Structure and Life Cycle of Microburst Outflows Observed in Colorado

Intense, small-scale divergent outflows known as microbursts are held responsible for a number of aircraft accidents. This paper describes the morphology of microburst outflows observed in Colorado. Outflows are categorized into morphological types based on analysis of observation by Doppler radars and a surface meteorological network. Outflow life cycle is discussed, and the vertical and horizontal structure is described. Basic characteristics of microburst outflows are summarized from statistics compiled using both single and multiple Doppler analyses. The microburst outflows are classified into two types: individual microbursts and microburst lines. Examples of observations of each type are shown. Organization of microbursts into microburst lines results in much longer-lasting wind shear than exists with isolated microbursts. The greater lifetime of microburst lines, combined with the much larger area of divergence, can create a much greater potential for hazard to aircraft than is the case for individual microbursts. Outflow structure was found to resemble many features of the laboratory wall jet. Vertical profiles of horizontal velocity follow curves similar to those of the wall jet. Radial profiles of horizontal velocity through the microburst center also agree with the velocities predicted from wall jet theory out to the velocity maximum. Beyond the velocity maximum, microburst outflow velocities decay more rapidly than wall jet velocities. Studies of microburst symmetry, as measured across the maximum velocity differential, reveal that the minimum shear is, on the average, only about 60% of the maximum. Implications of outflow structure and symmetry for aviation safety are discussed.