The Thunderstorm Microburst

© Fernando Caracena, 2015

Oddly, severe weather extremes happen in the arid and semi arid parts of the United States, where a very clear atmosphere gives them maximum visibility. A variety of convective environments are generated by a seasonal shift in the airflow patterns over the area that is called the North American Monsoon. A convective feature commonly seen in this part of the country is the severe convective downdraft, which is made visible by both precipitation and blowing dust. Small scale sever downdrafts, called microbursts, have played a significant role in past landing and take-off aircraft accidents.

The North American Monsoon

I grew up in the Desert Southwestern part of the United States where annual precipitation averages about 10 inches or less. Most of the year, the days are dry and sunny. Precipitation happens in rare extremes, most of it when "wet" season begins around the 4thof July. I remember this very clearly, because as a child I always looked forward to the big fireworks display on this national holiday. Up to that time, the weather had been bone dry, but on the evening of the 4th of July, thunderclouds would sometimes move in causing the fireworks display to be cancelled because of rain. The sudden onset of wet weather in the Desert Southwest is part of a systematic reversal of the airflow pattern over Mexico and the Southwest, which redirects the trade wind belt northward  around a large anticyclone that anchors itself near the Four Corners region. This large-scale circulation pattern is known as the North American Monsoon.

Intrusions of moisture from the monsoon set up a variety of convective environments over the western United States that vary from a wet to a dry extreme. Both these convective extremes are marked by the occurrence of strong convective downdrafts that cause damaging surface winds. In some of these environments, cumulus clouds just barely form at a high enough above the ground that precipitation develops as ice crystals. As a result, these particles falling from very high based clouds are largely consumed by evaporation before reaching the surface. The melting and evaporation of this precipitation causes a strong cascade of rain cooled air, which pushes strong, destructive winds outward about its surface impact center. A similar phenomenon occurs during heavy rain events, in which evaporative cooling is reinforced by water loading and melting of hail to produce the same dynamic effect.

Early Research on Microbursts

In the latter half of the 1970s, Dr. Theodore T. Fujita and I were independently researching strong downdraft effects on aircraft during take-off and landing. Both of us were motivated in this study by a rash of jetliner crashes that were happening at airports. Fujita had analysed the weather situation  at John F. Kennedy Airport in New York City, which had resulted in an air disaster during an attempted landing. At about the same time, I was analysing a crash on take-off at Denver' s Stapleton International Airport in Colorado, which happened under a high based shower. In both cases, the ill fated aircraft flew through a rain shower. The New New York rain shower was a heavy. The one in Denver was a very light, but accompanied by a lot of blowing dust. At the suggestion of an editor of a special issue of the Bulletin of the American Meteorological Society,  Fujita and I combined our research efforts and wrote a landmark paper on the subject (Fujita, T. T., and Caracena, F., 1977: "An Analysis of Three Weather Related Aircraft Accidents," Bulletin of the American Meteorological Society, 58, 1164 1181.).   Fujita eventually named the sever downdraft phenomenon that acted on a scale that precipitated airport accidents, microbursts. As a result of extensive field research, Fujita (1985) came up with several, scale-related terms for severe downdraft phenomena in "The downburst. " SMRP Res. Paper No. 210, NITIS PB 85-148880. 122 pp. Below are definitions quoted from this paper:

A downburst is a strong downdraft which induces an outburst of damaging winds on or near the ground. Damaging winds, either straight or curved, are highly divergent
MACROBURST: A large downburst with its outburst winds extending
in excess of 4 km (2.5 miles) in horizontal dimension. An intense
macroburst often causes widespread, tornado-like damage. Damaging
winds, lasting 5 to 30 minutes, could be as high as 60 m/sec (134 mph).

MICROBURST: A small downburst with its outburst, damaging winds
extending only 4 km (2.5 miles) or less. In spite of its small
horizontal scale, an intense microburst could induce damaging winds as high as 75 m/sec (168 mph).

My concern for the phenomenon was not so much scale related, at least not directly so, but rather I was interested in the fluid dynamic structure of the phenomenon.  On my web page at NOAA,  I gave the following definition of a microburst:

A microburst is a three-dimensional circulation pattern of damaging winds driven outward near the surface by the ground impact of an unusually strong convective downdraft. Its horizontal extent is 5 km or less; and its lifetime is only a few minutes. It may contain embedded and leading edge vortices that rotate along a horizontal axis, reaching tornadic strength, presenting an extreme hazard to aircraft taking off and landing. The entire structure of downdraft, severe winds, and imbedded and leading edge vortices constitutes the microburst¹s circulation pattern.

The dynamical structure that I had in mind as a model for a microburst was a descending vortex ring that may form about the base of a surface-impacting downdraft. I had noticed during the JAWS field experiment that not every promising-looking rain shaft or virga shaft produced a ring shaped dust cloud or precipitation spray that revealed the presence of a microburt. There must have been a special dynamical link that caused the microburst.

 photograph of a dry thunderstorm

A photograph of a dry thunderstorm east of Boulder, Colorado taken by the author in August, 1980. These types of high based storms over the plains of Colorado are characterized by very light rain, which is vanishingly small, but yet, may result in severe surface winds.


Bryan  Snider is an Arizona weather photographer who has taken photographs of wet microbursts from Arizona monsoon thunderstorms. He post many interesting pictures of Monsoon thunderstorms in  Arizona.

I developed a fluid dynamical model of a microburst, which I never formally published that links instabilities associated with concentrated downdrafts and the formation of ground impact vortices. In the following section, I reproduce part of my NOAA web page (now removed) that models the microburst as a large vortex ring convected against the ground.

The Microburst as a Vortex Ring

The a vortex-ring stretched at the base of an intense downdraft explains the observed behavior of a microburst: intense, narrowly focused surface winds expand in a ring, crest and abruptly dissipate (see Fujita, 1985). The stretching of the vortex transfers energy downscale from the larger downdraft to the smaller vortex circulation, spinning it progressively faster until instabilities rapidly break the vortex down into turbulence.

Vortex Ring Model of a Microburst

A depiction of the Vortex Ring Model of a Microburst proposed that the 1980 American Geophysical Union meeting. A large vortex ring forms around a thunderstorm downdraft, which on impact at the surface produces strong outflow winds of brief duration.

Fujita (1986) analyzing flight recorder data of Delta Flight 191, which crashed at Dallas-Fort Worth International Airport (DFW) the afternoon of  2 August 1985, found that the aircraft encountered not only a microburst, but that this microburst contained imbedded horizontal vorteces.

Brain Waranauskas' s photographs of a dry microburst during the JAWS project taken in rapid succession show the rapid decay of its vortex circulation.


Fujita (1985) observed that somtimes an in-cloud mesocyclone forms above a developing microburst. Roberts and Wilson (1989) report that vorticity about a vertical axis and in-cloud convergence are Doppler radar precursors of a microburst (see also, Wilson et al., 1984).

Multiple microbursts

Some investigators have reported that not only one, but several microburst have happend in succession over the same area minutes apart. A microburst periodicity was observed in the crash of Pan American Flight 769 (see Caracena et al., 1983; Fujita, 1983), and has also been observed in field experiments. The periodicity of the vortex ring instability explains the wide variety of observed microburst life times and characteristics. Most microbursts last less than 5 min, but others have been observed to last four or even six times as long (Wilson et al, 1984). Long-lived microbursts may be simply a series of discrete micobursts forming over the same site.


This entry was posted in metrology, Thermodynmaics, weather. Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *