The Impacts of Precipitation Efficiency and Water Budget of Squall-Line Mesoscale Convective System by Terrain

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Ming-Jen Yang (National Central University)

There have been many observational and modeling studies on precipitation efficiency of convective systems, but our understanding of the impacts of precipitation efficiency of a squall-line type mesoscale convective system (MCS) is still quite limited. In this study, the Weather Research and Forecasting Model (Version 3.5) with a horizontal grid size of 2 km is used to explicitly simulate the convection and precipitation of a squall-line type MCS and its interaction with an idealized bell-shape mountain. The thermodynamic environment condition is taken from a sounding in southern Taiwan during the SoWMEX/TiMREX project. The simulated squall line with a leading line and trailing stratiform precipitation reaches its mature stage before encountering the terrain of 2-km height. Convection is enhanced during the windward slope and suppressed on the lee side. Following the motion of major convective cells at the leading convective line, both the precipitation efficiency and condensation ratio are increased on the upwind slope and decreased on the lee side. The reverse trend is found for evaporation ratio and deposition ratio. Water vapor and condensate (liquid and ice phases) budgets are examined to explain for changes of precipitation efficiency and microphysical ratios of the squall-line MCS during the windward ascent and downslope descend. When the squall line climbs up the mountain, horizontal vapor convergence increases and updrafts at the leading line strengthen significantly, resulting in more rainfall (also more precipitation efficiency) by enhanced vapor condensation and raindrop collision and coalescence. On the other hand, as the squall line descends down the hill, horizontal vapor divergence increases and updrafts at the leading line further weakens, producing less rainfall (also less precipitation efficiency) by enhanced raindrop evaporation and ice-phase deposition. Similar evolutions of water budget and precipitation efficiency are also found for principal rainbands of Typhoon Morakot (2009) over the Central Mountain Range on Taiwan.

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