A Comparison of Model and Observed Turbulent Kinetic Energy Within Coastal Barrier Jets Forced By Landfalling Cyclones

Abstract

The coastal region of western North America frequently experiences intense orographically enhanced wind and rainfall when cyclones make landfall. The prefrontal lowlevel winds can undergo upstream deceleration and deflection in the coastal regions, producing strong barrier jets (Macklin et al. 1990; Overland and Bond 1993 and 1995; Loescher et al. 2006; Olson et al. 2007). Inland cold pools (Colle et al. 2006), which are common to this region, can be accelerated through the coastal mountain gaps (Lackmann and Overland 1989; Colle and Mass 2000); thus, further impacting the development of barrier jets along the coast of Alaska (Olson et al. 2007; Olson and Colle 2008). The ability of current and nextgeneration operational weather forecast models to simulate the inland cold pools, their subsequent formation of coastal gap outflows, and their interaction with coastal barrier jets produced by landfalling cyclones needs to be assessed. New PBL schemes are being developed and compared to current operational schemes in order to test and improve our ability to predict the strong turbulence (Smedman et al. 1995; Bond and Walter 2002) and wind stress forcing of local currents and storm surges (Orr et al. 2005) that are associated with these coastal jets. The National Center for Environmental Prediction (NCEP) currently employs the Rapid Update Cycle (RUC; Benjamin et al. 2004), which uses 1h assimilation at 13km grid spacing to provide mesoscale guidance for shortrange forecasts. Among the many significant weather phenomena, the RUC is relied upon for the prediction of turbulence and high speed wind events by the marine and aviation industries. The current RUC modeling system will be replaced with a new highfrequency system, the Rapid Refresh (Benjamin et al. 2006), with operational transition at NCEP planned in 2009. The Rapid Refresh (RR) forecast model component will be the Advanced Research version of the Weather Research and Forecast model (WRFARW; Skamarock et al. 2005). The current RUC domain will be expanded in the Rapid Refresh to cover a larger North American domain, which will include the complex orography of Alaska and Canada. As part of the development and testing of the RR, new planetary boundary layer (PBL) schemes are being developed and compared to current operational schemes. Model errors over this expanded region are relatively unknown, especially for shortrange forecasts as produced by the RUC or RR, and need to be identified and diagnosed in order to further system development. This study evaluates the performance of the WRFARW and investigate the model errors over the complex coastal orography of Alaska, with focus on the spatial and temporal structure of the turbulent kinetic energy (TKE) and the fluxes of heat and momentum within coastal barrier jets. Highresolution model simulations are compared with TKE and flux measurements within a barrier jet sampled by the Wyoming KingAir research aircraft during the Southeastern Alaskan Regional Jets (SARJET) field experiment. Some questions this study attempts to address are the following: · How well can the WRFARW simulate the coastal barrier jets and the associated TKE along the coastal orography of Alaska? · How is the TKE distributed relative to the jet for both classical and hybrid barrier jets? · How do the simulated TKE structures change with resolution? · What are limitations the 13km grid spacing planned for the WRFRapid Refresh on model errors adjacent to steep coastal terrain?