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 -- reducedlift and the plane istoo low to recover.

Headwind -- Lift increasedand the pilot drops the noseto stay on glide slope.

DOWNBURSTDOWNBURST

Gust Front & MicroburstGust Front & MicroburstTurbulent air Turbulent air forms along the forms along the leading edge of the leading edge of the gust front, which gust front, which can generate can generate tumbling dust tumbling dust clouds.clouds.

Such gust fronts Such gust fronts and associated and associated cold dense air cold dense air often feel like a often feel like a passing cold front, passing cold front, and may cause a 1 and may cause a 1 to 3 mb local rise to 3 mb local rise in pressure, called in pressure, called a mesohigh.a mesohigh.

Figure 15.6Figure 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 ThunderstormDissipating Stage of Thunderstorm

Figure 15.3Figure 15.3

Once downdrafts Once downdrafts dominate dominate updrafts, the updrafts, the storm ends as storm ends as precipitation precipitation leaves the cloud leaves the cloud faster than it is faster than it is replenished by replenished by rising, condensing rising, condensing air.air.

Often, lower level Often, lower level cloud particles cloud particles will evaporate will evaporate leaving an isolate leaving an isolate cirrus anvil top cirrus anvil top section.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 DaysAverage Thunderstorm & Hail Days

Figure 15.20Figure 15.20Figure 15.21Figure 15.21

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

1 1

2 2

3

1

3

4

2

4

5

3Each 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.