Lee Cyclogenesis: Its Role in Tropical Storm Development in the Eastern Pacific

东太平洋的一个小区域，位于墨西哥马德雷山脉以西，是热带环流发生的有利位置。目前公认的热带气旋形成理论认为，在足够温暖的海面上，盛行的东热带气流中存在气旋扰动，这是飓风形成的必要条件(但不是充分条件)。Zehnder和Gall提议继续探索他们的假设，即在适当的攻角下，墨西哥中部山区的气流提供了地形障碍物下游气流的气旋扭曲。他们之前的研究表明，某些驻波和缓慢行波与地形相互作用，在该地区热带气旋数量高于平均水平的年份更为活跃。他们还从理论上证明，这种行星尺度的波动可以产生一个稳定的背风槽和次级行进天气尺度波列，这可能是热带气旋的初始位置。他们建议采用数值模式模拟以及对观测数据集的进一步分析，包括将于1991年夏季在该地区获得的数据集，以继续进行研究。为什么热带气旋喜欢在墨西哥西海岸附近形成，这个问题不仅因为它本身而有趣。它的答案应该也能从总体上阐明热带气旋的发生，并可能导致飓风预报的改进。

A small area of the eastern Pacific, to the west of the Sierra Madre Mountains of Mexico, is a favored location for tropical cy- clogenesis. Currently accepted theories of tropical cyclone form- ation involve the preexistence of a cyclonic disturbance in the prevailing easterly tropical flow, over an adequately warm sea sur- face, as a necessary (but not sufficent) condition for the incep- tion of hurricanes. Zehnder and Gall propose to continue to explore their hypothesis that flow over the mountains of central Mexico provides, under suitable angles of attack, cyclonic distortion of the flow downstream of the orographic obstacle . Their previous research has shown that certain standing and slowly traveling waves, interacting with the topography, are more active in years with above average numbers of tropical cyclones in this region. They have also demonstrated theoretically that such planetary-scale waves can generate a steady lee-side trough of realistically small scale and trains of secondary traveling synoptic-scale waves which may serve as the initial locations of tropical cyclones. They propose to employ numerical model simulations as well as further analyses of observational data sets, including one to be obtained in the area during the summer of 1991, in their continuation of the research. The question as to why tropical cyclones like to form off the west coast of Mexico is not only interesting for its own sake. Its answer should also shed light on tropical cyclogenesis in general, and could lead to improvements in hurricane forecasting.