Small scale simulation and lidar validation of a shallow land-breeze front

On December 21, 1997, as part of the Lake-ICE experiments, the University of Wisconsin Volume Imaging Lidar (UW-VIL) observed a persistent, shallow land breeze circulation off of Sheboygan (WI) point. VIL imagery shows the 50-100 m deep circulation coming to a head against the prevailing easterly (onshore) flow in a frontal zone undulating between 1.5 and 4 km offshore. The visually intriguing nature of this circulation, as well as the influence that similar circulations have been thought to have on shore parallel lake effect snow bands and a curiosity as to whether events at this scale could be accurately replicated with today’s mesoscale models influenced the decision to simulate this circulation using the University of Wisconsin Non-Hydrostatic Modeling System. Using the two-way grid nesting available in the model, a horizontal resolution of 32 meters was utilized over the inner grid in order to try to capture boundary layer structures of 100-150 m in size, and a 20 meter vertical resolution was utilized in order to capture the shallow nature of this circulation. From the simulation, front position, circulation depth, wind speed and direction, boundary layer depth, as well as internal eddy size and structure were examined and compared to lidar data using vector field analysis, and a virtual scattering parameter implemented in the model. The simulation output bears striking resemblance to the lidar imagery in all of these areas, likely due to the fact that the simulation itself is a product of the synoptic situation, in conjunction with local thermal forcing of the land-lake boundary. The ability to replicate small scale features of this scale from a more easily simulated larger scale has interesting implications on the merits of microscale forecasts in the 24-48 hour time frame, and similar simulations could be of great importance to small scale pollution dispersion forecasts, wind energy forecasts, and boundary layer forecasts for aviation applications, to name a few.