Underprediction of Precipitation 

Field observations from the Olympic Mountain Experiment (OLYMPEX) around western Washington State during two atmospheric river (AR) event in November 2015 were investigated within the Weather Research and Forecasting (WRF) Model. 

Impacts of Large Turbulent Eddies on Orographic Precipitation

Large-eddy simulations (LESs) from the Weather Research and Forecasting (WRF) model and field observations from the Olympic Mountain Experiment (OLYMPEX) are used for an atmospheric river (AR) event on 12-13 Nov 2015 in order to investigate whether turbulence may be enhancing the precipitation and thus help explain the underprediction of precipitation in weather forecasting models as non-LES (~1-km grid spacing) scales. The period of interest is during the prefrontal period from 1800 UTC 12 Nov to 0300 UTC 13 Nov when there were well-defined shear eddies and convective cells in the radar observations. LES with 111-m grid-spacing shows an average of about 13.4% more precipitation at the coastal/lowland sites and 5.83% over the windward sites in comparison to WRF with 1-km grid-spacing (1km-WRF). Thus, the average precipitation deficits were effectively reduced by 30.64% and 57.6%, respectively.

We hypothesize that the poorly resolved shear-driven large turbulent eddies and small-scale convective cells (~ 1km) in weather forecasting models that may be leading to the under-prediction of precipitation. The southwesterly low-level jet (LLJ) associated with a warm front rapidly transports moisture and warmer temperatures to the mountain at speeds over 25, whereas airflow underneath the LLJ is trapped by terrain. Strong vertical wind shear associated with the strong stratification results in layers of shear-driven turbulent eddies overlying and underlying the LLJ. Simultaneously, the melting-induced diabatic cooling effects gradually destabilize a thin depth of atmosphere underneath the melting layer. After 2300 UTC 12 Nov, shear eddies become convectively developed. These observed shear eddies and small convective cells are well resolved using LES whereas are poorly simulated by 1km-WRF. The shear-driven turbulent eddies themselves in a stably stratified flow produce competing effects through enhanced microphysical processes in updrafts, while this can be offset by the compensating subsidence, and thus result in little changes of volume-integrated net condensation rate, precipitate formation rate, etc. Hence it shows little impacts on area-averaged precipitation. However, the eddies are shown to produce convective precipitation cells, and the corresponding microphysical processes (e.g. condensation, accretion, etc.) in updrafts are significantly amplified and overwhelm and yield significantly enhanced precipitation by about 30%. Additionally, the stationary turbulent eddies triggered by terrain reinforce the intensity of convective cells advected from upstream and produce 25% more precipitation over the windward slopes.