Notes

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Outline

1	Chapter 18: Thunderstorm
2	Thunderstorm Thunderstorms, also called cumulonimbus clouds, are tall, vertically developed clouds that produce lightning and thunder. The majority of thunderstorms are not severe. The National Weather Service reserves the word “severe” for thunderstorms that have potential to threaten live and property from wind or hail. A thunderstorm is considered severe if: Hail with diameter of three-quarter inch or larger, or Wind damage or gusts of 50 knots (58mph) or greater, or A tornado.
3	Locations of Severe Weather Events (in 2006)
4	Four Elements for Severe Thunderstorms A source of moisture A conditionally unstable atmosphere A mechanism to tiger the thunderstorm updraft either through lifting or heating of the surface Vertical wind shear: a rapid change in wind speed and/or wind direction with altitude
5	Lifting In cool season (late fall, winter, and early spring), lifting occurs along boundaries between air masses è fronts When fronts are more distinct, very long lines of thunderstorms can develop along frontal boundaries è frontal squall lines. In warm season (late spring, summer, early fall), lifting can be provide by less distinct boundaries, such as the leading edge of a cool air outflow coming from a dying thunderstorm. Thunderstorms developed along these subtle boundaries in the warm season often undergo a self-organized process and lead to the a kind of severe storm called “mesoscale convective system” (MCS). “Meso-scale” refers to atmospheric processes that occurs on a scale of a few hundreds hundreds kilometers.
6	Wind Shear If winds increase rapidly with height ahead of a strong front or a less distinct boundary, thunderstorms triggered along that boundary may organize into violent storms call “supercell thunderstorms”. Supercells can occur along a line or individually, but always have their own circulations that are related to the storm’s rotation and the vertical shear in the atmosphere. Supercells develop most often when strong winds are present in the upper troposphere and winds in the lowest kilometers or two of the atmosphere increase rapidly and change direction with altitude. Supercells can occur in both cool and warm seasons
7	Three Types of Severe Thunderstorms Frontal Squall Lines Mesoscale Convective Systems Supercell Thunderstorm
8	Airmass Thunderstorm (non severe)
9	Lifecycle of a Non-Severe Airmass Thunderstorm
10	Mesoscale Convective Systems
11	Development of Mesoscale Convective System
12	Squall Line
13	Another View of the Development of MCS
14	Another View of the Development of MCS
15	Gust Front
16	A Bow Echo shown in Radar Reflectivity
17	Front-to-Rear Flow
18	Key Features of a Thunderstorm in a Mature MCS
19	Frontal Squall Lines
20	Typical Environment for Frontal Squall Line
21	Key Features of a Frontal Squall Line
22	Supercell Thunderstorms
23	Four Key Ingredients for Supercell
24	Convective Available Potential Energy (CAPE)
25	A Typical Sounding in the Supercell Environment
26	Supercell Environment: CAPE and Energy
27	Supercell Environment: Wind Shear
28	Triggering Mechanisms for Supercell Thunderstorm
29	A Line of Supercell in eastern Kansas and Nebraska
30	Cross Section of a Classic Supercell Thunderstorm
31	Mesocycline
32	Mesocycline
33	Wall Cloud and Tornado
34	Precipitation
35	Precipitation in radar Image
36	Movement of Supercell
37	3D View of Supercell – Initial Development
38	3D View of Supercell
39	Horizontal View of the Supercell