Session 6.2 The interaction of simulated squall lines with idealized terrain

Tuesday, 5 October 2004: 1:45 PM
Jeffrey Frame, Penn State University, University Park, PA; and P. Markowski

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We present simulations of squall lines traversing idealized terrain features, including a detailed examination of a convective line moving over a 900 m high and 20 km wide sinusoidal ridge. The simulations show that the terrain weakens the convection, but that it is able to quickly restrengthen downstream of the ridge due to dynamical processes at the gust front. The sloping terrain causes the cold pool nose to become flatter and shallower, creating less lift at the gust front and weaker convective cells. If the slope is steep enough, the rear inflow jet is forced to ascend orographically, disrupting inflow to the updrafts and further weakening the convection. As the squall line begins to descend the ridge, the flow in the cold pool head transitions from subcritical to supercritical, then back to subcritical at the bottom of the hill. A hydraulic jump forms when the flow transitions the second time, enabling the development of a new convective line on the lee side of the downslope. These new updrafts deprive the older ones of inflow and eventually replace them. A higher mountain makes the squall line weaken more, while a wider ridge forces the squall line into a weaker state for a longer period of time.
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