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Ching-Sen Chen, Wen-Sheen Chen, and Zensing Deng

Abstract

The field program TAMEX (Taiwan Area Mesoscale Experiment) was held during May and June 1987. One of its objectives was to study the cited of terrain on precipitation systems. On 7 June 1987 a band of radar echo, orientated from north to south, developed during the afternoon along the western slope and mountainous area of Taiwan island. Before this system moved eastward toward the Pacific Ocean in the late afternoon, it dumped more than 100 mm of precipitation at a few stations in only a few hours. The analysis of radar data from CAA radar revealed that the precipitation occurred over western-sloped terrain and a mountain plateau in the early afternoon. The system was wider than 60 km in the east-west direction, and the echo top was higher than 10 km. The maximum reflectivity was over 50 dBZ along the steep slope and near the mountain peak. The precipitation system over the mountain area extended eastward with the passage of time; meanwhile, new echoes continually formed along the western-sloped area and moved eastward. They intensified as they moved toward the mountain peak merging with the precipitation system. Through this mechanism the precipitation system could maintain itself for several hours and produce a large amount of rainfall.

A two-dimensional numerical cloud model with a terrain-following coordinate system, similar to the one developed by Durran and Klemp, was used to investigate the topographic effect on the precipitation system. A smoother terrain feature was used for the lower boundary, with a 30-km-wide mountain plateau (of less than 1 km in height) and sloped terrain on the western and eastern sides. Surface heating and boundary-layer moisture supply were parameterized in the model. Simulation results indicated that during the early simulation a cell formed near the foothills of the west slope and moved eastward. As it climbed up the sloping terrain it intensified. Its speed decreased and its high intensity was maintained over the slope and the mountain plateau. At the same time, a new cell formed west of the older cell and moved eastward. Finally this new cell merged into the western side of the older one near the mountain peak to form one precipitation system and moved eastward slowly. Thus, the intensity of the merged system was enhanced over the mountain plateau. While this system maintained its high intensity and moved eastward, new cells continually formed along the western slope and moved eastward to merge into the western side of the precipitation system over the mountainous area. The intensity of the precipitation system was enhanced for a few hours over the mountain itself and became a long-lasting system. Toward the end of the simulation, this long-lasting system had moved near the eastern slope and had still maintained its intensity. At the same time, the low-level temperature decreased over the mountainous area as a result of precipitation evaporation. When new cells, forming over the western slope, moved toward the mountain plateau, they entered their decaying stage 45 min after their occurrence. They did not merge into the existing system on the eastern part of the mountain; therefore, the precipitation over the mountain plateau became weaker.

Several sensitivity tests have been made to study the effect of varying the magnitude of surface heating, the boundary-layer moisture supply, the height of the terrain, and the temperature, moisture, and wind profiles on the simulation result. The result indicated that low-level and midlevel moisture were important for the formation of new cells over the western slope and a long-lasting system over the mountain area, respectively. The initial wind speed of 7 m s−1 below 4 km and calm wind above 4 km was used in the model; then a long-lasting precipitation system over the mountainous area appeared. If the wind speed was reduced to 3.5 m s−1, only new cells formed over the western slope. If the maximum height of the terrain was decreased from 1 to 0.5 km, then only new cells formed over the slope area. Hence, sensitivity tests indicated that the combination of the adequate thermodynamic structure, the westerly wind pattern, and the correct size of the mountain could help form both the new cells over the sloped terrain and a long-lasting system over mountain areas as in northern Taiwan on 7 June 1987 during TAMEX. The surface heating effect played the role of creating the upslope wind and augmentation of this precipitation system.

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Yeong-Jer Lin, Hsi Shen, Tai-Chi Chen Wang, Zen-Sing Deng, and Robert W. Pasken

Abstract

A thermodynamic retrieval method was used to study the dynamical and thermodynamical structure of a subtropical squall line, which occurred on 17 May 1987 over the Taiwan Straits. Three-dimensional wind fields were derived from the dual-Doppler data based on the methodology presented in Part I of this paper. Subsequently, fields of perturbation pressure and temperature were retrieved from the detailed wind field using the three momentum equations.

Results show that the overall structural features of this subtropical squall line are similar to those of a tropical squall line. The orientation of the squall line is almost in a north–south direction. In the lowest layer, the gust front is located to the immediate east of the main convective updrafts. High pressure occurs behind the gust front with low pressure to its east. A buoyancy-induced low pressure area lies beneath the convective updraft corresponding to the ascending warm environmental air. In the middle and upper layers, high pressure forms on the upshear side with low pressure on the downshear at the leading edge. The orientation of horizontal pressure gradients is approximately in the direction of the average shear vector in the domain. The retrieved temperature field agrees well with the updraft–downdraft structure. The convective updrafts are warmed by the release of latent heat by condensation. Conversely, cooling prevails in the convective downdrafts due, in part, to evaporation. Precipitation loading further decreases buoyancy of the downdraft air in the high reflectivity areas. To the rear of the main (old) cells the rear-to-front air is negatively buoyant resulting in a sloping downdraft. As the cool descending midtropospheric air approaches the surface, it spreads out to form a cold outflow behind the gust front. Part of the descending air moves forward, colliding with the advancing environmental warm air at the leading edge to form new cells ahead of the old cells. The interplay between a cell's cold surface outflow and the low-level shear within the system contributes to the maintenance of the subtropical squall line. The momentum budget calculation shows that the horizontal and vertical flux convergences/divergences of horizontal momentum by the mean and eddy motions are the major contributor to maintain the mean momentum.

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