Wet and Dry Microbursts Daniel R. Adriaansen & Baylee A. Balschmiter SUNY Brockport Department of the Earth Sciences ESC 452 Mesoscale Meteorology 20 April 2006

What is a microburst? Downburst: Strong downdraft which induces an outburst of damaging winds on or near the ground. (Fujita 1985) - 2 types: Macroburst: A large downburst with its outburst winds extending in excess of 4km (2.5 mi) in the horizontal with maximum wind speeds of 60 m/s (134 mph). (Fujita 1985) Microburst: A small downburst with its outburst, damaging winds extending only 4 km (2.5 miles) or less with maximum wind speeds of up to 75 m/s (168 mph). (Fujita 1985)

Conceptual Model - Vortex ring model - Meso Low on either side of meso High http://www.cimms.ou.edu/~doswell/microbursts/Figure_04.JPG

Two (three) varieties (Intermediate?) http://www.cimms.ou.edu/~doswell/microbursts/Figure_08.JPG (Intermediate?)

Dry Microburst A microburst that is accompanied by little or no rain between the onset and the end of the high winds. (Wakimoto 1985) http://www.cimms.ou.edu/~doswell/microbursts/Figure_08.JPG

Dry Microbursts - JAWS (Joint Airport Weather Studies, 1982) - Further defined microburst characteristics, particularly involving dry microbursts. - Denver, CO

Physical Characteristics - Time scale 2-5 minutes - Peak wind speeds exceeding 30 m/s (67 mph) - Reflectivities of ≤ 30 dBz - Precipitation ≤ 0.25 mm (0.01 inches) - Lightning often limited - Limited synoptic forcing/influence - Sometimes associated with fair weather

Thermodynamic environment - Morning Inversion - Inverted V signature - Nearly dry-adiabatic ABL - Large Dew-point depressions in ABL - Moist layer located ~ 500mb (DPD  0) - Light ABL winds

http://www.cimms.ou.edu/~doswell/microbursts/Figure_09b.JPG

http://www.cimms.ou.edu/~doswell/microbursts/Figure_09a.JPG

Visual Identification - Virga is defined as wisps or streaks of water or ice particles falling out of a cloud but evaporating before reaching the earth's surface as precipitation. (NOAA 2001) - Blowing dust/dust rings at surface - Very good indicators of dry microburst potential

http://www.cimms.ou.edu/~doswell/microbursts/Figure_17a.JPG

http://www.cimms.ou.edu/~doswell/microbursts/Figure_17b.JPG

http://www.cimms.ou.edu/~doswell/microbursts/Figure_13.JPG

http://www.cimms.ou.edu/~doswell/microbursts/Figure_14.JPG

http://www.cimms.ou.edu/~doswell/microbursts/Figure_16.JPG

Wet microburst A microburst accompanied by heavy precipitation at the surface. A rain foot may be a visible sign of a wet microburst. http://www.cimms.ou.edu/~doswell/chasesums/chase97_18.JPG

Wet Microburst Formation There is a layer of nearly saturated air that is close to Γm Above this layer is an elevated dry layer When Tw is brought to saturation and mixed with warm air, it creates a negatively buoyant atmosphere that leads to a severe downdraft

http://www-frd.fsl.noaa.gov/~caracena/micro/FACEsons.gif

Characteristics Little or no capping inversion. A moist mid-tropospheric layer between 5000 ft and 15,000 ft. An elevated dry layer above 15,000 ft. When combined there is a high potential for a wet microburst

http://www. cimms. ou. edu/~doswell/microbursts/SW_Texas_rainfoot_c http://www.cimms.ou.edu/~doswell/microbursts/SW_Texas_rainfoot_c.JPG

http://www. nssl. noaa. gov/~doswell/microbursts/SW_Texas_virga_sm http://www.nssl.noaa.gov/~doswell/microbursts/SW_Texas_virga_sm.JPG&imgrefurl

References 1. Caracena, F., Holle, R. L., & Doswell III, C. A. (2001, June). Microbursts: A Handbook for Visual Identification. Retreived April 10th, 2006, from http://www.cimms.ou.edu/~doswell/microbursts/Handbook.html 2. Fujita, T. (1985). The Downburst: Microburst and Macroburst. Chicago: The University of Chicago. 3. NOAA. (2001). Question of the Month. Retreived April10th, 2006, from http://www.noaa.gov/questions/question_081701.html 4. Wakimoto, R. M. (1985). Forecasting Dry Microburst Activity over the High Plains. Monthly Weather Review, 113, (1131-1143).

Questions? Comments? Rude Gestures?