Perspective

Hot byproducts of thunderstorms

Interview | Kevin Knupp, Univ. of Alabama in Huntsville

2 November 2010  •  From snowstorms to thunderstorms, Kevin Knupp’s group at UAH has investigated a wide range of weather phenomena using MIPS, the university’s Mobile Integrated Profiling System. Knupp, a newly named fellow of the American Meteorological Society, presented an analysis of heat bursts in October at the AMS Conference on Severe Local Storms in Denver. Heat bursts are unusual pockets of very warm, dry air descending from thunderstorms, often at night, that can push temperatures up to 35–40°C (95–104°F). Relative humidities can drop below 10%, and winds may exceed the severe criterion of 26 meters per second (58 miles per hour).

 

 

Kevin Knupp, University of Alabama in Huntsville

When were heat bursts first documented?

The first scientific descriptions of heat bursts appeared in the 1960s in technical papers published by the National Weather Bureau [now the National Weather Service] and the National Severe Storms Project [now the NOAA National Severe Storms Laboratory]. Much shorter descriptions of heat-burst phenomena are recorded as early as 1909 in Monthly Weather Review. In 1996, wind gusts of 40 m/s (93 mph) were documented in a heat-burst event in Oklahoma. More recently, instruments such as Doppler radars, wind profilers, and passive microwave profilers have disclosed the complexities of heat-burst flows and thermodynamics.

How have you used MIPS data to study heat bursts?

During the International H2O Project (IHOP) in 2002, a heat burst in western Kansas struck our MIPS site. It produced a temperature of 36°C (96°F), a relative humidity of 9%, and a peak wind gust of 33 m/s (74 mph). We discovered that the heat burst was composed of small-scale microbursts and intense rotational circulations. One year later, during the Bow Echo and MCV Experiment (BAMEX), MIPS recorded a longer-lasting heat burst in extreme western Iowa that persisted for eight hours and produced wind gusts of 31 m/s (69 mph) north of Omaha.

Is there a particular type of thunderstorm, or a location within a storm, where heat bursts are most likely?

It appears that the most widespread and intense heat bursts are generated by mesoscale convective systems (MCSs) that produce large trailing anvils from which precipitation falls. Heat bursts are most likely at the back edge of MCSs such as squall lines after the main part of the storm has passed. This portion of the squall line is typically not associated with severe weather, but during intense heat bursts, severe winds may be widespread.

What can heat bursts tell us about thunderstorm dynamics in general?

When precipitation falls into a dry, unstable layer, sublimation and evaporation produce modest cooling and force the dry air down to lower levels. If this descent occurs over a large region, several smaller-scale microbursts may form and descend at speeds of 10–20 m/s (22–49 mph). If these reach the surface, they may collide and produce intense rotational circulations. Also, when the pre-existing air near the surface is cool and stable, strong gravity waves (similar to waves on a water surface) tend to form and produce strong, variable surface winds. We hypothesize that the most intense heat bursts occur when an intense low-pressure region called a wake low, formed by the warm descending air aloft, accelerates air toward the rear of an MCS. The strongest surface winds occur when the microbursts and gravity waves are superimposed on this background current.