Impact of Assimilating Pre-convective Upsonde Observations on Short-term Forecasts of Convection Observed during MPEX

Convection-permitting numerical weather prediction (NWP) models are useful to forecasters tasked with alerting the public of the threat for severe weather. Characteristics of convective storms are strongly tied to the meso- and synoptic-scale environment in which they develop, so it is important to continue to explore ways to improve the depiction of the mesoscale environment in model initial conditions, even for short-term forecasts. Over multiple days in 2013 the mesoscale environment preceding severe convective events was sampled by balloon-borne radiosonde (upsonde) observations released from multiple ground-based mobile facilities as part of the Mesoscale Predictability Experiment (MPEX).

This study addresses a goal of MPEX to explore the impacts of assimilating afternoon pre-convective upsonde observations on the analysis of the mesoscale environment, as well as their impacts on subsequent short-term (0-9 h) convection-permitting forecasts. For eight MPEX cases, ensemble data assimilation is performed on a mesoscale (15-km) grid and the resulting analyses are downscaled to produce forecasts on a convection-permitting grid (3-km). The ensembles of forecasts are evaluated through their depiction of radar reflectivity compared to observed radar reflectivity.

Examination of fractions skill scores over eight cases shows that, for four of the cases (18 May, 23 May, 31 May, and 8 June), assimilation of radiosonde observations nearby to subsequent convection has positive impacts on the initiation and early evolution during the first 3 to 4 h of the forecasts (and longer for the 18 May case), even for the smallest resolvable scales of the 3-km grid. For the four cases in which positive impacts near the smallest resolvable scales of the grid are not seen (19 May, 20 May, 27 May, 28 May), analysis of the changes to the pre-convective environment suggests that sub-optimal locations of the soundings compared to the location of convective initiation are to blame. Regardless, aggregate positive impacts on forecasts of convection is apparent when spatial scales larger than individual thunderstorms are examined.