Downbursts and dust storms

Review of last lecture 1.3 stages of supercell tornado formation. 2.2 types of non-supercell tornado formation. 3.Tornado outbreak (number>6) 4.Which state has the largest number of tornadoes in U.S.? 5.Tornado outlook 6.2 types of mesoscale convective systems

Downbursts: Introduction Downbursts are gusts of wind that can reach speeds in excess of 270km/hr (165mph), and are potentially deadly. Three common types: Derechos Haboobs Microbursts

Different downbursts 620 miles or 1000 km 620 miles or 1000 km 62 miles or 100 km 62 miles or 100 km 6.2 miles or 10 km 6.2 miles or 10 km 0.62 mile or 1 km 0.62 mile or 1 km

Derechos A derecho (spanish word meaning “straight ahead”) is a widespread, long-lived wind storm that is associated with a band of rapidly moving showers or thunderstorms (often a squall line). By definition, the wind damage swath extends more than 240 miles and wind speed > 57 mph. Can produce destruction similar to that of tornadoes. The damage typically is directed in one direction along a relatively straight swath. From NOAA SPC Derecho website

Video: Derecho KltL8 KltL8 KltL8 KltL8

Development of a derecho Thunderstorm stage Thunderstorm stage Bow echo stage Bow echo stage Comma or line echo stage Comma or line echo stage

Clouds associated with a derecho From NOAA SPC Derecho website

"The Line Storm," by John Steuart Curry,

Different types of derechos Serial derechos Serial derechos Progressive derechos Progressive derechos Hybrid derechos Hybrid derechos Low dewpoint derechos Low dewpoint derechos

Serial derecho typically is associated with an extensive squall line or quasi-linear convective system with multiple embedded bow echoes. typically is associated with an extensive squall line or quasi-linear convective system with multiple embedded bow echoes. usually occur in conjunction with well-defined, migratory low pressure system. usually occur in conjunction with well-defined, migratory low pressure system. Overall convective system (and, therefore, derecho) motion is dominated by advection --- the downstream movement of individual thunderstorm cells by the environmental wind. Overall convective system (and, therefore, derecho) motion is dominated by advection --- the downstream movement of individual thunderstorm cells by the environmental wind. From NOAA SPC Derecho website

Progressive derecho typically is associated with a squall line of restricted length that may involve a single bow echo or multiple bows. typically is associated with a squall line of restricted length that may involve a single bow echo or multiple bows. usually are associated with a stagnant large scale weather pattern. usually are associated with a stagnant large scale weather pattern. Overall convective system (and, therefore, derecho) motion is dominated by thunderstorm propagation -- - that is, the development of new storm cells downstream from existing ones. Overall convective system (and, therefore, derecho) motion is dominated by thunderstorm propagation -- - that is, the development of new storm cells downstream from existing ones. June 2012 derecho over the Midwest and Ohio Valley From NOAA SPC Derecho website

Annual number of derecho events

Timing of derecho events From NOAA SPC Derecho website

Microburst Downbursts with diameteres of less than 4km (2.5mi) called microbursts Two types: wet and dry can produce dangerous situations at airports, as they impede air travel. Three stages of development: Contact, Outburst, Cushion

Microburst and air travel

Haboobs Very strong horizontal winds over desert regions create sandstorms called haboobs (from the Arabic word meaning “wind”).

Video: Haboobs 4knF0 4knF0 4knF0 4knF0 A movie’s depiction: A movie’s depiction: A movie’s depiction: A movie’s depiction: mDBLWg （ at 1 ： 33 ： 59-1:35:00 ） mDBLWg （ at 1 ： 33 ： 59-1:35:00 ） mDBLWg mDBLWg

Global distribution of aerosols

Global impact of dust storms

Desertification Caused mainly be human activities and climate change Caused mainly be human activities and climate change Is one of the most significant global environmental problems Is one of the most significant global environmental problems About a billion people are under threat About a billion people are under threat

Global desertification vulnerability

Palmer drought severity index (PDSI) was developed by Wayne Palmer in the 1960s and uses temperature and rainfall information in a model to determine dryness of soil moisture. was developed by Wayne Palmer in the 1960s and uses temperature and rainfall information in a model to determine dryness of soil moisture. is most effective in determining long term drought (a matter of several months) and is not as good with short-term forecasts (a matter of weeks). is most effective in determining long term drought (a matter of several months) and is not as good with short-term forecasts (a matter of weeks). It uses a 0 as normal, and drought is shown in terms of minus numbers; for example, minus 2 is moderate drought, minus 3 is severe drought, and minus 4 is extreme drought. It uses a 0 as normal, and drought is shown in terms of minus numbers; for example, minus 2 is moderate drought, minus 3 is severe drought, and minus 4 is extreme drought.

PDSI for US in December 2011

Change of PDSI in the last 100 years

Summary 1.3 types of downbursts. 2.4 types of derechos. Which U.S. state has the largest annual mean number of derecho events? Which season has the largest occurrence of derechos? 1.Haboobs. 2.Global desertification. 3.What is PDSI? Which two variables is PDSI dependent on? How has the PDSI been changing in the last century?