Heat Burst Phenomena Experienced in Nebraska

Heat Burst Phenomena Experienced in Nebraska

By: Paige Hymson

A rare and powerful heat burst hit the south central part of Nebraska in early June, causing temperatures to rise from 73°F degrees to boiling 99°F degrees in just two hours.

The event followed dissipating thunderstorms that created conditions conducive to the formation of heat bursts.

These unusual phenomena are associated with the decline of thunderstorms, as rain evaporates and creates a dry dense air. As the last of the precipitation from the storm passes through that layer of dry air, the evaporation causes the air to become cool and denser than its surrounding warm air. They are generally characterized by a drastic jump in temperature accompanied by strong dry winds.

Reports of extreme heat bursts have been well documented in various parts of America. One occurred in South Dakota during May of 2013 and one in Wichita, Kansas in 2011, when temperatures rose 20F degrees during one over-night hour.

On August 23, 2011, a strong heat burst greatly affected the southwestern part of Iowa. There were reports of downed trees and power lines, caused by gusty winds that accompanied the event.

There have been a number of unconfirmed reports of extreme heat burst events in other parts of the world that have resulted in astronomically high temperatures. Some of the most notable events include the following: Antalya, Turkey – 152°F (1977), Lisbon, Portugal – 158° (1949), Abadan, Iran – 188° (1967).

These events open discussion for meteorologists who question if the high temperatures experienced during heat bursts should be considered within the context of historical temperature measurements and records.

Last week, the Capital Weather Gang described a late night heat burst in Nebraska – temperatures at several automated weather stations rose 5-15° (I converted these to Celsius) in a few hours.  This phenomenon has been observed quite frequently since automated observations became more common, with a number of examples listed in the Wikipedia article.  The generally offered explanation is that precipitation from a decaying thunderstorm falls into a very dry air at a high altitude and evaporates, cooling the air so that it becomes much denser than surrounding air and accelerates downward.  As it falls into higher pressures, compression warms the parcel.  Once it reaches equilibrium, it stops accelerating, but its momentum continues to drive it toward the ground.  If it reaches the ground before the buoyancy stops its motion, the air is both hotter and drier than the air it displaces and a heat burst results.  This explanation, cribbed from the sources I linked to above, is plausible but not rigorously documented and/or modeled as far as I can tell.

Observations from automated weather stations, or National Weather Service sites, lend great confidence to the existence of heat bursts.  However, there are many stories of much more extreme events, with temperatures as high as 87°C (see, for example this blog post).  Since I don’t understand the dynamics underlying the phenomenon well enough to guess whether such extreme temperatures are even possible, I will remain skeptical until one is seen by the observing systems I trust.

However, the existence of heat bursts of more modest magnitude seems to be quite clear, and that leads to an interesting question regarding climate data.  A little background first: everyone is familiar with the global temperature time series, generally presented as a monthly average departure from some base period.  That time series, over land, is constructed from individual daily mean temperatures at a number of observing stations, and the mean temperature for a day is simply the average of the highest and lowest temperatures observed during the day.  The connection to heat bursts is in the sampling – until recently, these small scale transient events were rarely observed and so didn’t affect the daily mean temperature often if ever.  Since the heat bursts are clearly part of the climate, they certainly should be included in the temperature data.  However, the conventional method of simply averaging the high and low daily values might give an extreme value that occurs during a heat burst undue weight in the daily mean – the time series from the Nebraska event illustrates this nicely. A more common occurrence with a potentially similar impact on the climate record is relatively cold temperatures in the downdraft of a thunderstorm.  The difference between the two is that the downdraft-related cold pools are quite a bit more extensive and much more likely to be detected in conventional observations, and so time-dependent bias is less likely.  I don’t know for sure whether any of the groups creating the global temperature data have figured out a way to deal with this issue, but I presume that they will eventually have to, since automated observations are likely to become more and more common.

Postscript

This will be my final post under the Climate Today label.  When I began, I hoped to keep to a weekly posting schedule; that lasted exactly 3 weeks, and then I managed 8 more in the following 15 months.  Professor Fernando Miralles-Wilhelm, who arrived in ESSIC and AOSC in May, will be taking over this space and I look forward to seeing his insights and commentary.  I hope to post occasionally when I feel I have something to say, probably in the Scientists’ Soapbox section.