations for improving the knowledge of the physical status of a cloud thus allowing a step forward in estimating rainfall.