Multi-Year Analysis of Rain-Snow Levels at Marquette, Michigan

This study uses observations from a ground-based instrument suite to investigate the rain-snow level (RSL) in stratiform rainfall from January 2014 to April 2020 in the Upper Great Lakes Region. The height above the surface where ice melts to rain, the rain-snow level, influences microphysical assumptions in remote sensing precipitation retrievals and the ability of space-based radar to discriminate surface precipitation phase because of ground clutter. The instrument suite is installed at the Marquette, MI (MQT) National Weather Service station adjacent to Lake Superior. Rain events and the RSLs are studied through a ground-based vertically profiling radar (Micro Rain Radar), a custom NASA-developed video disdrometer (Precipitation Imaging Package), and reanalysis products from ECMWF ERA5 and NASA MERRA-2. Distinct macro and microphysical characteristics are observed in precipitation events with shallow RSLs (<1.8 km above ground level [AGL]) and intermediate RSLs (>1.8 km AGL). Intermediate RSLs correspond to rain events with relatively higher rain rates and a higher concentration of small drops in the drop size distributions (DSDs). Shallow RSL DSDs contain relatively higher concentrations of large drops with lower fall speeds suggesting that partially melted snowflakes may be reaching the surface. Reflectivity-rain-rate relationships are also impacted by microphysical differences associated with RSL regimes. Radar-detected RSLs agree with reanalysis-derived melt levels-especially at wet-bulb isotherms of 0.5 degrees C and 1 degrees C. Seasonal differences such as shallow RSLs in winter, fall, and spring have subsequent implications for satellite detectability. Plain Language Summary The height above the surface where falling snow melts to rain, the rain-snow level (RSL), can be detected by both ground-based and space-based radars. However, space-based radars are limited in their ability to capture precipitation near the surface due to interference. This work investigates RSLs between January 2014 and April 2020 from ground-based observations at the Marquette, MI National Weather Service office. This work includes observations from ground-based profiling radar and a custom NASA-developed instrument that records high resolution videos of precipitation at the surface. In addition, profile temperature and moisture data are used. These products are commonly used with satellite observations to determine the surface precipitation phase. The results show different characteristics for rain events with shallow and intermediate RSLs. The radar-detected RSLs illustrate good agreement with melt levels derived from reanalysis profile data, which is useful for satellite retrievals of precipitation. Seasonal differences such as shallow rain-snow levels in winter, fall, and spring have subsequent implications for satellite detectability.