MICROBURSTS
Nolan T. Atkins
Lyndon State College
Prepared for COMET
Mesoscale Analysis and Prediction Course 2002
(COMAP 2002)
13 June 2002
OUTLINE
1. Introduction – Early Discovery
2. Climatology
3. Forcing Mechanisms
4. Microburst Conceptual Models
5. Wet Versus Dry Microbursts
6. Detection
7. Forecasting
INTRODUCTION – EARLY DISCOVERY

Aerial damage
surveys by
Fujita of 3 April
1974 super
outbreak
revealed
unusual
“starburst”
surface wind
damage pattern

315 fatalities,
5484 injuries

15% of damage
paths were
caused by
outburst winds
“Starburst” wind damage pattern
Figure from Fujita 1985
INTRODUCTION – EARLY DISCOVERY

“Starburst”
damage pattern
was very much
different than
swirling damage
left behind in
wake of tornado

Idea of “down
burst” was
conceived

Much like
“pointing the
nozzle of a
garden hose
downward”
“Starburst” wind damage pattern in corn field
Figure from Fujita 1985
INTRODUCTION – EARLY DISCOVERY

On 24 June 1975, Eastern Airlines Flight 66 (Boeing 727) crashed
while attempting to land at New York’s JFK Intl airport

112 fatalities, 12 injuries

Cause of crash was unknown, though thunderstorms were
observed in the area

In an attempt to unravel the mystery behind the crash, Captain
Homer Mouden (from the Flight Safety Foundation at the time)
approached Fujita and asked him to investigate reasons for the
crash
INTRODUCTION – EARLY DISCOVERY

After analyzing only
flight data recorders,
pilot reports and an
airport anemometer,
Fujita hypothesized
that Flight 66 flew
through a low-level
diverging wind field
– downburst

First suggestion that
a “starburst” wind
pattern may be a
cause for airline
crashes
Figure from Fujita 1985
INTRODUCTION – EARLY DISCOVERY

Fujita’s concept of a downburst, a strong downdraft which
induces an outburst of damaging winds on or near the ground,
was met with some skepticism

Many meteorologists at the time, believed that the downdraft
should be relatively weak by the time it reaches the ground

Resolution of Fujita’s downburst theory ultimately led to the
creation of the Northern Illinois Meteorological Research on
Downbursts (NIMROD) field program employing NCAR
Doppler radars
INTRODUCTION – EARLY DISCOVERY
Radial velocities from first radar-detected downburst

On 29 May, 1978, the
first radar-detected
downburst was
observed by the NCAR
CP-3 Doppler radar by
Fujita and Jim Wilson

The existence of the
downburst had been
verified.

Since then, a flurry of
observational, applied
and theoretical work
surrounding the
downburst has been
pursued
Figure from Wilson 2001
Climatology


A national climatological
summary of downbursts,
unfortunately, does not exist
Kelly et al. (1985) have produced
a climatology of damaging wind
gusts.
 Based on 75,626 severe
thunderstorm reports from
1955-1983.
 Does NOT distinguish
damage created from
different convective modes
(for example, RIJ associated
with a bow echo)
 Three categories of wind
gusts were created 
Severe thunderstorm wind gusts, 1955-1983.
Gust
speed
Annual
number
Percent
Damaging
unknown 1114
70
Strong
25.8-33.5 375
m/s
23
Violent
> 33.5
m/s
7
Total
From Kelly et al. (1985)
113
1602
Climatology

Damaging wind gusts:
 Primarily a summer time
phenomena
From Kelly et al. (1985)
Climatology

Damaging wind gusts:


Most events occur during
late afternoon
However, a non negligible
number of events occur
between midnight and
noon
From Kelly et al. (1985)
Climatology


1.
2.
3.

