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An Analysis of Thermodynamics, Dynamics, and Environments of Microbursts
Atmospheric Dynamics, Storms and Radar
Kyle Ziolkowski
ID: 7700294
Due: April 4, 2013
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Introduction
Microbursts are small but powerful downdrafts that descend from
convective clouds and radiate outward in all directions. They are a mesoscale
atmospheric phenomenon, averaging around 4 kilometres in diameter or less and
can produce damaging winds as high as 75 m s-1 (270km/hr) (Rinehart, 2010). With
such high and dynamic winds, microbursts can be a potential threat to life and
property and are especially dangerous to aviation. Since 1975, more than 500
people have been killed in aircraft accidents directly caused by microburst activity
(Marilyn M. Wolfson, Richard L. Delanoy, Barbara E. Forman, Robert G. Hallowell,
Margita L. Pawlak, and Peter D. Smith, 1994). For these reasons, it is important to
understand the fine processes such as the dynamics and thermodynamic structure
of microbursts to characterize their evolution and environment. By further
understanding these characteristics we are now able to forecast and detect
microbursts in real time, as well as warn and educate the aviation community for
potential wind shear threats. The focus of this paper will be on the thermodynamics,
dynamics and environment in which microbursts form, as it relates to the
forecasting and detection of microbursts.
Thermodynamics and Dynamics
There are three primary processes that lead to the formation of microbursts,
which include evaporative cooling / sublimination, melting, and liquid water loading
(Rinehart, 2010). All of these processes lead to negative buoyancy of the air within
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the thunderstorm and develop a powerful concentrated downdraft that penetrates
the base of the storm and rushes towards the surface. The root cause of a
microburst comes down to buoyancy and how buoyancy is shape shifted within a
thunderstorm. It is evaporative cooling, melting and liquid water loading which
causes these changes. Below the buoyancy equation is given,
𝜕𝑤
𝑑𝑇𝑣
=𝑔
𝜕𝑡
𝑇𝑣𝑒
Where the change in virtual temperature (dTv) is the parcel’s virtual temperature
minus the environmental virtual temperature divided by the environmental virtual
temperature (Tve) and g is the gravitational constant. From this one should notice
that the causes of vertical acceleration are attributed to buoyancy differences within
the storm (Medlin, Cullen 2006). Since we now know that buoyancy is a major factor
in vertical accelerations within storms, we can now examine the processes that lead
to negative buoyancy that ultimately cause the formation of enhanced downdrafts.
Evaporative cooling is one of the processes that attributes to cooling of the
air and thus negative buoyancy. Evaporative cooling is a process whereby the
surrounding air is cooled by the change of state from liquid water to water vapor.
This process requires energy, because going from a liquid to a gas means that there
is an increase in molecular energy, and thus energy is required to go from a liquid
state to a gaseous state (Rinehart 2010). So as water droplets descend into a subsaturated layer of dry air, they begin to evaporate and cool the surrounding air by
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removal of sensible heat. Since cold air is more dense than the surrounding warm
air, it will begin to descend.
Melting works in a similar way as evaporative cooling except the change of
state is from ice to water. Within thunderstorms, saturated air is carried to high
altitudes via strong updrafts and is transported above the freezing level allowing ice
particles to form as hail, snow and graupel. As the ice particles begin to descend
they will begin to melt, attributing to cooling of the surrounding air within lower
parts of the storm below the 0℃ isotherm line (Pryor, Ellrod, 2004). Similar to
evaporative cooling and melting is sublimination. This is the transformation of ice to
a gas and generates more cooling because of the greater amount of energy required
to go from a lower energy state of a solid to the higher energy state of a gas
(Vasquez, 2009).
Liquid water loading also contributes to the formation of microbursts and is
a process that operates by descending precipitation within a thunderstorm. As the
precipitation falls, it drags the air along with it creating a downdraft (Rinehart,
2010). Most precipitation that falls within clouds produces some sort of downward
motion associated with it due to precipitation loading or drag (Rinehart 2010).
