Passive Microwave Observations of Mesoscale Snow Bands from NASA IMPACTS

Mesoscale snow bands are common features in winter storms. They are usually identified as thin bands of locally higher radar reflectivity that are tens of kilometers long along their major axis. Several past and current satellites (e.g. GPM, TRMM, AMSR-E) have passive microwave radiometers with swath widths of 750 km or more. Broad geographic analysis of snow bands and their trends and climate variability outside of regions with radar observations, including sparsely populated higher latitudes and the oceans, is of necessity heavily reliant on use of these satellite observations. Hence it is worthwhile to determine to what degrees and in what conditions snow bands are detectable in passive microwave observations.

The NASA IMPACTS field campaign used coordinated flights of two NASA aircraft to collect detailed observations of northeastern US impacting winter storms including their mesoscale snow bands. The NASA ER-2 aircraft flew with multiple radars and passive microwave radiometers onboard suitable for observing the vertical dynamical and hydrometeor structures of snow bands.

The NASA ER-2 fine spatial resolution data sets illustrate the highly 3D nature of mesoscale snow bands. Shear, dispersion, and turbulence commonly produce mesoscale snow bands with a tilted and smeared structure. The coarser vertical resolution of WSR-88D observations can only partially capture these structures. We examine the implications of tilted and smeared 3D snow band structures for the detection and identification of mesoscale snow bands in ER-2 passive microwave data from the AMPR and COSMIR radiometers. These ER-2 sensors are proxies for satellite borne sensors on the GPM satellite. The vertically-integrated passive microwave measurements of ice water content from the radiometers tend to have diffuse and subtle signals of lower brightness temperatures associated with snow bands. Hence these signals are more difficult to distinguish from background values from more vertically upright convective cores in deep convection. The physical inferences from these subtle signals of snow bands have further implications for the impacts of mesoscale snow bands on surface snowfall amounts.