Browsing by Subject "Thunderstorms"
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Item An evaluation of lightning flash characteristics using LDAR and NLDN networks with warm season southeast Texas thunderstorms(Texas A&M University, 2008-10-10) Jurecka, Joseph WilliamA comparison of flash parameters from the National Lightning Detection Network (NLDN) is made with data obtained from the Houston Lightning Detection and Ranging II (LDAR) network. This research focuses on relating the peak current and number of strokes in a negative flash (multiplicity) of lightning with the spatial extent and mean altitude of three-dimensional lightning in 1407 flashes as mapped by the LDAR network. It is shown that increasing negative multiplicities over the range two through ten exhibit, on average, a higher flash extent with higher multiplicities. Singlestroke flashes have mean heights of nearly 2 km greater. Higher order multiplicities (2 to 10+) were correlated with mean source heights near 8 km. Increasing multiplicity tends to be associated with greater flash extents increasing more horizontally than vertically with a 50% to 70% increase in flash extent. No obvious relationship between peak current and flash extent was observed. Examining peak current and mean height shows that low current flashes (<10kA) exhibit higher mean heights. However, this may be due to intra-cloud only flashes being reported as cloud to ground events by the NLDN. Bipolar flashes do not show much variation with height and flash extent with the exception of negative-first bipolar flashes, which exhibited mean flash extents twice that of other types. Finally, the flash detection efficiency is 99.7% within 60 km of the network center.Item Surface layer characteristics of thunderstorm outflow(Texas Tech University, 1997-12) Weinbeck, Stephen W.The objective of this study is to define the turbulent characteristics of thunderstorm outflow winds in the surface layer. Thunderstorms are one of the most common types of severe weather, and the climatological record for many stations shows many extreme wind events can be identified as having been generated by thunderstorms. However, until recently it has proved difficult to separate which high wind events are caused by thunderstorm outflows, and which are generated by typical extratropical low-pressure systems. The advent of the Doppler weather radar and the deployment of a national network of these radars across the continental United States provides a powerful new tool for the classification and analysis of thunderstorm outflow events. Studies have shown that in many locations the strongest winds on record are generated by thunderstorms (Twisdale and Vickery, 1992, 1995). In many cases, these straight-line winds can cause damage similar to that caused by tornadoes. Besides causing structural damage near the surface, the lifting of less dense ambient air by thunderstorm winds can be the focusing and/or riggering mechanism for further thunderstorm development to occur. It is the modification of the environment surrounding the storm, and the localized nature of the outflow and cloud system that has made the investigation and forecasting of thunderstorms difficult. It is possible to be very near a severe thunderstorm and yet not feel the effects of the rain, hail, or outflow winds. In many cases the thunderstorms are not captured by the surface observation network that the National Weather Service (NWS) has set up to observe weather systems that occur on much larger scale.