Abstract
[1] Measurements of down‐welling microwave radiation from raining clouds performed with the Advanced Microwave Radiometer for Rain Identification (ADMIRARI) radiometer at 10.7–21–36.5 GHz during the Global Precipitation Measurement Ground Validation “Cloud processes of the main precipitation systems in Brazil: A contribution to cloud resolving modeling and to the Global Precipitation Measurement” (CHUVA) campaign held in Brazil in March 2010 represent a unique test bed for understanding three‐dimensional (3D) effects in microwave radiative transfer processes. While the necessity of accounting for geometric effects is trivial given the slant observation geometry (ADMIRARI was pointing at a fixed 30° elevation angle), the polarization signal (i.e., the difference between the vertical and horizontal brightness temperatures) shows ubiquitousness of positive values both at 21.0 and 36.5 GHz in coincidence with high brightness temperatures. This signature is a genuine and unique microwave signature of radiation side leakage which cannot be explained in a 1D radiative transfer frame but necessitates the inclusion of three‐dimensional scattering effects. We demonstrate these effects and interdependencies by analyzing two campaign case studies and by exploiting a sophisticated 3D radiative transfer suited for dichroic media like precipitating clouds.
Highlights
[2] three‐dimensional (3D) radiative transfer (RT) effects within cloudy atmospheres have been theoretically quantified via sophisticated radiative transfer tools [e.g., Marshak and Davis, 2005], their observation has been always extremely elusive
Observational studies toward 3D effects have been statistical in nature, for example, by analyzing satellite measurements in ways that illustrate dependencies that are inconsistent with the assumption of 1D RT
Thanks to their proximity to the target which results in narrow field of view (FOV), ground‐based radiometry has a huge potential in that respect because polarization features produced by 3D structures can be observed without having to contend with nonuniform beam filling (NUBF) effects, which tend to smooth them out
Summary
[2] three‐dimensional (3D) radiative transfer (RT) effects within cloudy atmospheres have been theoretically quantified via sophisticated radiative transfer tools [e.g., Marshak and Davis, 2005], their observation has been always extremely elusive. In particular we aim at validating the conjectures and predictions proposed by notional RT studies with field measurements Thanks to their proximity to the target which results in narrow FOV, ground‐based radiometry has a huge potential in that respect because polarization features produced by 3D structures can be observed without having to contend with NUBF effects, which tend to smooth them out. ADMIRARI measurements comprise TBs at vertical and horizontal polarization at its three frequencies (10.7–21.0–36.5 GHz); the TBs were complemented by slant reflectivity profiles observed at 24.1 GHz by a Micro Rain Radar (MRR) [see, e.g., Peters et al, 2002] at 30° elevation angle with 300 m range resolution and 31 bins.
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