Boy is it hot and windy!

A Review Of A Possible Heat Burst Affecting Pierre, South Dakota
Ken Harding, SOO, NWSO ABR

 

Introduction
 

In the early morning hours of September 6, 2000 Pierre, South Dakota received wind gusts in excess of 50 knots. Winds were measured in excess of 40 knots for over 90 minutes. These surface winds were associated with a temperature increase of almost 10 degrees F. and dewpoint falls of 5 degrees F. Initial reviews of the data by the operations staff on duty at the time of the event showed very weak convection moving south of Pierre, and relatively strong pressure gradient induced winds of around 30 knots. Upon further discussion, the possibility of a heat burst was raised.
 

A heat burst (HB) (Johnson 1982) is described as localized temperature rises and dewpoint falls associated with a strong downdraft penetrate a shallow surface inversion. HB's have been identified in the literature as early as a reference by Williams (1963) as a 'warm wake' located in or near a trailing stratiform region of  mature mesoscale convective systems (MCSs). Numerous studies of this region of MCSs have been documented in the journal articles so I won't repeat them here . If you are interested, I can provide you with some reading material, or you can learn more online by visiting:

/fsd/soo/heat/heat_burst.htm
http://www.nssl.noaa.gov/~mackeen/heatburst/heatburst.html

Discussion

The time series of observations from the PIR ASOS was as follows:
 

Time UTC Temp 0C Dewpoint 0C SLP Peak Wind
0500 27 14 1009.3 15027
0600 27 14 1006.6 16040
0700 31 12 1005.4 19055
0800 23 18 1008.6 12018

The temperature and dewpoint changes all agree well with a well documented HB event in Kansas in 1985 (Berstein et al., 1994). In this study, several stations in a mesonet measured HB events that had temperature changes from +1.2 0C to +3.5 0C in around 30 minutes, with corresponding dewpoint drops. Without the 5-minutes ASOS data from PIR, it is hard to know for sure the time scale of the temperature rise in this event, but it occurred in under one hour as can be seen from the routine hourly reports.

Details

HB events are often associated with atmospheric profiles characterized by nearly dry adiabatic profiles above a low level stable layer. This dry adiabatic profile allows the downward moving air that is initially cooler aloft to begin to accelerate downward and warm adiabatically. If the acceleration is strong enough aloft, the air may become positively buoyant before it reaches the surface, but it's momentum will allow it to penetrate the shallow stable layer, and arrive at the surface warm. This illistrates an important point about atmospheric stability that we discussed in our review of QG theory last fall - in an unstable atmosphere, small changes in density can result in large vertical responses. In the case of the HB, our response is downward, not up as in the synoptic cases we discussed.

Several soundings were archived to investigate the stability profiles around the MCS. Figure 1a-d are LAPS soundings from 0400 UTC through 0700 UTC at Pierre. (Click for larger Images)
 
 

Fig 1a. 0400 UTC LAPS Sounding  Fig 1b.  0500 UTC LAPS Sounding
Fig 1c.  0600 UTC LAPS Sounding Fig 1d.  0700 UTC LAPS Sounding

Goes derived soundings from both Chamberlin  at 0300 UTC and Ainsworth, NE  (0300 UTC0600 UTC, and 0700 UTC) also show nearly dry adiabatic lapse rates over a lower level stable airmass. Even the GOES sounding from PIR at 0600 UTC shows these steep lapse rates, although the low level stable air is not as evident.  Another interesting feature of the Ainsworth sounding is the amount of CIN (nearly 700 J/Kg). This is another clue to the strength of the low level stable air.

HEAT BURST OR WAKE LOW?

    A wake low is a surface feature often associated with long-lived MCSs.

As the above figure illustrates, the wake low is a hydrostacially lowered pressure induced by subsidence warming from the rear inflow jet in the vicinity of the trailing stratiform region of an MCS.  The resulting pressure gradient from the wake low to the mesohigh under the precip can cause winds to blow at 30 to 50 knots for several hours after the convection passes. Wake lows are difficult to forecast, but can be nowcast quite easily using LAPS surface pressure analysis and WSR-88D cross sections  (both velocity and reflectivity) to monitor the rear inflow jet's position and strength.

The following series of metars, LAPS SLP analysis, and IR satellite show the evolution of a wake low near PIR and the resulting intense pressure gradient. At 0400 UTC the pressure gradient from PIR to 9V9 was approximately 4.5 millibars. By 0600 UTC, the gradient had reached 8 mb, and achieved a maximum of 10 mb by 0700 UTC.

(Click Here for a high resolution loop)


 

Was a 10 mb gradient in about 65 miles enough to support a 50+ knot wind? For more information about wake lows, read an excellent case study HERE

RADAR DATA

Here is a radar loop of base reflectivity from 0632 UTC to 0717 UTC. As you can see from the loop, the convection was unimpressive. It was, however, similar to the other case studies mentioned earlier, in that the trailing stratiform region is in the area of the strong pressure gradient force. One other interesting feature in the radar data is the 0656 UTC 0.5 degree SRM. A broad cyclonic circulation is evident. This circulation leads support to the presence of a wake low, as upper level cyclonic circulation's (often called Mesoscale Convective Vorticies or MVC's) only form after the system has survived for 2 to 4 hours, which is the time scale usually needed to see a wake low develop.

CONCLUSIONS

I would have to call this event a weak HB superimposed in a wake low. The Berstein et al. study in 1994 documented several HB events associated with wake lows. I think the majority of the wind speed increase can be explained by the pressure gradient alone, but the warming and drying temperatures imply strong subsidence, and are consistent with a HB.
 

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