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An Analysis of Gust Front Activity on 14 June 2006Chris BonannoFlorida Institute of Technology Rain Shaft
Overview • On June 14th, 2006 FIT Marine Field Projects class collected weather observations at Deseret Ranch in eastern Osceola County off of State Road 192. • Data collected from approximately 12:10 to 1:50 LST • Observation site was suitable (i.e., far away from significant structures). • Of particular interest were measurements associated with the passage of a thunderstorm gust front including: pressure, temperature, wind speed, precipitation, and relative humidity.
Why are gust fronts important? • Gust fronts are accompanied by strong and sometimesdamaging winds up to 100 m.p.h. (~ cat. 2 hurricane). • High winds are a hazard to boating, aviation, agriculture, etc. • Gust front is a region of low-level convergence (air coming together) which can act as a trigger mechanism (i.e., generate additional storms). • Can issue relief (cooling) in the presence of heat stress
Gust Front Anatomy • Gust fronts are the leading edge of cold air associated with the downdraft which can often be visualized by presence of “shelf cloud”. • Gust front passage is noted by: 1. initially light winds, followed by 2. sudden increase in wind speed, 3. temperature drop, 4. onset of precipitation
Example of a Shelf Cloud rain-cooled air warm/moist
Other Gust Front Characteristics • “Mesohighs” often accompany gust fronts. Surface high pressure created by the rain-cooled (heavy) pool of air associated with the downdraft. • Though gust front winds can be relatively strong in Florida, they are most commonly found to be of greatest intensity in the mid-western and plains regions of the United States. • One measure of gust front intensity, in part, can be obtained via the wet-bulb temperature. • Wet-bulb temperature essentially indicates how cool the temperature will be at a given level (in this case the surface) associated with the precipitation-induced gust front.
temperature dew point Wet Bulb Temperature Tw: ~ 23 C or 74 F The wet-bulb temperature is the temperature resulting from evaporational cooling at constant pressure. On a sounding, it will always fall between the dew point and temperature lines.
Thunderstorm Development • Combination of surface heating, instability and an approaching surface trough extending SW from TD Alberto.
Radar Imagery • What are reflectivity images? • radar sends out an electromagnetic pulse and then listens for a back-scattered signal. The energy received by the radar is proportional droplet size and number. This energy is converted to reflectivity which is color coded based on intensity.
Observed Gust Front Characteristics • temperature (recall wet-bulb) • precipitation versus temperature • humidity • precipitation and humidity • wind speed • precipitation and wind speed • pressure • precipitation and pressure Order of events (non-simultaneous)
Conclusions/Issues • wet bulb temperature is a relatively good predictor of actual gust front temperature (lower bound) • surface temperatures dropped on three distinct occasions two of which mark the passage of gust fronts • barometric pressure increased with the gust front (meso-high) • humidity initially decreases (~10%) – cloud cover? • humidity increases with the advent of precipitation • wind speed and precipitation are highly correlated (precipitation drag + evaporative cooling)
References • apollo.lsc.vsc.edu/.../chapter15/gustfront.html • www.mnchasers.org/june172002/undergust-tb.html • http://www.usatoday.com/weather/tg/wgfront/wgfront.htm Kathryn Shontz