Scale Dependence of Model Simulated Coastal Low-Level Wind Jets

Meteorological conditions in the coastal regions are different from both the inland and marine conditions. The flow pattern in coastal regions is impacted by land–sea temperature contrasts, complex terrain, shape of the coastline etc. Along the US west coast the large-scale flow is determined by the summertime position of the North Pacific subtropical high pressure, located at about 1000 km west of California and a thermal low over the continental USA. Winds in the boundary layer are mainly from a north or northwesterly direction, roughly parallel to the coastline. Frequently, the coastal low-level wind field is characterized by a sharp wind maximum. This phenomenon, commonly referred to as a Low-Level Coastal Jet (LLCJ), have significant impact on the climatology of the coastal region and influences aviation, shipping, fishing and a number of human activities along the littoral zone and better understanding and forecasting of LLCJ features is vital.

The purpose of this study is to illustrate the effectiveness of the US Navy's COAMPS (Coupled Ocean-Atmosphere Mesoscale prediction System) in resolving this type of marine boundary weather phenomenon. The study will focus on 1) overall skills of COAMPS in capturing the temporal and spatial structure of the LLCJ and its characteristic features, and 2) significance of the model horizontal resolution in terms of the realistic representation of captured features. Although, it is well established that higher resolution produces more realistic small-scale features knowing a lower bound for necessary resolution can guide the applicability of the model for real time situations. The model is run in a one way nested setup of 54 km, 18 km, 6 km and 2 km horizontal resolutions respectively, along the California coast for a period of 4 to 7 August 2004. Model results are verified against observed vertical profiles of wind speed and direction and temperature from radiosondes from an area of supercritical flow adjustment between Pt. Sur and Pt. Conception.