When one or all of these processes is present within a thunderstorm, a
microburst is formed, often moderately small in spatial scale due to the
concentration of these processes (Rinehart 2010). Microbursts, in general, fall
almost vertically. Consider the analogy to that of turning on your bathroom water
tap. Imagine the water leaving your tap, and once the water hits the bottom of the
sink it spreads out in all directions, microbursts work in a similar manner. When the
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air reaches the ground it cannot go down any further and thus radiates outwards in
all directions and at equal speeds (assuming static conditions of the storm and
microburst).
Now consider the dynamics of a travelling microburst on its descent,
radiating outwards once in contact with the ground, as well as the dynamics of
potential wind speeds. If we consider flow against a flat plain, as the air approaches
the surface, the vertical component of the wind velocity will begin to decelerate
because of interference with the ground and the horizontal component will begin to
accelerate outwards from the impact centre (Caracena, Holle, Doswell, 2005). This
model paints a picture for the microburst to ground interaction and can be seen in
figure 1. Once the horizontal component begins and radiates outwards, past studies
have observed pressure noses or “mesohighs” that have developed within the
microburst. Different from other downdrafts, microbursts pressure noses have a
tighter stronger gradient with the outside environment and are relatively short in
duration (Caracena, et. al 2005). Often a 2 to 5 mb increase is noted with the passing
of the microburst outflow and due to the relative high pressure ring that is
developed at the centre of impact, flow is directed outwards towards a ring of lower
pressure around the microburst outflow (Caracena, et al. 2005).
Another dynamical feature important to microbursts is horizontal vortex
tubes (or rotors) that develop on the leading edge of the microbursts outflow and
can be seen in figure 1. This horizontal vortex tube has a cyclonic curvature and low
pressure develops within the axis of rotation, which dynamically enhances the
pressure gradient across the microburst (Caracena, et. al 2005). The cyclonic flow
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develops a return flow on the backside of the ring which enhances the outflow,
while upward motion is found on the leading edge and may be visible by curling
rings of dust or precipitation spray in ideal conditions (Caracena, et. al 2005). This
phenomena may explain why microbursts intensify as they expand with distance
from the centre of impact due to stretching of the horizontal vortex ring which
would intensify the low pressure at its axis while, theoretically, the outflow should
weaken due to friction (Caracena, et. al 2005). It is these circulations in combination
with the downdraft that become problematic for aircraft. Rapid changes in wind
speed and direction will cause complications for aircrafts taking off or landing and
can be a potentially deadly situation (Keen, Thatcher, 2010). This highlights the
importance and research of conditions necessary for microburst generation.
Environment and Forecasting
There are three main types of microbursts which are wet, dry and the hybrid
of both wet and dry microbursts, each forming under their own specific conditions
(Caracena, et al, 2005). Each of these microbursts are formed in different
environments and thus different conditions need to be recognized in order to
evaluate which type of microburst a forecaster will be dealing with. Each type of
microburst will be discussed individually about their specific environments and
conditions that will alert forecasters to their potential.
a. Wet Microbursts
Wet microbursts are primarily caused by dry air entrainment into the mid
levels of the storm, as well as precipitation loading, and are often observed in moist,
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tropical environments (Rinehart, 2010). These types of microbursts are
characterized by precipitation fully extending down to ground level, accompanied
by high level winds and torrential rain (Rinehart 2010). Figure 2 shows a sample
sounding for a typical wet microburst situation. Wet microburst environments are
often associated with deep low level moisture (high theta-e), anywhere from 800 to
700mb deep with a region of dry air above (low theta-e), and high precipitable
water (Atkins, Wakimoto, 1991). The low level lapse rates are nearly pseudoadiabatic with steep mid level lapse rates above, and high values of CAPE are needed
to ensure sufficient penetration of moisture into the dry air aloft and to enhance
precipitation loading within the storm (Pryor, et al, 2004). The dry air aloft is
instrumental to the development of negative buoyancy to initiate a downburst due
to evaporation and (or) sublimination of precipitation particles, which will draw
sensible heat from the surrounding atmosphere (Vasquez, 2009). Hail/graupel have
often been observed and are assumed to be a major contributor to negative
buoyancy by sublimination aloft and melting in the lower parts of the storm
(Scharfenberg, 2003). So vertically deep updrafts are vital to wet microburst
formation and 50+ dBZ cores are prime areas for wet microburst formation where
hail and heavy precipitation is likely (Medlin, 2006).
