Microbursts Hazards of air mass thunderstorms

Today Mature phase Downdraft

Hazards of Air Mass Thunderstorms Heavy Rain Hail –Usually not large –May be numerous Downbursts or Microbursts –Exceptionally strong downdrafts that, when they hit the earth, may have potentially destructive winds associated with them.

Downbursts Exceptionally strong downdrafts Very localized Upon striking the earth, the downbursts will spread out Can cause “tornado-like” damage Straight-line wind damage Smaller scale downbursts are called microbursts

Downbursts Have been known to bring down commercial aircraft –New Orleans -- July 1982 Boeing 727 flew into a microburst after takeoff 145 killed –Dallas/Fort Worth -- August 1985 Aircraft encountered a microburst upon landing Over 100 killed –Denver -- August 1975 Aircraft encountered a microburst during takeoff

Downbursts Tailwind -- reduced lift and the plane is too low to recover. Headwind -- Lift increased and the pilot drops the nose to stay on glide slope. DOWNBURST

Gust Front & Microburst Turbulent air forms along the leading edge of the gust front, which can generate tumbling dust clouds. Such gust fronts and associated cold dense air often feel like a passing cold front, and may cause a 1 to 3 mb local rise in pressure, called a mesohigh. Figure 15.6

Downbursts and Microbursts Microburst –An anomalously strong, concentrated downdraft that produces a pocket of dangerous wind shear near the ground over an area of 4 km or less in horizontal extent. Associated with cumulonimbus clouds –Can have heavy rain (Wet microbursts) –Can have vanishing sprinkles (Dry microbursts)

Microburst Damage

Thunderstorms Dissipating Stage The downdraft and precipitation fall down through the updraft cutting off the updraft. The gust front surges out in front of the storm cutting off the inflow of warm, moist air. The storm begins to die of “starvation.”

Dissipating Phase Eventually the downdraft overwhelms the updraft and convection collapses. Precipitation becomes lighter and diminishes. Cloud begins to evaporate from the bottom up often leaving behind an “orphan anvil.”

Dissipating Stage of Thunderstorm Figure 15.3 Once downdrafts dominate updrafts, the storm ends as precipitation leaves the cloud faster than it is replenished by rising, condensing air. Often, lower level cloud particles will evaporate leaving an isolate cirrus anvil top section.

Thunderstorms The previous discussion was a description of an ideal single cell thunderstorm. These are the common variety of thunderstorms seen in the summer. Warm Humid Air Mass Widely Scattered Usually Weak Usually Short Lived Produce a good portion of summertime rainfall. Produces temporary cooling Slight potential for producing severe weather.

Air Mass Thunderstorms Usually weak (but can produce heavy rain in a short period of time). Usually not severe. Usually move slowly. Often develop and dissipate in less than one hour. Form in a weakly sheared environment.

Average Thunderstorm & Hail Days Figure Figure Observed frequency in the pattern and occurrence of thunderstorms does not overlap with hail frequency, possibly because hail falling into the thick layer of warm Gulf air will melt before reaching the ground.

Air Mass Thunderstorms Outflow may trigger additional storms nearby. New Storm

Thunderstorms Multicell Thunderstorms –A storm that has a succession of cells in various stages of their life cycle. –Most thunderstorms are multicellular Each cell moves: The group moves:

Thunderstorms Multicell Thunderstorms –Each cell goes through the complete life cycle of a single cell. –Each cell is often unique and can be identified on radar. –Although each cell grows and dies, the group of storms can last for over an hour. –Has the potential for producing severe weather.

Multicellular Thunderstorms A cluster of short lived single cells. Cold outflow from each cell combine to form a much larger and stronger gust front. Convergence along the gust front tends to trigger new updraft development. This is the strongest in the direction of storm motion. New cell growth often appears disorganized to the naked eye.

Multicellular Thunderstorms Individual cells typically move with the mean (average) wind flow. Multicell storms may last a long time. They constantly renew themselves with new cell growth.