CloudSat spaceborne 94 GHz radar bright bands in the melting layer: An attenuation‐driven upside‐down lidar analog

Abstract

The CloudSat satellite supports a W‐band (94 GHz) cloud profiling radar. At this 3.2 mm wavelength, ground‐based measurements of rainfall associated with melting snowflakes do not show the radar reflectivity peak that is characteristic of bright band measurements at longer (Rayleigh scattering‐dominated) wavelengths. Nonetheless, examination of downward‐looking CloudSat returns in precipitation often indicate an obvious signal peak in the melting region. Through melting layer microphysical and scattering model simulations, we demonstrate that this downward‐viewing radar feature is analogous to the lidar bright band observed from the ground in that it owes its existence to strong attenuation. In the upward‐looking lidar case, the strong attenuation comes from large low‐density snowflakes. In the downward‐looking 94 GHz radar case, it is due to the effects of the greater refractive index of water particles compared to ice: it is comparable to an upside‐down lidar bright band. A W‐band radar dark band, which contributes to the visibility of the bright band, is shown to be due to attenuation in the snowfall. For comparison, the bright and dark bands for an upward viewing lidar are also modeled: the latter is simulated by a reduction in light backscattering efficiency of ice‐containing raindrops.