P2b.4 a Comparison of Two Mellor-yamada-based Pbl Schemes in Simulating a Hybrid Barrier Jet

The coastal meteorology of southeastern Alaska frequently consists of intense orographically enhanced winds. Inland cold pools develop as air cools over the interior and accelerates downs the terrain, resulting in strong winds manifested as gap and channel flows, and downslope windstorms depending on the location and synoptic situation. The latter are locally known as “Taku” windstorms (Dierking 1998; Colman and Dierking 1992; Bond et al. 2006). High wind events can extend from the inland waterways to the coast when a low pressure system over the Gulf of Alaska is combined with a high pressure system over northwestern Canada, producing an ambient pressure gradient conducive for accelerating the inland cold pools through the coastal mountain gaps (Colle et al. 2006). Furthermore, coastal gap outflows can then merge with ambient coastal barrier jets to form hybrid barrier jets (Loescher et al. 2006; Winstead et al. 2006). Strong turbulent mixing can occur at the confluent interface between the gap outflow and the ambient onshore flow (Olson et al. 2008). To our knowledge, there have been no detailed studies of the TKE within statically stable gap flows or hybrid barrier jets. The turbulent mixing within these jets will likely regulate their strength, structure, and duration. Accurate modeling of stable boundary layers within mesoscale models has been quite challenging, since commonly observed intermittent turbulence (Nappo 1991; Mahrt 1999, 1998; Howell and Sun 1997; Blumen et al. 2001) violates the assumption of steady-state theory. The ability of current and next-generation operational weather forecast models to simulate the formation of an inland cold pool, their subsequent contribution to coastal gap outflows, and their interaction with coastal barrier jets needs to be assessed. The Advanced Research version of the Weather Forecasting Model (WRF-ARW) (Skamarock et al. 2008) has become host to several planetary boundary layer (PBL) schemes in the past few years. The testing and validation of these and older PBL schemes in the WRF-ARW is important because of the increased emphasis on the prediction of near-surface variables such as wind, ceiling and visibility, as well as the increasing use of WRF-ARW for operational forcasting. A TKE-based schemed called the Mellor-Yamada-Nakanishi-Niino (MYNN) (Nakanishi and Niino 2004) has recently been integrated into the WRFARW. This scheme has potential to help reduce some of the common biases associated with the Mellor-Yamada-Janjić (MYJ) (Janjić 2002) scheme, such as shallow PBL height and low temperature bias (Zhang and Zheng 2004). Model biases of both schemes are relatively unknown in regions of complex terrain and need to be identified and diagnosed in order to further system development. The case study chosen for analysis was taken from the Southeastern Alaskan Regional Jets (SARJET) experiment, which investigated the structures and physical processes of barrier jets along the coastal mountains near Juneau, Alaska (Winstead et al. 2006). This study investigates the performance of the WRF-ARW over the complex coastal orography of Alaska, with focus on the spatial and temporal structure of the hybrid barrier jet and TKE produced by two different TKE-based PBL scemes. The intent is to uncover the reasons why each scheme produces a different structural evolution. To do so, each component of the eddy diffusivity coefficient, Kφ,