Microbursts

Microbursts are strong winds concentrated in an area less than 4 km in diameter, beneath a convective downdraft, usually associated with a cumulonimbus cloud. The column then ‘splashes’ onto the ground (Fig 1), creating winds outwards of up to 70 m/s. Microbursts are either dry (i.e. little or no rain reaches the ground) or wet (usually within a downpour). The radial pattern means winds of various directions within a small area, and hence considerable wind shear near the ground, for up to ten minutes.

Fig 1. The top three images show a wet microburst touching down and spreading. The interval between the images is just 40 seconds. The three schematics below show the airflow and the regions of high wind around the time of touchdown. The two schematics at the bottom illustrate the ring vortex, with a horizontal 'rotor' axis around the microburst. This vortex spreads and therefore (temporarily) intensifies after touchdown. (1)

This wind shear can cause havoc to aircraft in the vicinity of the runway. A few aircraft accidents have been attributed to microbursts. For instance, Panam flight 759 in New Orleans on 9 July 1982 crashed shortly after take-off because of a microburst at the end of the runway (Fig 2). Accidents are the result of a sudden change from headwind to tailwind. This reduces the lift of the aircraft, which may force the aircraft down, either during take-off or landing. The downdraft itself and the increased drag and weight of heavy rain are less important factors.

Fig 2. Flight trajectory (red line), winds (green arrows), and indicated airspeed of PAA 759 (1).

• evaporative cooling: this is the most important mechanism, even for dry microbursts. Thunderstorms entrain ambient air, and when the drier and cooler ambient air is entrained into a cumulonimbus, the droplets or ice crystals within will rapidly evaporate, cooling the mixed air further and making it more dense, triggering a descent. Descending air will follow the moist adiabatic lapse rate as long as water is available to keep the air saturated. If the environment below the cloud base has a dry adiabatic lapse rate, the descending air parcel will become increasingly negatively buoyant, accelerating towards the ground. Therefore microbursts are more likely and/or more intense in environments with a deep well-mixed convective boundary layer.
• hydrometeor loading: the sheer weight of rain, snow, graupel or hail may trigger or support a downdraft. However, even the highest hydrometeor concentrations ever measured (by aircraft or ground radar) do not exceed 6 g/kg, which equivalent to a negative buoyancy of only 2K.
• cooling by melting: convective clouds produce graupel or even hail, and this precipitation does not all melt at the freezing level. However with the heat required to evaporate a liter of water one can melt eight times as much ice.
• downward push: in the vicinity of thunderstorms the atmosphere is not hydrostatically balanced and one can have perturbation pressure gradients (ppg) up to a few hPa per km. (the word 'perturbation refers to the departure from hydrostatic balance.) Strongly buoyant cells always experience a downward ppg force. This force allows compensating downward motion around the buoyant bubble. This downward motion may locally accelerate into a microburst by any of the 3 cloud processes mentioned above, mainly evaporative cooling.

(1) Fujita, T.T., 1985. Downburst: microburst and macroburst. Report of Projects NIMROD and JAWS. Satellite and Mesometeorology Research Project paper #210, Univ. of Chicago, Chicago, IL, 122 pp.