Geographical
Distribution of
damaging wind
gusts:
Two major
frequency axes:
Southern MN – IA
– IL – IN – OH
(NW flow events)
NW IA – Kansas
City, MO – KS –
OK – TX
Possibly a third
from eastern TX –
AL – up to New
England
High probability
of a population
bias in data
From Wakimoto (2002)
Climatology
Kelly et al. results are similar to those by Fujita
(1981) for the year 1979
From Fujita (1981)
Climatology

1.
2.
3.
4.
Data from Downburst field
programs:
Northern Illinois
Meteorological Research on
Downburst (NIMROD) –
1978
Joint Airport Weather Studies
(JAWS) – 1982
FAA/Lincoln Lab Operational
Weather Studies (FLOWS) –
1985/86
Microburst and Severe
Thunderstorm (MIST) project
- 1986
From Wakimoto (2002)
Climatology

186 microbursts during JAWS over 86
days

Diurnal variation similar to Kelly et al.
(1985)
Figures from Wakimoto (1985)
Climatology



62 microbursts during MIST over 61 days
Diurnal variation similar to Kelly et al.
(1985)
Data from field programs suggest
downbursts occur frequently
Figures from Atkins and Wakimoto
(1991)
Forcing Mechanisms – Updrafts and Downdrafts
UPDRAFT
DOWNDRAFT
Ascends supersaturated
rc+rr+ri negate updraft
Latent heat release enhances UD
Microphysical details not that important
Entrainment is detrimental
descends largely subsaturated
rc+rr+ri enhance downdraft
evap cooling/sub/melt enhances DD
microphysics can be very important
mid-level entrainment can enhance DD,
low-level entrainment can be detrimental
Forcing Mechanisms


Q: What physical processes are responsible for generating
strong, low-level downdrafts?
The answer can be found in the vertical momentum equation:

  v cv p

dw
1 p

 g

 rc  rr  ri 
dt
 z
 vo c p po

I
II
III
IV
I – Vertical gradient of perturbation pressure
II – Thermal buoyancy (parcel theory)
III – perturbation pressure buoyancy
IV – Condensate loading of cloud, rain and ice water
Forcing Mechanisms
I – Vertical gradient of perturbation pressure
1 p

 z

In
weakly sheared environments promoting the formation of
ordinary cells, the vertical perturbation pressure gradient force
tends to be weak
This
force becomes more important in more strongly sheared
environments
Example: occlusion downdraft within supercell thunderstorms
Forcing Mechanisms
II – Thermal buoyancy
 v
g
 vo
process in convective downdrafts – is the most important
forcing mechanism for most convective downdrafts
Well-understood
Created
by the evaporation, melting and sublimation of cloud and precipitation
particles within a sub saturated parcel of air
In
weakly precipitating downdrafts:
The downdraft can simply be though as the competing processes of
negative buoyancy generation through condensate phase changes and
adiabatic compressional warming
Note
the use of the virtual potential temperature
Downdraft intensity has been shown to increase within higher relative
humidity environments at low levels by increasing the v difference between
the sub saturated downdraft parcel and the environment (e.g., Srivastava
1985; Proctor 1989)
Forcing Mechanisms
Yes,
observational and modeling studies (e.g., Kamburova and Ludlam 1966;
Leary and Houze 1979; Srivastava 1985; Proctor 1989) have shown that the
downdraft often descends sub saturated.
Cooling due to condensate phase changes does not completely
compensate for adiabatic compressional warming
This
may be true even with heavier precipitation events:
Byers and Braham (1949) noted “humidity dips” associated with
Florida and Ohio thunderstorm downdrafts
Thus,
microphysical details, while not as important for updrafts, appear to be
quite important for generating stronger downdrafts:
Numerical
calculations (e.g., Kamburova and Ludlam 1966;
Srivastava 1985, 87; Proctor 1989) suggest that the maintenance and
intensity of a downdraft by falling precipitation is a function of:
Precipitation
type (i.e., rain, snow, hail or graupel)
Precipitation
size
Precipitation
intensity and duration
Forcing Mechanisms
III – Perturbation pressure buoyancy
This
cv p
g
c p po
term is ignored in Parcel Theory
Has
been shown to be relatively weak in comparison to the
thermal buoyancy and vertical perturbation pressure gradient
terms within convective storms (Schlesinger 1980)
Perturbation
pressure buoyancy term has been shown to have
appreciable magnitudes where the updraft penetrates the
tropopause
Forcing Mechanisms
IV – Condensate Loading
 g rc  rr  ri 
Long
been recognized as an important process for the initiation and
maintenance of downdrafts (e.g., Brooks 1922)
Compared
to thermal buoyancy, this term is often of secondary importance
for downdraft maintenance and intensity (but not always).
It
is, however, important for downdraft initiation
Forcing Mechanisms
Entrainment
Entrainment
has long been recognized as an important process
affecting the strength of updrafts within convective storms
Weakens
the updraft by mixing environmental air into buoyant
parcels
Largely explains why Parcel Theory over estimates the maximum
vertical velocity expected for a surface-based ascending parcel, i.e.,
Wmax  2  CAPE
For
downdrafts, it is generally thought that entrainment of dry
environmental air promotes downdraft initiation and maintenance by
increased evaporation, melting and sublimation of cloud and
precipitation particles within sub saturated downdraft parcels of air.
However………..
Forcing Mechanisms
Entrainment
Numerical
simulations by Srivastava (1985) and Proctor (1989)
suggest that entrainment can be detrimental to downdraft strength!
Srivastava’s
1-D,
Model configuration:
time-dependent model of evaporatively driven downdraft
Initial
downdraft at top of model domain specified by P, T, RH, W,
DSD
Environmental
RH = 70%
From Srivastava (1985)
Forcing Mechanisms
Entrainment
Resolution
of these two conflicting ideas may be related to where and
when entrainment is occurring:
Entrainment may be beneficial for downdraft initiation and
subsequent maintenance say near cloud base.