By observing soundings we can pinpoint the potential for wet microburst
formation. Additionally, algorithms have been developed to further verify wet
microburst potential and give the forecaster additional information on maximum
wind gust potential. Such an algorithm is the GOES sounder derived product Wet
Microburst Severity Index (WMSI). This index incorporates theta-e deficit and CAPE
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to alert forecasters to the potential of wet microbursts and fill in gaps where
soundings are unavailable.
𝑊𝑀𝑆𝐼 =
(𝑀𝑈𝐶𝐴𝑃𝐸)(𝜃𝑒𝑚𝑎𝑥 − 𝜃𝑒𝑚𝑖𝑛 )
1000
Where, theta-e max is the maximum theta-e at the surface minus the minimum
theta-e in the mid levels and MUCAPE represents most unstable CAPE (Pryor, et al,
2004). Theta-e deficit is important for differences in moisture between the two
levels and helps characterize the thermal energy of latent heat (Pryor, et al, 2004).
Significant statistical evidence has shown that there is a correlation between GOES
WMSI values and surface convective wind gusts associated with wet microbursts
(Pryor, et al 2004). An example of the GOES WMSI product can be seen in figure 3
with a corresponding table of values showing the maximum expected wind gusts.
b. Dry Microbursts
Dry microbursts differ from wet microbursts such that at the surface they are
unaccompanied by precipitation. Virga is often observed beneath the thunderstorm
base, which is a visual cue that evaporation/sublimination of rain and ice particles is
taking place beneath the base of the storm (Rinehart, 2010). The only visual
evidence of the presence of a dry microburst at ground level is dust and debris that
is being lofted by the leading edge of the microburst. These types of microbursts are
most common in the high plains and higher altitude locations where drier
conditions exist (Pryor, et al, 2004).
Figure 4 shows a typical dry microburst sounding, also referred to as an
“inverted-V” sounding (Vasquez, 2009). Dry microbursts occur in dry environments
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where the convective condensation level is high (as high as 500mb), meaning high
cloud bases, low surface dewpoints extending at least 3km deep, low CAPE, dry
adiabatic lapse rate in the low levels, shallow moisture in the mid levels, and
convective instability (NOAA, 2011). The primary generation of a dry microburst is
evaporative cooling in the sub-cloud layer, which induces negative buoyancy (Pryor,
et al, 2004). Due to the dry air in the low levels of the atmospheric boundary layer,
precipitation particles will evaporate, cooling the surrounding air. This is the
reverse of the wet microburst environment to a greater degree where all of the
precipitation is evaporated, thus virga is visible when a dry microburst is occurring
(NOAA, 2011). An algorithm commonly used in the forecasting of dry microbursts is
the Dry Microburst Index (DMI).
𝐷𝑀𝐼 = Γ + (𝑇 − 𝑇𝑑 )700 − (𝑇 − 𝑇𝑑 )500
Where Γ is the 700-500mb lapse rate and the other terms are the dewpoint
depressions for the 700 and 500mb levels respectively. This index assists in locating
areas for dry microburst potential and is also a GOES sounder product similar to
that of the WMSI (Pryor, et al 2004).
c. Hybrid Microbursts
Dry air aloft and dry air in the low levels are favorable environments for wet
and dry microbursts respectively. However, a different environment exists where a
combination of both wet and dry microbursts have been observed generating a
downdraft known as a hybrid microburst (Pryor, 2011). These types of microbursts
develop in deep convective situations that will generate heavy precipitation but also
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have a dry sub-cloud layer beneath the base of the thunderstorm which significantly
helps in evaporation of precipitation (Pryor, 2011). This type of intermediate
environment requires significant CAPE values, a deep dry adiabatic lapse rate below
the cloud base typically near the 700mb level, and a dry (low theta-e) layer above a
moist mid level layer (Caracena, et al, 2005). A sample sounding for this type of
environment is shown in figure 5 where there is an inverted-V shape with an area of
high CAPE. Strong buoyancy that is associated with this type of environment will
likely be associated with large hail and graupel which can further complicate and
augment the negative buoyancy with the elevated dry layer being a possible source
of microburst initiation (Caracena, et al, 2005).
