The Influence of Mesoscale Orography on a Coastal Jet and Rainband

Abstract

The role of mesoscale orography along the central California coast in the development and evolution of a coastal jet and rainband is investigated using a high-resolution, triply nested, nonhydrostatic numerical model. Comparison of the model simulations, which use horizontal grid increments of 5 and 2 km on the inner computational meshes, with a coastal mesoscale observation network indicates that the finescale structure of the jet and rainband dynamics are adequately simulated, although phase and orientation errors occur. The observed and simulated near-surface winds have maximum speeds that exceed 22 m s⁻¹ and a direction nearly parallel to the coastline and topography. Force balance analysis indicates that blocking in the lowest 500 m and flow over the coastal range above this layer contribute to mesoscale pressure perturbations, including pressure ridging upstream of the coastal mountains, which forces the ageostrophic dynamics of the coastal jet. Pressure perturbations associated with the topographic flows induce a complex mesoscale response that adds rich mesoscale structure to the jet including a wake region that forms on the lee side of the coastal range that limits the horizontal scale of the jet. Sensitivity test results underscore the multiprocess character of the coastal dynamics and the importance of the coastal topography and differential frictional drag at the land–sea interface for the formation and amplification of the jet. The mesoscale response to steep coastal topography results in a 45% enhancement to the near-surface jet strength. The onshore movement of line convection at the leading edge of a weak front is impeded by steep coastal topography in both the radar observations and numerical simulations. Low-level blocking forces the rainband to emulate a wedge-shaped structure with a coastal jet that is dynamically trapped between the steep coastal topography and the front.