Polarimetric Radar Observations of Bow Echoes with Descending versus Non-Descending Rear Inflow Jets

Bow echoes are a sub-classification of a Mesoscale Convective System (MCS) characterized by an intense line of convection that takes on a bow-like shape when viewed on weather radar.  While these systems can produce a wide range of severe weather hazards (including large hail, tornadoes, and flash flooding), they are typically recognized to be producers of straight-line damaging winds.  One mechanism by which these storms can produce damaging winds is through the descent of a rear inflow jet (RIJ) near the leading edge of the system.  While this mechanism is well understood, bow echoes with descending versus non-descending RIJs can be difficult to discern operationally due to issues with radar sampling in the near-surface layer.  Unless the storm is very close to the radar site, a lack of data near the surface makes it difficult to determine if the RIJ is descending in a manner that would produce damaging winds at the ground.

This study is using polarimetric radar data to assess the microphysical characteristics of bow echoes with descending and non-descending RIJs.  In a convective storm negative buoyancy, induced by cooling from melting and evaporation, can create a strengthened downdraft and force the descent of a RIJ.  With the recent upgrade of the National Weather Service WSR-88D network to dual polarization, information related to these processes may be available in an operational framework.  For instance, regions of melting hydrometeors  can be identified using the specific differential phase and correlation coefficient products.  Similarly, regions of small liquid drops that may favor enhanced evaporation can be identified using differential reflectivity and specific differential phase. If patterns in these fields unique to descending RIJ cases are identified, they could be used to detect a descending RIJ in the absence of near-surface Doppler wind data.  

Archived polarimetric radar data for bow echo cases from the years 2013-2015 have been gathered and are being analyzed for common polarimetric signatures. Bow echoes that occurred close to radar sites were selected so that Doppler velocity data can capture  RIJ descent, if present.  Additionally, Storm Prediction Center severe weather reports are being analyzed to help confirm likely cases of RIJ descent, based on reports of widespread damaging winds. This presentation will focus on initial results from comparing a preliminary set of descending and non-descending RIJ cases, and lay the groundwork for a more extensive study that will build upon these results.