WRAL WeatherCenter Blog

What is the difference between a wet mircoburst and a mircoburst? Then...... How does a gust front form and is it similar to an outflow boundary?

Posted September 1, 2007 11:59 a.m. EDT

MIKE MOSS SAYS:     Blake,      A microburst in general is a strong wind that occurs due to downdrafts in a thunderstorm striking the ground and spreading outward at speed sufficient to cause damage or interfere with aviation, with the affected area coveraing a diameter of 2.5 miles or less (larger events, like the one in Goldsboro a few weeks back, are called macrobursts or sometimes just downbursts). Microbursts can be wet, dry or a hybrid in between the two extremes. A wet microburst occurs when a sizable depth of the atmosphere is quite humid,  including the layer of air between the cloud and ground, and in a classic wet microburst a  large part of the downward momentum that is imparted to the air is due to precipitation drag, in which case the air is literally pulled along by large numbers of falling raindrops and hailstones - in these storms, the strong winds are accompanied by heavy precipitation at the surface. Dry microbursts, on the other hand, occur when the storm environment includes a deep layer of dry air below the storm, and sometimes a layer or two of dry air  aloft that can mix with the storm cloud or it's precipitation. In these cases, a significant fraction of precipitation falling through and mixing with the layers of dry air evaporates, which results in less precipitation at the surface (and in some cases, no precipitation reaching the ground), but the evaporation sharply cools the descending air, making it much denser than surrounding air and causing it to be accelerated toward the ground by negative buoyancy. Of course, like many atmospheric processes, there aren't just these two extremes that exist in nature, and there is a contimuum between the wet and dry ends of the spectrum in which some portion of the downard momentum is imparted by precipitation drag and some other fraction is due to evaporative cooling.

As for gust fronts, they are indeed a form of outflow boundary, with the front moving outward from the thunderstorm or sometimes trailing along beside it. The gust front separates cool, dense air that descended from the storm from (usually) warmer, moister air outside the base of the thunderstorm and is oftena zone of sharp changes in wind speed and direction, leading to converging air near the front and sometimes to new thunderstorm cell development as warm, unstable air is lifted by the denser outflow air that undercuts it.