High-Resolution Polarimetric Radar Observation of Snow- Generating Cells Karly Reimel May 10, 2016

Front Range Orographic Storms (FROST) Project Goals: Investigate fine scale structure of winter storms along the Colorado Front Range Document and quantify the microphysical processes occurring in winter storms Relate remotely sensed quantities to the surface precipitation type and intensity

Project Instruments CSU-CHILL radar in X-band only mode NCAR-XPOL mobile radar WSR-88D near Denver (KFTG) National Weather Service soundings Soundings launched from the Marshall Field Site (MFS) Automated snow gauges at MFS through the Solid Precipitation Intercomparison Experiment (SPICE) WHY X-BAND? 1.Attentuation and non-Rayleigh effects are not as prevalent in winter storms 2.Shorter wavelength allows for larger K DP values, which are necessary for detecting dendritic growth zones

Previous studies have shown: Generating cells are small scale convective towers that are usually located at or near cloud top Horizontal scale is 1-2km, with the vertical scale slightly less than this Updraft speeds are generally around 1m/s What is not as well understood: characteristic spacing between generating cells (ranges from 5 to 32 km) What height or temperature level they generally form (ranges from -12C to - 40C) Source of convective instability

Project Results: 19 snowstorms were analyzed Every case included generating cells, even though RHI scans were limited No preferred height or temperature level Frequency and spacing varied greatly Widths were generally ~1km, some reached 1km in height as well Complete picture of microphysics uncertain in generating cells since supercooled liquid cloud droplets may not be seen by radar

Shallow Upslope Flow Layer Enhanced Z DR and smaller Z H indicate pristine anisotropic ice crystals Moving towards the surface Z H increases while Z DR decreases, indicating aggregation of descending crystals

Storm Structure Shallow Upslope Layer Enhanced Z DR and smaller Z H indicate pristine anisotropic ice crystals Moving towards the surface Z H increases while Z DR decreases, indicating aggregation of descending crystals Generating Cells Similar set up to shallow upslope but more convective Strong updrafts could also support supercooled liquid water and thus the chance for riming to occur

Shroud Echoes Enhancement of Z DR and reduced Z H along the periphery of generating cells Most likely indicative of pristine anisotropic crystal-like plates or platelike dendrites This may be due to larger particles falling out, leaving these particles behind Not found in every generating cell- highly dependent on crystal growth habits Radar might not be able to see shroud echoes due to crystals being very small or due to beam broadening as you move away from the radar

Life Cycle of a Generating Cell Echo top increases in height at a rate of 2m/s Echo top becomes quasi-stationary while the 10dBZ contour moves upward in height This increased gradient is possibly caused by particles growing in size within the generating cell 20dBZ echo appears, also indicating growth

Life Cycle of Generating Cells Echo top remains constant while 20 and 10 dBZ contour descend Indicates fallout of particles Generating cell diminishes vertically Z H enhancement moving towards the surface suggests aggregation/riming

For some of the cases, soundings were launched from MFS and compared to RHI radar profiles taken near the same time over MFS Potential instability was calculated by determining where equivalent potential temperature decreases with increasing height Regionas of potential instabilty corresponded to the location of generating cells Potential Instability

Sources of Potential Instability Potential instability is very localized, occurring in MSF soundings but not necessary DNR soundings Cloud top cooling paired with heating associated with depositional and riming growth of snow crystals in the cloud could create steep lapse rates conducive for convection Advection of drier and cooler air aloft due to synoptic set up could also play a role

Vertical Shear Previous studies suggest vertical shear as a source of instability Upright stature of generating cells suggests little shear substantial low level shear At upper levels, where generating cells are located, the shear is much smaller Radar indicates vertical motion above shear layers, indicating that shear is not the source of instability

Conclusions Generating cells were found in every case, suggesting that they are common in winter precipitation Horizontal and vertical dimensions were ~1km and updrafts were 1-2m/s in general, consistent with literature Potential instability was found within generating cells when soundings were launched through storms Vertical wind shear was not significant in these cases Some generating cells had shroud echoes characterized by enhanced Z DR and weakened Z H suggesting riming/aggregation occurs within the core of the cell Implied presence of supercooled liquid water is important for numerical modeling and aviation icing concerns.