Tuesday, 4 August 2015: 5:30 PM
Republic Ballroom AB (Sheraton Boston )
The distribution of radar-estimated precipitation from long-lake-axis-parallel (LLAP) bands over and downwind of Lake Ontario shows larger snowfall amounts in downwind areas than over the lake itself (Veal and Steenburgh 2015). Here we examine two possible mechanisms for this, i.e. the collapse of convection that keeps hydrometeors lofted over the lake, and isentropic lifting over land, aided by a hill downwind of Lake Ontario (Tug Hill). This study mainly uses data from an airborne Doppler cloud radar, the Wyoming Cloud Radar (WCR), as well as from an array of four Ka-band Micro Rain Radars (MRRs), an X-band Profiling Radar (XPR), and a Doppler on Wheels (DOW) scanning X-band radar. Vertical profiles of reflectivity (VPRs) and vertical velocity (VPVs) from the radars in a LLAP band are examined, from offshore to onshore. This LLAP band occurred on 11 December, 2013 during the field operations deployment for the Ontario Winter Lake-effect Systems (OWLeS) project. The standard deviation, skewness, and peak values of the WCR vertical velocity are larger over the lake, and decrease moving inland, suggesting convection collapse. This decrease is consistent with the change in precipitation texture, mostly convective offshore and more stratiform over land, which is consistent with isentropic lift there. The latter mechanism also explains the increase in mean WCR vertical velocity over land. Particles tend to have a greater fall speed in the LLAP band over the lake, although the signal is weak compared to the noise in terminal velocity estimation, i.e. the difference between the WCR (hydrometeor) vertical velocity and the gust probe (air) vertical velocity. This suggests more riming over the lake, consistent with the convective updrafts there. The level of maximum reflectivity increases moving east over the lake, and then decreases with inland progression, again consistent with convection collapse. The difference in reflectivity and X, Ka, and W bands at nearly the same time and location indicates that snow diameter increases at low levels over land areas nearest the lake, yet decreases further inland, again consistent with the transition from convective to stratiform precipitation.
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