Entrainment may be detrimental for downdraft maintenance at low
levels since the virtual potential temperature difference between the
sub saturated negatively buoyant downdraft parcel and the
environment will decrease, particularly if the mixing ratio of the
environment is larger than that of the downdraft parcel.

Microburst Conceptual Models

Fujita defined a downburst as a strong downdraft which induces an
outburst of damaging, highly divergent winds on or near the ground.

The scale of the downburst varies from less than 1 km to 10s of km.

Thus, he subdivided downbursts into macrobursts and microbursts
according to their horizontal scale of damaging winds:

Macroburst: A large downburst with its outburst winds extending
in excess of 4 km in horizontal dimension. An intense
macroburst often causes widespread, tornado-like damage.
Damaging winds, lasting 5 to 30 minutes, could be as high as 60
m/s.

Microburst: A small downburst with its outburst, damaging winds
extending only 4 km or less. In spite of its small horizontal scale,
an intense microburst could induce damaging winds as high at
75 m/s.
Microburst Conceptual Models

The F2 Andrews Air Force Base Microburst on 1 August 1983
Figure from
Fujita 1985
Microburst Conceptual Models

One of the earliest conceptual models was put forth by who else….,
yes, Fujita (1985).



The midair microburst may or may not reach the ground
At touchdown, the microburst is characterized by a shaft of strong
downward velocity at its center and strong divergence.
Soon thereafter, an outburst of strong, accelerating winds within a rotor
circulation spreads outward.

The strongest winds are generally found in the base of the rotor
circulation and can have a significant impact on aviation operations
Figure from Fujita 1985
Microburst Conceptual Models

Numerical Simulations of a microburst and associated rotors
Figure from Orf et al.
(1996)
Figure from Proctor et al. (1988)
Microburst Conceptual Models

Observations of a microburst and associated rotor
Figure from
Kessinger et al. (1988)
Also see Wilson et al.
(1984)
Presumably,
the rotor is generated through tilting of vertical vorticity
and/or baroclinically along the leading edge of the outflow
As
the outflow and rotor spreads out, the rotor circulation is enhanced
through vortex stretching
Microburst Conceptual Models

3-Dimensional conceptual model of a microburst (Fujita, 1985)


Notice the intense small-scale (< 4 km; misocyclone) rotation
associated with the microburst
This rotation is a relatively common feature associated with microbursts
Some
studies suggest
the rotation enhances
microburst strength (e.g.,
Rinehart el al. 1995;
Fujita 1985; Wakimoto
1985)
Other
studies suggest
that the rotation weakens
the microburst (e.g.,
Kessinger et al. 1988;
Proctor 1989)
Figure from Fujita (1985)
Microbursts – Wet and Dry

A large number of studies have shown that microburst winds are
associated with a continuum of rain rates, ranging from heavy
precipitation from deep cumulonimbi to virga shafts from altocumuli
or high-based cumulonimbi.

