Sensitivity of Boundary Layer Characteristics and Related Low-Level Jet Behavior to Planetary Boundary Layer Schemes in the WRF Model for Several MCS Cases

The Great Plains low-level jet (LLJ) is an important influence in the development of nocturnal convection through the transfer of heat and moisture. It is well known that simulations of nocturnal convection pose a particular challenge for numerical models, but it is less clear what role shortcomings in the simulation of the LLJ play in errors in the depiction of MCS characteristics such as precipitation and morphology. Errors in the simulation of the LLJ can stem from errors in depiction of the planetary boundary layer (PBL), and these errors would likely vary among the different PBL schemes used within forecasting models.

This study investigates the impact of the use of several different PBL schemes in the WRF model on boundary layer characteristics, and how these differences in boundary layer characteristics during the afternoon affect the formation and evolution of the nocturnal LLJ on nights when MCSs were present in the central United States. Three PBL schemes are investigated: Mellor-Yamada-Janjic (MYJ), Mellor-Yamada-Nakanishi-Niino (MYNN), and Yonsei University (YSU). A set of 30 LLJ cases previously used to explore relationships between errors in LLJ properties and errors in MCS QPF is used to examine the differences in afternoon PBL behavior as different PBL schemes are used, along with differences that occur in the LLJs several hours later. The LLJ cases were split into two groups: stronger, synoptically-forced events and weaker, locally forced events in order to see how the amount of larger-scale forcing might affect the relationship between PBL properties and the LLJs that develop.