Combined radar-radiometer retrievals from satellite observations

Abstract

A major challenge in precipitation estimation from satellite microwave observations stems from the fact that the distributions of precipitation particle sizes are highly variable in time and space. Mathematically, the particle size distribution (PSD) variability can be expressed through analytical functions (such as Eulers gamma function) of at least two or three independent parameters. Even for short time periods and atmospheric volumes small enough to be characterized by constant values of these parameters, precipitation retrieval is subject to uncertainties because the information necessary to estimate the parameters associated with the PSD within such volumes is incomplete. For example, single frequency spaceborne radars provide only reflectivity observations, while the PSDs on which these observations depend are functions of three variables. The problem of determining three independent variables from a single observation is mathematically ill-posed (i.e., it does not have a unique solution, insensitive to small variations in the input data). In practice, it is customary to determine the sensitivity of the observations with respect to the PSD parameters and solve for the parameter that has the largest impact on the observations, while setting the other parameters equal to constant values determined from independent observations. When independent observations are available, it is possible to derive more accurate solutions simultaneously solving for more parameters and reducing the numbers of parameters that have to be set to a priori values. This kind of approach can be applied to dual-frequency radar observations or to combined radar radiometer observations. Recent developments in the area of precipitation retrieval from combined radar and radiometer observations have been motivated by the deployment of the Tropical Rainfall Measuring Mission (TRMM) satellite. TRMM features a Precipitation Radar (PR) operating at 13.8 GHz and a nine channel TRMM Microwave Imager (TMI) (Kummerow et al. 1998). Inconsistencies in the early version of TRMM products (Kummerow et al. 2000) suggested that both the PR-only and TMI-only estimates might be subject to systematic errors. Combined radar radiometer retrievals are deemed to be less prone to systematic errors than retrievals from individual instruments because the number of assumptions that need to be made to make the retrieval problem mathematically well-defined is smaller. This is the reason why combined retrievals drew considerable attention in the years immediately preceding and following the TRMMs launch in 1997. Here, the most recent developments in the area of satellite combined and radiometer retrievals are described. Strengths and limitations of combined retrievals are discussed. Conclusions and recommendations on further work are presented as well.