Mesoscale Thermally-driven Circulations and Their Associated Moist Convection PDF Download

Author: Chun-Chih Wang
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Languages : en
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Book Description
"Mesoscale thermal circulations regulate important meteorological phenomena like convection initiation and boundary-layer pollutant transport. Due to their turbulent and complex nature, these circulations and their effects remain inadequately understood. This dissertation advances this understanding by utilizing intensive field-campaign observations and complementary cloud-resolving numerical simulations of two such circulations (mountain-plain solenoids and lake/sea breezes) and the cumuli they initiate. The first objective was to quantify different types of thermal circulations and their associated moist convection, which was addressed by intensive analysis of observations and numerical simulations of multiple real-world events. In two case studies of lake breezes over southern Ontario, it was found that the ascent along the lake-breeze front increased substantially (by around 40%) after two opposing lake breezes merged over the Niagara Peninsula. This updraft enhancement was necessary to generate precipitating convection in both cases. However, the depth, intensity, and duration of this convection differed greatly between the two cases. In a case study of diurnally forced cumulus convection over the mountainous Caribbean island of Dominica, the onset of diurnal heating quickly generated localized upslope flows and shallow cumuli over small-scale ridges, followed by an 'island-scale' solenoidal circulation. As the day progressed, the island cumuli deepened and multiplied, reaching peak heights of around 4 km and covering up to 40% of the island, but generated little to no radar-observed precipitation (less than 1 mm/day).The second objective was to evaluate the sensitivity of thermal circulations and associated convection to environmental and terrain-related parameters, again by synthesizing observations and numerical simulations. In the lake-breeze studies, subtle differences on the larger-scale (e.g. horizontal advection, synoptic descent) were found to be the dominant factors behind the large difference in convection intensity between the two cases. By contrast, the mesoscale subcloud forcing of the lake breezes was very similar in the two cases. In the Dominica study, stronger or more cross-island ambient winds weakened the island circulation by reducing the thermal contrast that drove it. Compared to simulations of a flattened Dominica, the presence of the rugged Dominica terrain increased the island cloud fraction by creating more localized convergence pockets. However, contrary to previous findings that higher terrain tends to produce stronger thermal circulations, the presence of the island orography weakened the island-scale circulation by about 15%.The final objective was to better understand the complex interactions between thermal circulations and their associated moist convection and between different types of thermal circulations. Numerical sensitivity experiments were conducted to isolate the impacts of three moist-convection feedbacks on the parent thermal circulations and the effects of up-mountain flows on sea breezes. The findings revealed that moist convection could feed back either positively (cloud latent heating) or negatively (cloud shading and precipitation) onto the parent circulation. Ultimately, the negative effects dominated to weaken the thermal circulation that triggered the convection. In addition, the inland propagation speed of the sea-breeze front (SBF) increased over taller islands due to increased advection by the stronger up-mountain flow. However, both the SBF updraft and sea-breeze circulation weakened over taller islands, similar to the above-mentioned tendency for the island-scale circulation to weaken over a more mountainous Dominica. Using scaling arguments, it was shown that the weakening of thermal circulations over taller islands stemmed from the mountain protrusion into the free troposphere, which weakened the pressure gradient driving the onshore flow"--