Design and implementation of a convection-permitting GSI-based EnKF data assimilation and forecast system for the PECAN field experiment

Aaron Johnson* and Xuguang Wang, Samuel Degelia
University of Oklahoma, Norman, OK

A real time GSI-based EnKF data assimilation (DA) and ensemble forecast system was implemented by the Multiscale data Assimilation and Predictability (MAP) group at the University of Oklahoma during the Plains Elevated Convection At Night (PECAN) field experiment. Since the focus of PECAN is elevated nocturnal convection, extensive experiments of the system configuration were conducted to optimize the system design for this focus. Pre-field phase experiments were conducted using 20 cases of nocturn al convection from the previous year (2014). The experiments focused on the configuration of the cycled GSI-based multi-scale EnKF and the physics configuration of the forecast ensemble.

The advantage of EnKF assimilation of radar observations in 1300 UTC analyses is found to extend through the following night. While sensitivities are found to whether the radar DA is cycled every 5, 10 or 15 minutes, these differences generally do not persist beyond the 12hr lead time, into the nocturnal period of interest. However, 60 minutes of radar DA does show advantages over 30 minutes of radar DA during this period. Sensitivities to the microphysics and PBL schemes used during DA are demonstrated and explained. The impact of different ensemble physics configurations are also evaluated in the specific contexts of nocturnal MCS, nocturnal CI, atmospheric bore environment, and nocturnal LLJ prediction.

The results from the 2014 cases are used to guide the implementation of the real time GSI-based multi-scale EnKF and ensemble forecast system for the PECAN field experiment in 2015. The system consists of a 40 member multi-scale EnKF and 20 member ensemble 2-day forecasts with 4 km grid spacing initialized twice daily. A deterministic 1-day forecast with 1-km grid spacing is also initialized once daily. Subjective and objective verification of these forecasts will be presented with an emphasis on the same nocturnal convective phenomena as the 2014 forecasts.

*Preference: Oral