Sam Degelia-February 23

A Study of the Impact of PECAN Field Observations on the Numerical Prediction of Nocturnal Convection Initiation using a GSI-based Ensemble Data Assimilation System

Both the thermodynamic environments and kinematic forcing mechanisms responsible for producing convection are fundamentally different between day and night. Thus, the impacts of storm-scale data assimilation (DA) on forecasts of nocturnal convection initiation (CI) cannot be assumed to be the same as those found during the daytime. The Plains Elevated Convection at Night (PECAN) field project provided a unique observation set related to nocturnal convection, including high-frequency thermodynamic and kinematic profilers and mobile surface observations. This study applies a GSI-based EnKF DA system to evaluate the impact of assimilating the PECAN dataset for a nocturnal CI event on 26 June 2015. CI occurred above an elevated moisture layer located just north of the intersection of a nocturnal low-level jet (LLJ) with a quasi-stationary surface boundary; such features are commonly observed during nocturnal CI events.

Data denial experiments were performed to evaluate the forecast impact of assimilating each PECAN observation type on the CI initiation mechanisms, elevated moisture layer, and the associated nocturnal CI event. A successful forecast of CI timing, location, and orientation was only achieved when assimilating all available PECAN observations. Assimilating PECAN observations had little impact on the initiation mechanisms (isentropic ascent and LLJ). Instead, improvements in the CI forecast largely resulted from increased moisture content above the stable boundary layer by the assimilation of thermodynamic profilers, radio wind profilers, and mobile surface observations. These results indicate that even a small network of non-conventional profilers can improve forecasts of severe convective events at night.