Degradation of Radar Reflectivity by Cloud Attenuation at Microwave Frequency
640 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 24 Degradation of Radar Reflectivity by Cloud Attenuation at Microwave Frequency OLIVIER PUJOL AND JEAN-FRANÇOIS GEORGIS Laboratoire d’Aérologie, Observatoire Midi-Pyrénées, Université Paul Sabatier, Toulouse, France LAURENT FÉRAL Laboratoire AD2M, Université Paul Sabatier, Toulouse, France HENRI SAUVAGEOT Laboratoire d’Aérologie, Observatoire Midi-Pyrénées, Université Paul Sabatier, Toulouse, France (Manuscript received 2 November 2005, in final form 7 June 2006) ABSTRACT The main object of this paper is to emphasize that clouds—the nonprecipitating component of condensed atmospheric water—can produce a strong attenuation at operational microwave frequencies, although they present a low reflectivity preventing their radar detection. By way of a simple and realistic model, simu- lations of radar observations through warm precipitating targets are thus presented in order to quantify cloud attenuation. Simulations concern an airborne radar oriented downward and observing precipitation at four frequencies: 3, 10, 35, and 94 GHz. Two cases are first considered: a convective cell (vigorous cumulus congestus plus rain) and a stratiform one (nimbostratus plus drizzle) superimposed on the previous one. Other simulations are then performed on different types of cumulus (congestus, mediocris, and hu- milis) with various thicknesses characterized, in a microphysical sense, by their maximum liquid water content. Simulations confirm the low cumulus reflectivity ranging from Ϫ45 dBZ for the weakest cumulus (i.e., the humilis one) to Ϫ5dBZ for the strongest one (i.e., the vigorous cumulus congestus). It reaches Ϫ35 dBZ for the nimbostratus cloud. On the other hand, cumulus attenuation [precisely path-integrated cloud at- tenuation (PICA)] is not negligible and, depending on the frequency, can be very strong: the higher the frequency, the stronger the PICA.
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