台灣中部山區中尺度對流系統之雷達分析

• Main Authors: Mou-hsiang Chang, 張茂興
• Other Authors: 周仲島
• Format: Others
• Language: zh-TW
• Published: 1998
• Online Access: http://ndltd.ncl.edu.tw/handle/47164514907904469788

博士 === 中正理工學院 === 國防科學研究所 === 86 === On 20 June 1987 during the TAMEX (Taiwan Area Mesoscale EXperi- ment) IOP11, several mesoscale convective systems occurred at the central part of Taiwan Island with weak vertical wind shear and convective instability. These mesoscale convective systems lasted for more than six hours. The evolution of mountainous convective systems can be separated into three different stages. In the first stage, the isolated thunderstorm cells and merged cells dominated and occurred over the upslope wind region at altitude of 200m-500 m. In the second stage, the system was organized by merging isolated convective cells into a north-south oriented line system along the mountain slope, propagating toward northeast along upslope. During the last stage, the new convective cells formed over the sloping area at altitude of 200m-500m on the southwest flank of the lined parent system and were merged with it. The entire system was quasi-stationary, and oriented from the north-south to the northeast-southwest. The rainfall distribution on the ground was apparently different in the three stages due to the different structures and movements of the convective systems. Specially, the rainfall rate at Tai-Chung and Miao-Li areas had reached the extent of heavy rainfall (more than 50mm/day). The precipitation and kinematic structures of the mountainous multicell thunderstorm and organized mesoscale convective systems were analyzed by using the high resolution data of NCAR CP4 and NOAA TOGA Doppler radars. The development and organi- zation of the storms were analyzed and the possible effects of the prevailing wind, complex terrain and local circulation were discussed. In the first stage, the radar reflectivity showed new thunderstorm cells formed over the sloping area around 500 m altitude and moved toward higher terrain. From dual-Doppler analysis, the isolated thunderstorm cells were dominated by single updraft. The updraft was located in the front of the storms with maximum intensity of 4 ms at 5km height. Low level convergence and upper level divergence were shown in the flow structure. Weak downdraft was found at the rear portion of the storms and separated from the major echo region. No cold pool was observed at low level. These results suggest that the downdraft was not caused by water loading. The entrained air from the unsaturated environment probably played the key role. The system was organized by merging isolated convective cells at different life stages. The cell had distinct downdraft in its mature stage, but no pronounced precipitation accompanied, so as no obvious gust front at the surface. This results suggest that the development of the storm was not owing to the local convergence between the outflows produced by the precipitating thunderstorm and the impinging moist enviromental flow. It also suggests that the lifting mechanism for the new cell development is possibly due to the upslope motion caused by the uneven heating at the terrain slope. This is consistent with the observations that new cells developed first at the slope area. The organized mesoscale convective system in the third stage was analyzed by using the dual-Doppler and single Doppler radar data. The results showed the system was quasi-stationany possibly due to the environment wind and terrain effect. The mainteinance and intensification of the storm were owing to the penetration of sea breeze circulation from coastal area into sloping area and provided the extra moist air. The supply of the moist air and the terrain effect made the great release of latent heat, intensified the buoyancy and increased the upslope winds. Eventually, the intensity of the system was appreciated greatly and it reached higher than 8 km in altitude and 18 m/s in vertical velocity. Then, the horizontal wind and relative vorticity became more intense and well organized, accompanying the feature of vorticity dipole. It seems that the tilting term played a key role on the organized and intensified vertical vorticity .The retrieved perturbation pressure field from dual-Doppler data for the organized mesoscale convection appeared a pair of high and low pressure systems. The pressure gradient force of the storm was influenced by the environment wind and terrain effect. This arrangement of the pressure systems didn''t take advantage to the convective system propagation toward northeast of the high sloping area. The major mechanism for the formation of perturbation pressure was buoyancy force. The maximum vertical velocity was located at the maximum perturbation temperature area. It suggests that the lifting mechanism for the third stage of mesoscale convection is due to the latent heat release.