The Great Plains Low-Level Jet During PECAN: Initial Comparisons of Profiling Observations with WRF Model Predictions

During the 2015 Plains Elevated Convection At Night (PECAN) field campaign, several nocturnal low-level jets (NLLJs) were observed with state-of-the-art integrated profiling systems. One such system, the Collaborative Lower Atmospheric Boundary Layer Profiling System (CLAMPS), was developed and operated during PECAN in collaboration between the University of Oklahoma and the National Severe Storm Laboratory. CLAMPS integrates a Doppler lidar, a microwave radiometer, an Atmospheric Emitted Radiance Interferometer (AERI), and a radiosonde system. During PECAN NNLJ intensive observation periods (IOPs), the CLAMPS remote sensing instruments typically started to collect data before sunset at around 0 UTC and operated continuously until an IOP ended between 06-09 UTC, capturing in detail the evolution of the NNLJ and nocturnal boundary layer (NBL). Additionally, during each IOP five or more radiosondes were released to monitor the conditions above the boundary layer and to evaluate the accuracy of the remote sensing data sets. In this study, CLAMPS data sets from three PECAN NLLJ IOPs are interrogated to describe and contrast the evolution of NLLJs and study their role in modulating the structure of the NBL.

In addition to observations, numerical simulations with the Weather Research and Forecasting (WRF) model are used in this study to further investigate the role of mesoscale processes in the formation and evolution of the NLLJ cases observed during PECAN. Previous work identified the optimal horizontal grid spacing, vertical grid spacing, and planetary boundary layer scheme for reproducing Great Plains NLLJs with the WRF model. Comparisons between the observed and WRF simulated NLLJs allow for validation of the model configuration. Validated WRF model outputs then provide more detailed information about the observed NLLJ structure and evolution, as the increased spatial and temporal coverage of the numerical simulations permits a more thorough description of the events. This work gives an overview of the selected PECAN NLLJ cases and presents an initial comparison of the WRF simulations with high-resolution observations from the CLAMPS system. Analyses suggest that the simulated NLLJ typically forms earlier than the observed NLLJ. The WRF simulations produce NLLJs that are stronger than the observed counterparts early in the event, but weaker than the observed NLLJs later on. Sudden variations in the boundary-layer winds seen in the observations on individual nights are well captured in the WRF simulations. The WRF model predicts the height of the NLLJ maximum on a case-to-case basis; sometimes the height is captured quite well while other times the height is overestimated by the model. Potential temperature fields are well represented by the WRF model, as indicated by comparisons with the observed data. Finally, turbulent bursts often appear in the simulated flow fields very similarly to how they appear in the observed data.