WMOD00311

• Title: Harold D. Orville Symposium - Forty Years of Modeling Clouds and Weather Modification
• Date: 4/26/2003
• Weather Modification - Doc Type: Report

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Microbursts and Their Numerical Simulation

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Mark R. Hjelmfelt
Institute of Atmospheric Sciences
South Dakota School of Mines and Technology
Rapid City, SD 57701

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Introduction

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I first encountered Dr. Orville at the International
Cloud Physics Conference in London, England in
August 1972. I had begun a Master's Program in
Meteorology at Purdue, but this was during the Viet
Nam war and I received my draft notice and joined the
Air Force. I was stationed at a small base about 40 mi
north of London and somehow managed to talk the
Lieutenant into letting me have a pass to go to London
for a couple days of the Conference. Harry gave an
invited talk "The structure and dynamics of clouds". He
centered his talk on the equation of motion for vertical
velocities in convective clouds, a very pertinent topic
for the present discussion. Beside my notes I wrote,
"He would be a good teacher." Little did I know that
less than a year later I would be reassigned to Ellsworth
AFB and attending SDSM&T. "He was a good
teacher!"

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I completed my degree in 1975 and went on to the
University of Chicago. In the fall semester of 1976, I
took an independent study course in Mesoscale
Analysis from Dr. Ted Fujita, which included working
through analysis of the data he used in his aircraft
accident papers (Fujita and Byers 1977; Fujita and
Caracena, 1977). Somehow I got included in the long
list of people acknowledged in his Manual of
Downburstldentification (Fujita, 1978).

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The emphasis of my efforts over the next several
years had little to do with microbursts or downbursts,
until I received an offer in spring 1983 to join the Field
Observing Facility at NCAR to serve as a project
scientist and work on the JAWS project, the field phase
of which had been carried out in summer 1982.

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Since that time, I have worked on aspects of
microbursts and downbursts off and on for the past 20
years. Dr. Orville also became interested in the subject.
He has been directly involved in at least three refereed
papers (Tuttle et aI., 1989; Hjelmfelt et aI., 1989; and
Orville et al., 1989), four theses (J-P. Chen, 1986; Y -C.
Chi, 1989; D. Todey, 1990; and J. Searles, 1992) and a
number of conference papers dealing with microburst
dynamics. Of course his work on cloud

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Author address: Dr. Mark R. Hjelmfelt, Institute of
Atmospheric Sciences, South Dakota School of
Mines and Technology, 501 E. SaintJoseph Street,
Rapid City, SD 57701-3995.

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thermodynamics and the interactions of microphysics
and dynamics strongly influenced the field. Also, most
of the models discussed below used the IAS
microphysical scheme.

Knupp and Cotton (1985) reviewed the general
subject of convective downdrafts. Downbursts and
microbursts have been described by Fujita (1981, 1985)
and others. Most recently Wakimoto (2002) has
provided an excellent comprehensive review of
convectively driven high wind events.

The general topic of "downbursts" - a localized
current of air descending from a cumulus cloud which
induces an outward burst of damaging wind on or near
the ground (Fujita and Byers, 1977) can be thought of
as consisting of:

a) More-or-less isolated incidences of individual
pulses of strong downdraft and outflow. Depending on
the size of the outflow, these may be classified as
"micro bursts" (outflows for which the distance between
the peak winds on either side is less than 4 km) or
larger scale "macrobursts." Sometimes these occur in
sequences along a line of storms, giving rise to a
continuous line of divergence, "microburst lines."
Figure 1 shows Fujita!s (1978, 1985) conceptual model
of a microburst downdraft and outflow and an example
of forest blowdown damage he attributed to a rapidly
moving microburst.

Examples of Doppler radar observed microburst
and microburst line outflows are shown in Fig. 2.

b) Larger scale, organized severe-wind producing
systems, which may produce winds over a sizable area
for an extended path of many tens of kilometers. These
are typically associated with supercell thunderstorms,
squall lines, line echo wave patterns (LEWP), and
especially Bow Echo storms, see Fig. 3. At the largest,
these may be classified as "derechos," which have been
defined by Johns and Hirt (1987) to have path lengths
of high surface wind extending over 400km. An
example of the interest in these storms is the Bow Echo
and MCV Experiment (BAMEX, Weisman and Trapp,
2002), which is about to start in the central U. S.

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