Abstract
Abstract. Falling raindrops undergo a change in morphology as they grow in size and the fall speed increases. This change can lead to significant effects in passive and active microwave remote sensing measurements, typically in the form of a polarization signal. Because previous studies generally only considered either passive or active measurements and a limited set of frequencies, there exist no general guidelines on how and when to consider such raindrop effects in scientific and meteorological remote sensing. In an attempt to provide an overview on this topic, this study considered passive and active remote sensing simultaneously and a wider set of frequencies than in previous studies. Single-scattering property (SSP) data of horizontally oriented raindrops were calculated using the T-matrix method at a large set of frequencies (34 in total). The shapes of the raindrops were calculated assuming an aerodynamic equilibrium model, resulting in drops with flattened bases. The SSP data are published in an open-access repository in order to promote the usage of realistic microphysical assumptions in the microwave remote sensing community. Furthermore, the SSPs were employed in radiative transfer simulations of passive and active microwave rain observations, in order to investigate the impact of raindrop shape upon observations and to provide general guidelines on usage of the published database. Several instances of noticeable raindrop shape-induced effects could be identified. For instance, it was found that the flattened base of equilibrium drops can lead to an enhancement in back-scattering at 94.1 GHz of 1.5 dBZ at 10 mm h−1, and passive simulations showed that shape-induced effects on measured brightness temperatures can be at least 1 K.
Highlights
Hydrometeors are important components in virtually all applications involving microwave radiation in the atmosphere
This is especially true for frozen hydrometeors, as in recent years there has been a trend towards more sophisticated representations of ice particle single-scattering property (SSP) data (Liu, 2008; Hong et al, 2009; Kuo et al, 2016; Ding et al, 2017; Eriksson et al, 2018)
Show the differences TBv in vertically polarized brightness temperatures between the cloud and rain cases compared to clear-sky case
Summary
Hydrometeors (i.e. atmospheric liquid or frozen water particles) are important components in virtually all applications involving microwave radiation in the atmosphere (microwave communications and remote sensing). The utilization of non-spheroidal raindrop models for passive microwave remote sensing applications is much more limited This is especially true for satellite-based applications where raindrops are generally assumed to be spheres. This limitation in treatment of raindrops comes despite the availability of polarimetric sensors and the fact that several modelling and measurement studies have shown that passive microwave measurements at frequencies up to 40 GHz are influenced by oblate raindrops (Czekala et al, 2001a, b; Battaglia et al, 2009). The impact of rain-induced polarization on sensors that operate at higher microwave frequencies is largely unexplored This is especially problematic with respect to the multitude of satellite-borne sensors in operation, e.g. the CloudSat radar at 94.1 GHz and the GPM (Global Precipitation Measurement) microwave imager (GMI) up to 190.31 GHz, highly important sensors for weather forecasting and climate research. This study contributes to a more realistic representation of liquid hydrometeors and provides guidance on the suitability of accounting for rain-induced polarization in microwave remote sensing
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