High Resolution Simulation Using the VLES Configuration of WRF-ARW of a Low Level Jet Observed over the Vale Do Cobrão during the Perdigao Microscale Field Campaign of 2017

During the months of May and June of 2017, the Perdigao (Portugal) field program was executed as a collaborative effort between the University of Notre Dame, Colorado University, Cornell University, University of California Berkeley and the University of Oklahoma, in partnership with NOAA National Severe Storms Laboratory and the Army Research Laboratory, to study atmospheric dynamics in complex terrain with applications to wind energy, air pollution in urban basins, aviation, alpine warfare and firefighting. The main focus area for this microscale study was Vale do Cobrão, a valley in eastern Portugal. Many different wind flow patterns forced by the complex terrain were observed throughout the field measurement campaign, and extensive measurements were made of velocity, turbulence, temperature, moisture and radiation.

Vale do Cobrão is nestled below two near parallel ridges, with land cover consisting of farmland, vegetation (including patches of eucalyptus), canyons, gullies, and a river. The wind tends to flow perpendicular to the ridges, although flow reversal and gustiness do occur quite frequently. During a number of the intensive observation periods (IOPs) of the study, a complex small-scale nocturnal low level jet structure was observed to develop over the valley. The valley spans about 6 kilometers long and 2 kilometers wide in dimensions, and the bounding ridgeline extends up to a couple of hundred meters above the valley floor. A typical mesoscale numerical weather prediction model (such as the WRF-ARW) may only offer one grid point to resolve the valley. However, in this study, the very large eddy simulation (VLES) nesting configuration of WRF-ARW, experimented with in recent years by NCAR, was used by ARL to resolve aspects of the low level jet structure. Similar to NCAR, the ARL implementation of VLES WRF-ARW was nested using echelons of 24.3 km, 8.1 km, 2.7 km, 900m and 300 m. The number of vertical levels employed was ninety. In terms of physics and dynamics/diffusion settings, values fairly close to those used by NCAR (with a few modifications) were used. Using a finest grid spacing of 300 m, it is hoped that some details of the structure can still be captured at this relatively coarse model resolution for capturing the presence of the valley. An IOP was selected for study and numerical simulation where observations from Perdigao indicated the presence of this structure. No data assimilation was used, so the simulation was essentially a downscaling from a recent NCEP global model GFS (1/2 degree resolution) forecast.