Numerical Simulations of the Barrier Jet over Northwestern Taiwan during the Mei-Yu Season

Abstract

Orographic effects on the airflow over Taiwan under the prevailing southwesterly monsoon flow are studied using the fifth-generation Pennsylvania State University?National Center for Atmospheric Research (PSU?NCAR) Mesoscale Model (MM5). In particular, the orographically induced low-level strong winds along the northwestern coast of Taiwan are simulated in the absence of radiative forcing, thermal forcing from the surface, and synoptic forcing. The momentum balance for the low-level strong winds along the northwest coast of Taiwan shows that, above the mixed layer, the alongshore force balance is dominated by the inertial advection term and the pressure gradient force term. The significant Lagrangian flow acceleration of the northern branch of the deflected airflow along the northwest coast is mainly related to the pressure gradient force as the airflow moves down the orographically induced pressure ridge along the windward coast. Even though the alongshore pressure gradient force along the northwest coast is largest at the surface, the observed and simulated wind profiles show that the wind speed maximum in the vertical of the barrier jet occurs at a height of ?800 m primarily due to frictional retardation in the lowest levels. Within the well-mixed layer, the alongshore force balance is a three-way balance between inertial advection, pressure gradient force, and frictional force. Normal to the northwest coast, the force balance is approximately geostrophic. In addition, the location and the strength of the orographically induced low-level strong winds along the northwest coast of Taiwan also depend on the impinging angle and the wind speed of the prevailing winds.