User:Jarjames
Jarjames Meteorology Student at the University of Utah
Downbursts
Downbursts are strong wind flows generated under cumulus clouds due to evaparative cooling and hydrometeor loading that thrust downward to ground level and then flow outward. Winds can reach speeds up to 75 m/s or 168 mph. Downbursts can be broken into two catagories, microbursts and macrobursts. Microbursts are on the scale of 2.5 miles (4 km) or less in diameter and lasts only a few minutes. Macrobursts are any downburst greater than 2.5 miles and can last from 5 to 30 minutes with wind speeds normally less than that of a microburst, usually on the order of 60 m/s (168mph) or less. Both types tend to leave circular damage patterns unless otherwise affected by topography.
The University of Illinois breaks the evolution of downbusrts into three stages, the contact stage, the outburst stage and the cushion stage. "A [downburst] initially develops as the downdraft begins its descent from cloud base. The downdraft accelerates and within minutes, reaches the ground (contact stage). It is during the contact stage that the highest winds are observed.
fig. 1
During the outburst stage, the wind "curls" as the cold air of the [downburst] moves away from the point of impact with the ground.
fig. 2
During the cushion stage, winds about the curl continue to accelerate (WW2010.atmos.uiuc.edu)". While the winds at the surface slow due to friction.
fig. 3
Microbursts come in two varieties dry and wet. Dry microbursts may occur when there is a relatively dry air mass at low levels followed by a relatively thin moist layer and dry air aloft. Wet (rain producing) microbursts may occur when there is thin dry surface layer followed by a thick saturated layer with dry air aloft that entrains (or mixes down) into the moist layer.
Driving forces and mechanisms of downbursts: "evaporative cooling: this is the most important mechanism, even for dry microbursts. Thunderstorms entrain ambient air, and when the drier and cooler ambient air is entrained into acumulonimbus, the droplets or ice crystals within will rapidly evaperate, cooling the mixed air further and making it more dense, triggering a descent. Descending air will follow the moist adiabatic lapse rate as long as water is available to keep the air saturated. If the environment below the cloud base has a dry adiabatic lapse rate, the descending air parcel will become increasingly negatively buoyant, acceleration towards the ground. Therefore microbursts are more likely and/or more intense in environments with a deep well-mixed convective boundary layer. hydrometeor loading: the sheer weight of rain, snow, graupel of hail may trigger or support a downdraft. However, even the highest hydrometeor concentrations ever measured do not exceed 6 g/kg, which equivalent to negative buoyancy of only 2K. cooling by melting: convective clouds produce graupel or even hail, and this precipitation does not all melt at the freezing level. However with the heat required to evaporate a liter of water one can melt eight times as much ice. downward push: in the vicinity of thunderstorms the atmosphere is not hydrostatically balanced and one can have perturbation pressure graadients (ppg) up to a few hPa per km. Strongly buoyant cells always experience a downward ppg force. This force allows compensating downward motion aroun the buoyant bubble. This downward motion may locally accelerate into a microburst by any of the 3 cloud processes mentiond above, mainly evaporative cooling. (www.das.uwyo.edu)"
Downbursts are often mistaken for tornados, but tornados have wind flows that are convergant (inward) where as downdrafts have divergant flows (outward). Although visual appearance can be very similar if viewed in a short time period.
Refrences
http://www.das.uwyo.edu/~geerts/cwx/notes/chap08/microburst.html http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/svr/comp/out/micro/home.rxml http://www-frd.fsl.noaa.gov/mab/microburst/micro_course.html