Shear-Driven Turbulent Eddies within Frontal Systems Approaching Steep Terrain and the Impact on Precipitation

Large-eddy simulations (LESs) from the Weather Research and Forecasting (WRF) model nested down to 111-m grid spacing and field observations from the Olympic Mountain Experiment (OLYMPEX) are used to investigate large turbulent eddies within landfalling frontal systems and their impacts on precipitation. Large shear eddies, some of which were coupled with shallow convective cells, were observed by NPOL radar at the Washington coast and the DOW radar over the windward valley within many precipitating frontal systems and were well simulated by WRF-LES. We hypothesized that flow blocking by the Olympics would enhance the vertical wind shear, which in turn would enhance the eddies and convective cell generation. We hypothesize that these eddy-driven convective cells would increase the accretional growth, thus explaining why the LES run has 10-15% more precipitation than the non-LES (1-km) grid.

Large turbulent eddies were observed within the prefrontal periods during several cases during OLYMPEX in 2015 (12-13 Nov, 16-17 Nov, 03 Dec, 05 Dec, 08 Dec, 12 Dec, and 17 Dec). Most of the cases were characterized by strong vertical wind shear and a stable layer at low levels as the front passed, followed by reduced stability above the shear but underneath the melting layer. The 12-13 Nov, 08 Dec, and 17 Dec events were simulated by the WRF LES and the eddies were realistically predicted. The large shear eddies are characterized by periodic and comparable updrafts and downdrafts. The temperature perturbations are in quadrature with the vertical velocity perturbations, resulting in enhanced water vapor transport associated with the updraft branch of the overturning eddies. Meanwhile, the shear-induced updrafts may penetrate into a less stable layer, which was enhanced by melting-induced diabatic cooling. The convective cells triggered by these shear eddies are characterized by filaments of significantly amplified updrafts but weak compensating downdrafts on the order of 1-km. WRF-LES experiments without Olympic mountain for 12-13 Nov, 08 Dec, and 17 Dec events show that the terrain-blocking effect detected in the upstream coastal, valley, and windward slope amplify the magnitudes of the shear-driven turbulent eddies and terrain-forced lifting favor the enhancement of convective cells. A microphysical budget highlights how the convective cells triggered by the shear eddies helped increase the accretional growth by 20-40%, thus reducing the precipitation underprediction in the LES run as compared to the 1-km grid.