Barrier Jets during TAMEX

A barrier jet is frequently found along the northwestern coast of Taiwan in the prefrontal southwesterly flow regime during the Taiwan Area Mesoscale Experiment (TAMEX). It has a maximum wind speed of 14 m s−1 at approximately 1 km above the surface with a vertical wind shear approximately 10 × 10−3 s−1 below and 4 × 10−3 s−1 above that altitude. During TAMEX, the southwesterly monsoon flow strengthens over Taiwan when the low-level pressure trough/surface front moves toward the southeastern China coast. The barrier jet is a result of the stably stratified airflow past an island obstacle under a small–Froude number [<O(1)] flow regime. During the occurrence of a barrier jet, a windward pressure ridge is observed along the southwestern coast due to island blocking. In low levels, the incoming southwesterly flow decelerates off the southwestern coast and moves around the island. Along the western coast, the deflected airflow accelerates northward with a large cross-contour wind component down the pressure gradient along the western coast, resulting in a barrier jet over northwestern Taiwan. The barrier jet is the strongest when the surface windward ridge–leeside trough pressure pattern is most significant and weakens after a surface front arrives.