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

“Starburst” wind damage pattern

 315 fatalities, 5484 injuries  15% of damage paths were caused by outburst winds

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

“Starburst” wind damage pattern in corn field

 Much like “pointing the nozzle of a garden hose downward”

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

On 29 May, 1978, the first radar-detected downburst was observed by the NCAR CP-3 Doppler radar by Fujita and Jim Wilson Radial velocities from first radar-detected downburst  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.

Damaging Strong Violent Total Gust speed unknown 25.8-33.5 m/s > 33.5 m/s

From Kelly et al. (1985)

Annual number 1114 375 113 1602 Percent 70 23 7

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

Ascends supersaturated r c +r r +r i negate updraft Latent heat release enhances UD Microphysical details not that important Entrainment is detrimental

DOWNDRAFT

descends largely subsaturated r c +r r +r i 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:

d w dt

  1  

p



z

 

g

      

v vo



c c p v p



p o

 

r c



r r



r i

     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

g

  

v vo

 Well-understood process in convective downdrafts –

is the most important forcing mechanism for most convective downdrafts

 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 

c g c p v p p o

  This 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



r c



r r



r i

  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.,

W

max  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 Model configuration:  1-D, 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 dry microburst outflow (also see Wilson et al. 1984)

figure from Hjelmfelt (1988)

Figure from Fujita (1985)

Figure from Fujita (1985)

Dry Microbursts - Observations

Figure from Wakimoto et al. (1994)

Dry Microbursts - Environment

 Deep, dry-adiabatic, well-mixed boundary layer.

 High cloud bases – 500 mb  Dry 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 low level 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 low-density snowflakes readily sublimate  Large 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

Figure from Atkins and Wakimoto (1991).

Wet Microbursts - Observations

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 dry adiabatic, 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 reflectivity cores  Increasing radial convergence within cloud  Rotation  reflectivity notches  These 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)

 Rotation

Microburst Detection - Examples

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 dry adiabatic 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 low levels up to 3-5 km, dry mid levels  Dry 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  The 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%

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 sensitivity to microphysics  Largely 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

 

H

2

m

    0  

H m



Q v

( 1

k m

)  1 .

5

Q v

(

H m

)  / 3  0 .

5 5 Where:  H m     o is the height of the melting level is the mean lapse rate from the ground to the melting level = 5.5 ºC/km  Q v is the mixing ratio  If  <  o , then I < 0  I is larger if:  H m   is large is large  Moist at 1 km and dry at the melting level  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 R Q

 G 2  30 

Q L

 2

Q M

  0 .

5 Where:  WI = Wind Index (WINDEX)  H m  G is the height of the melting level is the mean lapse rate from the ground to the melting level  Q L  Q M is the mean mixing ratio of lowest 1km is the mixing ratio at the melting level  R Q = Q L /12 but is set to 1 if Q L /12 > 1.

 WI is larger if:  H m  G is large is large (note G 2 dependence)  Moist at low levels and dry at the melting level  How well does WINDEX work?

24 August 1993

Microburst Forecasting

2000 UTC 2200 UTC

Figure from McCann (1994)

 Notice the outflow boundary moving into an area with high WINDEX values  Microburst damage in vicinity of DFW was observed on this day  Microburst 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 = a WI + 0.5U

500  Where a is a constant (he set it to 0.6)  WI = WINDEX  U 500 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.

WINDEX DMI = G  700-500hPa  + (T-T d ) 700 – (T-T d ) 500 > 6 for dry microbursts to occur (Ellrod and Nelson 1998); DMI 3.

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  Formed from negative thermal buoyancy and precip loading – again, ice phase is important!

 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