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Class #26: Friday October 30 Thunderstorms Class #26: Friday, October 30, 2009 1 Conditions for thunderstorm formation (continued) • For the more severe thunderstorms, vertical shear of the horizontal wind – The westerly (west to east) part of the wind increasing as height increases – Clockwise turning of the direction from which the wind blows as height increases • For example, southeasterlies at the surface, southerlies at 850mb, southwesterlies at 700mb, and westerlies at 500mb Class #26: Friday, October 30, 2009 2 Class #26: Friday, October 30, 2009 3 An ordinary or air mass thunderstorm Class #26: Friday, October 30, 2009 4 Life cycle of a thunderstorm cell/air mass thunderstorm Class #26: Friday, October 30, 2009 5 Class #26: Friday, October 30, 2009 6 Lifted Index • Is a way to describe stability with one number • Is not a perfect measure of stability • Is useful for forecasting, but not the only criterion forecasters use • Is a difference in temperature at 500mb • T(environment) – T(air parcel) • Negative values are unstable Class #26: Friday, October 30, 2009 7 Lifted Index (continued) • Where do the temperatures come from • For the environment, use a sounding • For the parcel – Start with surface conditions of temperature and humidity – Next, lift the parcel dry adiabatically (10°C/km) until it is saturated – Finally, raise the parcel moist (saturated) adiabatically to 500mb Class #26: Friday, October 30, 2009 8 Satellite-derived values of the lifted index Class #26: Friday, October 30, 2009 9 An environment that favors the formation of severe thunderstorms At the highest levels there is a jet stream with divergence aloft At 500mb there is a trough to the west At 700mb there is a warm dry air mass Between 700mb and 850mb there is convective or potential instability Just above the surface there is a low level jet of southerlies with warm moist air Overall, a clockwise turning wind in the warm sector Class #26: Friday, October 30, 2009 10 Drylines • Are not true fronts because there is no temperature contrast across the front. • Resemble fronts because there is a boundary between air masses, cT and mT. • Resemble fronts because there is a wind shift, pressure trough, convergence, and often convective clouds along the dryline. • Occur in West Texas in spring and early summer, and severe weather can occur. Class #26: Friday, October 30, 2009 11 Class #26: Friday, October 30, 2009 12 Drylines (continued) • Have a strong contrast in humidity and wind direction across the front • Can persist for several days. • Can move westward at night (called the dewpoint front) • Move from west to east during the day • Are a powerful source of convergence • The moister air is lighter, and rises Class #26: Friday, October 30, 2009 13 Schematic view of a multicell thunderstorm Class #26: Friday, October 30, 2009 14 The shelf cloud signifies the arrival of the gust front Class #26: Friday, October 30, 2009 15 Composite radar imagery of a squall line Class #26: Friday, October 30, 2009 16 Satellite image of the southern portion of the squall line Class #26: Friday, October 30, 2009 17 Infrared image of a supercell thunderstorm and squall line Class #26: Friday, October 30, 2009 18 Photograph of an overshooting thunderstorm cloud The updraft of the thunderstorm is so strong that its top pokes into the tropopause. The overshooting cloud updraft is decelerating and the updraft speed decreasing. Cloud top occurs when the updraft slows to a stop. Class #26: Friday, October 30, 2009 19 Visible satellite image of the same storms Class #26: Friday, October 30, 2009 20 Life cycle of the multicell thunderstorm A squall line would extend into the slide This storm is moving from right to left on the slide Class #26: Friday, October 30, 2009 21