||In this study, idealized numerical simulations of a squall line traversing a sinusoidal mountain ridge are conducted using the Weather Research and Forcasting model, version 3.2, with 2-km horizontal grid size. The vapor and condensate budgets are examined, and the temporal variation of four microphysics ratios, including precipitation efficiency(PE), condensation ratio(CR), deposition ratio(DR), and evaporation ratio(ER) are calculated during and after the period when squall line interacts with the terrain.|
In an Eulerian framework, the whole life cycle of the squall line can be divided into five stages, which include mature, over-windward-slope, over-mountain, over-lee-side and, dissipating stage. When the squall line moves from mature stage to over-windward-slope stage, the corresponding PE increases from 50.42% to 58.71%, due to the increasing horizontal flux convergence of vapor and strong condensation of liquid water. Then, a Quasi-Lagrangian framework is adopted to investigate the “in situ” orographic forcing of the mountain on the microphysics process by following the eastward propagation of the squall line. The result shows that the high PE observed on the windward slope is caused by the increase of cloud condensation and the orographic lifting. On the other hand, the low PE observed on the lee side is a result of strong increase of raindrop evaporation and the decrease of cloud condensation. The vertically propagating gravity waves above the terrain is helpful to transport hydrometeors upward and then let the hydrometeors transform into ice critical or snow, so the DR also shows an increasing trend on the lee side.
Two sensitivity experiments with different terrain height are performed to examine the effect of terrain on microphysics ratios. The half-terrain sensitivity experiment shows that because of the reduced orographic lifting effect, the condensation on the windward slope also decreases, which further results in lower PE. But the lower mountain height makes the blocking- effect occurred at mountain ridge become less significant, so the squall line can traverse the mountain ridge more smoothly and maintain a stronger convective system on the lee side compared to the full-terrain control run. Finally, the result from no-terrain sensitivity experiment shows that without the orographic lifting effect, all of the characteristics associated with the interaction between squall line and terrain disappear.
||Holton,2004 : An Introduction to Atmospheric Dynamic. Academic Press.(text book).|
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