Jordan Laser - September 1

School of Meteorology Colloquium

Evaluation of the Warn-on-Forecast System with Doppler Lidar and Mobile Radiosondes from TORUS 2019

Jordan Laser 

Tuesday, September 1^st

1:00 PM Central Time (US and Canada)

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Doppler Lidar and mobile radiosonde observations from the Targeted Observations with Radars and UAS of Supercells 2019 (TORUS2019) field project are compared to the environmental guidance NSSL Warn-on-Forecast System (WoFS). The aim of WoFS is to provide short-term, storm-scale, probabilistic guidance for severe thunderstorm hazards. As the evolution of convective storms are modulated by the properties of the inflow environment, an accurate prediction of these properties is important for forecasting the evolution of an individual thunderstorm. However, the scarcity of routine observations of the inflow environment of thunderstorms makes evaluation of the quality of WoFS environmental forecasts difficult. Therefore, this study will assess the accuracy of WoFS predictions of the inflow environment of supercells through comparisons to TORUS2019 Doppler lidar and radiosonde observations.

Three case studies from TORUS2019 are explored to evaluate WoFS forecasts: 17 May, 20 May, and 23 May. 16 radiosonde observations and 5 lidar deployments are available for comparison during WoFS forecasts. All 16 radiosondes made it above 3 km so SRH comparisons can be made with all. However only 5 radiosondes traveled high enough (equilibrium level, EL) to compute CAPE, a testament to the tumultuous environment in which they were released. Five lidar deployments are analyzed, three of which are about 1 hour long. Three out of four observed supercells on these days produced a tornado within 1 hour and 40 km of a lidar deployment.

WoFS over forecasted mixed-layer and surface based convective available potential energy (MLCAPE  and SBCAPE) in all 5 radiosonde observations available for comparison, owing largely to positive surface temperature error and negative temperature error above the PBL. The YSU planetary boundary layer (PBL) scheme consistently forecasted lower CAPE and thus had the lowest error. Similarly, YSU often forecasted lower storm-relative helicity (SRH), a common trend as Cohen et al. (2015) and (2017) found. SRH comparisons show WoFS typically under forecasting SRH at later times in the evening on 17 May and 23rd, owing to differences in storm motion and more backing in the observation. WoFS well forecasts SRH on 20 May, with small negative errors in YSU and MYJ; The observation falls within the 50th percentile of MYNN for 4 out of 6 observations.

The lidar deployments show WoFS modifying the near-storm environment as the storm approaches similarly to that of the observation, except when the intensity of the mesocyclone is variable. Following a drop in rotational velocity of the mesocyclone, WoFS backs the winds during the first deployment on 20 May, however the observations do not back, likely as a response to the variable mesocyclone intensity. The lidar deployment on 17 May observed a supercell with constant rotational velocity and WoFS forecasts backing of the winds, but not to the same extent as in the observation.

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