Degradation of Radar Reflectivity by Cloud Attenuation at Microwave Frequency

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, simulations 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 humilis) 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 Ϫ5 dBZ 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 attenuation (PICA)] is not negligible and, depending on the frequency, can be very strong: the higher the frequency, the stronger the PICA. At 3 GHz, the far less attenuated frequency, PICA for the vigorous cumulus congestus alone in the convective cell (embedded into the stratiform background) is on the order of 1.2 dB (1.5 dB) at 10 GHz, 16 dB (20 dB) at 35 GHz, and 80 dB (100 dB) at 94 GHz. For weaker cumulus, PICA is lower but, in certain cases, significant. All these results mean that it is necessary to be very careful about radar measurements if reliable information on precipitation-for example, the precipitation rate R-has to be deduced, particularly at high operational frequencies.

Comparisons between S‐, C‐ and X‐band polarimetric radar observations and convective‐scale simulations of the HyMeX first special observing period

“…Back‐scattering from cloud droplets is usually very small (less than −5 dBZ) and can be neglected. Attenuation by cloud has been found to be non‐negligible in the X band (up to 1.2 dB) by Pujol et al (), but in this study, as a first approximation, back‐scattering and attenuation by cloud water are ignored, as in CA06.…”

Simulation of airborne radar observations of precipitating systems at various frequency bands

“…Only the lower part of the nimbostratus is modified, suffering a general decrease of reflectivity of about 10 dBZ. At K a -band, degradation is worse and at W-band there is no more signal below 2 km of altitude due to liquid hydrometeors attenuation [7]. Bright band reflectivity is also considerably decreased, from about 50 dBZ at S-band to 25 dBZ at W-band.…”

L'estimation du risque météorologique par les radars embarqués sur les avions commerciaux

“…Sauvageot, 1992 ;Doviak et Zrnic, 2006 (Pujol et al, 2007), ce qui introduit une sérieuse difficulté dans la correction de l'atténuation. Quant aux gaz atmosphériques, ce sont des causes d'atténuation plutôt faibles, mais cependant non négligeables puisque le chemin optique est long, le risque devant être évalué à longue distance.…”