Downbursts

1
Downbursts

• 9.4 Convective downburstsThe turbulence
  associated with clouds types is St slight
  Ci, Cs, Cc, Ac, As nil or slight except when
  Ac cas or when merging into Cb Sc moderate Ns
  moderate but may be severe near base Cu, TCu,
  Cb Generally severe but may be catastrophic and
  include the downbursts described below and the
  internal up/downdraughts described in the
  following section 9.5.A microburst is a strong
  concentrated plunge of cold dense air from a
  convective cloud. Peak wind gusts usually last
  less than ten minutes, often 3 to 5 minutes, but
  extremely hazardous vertical and horizontal shear
  results. It may be dry or associated with
  precipitation ranging from virga showers to heavy
  rain showers wet. A curling outflow foot of
  dust or precipitation from the surface touchdown
  point may be visible near the surface. In
  bushfire conditions the firestorms associated
  with dry microbursts are particularly dangerous
  to firefighters.Microbursts are generally
  associated with hot and relatively dry conditions
  at low levels (such as found in inland
  Australia), convectively unstable moist air aloft
  with high (5000 to 15 000 feet) based Cu or TCu.
  If the cloud is forming when the surface
  temperature/dewpoint spread is 15 C to 25 C
  then the microburst potential is high. The high
  spread means the atmosphere can hold much more
  water vapour. Rain falling in, and from, the
  cloud is evaporating (virga), thus cooling the
  entrained air and resulting in downward
  acceleration of the denser air. Consequently
  flight through, or under or near, precipitation
  from a large Cu involves risk. Significant hail
  is unlikely. The most dangerous area is the
  horizontal density current vortex ring close to
  the touchdown point. The ring moves outward from
  the contact point at high velocity (up to 150
  knots) until it disintegrates into several
  horizontal roll vortices spread around the
  periphery and which may continue to provide
  extreme shear for several minutes. The maximum
  horizontal winds occur about 100 feet above
  ground level.Microbursts occur under 5 10 of
  Cb (refer 9.5 below) but a less concentrated,
  longer lasting gust front macroburst is normally
  associated with the entire cold air outflow of
  larger storm cells. The severe gust fronts from a
  microburst extend for less than 4 km, those from
  a macroburst extend for more than 4 km. The
  vertical gusts within the downburst, perhaps with
  a velocity twice the mean, may produce a
  microburst within the macroburst.