Predicting the precise location and timing of the initiation of convective showers and thunderstorms is one of the major uncertainties in numerical weather prediction forecasts of heavy precipitation:Th~ structure and dynamics of convective clouds have beem the focus of substantial research, but the mechanisms governing the initiation of the first cells are one of the least understood aspects. ':r:hree complementary field campaigns have taken place during the last five years, with the same overarching goal of improving forecasts of convective precipi~ation: the International H20 Project (IHOP) iIi 2002, the Convective Storm Initiation Project (CSIP) in 2005 and the Convective and Orographically-induced Precipitation. Study (COPS) in 2007. A major focus of all three projects was improving .the understanding of boundary-layer processes that lead to the formation and development of convective storms. . The main aim of this thesis is to present a detailed observational analysis of the development of the convective boundary layer (CBL) and to highlight precesses that are important for the initiation of convection. Case studies from CSIP and IHOP are examlned by integrating a large number oCdata sets from both routine measurements and state-of-the-art instruments. The work focuses on the variability in the temperature and humidity structure of the evolving CBL and its influence on the development of thermals and cumulus clouds. The first case presents observations of the gradual development of thermals and cumulus clouds as they are at first inhibited by and then grow through multiple stable layers of dry air, knoWn as lids. One particular lid inhibited the development of convection for about an hour, delaying the production of precipitation and causing it to fall about 30 km further north than it would have done without the influence of the lid. The initiation of convection during the second case is shown to be influenced by differential heating on sun-facing slopes of higher terrain and a mesoscale band of higher boundary-layer temperature and humidity. It . . is shown that the CBL is not well-mixed, especially below cumulus clouds. Large fluctuations in temperat~e and humidity are observed as a result of two interacting processes: thermals that carry warm, moist· air upwards, probably from the . surface layer, and descending intrusions of warm, dry air from above the CBL. Similar processes were observed during the third case, but within the framework of open-celled convection.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:487507 |
| Date | January 2007 |
| Creators | Bennett, Lindsay Joanne |
| Publisher | University of Leeds |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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