There is no positive correlation between downburst winds and
surface precipitation rates

Accordingly, microbursts are subdivided into wet/high reflectivity and
dry/low reflectivity events and are defined as follows (Fujita and
Wakimoto 1981; Wilson et al. 1984; Fujita 1985):

Dry/low reflectivity microburst: A microburst associated with < 0.25
mm of rain or a radar echo < 35 dBZ in intensity

Wet/high-reflectivity microburst: A microburst associated with > 0.25
mm of rain or a radar echo > 35 dBZ in intensity
Dry Microbursts - Observations
Produced
from innocuous
pendent virga shafts from
weakly precipitating altocumulus
Photographs taken by B. Waranauskas, from
Fujita (1985) of virga and curl of dust associated
with the rotor circulation with a dry microburst
Example of altocumuli producing dry microbursts
Photograph taken by B. Smith (from Wakimoto 1985)
Dry Microbursts - Observations
Dual-Doppler radar
observations of a drymicroburst outflow
(also see Wilson et al.
1984)
figure from Hjelmfelt (1988)
Figure from
Fujita (1985)
Dry Microbursts - Observations
Figure from
Wakimoto et al. (1994)
Figure from
Fujita (1985)
Dry Microbursts - Environment
Deep,
High
Dry
dry-adiabatic, well-mixed boundary layer.
cloud bases – 500 mb
sub cloud layer (3-5 g/kg) with mid-level moisture present
Figure from
Wakimoto (1985)
(Also see Krumm 1954;
Wilson et al. 1984;
McCarthy and Serafin
1984; Fujita 1985;
Mahoney and Rodi 1987;
Hjelmfelt 1988)
Dry Microbursts - Environment
Dry
microbursts are largely driven by negative thermal buoyancy created by
the evaporation, melting and sublimation of precipitation
When
a deep, dry adiabatic layer is present, only light precipitation is required
to generate strong downdrafts…., why?
Compressional
warming
can not counteract negative
buoyancy created by
precipitation phase changes
Parcel
accelerates to the
ground
Note
that surface parcel
temperature may not be
much different than
environment, may actually
be warmer! (Fujita 85;
Srivastava 85; Proctor 89)
Based on a figure from Wakimoto (1985)
Dry Microbursts - Environment
With
a slightly more stable layer just below cloud base, for example, it may not
possible to generate a strong downdraft.
Thus,
deep, dry-adiabatic sub cloud layers are crucial for producing strong dry
microbursts
Numerical
simulations
also suggest that lowlevel environmental
moisture helps produce
stronger downdrafts by
increasing the v
difference between the
sub saturated parcel
and environment (e.g.,
Srivastava 1985; Proctor
1989)
Based on a figure from Wakimoto (1985)
Dry Microbursts – Microphysical Considerations
In
addition to the environmental profiles of temperature and moisture,
dry microburst strength has been shown to be a function of:
Precipitation
intensity, size, and phase
In
particular, sublimation from snowflakes has been shown to very
very effective at generating strong dry microbursts (Proctor 1989;
Wakimoto 1994). Why?
Numerous
Large
low-density snowflakes readily sublimate
latent heat due to sublimation
Sublimation
cooling (also melting) occurs quickly at relatively
high altitudes (Srivastava 1987) – allowing the downdraft parcels
to accelerate through a deep dry-adiabatic layer.
Dry Microbursts –
Microphysical Considerations
Some
visual evidence of the sublimation
process was presented by Wakimoto et al.
(1994)
Figure from Wakimoto et al. (1994)
Wet Microbursts - Observations
Produced
by deep cumulonimbus with
warm cloud bases in more humid
environments
Figure from Fujita (1985) Photo copyrighted and taken by Mike Smith
Figure from Atkins and Wakimoto (1991).
Photo taken by K. Knupp
Wet Microbursts - Observations
Figure from Atkins
and Wakimoto
(1991).
Wet Microbursts - Observations
Figures from Kingsmill and Wakimoto (1991)
Wet Microbursts - Environments
Relative
to dry microbursts, wet events form in more stable
environments
Accordingly,
it is more difficult for negative thermal buoyancy to
counteract compressional warming
Thus,
more precipitation is required to enhance negative
thermal buoyancy production and increase precipitation loading
Figure from Atkins and Wakimoto (1991)
Wet Microbursts - Environments
Notice
that for lapse rates >
8.5 ºC km-1 , both wet and
dry microbursts are observed
to occur
However,
when the lapse
rate is < 8.0 ºC km-1 , only
wet microbursts occur
Virtually
no microbursts
occur when the lapse rate
was less than 7.0 ºC km-1 .
Figure from Srivastava (1985)
