Transcript Downbursts
Downbursts • 9.4 Convective downbursts The 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.