A differential attenuation based algorithm for estimating precipitation from dual-wavelength spaceborne radar Mircea Grecu and Emmanouil N. Anagnostou Abstract. An algorithm for estimating precipitation from dual-wavelength spaceborne radar measurements is formulated and investigated. The algorithm is based on a single-wavelength attenuation-correction procedure applied within discrete ranges [r 1 , r 2 ] along a radar ray, satisfying the condition r 2 – r 1 ≤ 750 m. The attenuation-correction procedure is based on two parameters, namely the integrated attenuation from the top of the atmosphere to r 1 and the intercept parameter (N 0 * ) of the normalized gamma drop size distribution model. The N 0 * value is assumed constant within the discrete range. A cost function is defined to evaluate the discrepancy between the two-wavelength precipitation estimates obtained from the corrected reflectivity profile within the discrete range. Consequently, the path-integrated attenuation to r 1 and N 0 * is determined through optimization, minimizing the cost function. The algorithm is investigated using synthetic data, i.e., derived from cloud model simulations, and airborne observations from the fourth convection and moisture experiment (CAMEX-4). Résumé. Dans cet article, on propose et on étudie un algorithme pour l’estimation des précipitations à partir de mesures radar satellitaires bi-fréquences. L’algorithme est basé sur une procédure de correction de l’atténuation à une longueur d’onde appliquée à l’intérieur de la portée radar [r 1 , r 2 ] le long du faisceau radar et satisfaisant à la condition r 2 – r 1 ≤ 750 m. La procédure de correction de l’atténuation est basée sur deux paramètres, l’atténuation intégrée du sommet de l’atmosphère jusqu’à r 1 et le paramètre de l’ordonnée à l’origine (N 0 * ) du modèle gamma normalisé de distribution de la taille des gouttelettes de pluie. La valeur de N 0 * est considérée comme constante à l’intérieur de la portée radar. Une fonction de coût est définie pour évaluer les différences entre les estimations de précipitation dérivées d’un modèle à deux longueurs d’onde obtenues à partir du profil de réflectivité corrigée à l’intérieur de la portée. En conséquence, l’atténuation intégrée sur le parcours jusqu’à r 1 et N 0 * sont déterminés par optimisation minimisant la fonction de coût. On examine l’algorithme à l’aide de données synthétiques, i.e., dérivées des simulations de modèles de nuages et d’observations aéroportées acquises dans le cadre de l’expérience CAMEX-4 (« fourth convection and moisture experiment »). [Traduit par la Rédaction] 705 Introduction Accurate estimates of global precipitation are essential for the success of a number of hydrologic, meteorologic, and climatic applications. For example, no rigorous diagnosis of the earth climate evolution can be achieved without accurate information about the global precipitation variability. Precipitation is probably the most important variable needed to evaluate the global water and energy cycle variability. This requirement has justified the launch of spaceborne missions dedicated to measuring precipitation over the globe, and in particular over oceans and remote tropical land that concentrate about two thirds of global rainfall. The joint National Aeronautics and Space Administration (NASA, USA) – Japan Aerospace Exploration Agency (JAXA, Japan) tropical rainfall measuring mission (TRMM) has been the first international effort in this direction (Kummerow et al., 2000). The TRMM satellite features several instruments, including a single- wavelength precipitation radar (PR) and a five-frequency TRMM microwave imager (TMI). The motivation behind this combination of instruments has been that the PR could provide high-resolution observations that may be used to improve the precipitation estimates from the more widely used, less expensive, microwave radiometers. Although this objective has been achieved to some extent (Shin and Kummerow, 2003), there are issues that need to be further investigated to achieve the accuracy required to make these spaceborne retrievals useful in climate and hydrometeorological applications. The two outstanding issues that require immediate attention are the impact of drop size distribution (DSD) variability on the radar (PR) rain retrieval, and the notable systematic and random differences between TMI and PR estimates (Grecu et al., 2004). The information used to mitigate the impact of DSD variability on the PR estimates is not always reliable (Grecu et al., 2004), and therefore future missions (such as the planed global precipitation measurement or GPM) would likely feature © 2004 CASI 697 Can. J. Remote Sensing, Vol. 30, No. 5, pp. 697–705, 2004 Received 30 November 2003. Accepted 1 April 2004. M. Grecu. Goddard Earth Sciences and Technology Center, University of Maryland, Baltimore County, and NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA. E.N. Anagnostou. 1 Department of Civil and Environmental Engineering, University of Connecticut, Storrs, CT 06269-2037, USA. 1 Corresponding author (e-mail: manos@engr.uconn.edu).