Wet Microbursts - Environments
Numerical
simulations
by Srivastava (1985) and
Proctor (1989) are
consistent with the
observations by
Srivastava (1985) that
suggest progressively
larger amounts of
precipitation are required
to form microbursts in
increasingly more stable
environments
Figure from Wakimoto (2002), based on figure from Srivastava (1985)
Wet Microbursts – Microphysical Considerations
Similar
to dry microbursts, the
ice phase has been shown
numerically (Srivastava 1987;
Proctor 1989) and
observationally (Wakimoto and
Bringi 1988) to be important
Hail
in particular, provides
cooling throughout the entire
depth of the downdraft extent –
very important at low levels
below cloud base!
Figure from Wakimoto and Bring (1988)
Wet Microbursts – Microphysical Considerations
Unlike
dry microbursts,
precipitation loading can be
important for the initiation and
initial maintenance of the wet
microburst at higher levels
Notice
that within the wet
microburst, parcels can be
warmer than the surrounding
environment! (also see Wei et al.
1998 and Igau et al. 1999 for
tropical downdrafts)
Below
cloud base in the dryadiabatic, well-mixed layer,
thermal buoyancy becomes very
important
Figure from Proctor (1989)
Microburst Detection
Wilson
et al. (1984) showed that Doppler radar could detect events at
close range. Events during JAWS showed:
Typical
downdraft is 1 km wide
Spread
out horizontally below a height of 1km AGL
Median
time from initial divergence at the surface to maximum
differential velocity across microburst is 5 minutes
Height
of maximum differential velocity is about 75 m AGL
Median
velocity differential was 22 m/s over an average
distance of 3.1 km
They
are short-lived, low-level, small-scale events.
Microburst Detection
Roberts
and Wilson (1989) suggest that the following radar attributes
can be used to detect microburst development:
Descending
Increasing
reflectivity cores
radial convergence within cloud
Rotation
reflectivity
These
notches
typically appeared 2-6 minutes prior to initial surface
outflow
Their
results suggest 0-10 minute microburst nowcasts are
possible
Microburst Detection - Examples
Descending
reflectivity cores
Figure from Wakimoto (2002), original figures from Kingsmill and Wakimoto (1991)
Microburst Detection - Examples
Increasing
radial convergence within cloud
Figure from Fujita (1985)
Microburst Detection - Examples
Rotation
Figure from Roberts and Wilson (1989)
Microburst Detection - Examples
Reflectivity
notch
Figure from Roberts and Wilson (1989)
Other
automatic detection schemes and algorithms are discussed in
Dance and Potts (2002)
Microburst Forecasting
When
the environmental wind shear is relatively weak, the vertical
profile of temperature and moisture can be used to assess microburst
potential (Johns and Doswell 1992)
Dry Microbursts:
Deep
dryadiabatic sub-cloud
layer to mid levels
Moist
mid
tropospheric layer,
dry low-levels
Marginal
updraft
instability
Updraft
sounding
indices can not be
used to forecast
microburst potential
or severity
Figure from Wakimoto (1985), also see Krumm (1954), Beebe (1955) and Caracena et al. (1983)
Microburst Forecasting
Wet Microbursts:
Moist
Dry
low levels up to 3-5 km, dry mid levels
adiabatic sub-cloud layer 1.5 km deep
Weak
capping inversion
Figure from Atkins and Wakimoto (1991) Also see Caracena and Maier (1987)
Microburst Forecasting
e difference from surface to emin (De) of 20 K or so appears to be a
characteristic of wet microburst producing environment
De values less than 13 K produced thunderstorms, but no wet microbursts
Cape Canaveral Air Station have developed the MDPI = De/30.
(Wheeler and Roeder 1998). MDPI > is interpreted as high wet microburst
probability, issued only when thunderstorm activity is forecast > 60%
The
Figure from Atkins and Wakimoto (1991)
Microburst Forecasting
While
sounding indices for predicting updraft strength work reasonably well,
the same can not be said for predicting peak downdraft strengths with sounding
indices:
Downdraft
Largely
sensitivity to microphysics
sub saturated descent
Nonlinear
relationship between maximum downdraft vertical velocity
and outflow speeds (it’s not 1:1!!).
That
said, previous investigators have developed potential microburst strength
indices that can be easily calculated with routinely collected sounding data.
Microburst Forecasting
Proctor
(1989) put forth the following “wet microburst potential intensity” index:
I

