Use of In-Situ Airborne Measurements of Cloud Microphysical Properties to Quantify Processes Occurring in Wintertime Snow Storms

Winter snowstorms, which are frequent in mid-latitudes and along the eastern seaboard of the United States and Canada, can have devastating economic impacts and can result in loss of life. In order to improve model simulations and produce better quantitative and precipitation type forecasts of such events, it is necessary to better understand the multi-scale dynamical and microphysical processes occurring in these storms. To accomplish this, synergistic use of remote sensing retrievals, in-situ microphysical observations and model simulations with various temporal and spatial scales is required. In this presentation, the use of airborne in-situ observations and derivations of cloud microphysical properties from a variety of field campaigns for hypothesizing processes occurring in wintertime snowstorms and for use in improving parameterizations of cloud microphysical processes is discussed.

Field projects studying wintertime snowstorms in which the lead author has been involved include the 1992 Canadian Atlantic Storms Program-2 (CASP-2), the 2003 Alliance Icing Research Study II (AIRS-II), the 2009-2010 Profiling of Winter Storms (PLOWS) project, a 2015 flight of the NCAR High-Performance Instrumented Airborne Platform for Environmental Research (HIAPER), and the 2020, 2022, and 2023 Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS). The first part of the presentation will summarize findings from in-situ cloud observations obtained during campaigns prior to the most recent IMPACTS campaign, particularly PLOWS, that identified some key features of wintertime snowstorms including a characterization of bands of precipitation that are ubiquitous in such system, including the following: mixed-phase cloud regions are typically dominated by either supercooled water or ice crystals, and instances of approximately equal contributions from different phases are rare; transitions between supercooled water and ice-phase regions occur on very small spatial scales, on the order of 100 m; small-scale generating cells are common near cloud top, especially in stratiform areas of such systems and are typically 1 to 2 km deep, 0.5 to 2 km wide, and with updrafts of 1 to 2 m/s; reflectivity fall streaks emanating from such generating cells are responsible for much of the banding of precipitation that is observed in winter storms; ice water contents, ice crystal median mass diameters, liquid water contents and ice crystal concentrations are larger in generating cells than the surrounding environments showing that generating cells provide protective environments favorable for enhanced particle growth even though turbulent mixing lessened the observed differences between cells and the surrounding environments; whereas the generating cells are critical for the nucleation of crystals, the majority of ice crystal growth occurs below the level of the generating cells; in regions of elevated convection, turbulence and mixing within updrafts effectively distributed particles through cloud depth so that there was statistically insignificant dependence of cloud properties with distance below cloud top with the exception of right at cloud top where entrainment effects dominated.

These observations and related modeling studies motivated the IMPACTS study that was designed to focus on East Coast United States snow storms in order to better characterize the spatial and temporal scales and structures of snow bands, to understand the dynamic, thermodynamic and microphysical processes that produce the observed structures, and to apply this understanding of structures and underlying processes to improve the remote sensing and modeling of snow fall. Although the analysis of IMPACTS data is incomplete, some initial findings from IMPACTS will be presented including the following: a unique fitting algorithm that automatically identifies the number of modes in an observed size distribution applied to the IMPACTS data shows that bimodal distributions are common in winter storms; using turbulence probe measurements to identify generating cells, updraft, downdraft and stratiform regions shows that differences between updraft and stratiform regions are not as distinct as observed during prior campaigns; a simple cloud phase identification scheme applied to probes with different resolutions provides information about the ability of this and prior campaigns to uniquely identify crystal habits; a wavelet technique applied to remote sensing and in-situ data allows diagnosis of regions of turbulence and gravity waves, showing turbulence existed on scales of 10 to 200 m and gravity waves on the order of 5 to 20 km for the winter storms sampled; local maxima of ice water content were more apt to occur in regions of statistically significant turbulence.

Needs for future observational campaigns are presented, noting that providing the remote sensing context of in-situ observations is absolutely critical for developing a process-oriented understanding. Further, comparisons of models and observations is critical for truly understanding the role of different processes in generating snow bands.