Orographic Effects on Cold Surges and Lee-Cyclogenesis as Revealed by a Numerical Experiment

Part II. Transient Aspects

Abstract

A numerical experiment using a 10-layer hemispheric dry model produced prominent low-level cold surges and subsequent lee-cyclogenesis on the east and southeast sides of a hypothetical facsimile of the Tibetan Plateau. These phenomena were preceded by a sequence of events that occurred along the western, northern, and eastern periphery of the prescribed mountain.
(1) When a major anticyclone approaches the western end of the mountain, cold air advected from the north accumulates in the lower troposphere due to the barrier effect of the east-west elongated mountain.
(2) The cold air pool is then advected rapidly eastward along the northern periphery of the mountain by the prevailing low-level westerly jet. Associated with this cold air pool is the development of a small-scale edge anticyclone trapped below 700 mb near the northeastern side of the mountain.
(3) The clockwise (southward) movement of the edge anticyclone is substantially reduced east of the mountain. However, it is still faster than the movement of the edge cyclone which has already passed the eastern periphery, and became nearly stagnant at the southeast corner of the mountain. The phase speed difference between the edge anticyclone to the north and the edge cyclone to the south, results in a sudden increase in the pressure gradient with prominent subgeostrophic northerlies east of the mountain. These subgeostrophic northerlies are responsible for the strong advection of cold air from the north. Here, the advected cold air pool takes a form similar to a gravity-type shock (cold) front.
(4) The edge cyclone begins to shift eastward by the prevailing southwesterlies near the southeast corner of the mountain. The subsequent phase is the amalgamation of the edge cyclone with a major trough which has travelled north of the mountain along about 50°N. Also occurring during this merging phase is the eastward extension of cold front from the southeastern corner of the mountain to as far downstream as eastern China. The merged lee-cyclone intensifies into a major mid-latitude system due to the active baroclinic conversion process occurring around the extended cold front. To the rear of this intensified lee-cyclone are widespread northerly cold surges, which penetrate southward to the South China Sea.