2
  0  H m Qv (1km) 1.5Qv ( H m )  / 3
Hm

0.5
5
Where:
Hm is the height of the melting level
 is the mean lapse rate from the ground to the melting level
o = 5.5 ºC/km
Qv is the mixing ratio
<o, then I < 0
I is larger if:
Hm is large
 is large
Moist at 1 km and dry at the melting level
If
Worked
well for modeled microbursts, but not for observed events
Microburst Forecasting
McCann
(1994) modified Proctor’s index in the following way:


WI  5 H m RQ G  30  QL  2QM
2

0.5
Where:
WI = Wind Index (WINDEX)
Hm is the height of the melting level
G is the mean lapse rate from the ground to the melting level
QL is the mean mixing ratio of lowest 1km
QM is the mixing ratio at the melting level
RQ = QL/12 but is set to 1 if QL/12 > 1.
WI
is larger if:
Hm is large
G is large (note G2 dependence)
Moist at low levels and dry at the melting level
How
well does WINDEX work?
Microburst Forecasting
24 August 1993
2000 UTC
2200 UTC
Figure from McCann (1994)
Notice
the outflow boundary moving into an area with high WINDEX
values
Microburst
Microburst
damage in vicinity of DFW was observed on this day
forecasting is intimately related to convective initiation
forecasting – monitoring low-level convergence boundaries
Microburst Forecasting
Recently,
Geerts (2001) has modified the WINDEX to account for other
processes that help to generate strong wind gusts such as the downward
transfer of horizontal momentum:
He
created the GUSTEX to include this process:
GU
= aWI + 0.5U500
Where
WI
a is a constant (he set it to 0.6)
= WINDEX
U500
is the 500 hPA wind velocity
For Australian
wind gust events, he showed a better correlation between
GUSTEX and observed gust speed than with WINDEX and observed gust
speed.
Microburst Forecasting

Ellrod (1989) and Ellrod et al. (2000) have shown the value of using GOES
satellite data form microburst forecasting.

Ellrod et al. (2000) tested the following indices derived from satellite data:
1.
2.
3.

WINDEX
DMI = G700-500hPa + (T-Td)700 – (T-Td)500 (Ellrod and Nelson 1998); DMI
> 6 for dry microbursts to occur
De
Products are creating hourly and have been shown to provide “information
useful in the preparation of short-range weather forecasts and advisories”.
Conclusions
First
discovered by Fujita in mid 70s while surveying tornado damage
Immediately
realized their significance in creating damage at the surface
(up to F3) and in impacting aviation operations
No
comprehensive microburst climatology exists
Data
from field programs suggest they are a relatively common
occurrence – summertime phenomena, most common mid-late afternoon
Primary
forcing mechanism is negative thermal buoyancy generated by
evaporation, melting and sublimation of cloud and precipitation particles
Precipitation
loading is also important, particularly with wet microbursts
Microphysics
are very important for the downdraft that quite often descends
subsaturated
Entrainment
can be beneficial or detrimental depending upon where/when it
occurs
Microbursts
events are associated with a continuum of rain rates and are thus
subdivided into “wet” and “dry” events
Conclusions, cont.
Dry
microbursts occur within deep, dry-adiabatic subcloud layers and originate
from innocuous virga shafts associated with altocumulus
Formed
from negative thermal buoyancy – ice phase is important!
Wet
microburst occur within more stable, humid environments and originate
from deep cumulonimbus
from negative thermal buoyancy and precip loading – again, ice
phase is important!
Formed
Detection
is challenging, they are short lived, low-level, small-scale in nature
There
are useful radar attributes that can detect their occurrence 2-6
minutes before damaging winds are observed at the surface
In
weakly sheared environments, soundings can be used to forecast their
occurrence.
Downburst
indices are problematic, though recent studies have shown they
are of some utility in predicting downburst potential and intensity