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

东太平洋的一小块区域，墨西哥马德雷山脉以西，是热带气候形成的有利地点。目前公认的热带气旋形成理论认为，在相当温暖的海面上，盛行的偏东热带气流中预先存在气旋扰动，这是飓风形成的必要条件（但不是充分条件）。Zehnder和Gall建议继续探索他们的假设，即在合适的攻角下，流经墨西哥中部山脉的水流会对地形障碍下游的水流产生旋风扭曲。他们之前的研究表明，某些驻波和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，这种行星尺度的波可以产生稳定的背风面槽和次级天气尺度的行波，它们可以作为热带气旋的初始位置。他们建议在继续研究时采用数值模式模拟和对观测数据集的进一步分析，包括1991年夏季将在该地区获得的数据集。为什么热带气旋喜欢在墨西哥西海岸附近形成，这个问题不仅因其本身而有趣。它的答案也应该揭示一般的热带气旋形成，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.