Planetary Boundary Layer Parameterization's Control on Ensemble Forecasts of Convection Initiation

In this study, we evaluate the influence of varying planetary boundary layer parameterization upon short-range (3-12 h) ensemble forecasts of convection initiation across the central United States for three aircraft reconnaissance missions conducted by the Mesoscale Predictability Experiment (MPEX). The three missions selected are each characterized by a different synoptic-scale flow configuration across the Great Plains: southwesterly flow (Research Flight 4; 19 May 2013), westerly flow (Research Flight 10; 31 May 2013), and northwesterly flow (Research Flight 12; 8 June 2013). The ensemble Kalman filter implementation within the Data Assimilation Research Testbed software package as coupled to version 3.4.1 of the Advanced Research version of the Weather Research and Forecasting mesoscale model is utilized to obtain thirty-member short-range ensemble forecasts.

All ensemble forecasts utilize initial conditions generated through a cycled data assimilation process that incorporates targeted MPEX dropsonde observations. Forecasts for a given mission begin at 1500 UTC, extend forward 15 h, and are conducted on a domain encompassing the conterminous United States with 3 km horizontal grid spacing and 51 vertical levels. For a given mission, five thirty-member sets of ensemble forecasts are conducted, each utilizing a different planetary boundary layer parameterization. The chosen parameterizations are identical to those considered by Coniglio et al. (2013, Wea. Forecasting): the local-mixing Mellor-Yamada-Janjic (MYJ), Mellor-Yamada-Nakanishi-Niino level 2.5 (MYNN2.5), and Quasi-Normal Scale Elimination (QNSE) parameterizations and the non-local-mixing Asymmetric Cloud Model version 2 (ACM2) and Yonsei University (YSU) parameterizations.

Convection initiation events across the central United States are identified from objects in the convectively-active field with observed or simulated radar reflectivity of 35 dBz or greater at the -10 C level for at least thirty minutes. All convection initiation events occurring between 1500 UTC and 0600 UTC are considered. For each case considered, all ensembles are found to overproduce the number of convection initiation events, with the ACM2-based ensembles exhibiting the smallest degree of overproduction (~1.5 times observed) and the MYJ-based ensembles exhibiting the greatest degree of overproduction (>2 times observed). For the cases considered, the overproduction of convection initiation is greatest during the local early afternoon hours and smallest during the local mid-to-late evening hours. The presentation will focus upon the evaluation of potential causes of this overproduction (including insufficient entrainment by simulated updrafts and overly-energetic turbulent circulations within the planetary boundary layer), why it preferentially occurs during the local daytime hours for the cases considered, and its implications upon the predictability of convection initiation.