. "Appendix A: A Rationale for Choosing the Spatial Density and Temporal Frequency of Observations for Various Atmospheric Phenomena." Observing Weather and Climate from the Ground Up: A Nationwide Network of Networks. Washington, DC: The National Academies Press, 2009.
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Observing Weather and Climate from the Ground Up: A Nationwide Network of Networks
Duration: typically from 1 to 10 minutes
Geographic preference: Downbursts can accompany any convective storm. The most dangerous to aircraft are those with low reflectivity, little or no precipitation at the ground, and no lightning, because they offer few visual cues. Such “dry” downbursts occur most often in summer from early afternoon until early evening in the dry climates of the western Great Plains and the Intermountain West (Caracena et al., 1989).
Downbursts pose a serious risk to an aircraft on takeoff roll or one about to land. An aircraft first experiences the downburst as a sudden headwind. Once past its nearest approach to the center of the downburst, it experiences a sudden tailwind. The loss of lift can cause departing aircraft to roll off the end of the runway and landing aircraft to crash short of the runway.8 Wet downbursts are associated with a descending core of heavy precipitation, perhaps mixed with unsaturated air from mid-levels. These downbursts look menacing, at least in the daytime, and are reliably detected by radar. They are not difficult to avoid.
Dry downbursts are more sinister. They are caused by the evaporation of droplets falling into a deep layer of fairly dry air below a high cloud base in the presence of a steep lapse rate (rapid decrease of temperature with height, about 1°C for each 100 m of altitude). If the droplets are numerous and small, but do not completely evaporate until reaching the ground, the downrushing air can accelerate enough to produce radial outflows exceeding 30 m s−1. Sometimes the only visual cue is a circular ring of blowing dust at the ground.
Early afternoon soundings would aid in the prediction of downbursts. Moist air in mid-troposphere to support high-based thunderstorms and a steep lapse rate below cloud base are the hallmarks of downburst conditions (Wakimoto, 1985). Mid-tropospheric winds of 20 m s−1 or so will strengthen the downburst if momentum at this level is incorporated in the descending precipitation. A single sounding such as a rawinsonde would suffice in the vicinity of the airport.
Once convective showers form, monitoring from the surface to 400 hPa (within the convective cloud) becomes critical. Given that most down-bursts last for only a few minutes and affect small areas (on the order of 10 km2), the appropriate sampling resolution is ∆x=1 km, ∆z=200 m, and ∆t=1 min.