Environmental Conditions of Lightning Events during the Ontario Winter Lake-effect Systems Project 2013-14
A major goal of this study was to determine under what conditions lake-effect snow clouds generate lightning. Previous studies have shown most lake-effect lightning associated with the Great Lakes occurs early in the cold season (November and December) and is confined to over or near the lake. Lightning was reported (by humans and/or computer detection systems) during 5 OWLeS events: 11 Dec 2013, 18 Dec, 7 Jan 2014, 20 Jan, and 27 Jan. The most prolific event occurred on 7 Jan, when the Earth Networks Total Lightning Network (ENTLN) detected 24 flashes between 0630 and 1130 UTC. The 850 hPa temperatures were between -20 and -25°C, with strong west-southwest winds of near 50 kt. A low-level shortwave passage forced the lake-effect band to move south and the reflectivity gradient became very sharp along the northern band edge with many vortices embedded near the time of peak lightning activity (0630 – 0700 UTC). The in-cloud layer where temperatures were between -10 and -25°C (layer where mixed-phase microphysics and charge separation is possible/likely) was 1750 m deep, but the -10°C level was below ground (i.e., very cold conditions). This begs the question: where is the cloud base in a lake-effect snowstorm when relative humidity with respect to ice is 100% from ground to cloud top? This event also featured a very deep boundary layer with tops near 550 hPa. Lake-induced CAPE values (calculated using NAM model soundings via the BUFKIT computer program) were greater than 2100 J kg-1 and the lake-induced equilibrium level was 5.2 km! In contrast, the 18 Dec event was much warmer and climatologically a more favorable time for lake-effect lightning. Five lightning flashes occurred between 2130 and 2230 UTC. 850 hPa temperatures were near -10°C and winds at this level were westerly at 35 kt. Surface temperatures were near 0°C. The in-cloud layer conducive to charging was only 1200 m deep and the boundary layer top was 725 hPa. It is also important to note the band structure on 18 Dec was quite broken (“convective”) while it was more solid during the 7 Jan case.