We next present an analytic analysis of microbursts, which shows that the contributions by evaporative cooling and momentum wall effects to the outburst flow are essentially independent. These contributions can be combined to produce a Bernoulli equation that can be used to examine microburst behaviour in a range of conditions. We examine sheared and unsheared flow, and the effects of a surface stable layer . Application of this quite simple analysis to a series of published numerical modelling studies explains nearly all the variance in maximum outflow winds that were produced. Wall-effect processes appear to dominate at small scales, whereas macrobursts seem to largely arise from hydrostatic adjustment to evaporative cooling. When a microburst is originated in a region of strong mid-level flow, we suggest that momentum, rather than buoyancy effects dominate its subsequent evolution. At the ground the tilted downdraft remains focused in a jet that can produce severe damage along a narrow streak, with high winds lasting from seconds to a minute or so.
The environmental conditions associated with the development of our observed jetbursts started with a large region of high DCAPE with a mid-level jet. An intense convective element developed on the edge of a bow echo region in a mesoscale squall line. The observed jetbursts appear to have been initiated in a narrow dry slot that developed in the bow echo adjacent to the severe convective cell. We suggest that the mechanisms to produce such narrow wind streaks are found in a variety of conditions, including the hurricane boundary-layer in the core region.
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