101: Gust Fronts

A gust front is propagating away from a cluster of storms. While very well defined, this front was likely harmless.

Gust fronts, also called outflow boundaries, are nothing more than the border between air from a downdraft spreading along the ground and the ambient air it is displacing. This basic definition belies the many different varieties of gust fronts and their myriad of size scales.



The above diagram depicts the airflow associated with a mature thunderstorm, one of  the most common sources of gust fronts. Thunderstorms, like all convective clouds, are powered by pockets of rising air. As the cloud ages it will inevitably mix in drier environmental air. Furthermore, the cloud may have begun to precipitate. Both of these factors will strengthen the downdraft within the cloud. In typical thunderstorms, the downdraft will occupy much of the same space as the updraft, causing the updraft to weaken. As this continues, the convection powering the cloud will be disrupted and the cloud will begin to dissipate, which is why thunderstorms are often said to "rain themselves out".





A gust front is made visible by the rain its carrying. This one is from a rather powerful storm and would likely have a real kick.

Normally, as air descends it dries and warms as it compresses under higher pressure. This does not happen in downdrafts because when rain, which is helping to power the downdraft, evaporates it takes in latent heat, thus cooling the surrounding air. The cooled air will be even less buoyant an descend even faster, allowing more rain to evaporate into it, cooling the downdraft, thus a positive feedback mechanism develops.





Dust kicked up by this gust front allows the chaotic airflow right at its edge to be seen.

When this cool, dense air descends out of the base of the cloud, it encounters the ground and must spread out. As the air spreads, it plows though the warmer air already at the surface, as something that vaguely resembles a tiny cold front. It is this air that produces the gust front.

A microburst occurring very near an airport. Notice the chaotic structure of the air flow as it spreads out.

Most of the time these gust fronts are benign gusts of wind on a stormy day. However, on occasion gust fronts and their parent downdraft become invisible killers. If the positive feedback mechanism described above is especially effective the downdraft can take on incredible speed. On impact with the surface this air spreads out in a powerful gust front as well as "rebounds" and shoots upward a short distance. This event is called a microburst and lasts less than a minuet. To an observer on the ground, it will likely look as though the entire core of the cloud has collapsed. Until the 1980s, the mechanics behind microbursts were poorly understood and they were responsible for countless plane crashes near airports. The culprit was the intense downward force of the downdraft combined with the sudden tailwind which caused aircraft to loose lift. This always happened during takeoff and landing as the proximity to the ground left no room for the plane to recover lift.





In this series of images, a storm complex is seen dissipating, while creating a series of gust fronts. As time progresses new storms form along the front. These eventually become a large storm complex themselves.

Gust fronts can also give storm systems a second life. As the front propagates away from the parent storm, it forces any air in its way upward. In some cases, this is just the kind of lift nearly unstable air needs to begin rising and start convection of its own. This process is one of the keys the longevity of squall lines. As one storm in the line begins to dissipate, its gust front may initiate the next storm in line, allowing the squall line as a whole to last many hours.

The storm system in the center of this radar image has created two very clear gust fronts, one to the north, and the other to the south.

On radar, gust fronts often appear as a well defined line of light "precipitation" spreading out from a region of heavy precipitation. What the radar is seeing as rain in the gust front is often dust being kicked up by the front. On occasion the radar signature may be of all the flying bugs, or even birds, caught in the gust front and forced to travel the same direction.





06/18/2013, Terra/MODIS captured this very large gust front in the NW Pacific.

Aqua/MODIS image from about three hours after the Terra image above. A small portion of the front can still be seen towards the bottom of the image.

The inspiration for this post came to me several days ago as I was observing tropical storm Leepi using Worldview. I noticed something peculiar in the storm's inflow, albeit far from the storm itself. Upon closer inspection I found that it was a large gust front produced by a large complex of convective clouds measuring several hundred kilometers across.

I made this animation from geostationary satellite imagery (MTSAT) at roughly thirty minute intervals. The front is propagating at around ten to fifteen knots and the clouds that are forming right on the front are about three kilometers across, which is typical of a small tropical cumulonimbus.

False color image of the Terra/MODIS shot. The deep orange/red on the left indicates the top of convective complex is very cold, and thus very high.

The false color image highlights the various cloud top temperatures (an indication of height). The color of the clouds at the front shows they are relatively high, but nowhere near as high as the parent convection mass. Between the convective complex and the front are tiny cumulus clouds that exhibit very little vertical extent. By the end of the satellite loop, the gust front was absent, save a few larger storm systems that it had kicked off, and the parent convection was entirely gone, leaving just the cirrus from its top.