A current index that is under development is the Hybrid Microburst Index,
which is once again a GOES sounder derived product. HMI incorporates sub-cloud
lapse rates (G), dewpoint depressions between the typical convective cloud base
(670mb) and the sub-cloud layer (850mb) (Pryor, 2011). This typical cloud base
was chosen to be the intermediate level between the wet microburst LCL (850mb)
and the LCL for dry microbursts (500mb).
𝐻𝑀𝐼 = 𝐺 + (𝑇 − 𝑇𝑑 )850 − (𝑇 − 𝑇𝑑 )670
Large values of HMI result from nearly dry adiabatic lapse rates in the low levels
and large dewpoint depression differences between the typical convective cloud
base and the sub-cloud layer (Pryor, 2011). This index has shown to be useful and
has a strong correlation between wet and dry microbursts as well and can be used
as a supplement for WMSI and DMI (Pryor, 2011).
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Radar is a useful tool for observing microbursts and can alert aviation to
their presence. Microbursts are reasonably symmetrical at the surface and will
exhibit a couplet on velocity imagery that is aligned perpendicular to the beam
(Rinehart, 2010). A strong, divergent signature (figure 7) will be visible on velocity
products accompanied by strong reflectivities on base reflectivity (Rinehart, 2010).
In the case of dry microbursts where no precipitation is visible, often debris that is
lofted, insects, or birds can make dry microbursts visible on radar (Rinehart, 2010).
Conclusion
To summarize, microbursts are dangerous meteorological phenomena that
can cause significant damage and danger to property, aviation and life. Past research
and current ongoing research is important to obtain a further understanding of
microbursts to ensure public and aviation safety through applying knowledge
obtained from studies to forecasting. Through working on this paper, applying my
own views, I find that possibly looking at short fat areas of CAPE and contrasting
them to longer broad areas of CAPE may differentiate severe microburst events to
general events. Reason being is short fat areas of CAPE have been linked to stronger
accelerating updrafts due to sooner realization of positive buoyant energy from an
ascending parcel. Also through researching I did not come across any papers that
looked at these situations for Northern Plains environments, which may differ from
more Southern Plains and Gulf coast situations. This reinforces that more research
is needed on such a phenomena as well as, education and public awareness of such
events is important for personal safety.
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Figure 1.
Figure 1: Structure of a microburst and the horizontal vortex rings. Source
http://www.propertyinsurancecoveragelaw.com/tags/microbursts/
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Figure 2.
Figure 2: Wet microburst sounding taken from Met Ed. Source
(http://www.meted.ucar.edu/mesoprim/skewt/micro2.htm )
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Figure 3.
Figure 3: WMSI and corresponding table example from the SE U.S.A. (Pryor, et. al 2004)
WMSI
< 10
10 - 49
50 - 79
> 80
Wind Gusts (kt)
Convection/Microbursts
Unlikely
< 35 kts
35 - 49 kts
> 50 kts
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Figure 4.
Figure 4: Dry microburst sounding. Source (http://www.meted.ucar.edu/mesoprim/skewt/micro3.htm)
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Figure 5.
Figure 5: Hybrid microburst sounding. Source (NOAA, 2011)
http://www.srh.noaa.gov/ama/?n=microbursts
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Figure 6.
Figure 6: HMI and corresponding table. This example is for a Great Plains event from 2005. (Pryor, 2011)
Table 1. Downburst risk corresponding to HMI
values
HMI
Box Color
Downburst
risk
<8
Red
Downbursts
Unlikely
> or =8
Green
Downbursts
Likely
> or =16
Yellow
Downbursts
Likely
Orange
High Risk
of
Downbursts
> 24
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Figure 7.
Figure 7: Divergence on base velocity imagery. This image was taken form the NWS archives in Kansas
City/Pleasant Hill MO from an event that took place on Wednesday August 11, 2010. (NWS, 2010)
http://www.crh.noaa.gov/eax/?n=microburst_aug112010
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References
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