The Science of Contrails

 
  Tree damage due to a microburst over Chicago

Tree damage due to a microburst over Chicago

 
 

by Jacoya Thompson

Introduction
One of nature's most dangerous wind storms created by severe thunderstorms is called a 'microburst'. Microburst winds effect areas less than 2.5 miles wide, last only 5 to 15 minutes, and can produce winds between 65-110 mph causing more damage than a weak tornado.

So, what exactly is a microburst?
It begins with the development of a severe thunderstorm in which water droplets/hailstones become suspended in the updraft of the cumulonimbus cloud. Mature thunderstorms have both an updraft and a downdraft. The updraft adds warm moist air into the storm, and the downdraft deposits cooled air with precipitation. Downdraft winds normally don't reach severe levels but strong wind flow can develop as more dry air is introduced to the middle and lower parts of the cumulonimbus cloud. For a microburst to form both a weakening of the updraft and evaporative cooling has to occur (see diagram below). The incoming dry air mixes in with water within the cumulonimbus cloud causing the water to evaporate and lowering the air temperature. This process is called 'dry air entrainment'. As the cool air sinks through the cumulonimbus cloud it gains speed as it falls. An updraft can be so strong it can transport large amounts of water to the upper part of the cumulonimbus cloud. Water is heavy and when combined with dry air entrainment, the weight of the water falling out of the cumulonimbus cloud drags the cooler air to the surface. This process is called 'water loading' or 'precipitation loading'.

 
 
  Evaporational cooling occurring in the cumulonimbus cloud causing a microburst.  Photo Dr. Jeffrey Frame @VORTEXJeff

Evaporational cooling occurring in the cumulonimbus cloud causing a microburst.  Photo Dr. Jeffrey Frame @VORTEXJeff

 
 

There are actually three types of microburst, dry, wet and a hybrid version with elements of both. As might be expected dry microbursts occur in drier regions. When they hit the ground they do so without any precipitation because the very dry air beneath the cumulonimbus cloud base evaporates the precipitation and creates a negative buoyancy situation. Wet microbursts on the other hand do have precipitation and are caused by dry air entrainment and water loading. Naturally, they are more likely to occur  in more humid regions. Hybrid microbursts possess characteristics of both wet and dry microbursts. They are forced in the mid-levels by dry air entrainment and/or precipitation loading.

Meteorologists have had some success forecasting microbursts by using Doppler Radar. If the weather environment allows it they can often issue warnings hours - or even minutes - before one occurs. But, even with today's advanced technology, radar has its limitations. What meteorologists are looking for is converging air streams in the middle section of the storm and a large core of precipitation being held up by a strong updraft. But they are still difficult to predict since not all severe thunderstorms will produce a microburst even with favorable conditions. Forecasters look for several factors, including "air instability, a prediction of precipitation levels based on moisture in the atmosphere, dry air in middle levels, and strong winds in the layer of dry air" (See 'What is a Microburst? below).

Microbursts can cause great damage to property and sometimes even tragic consequences to those in the air. It is estimated that microbursts have caused as many as 20 major airline accidents, resulting in over 500 deaths  (see 'Discovery of Microbursts Leads to safer Air Travel' below). In response there is an ongoing effort to improve prediction of microbursts using new algorithms and data retrieved from geostationary satellites (GOES), for example. However plenty of work remains to be done as new prediction and detection technologies evolve.  

Recommended Further Reading:

Discovery of Microbursts Leads to Safer Air Travel  National Science Foundation, Josh Chamot, 2003

Extreme Convective Windstorms  Current Understanding and Research, CHARLES A. DOSWELL III

What is a microburst?  NOAA

Progress and Developments of Downburst Prediction Applications of GOES  Kenneth L. Pryor, AMS, 2015

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Jacoya Thompson created this while a student in the Department of Earth and Planetary Sciences at Northwestern University