SR/SSD 98-31
8-1-98
Technical Attachment
PRELIMINARY DOWNBURST CLIMATOLOGY
AND WARNING GUIDELINES FROM A SINGLE CELL THUNDERSTORM
DATABASE
Ken Falk, SOO
Lee Harrison, MIC
John Elmore, Forecaster
NWSO Shreveport, LA
1. Introduction
A downburst
is defined as a rapid downdraft of wind from a single
cell thunderstorm which produces a sudden outflow of horizontal
winds at the surface (Fujita 1981). Beginning with a downburst-
producing single cell thunderstorm over Waskom, Texas
in July, 1996 (Falk and Harrison 1996), and continuing
through the summer of 1997, we compiled a database of
over 30 single cell thunderstorms which exhibited strong,
high-level reflectivity cores, and mid-level convergence
signatures. Many of these thunderstorms produced severe
downbursts.
Radial velocity
convergence was first proposed as a procedure for forecasting
microbursts by Roberts and Wilson (1989). Other research
(Eilts, et al. 1996) indicated values of radial velocity
con-vergence of greater than 44 kt at mid-altitudes in
a storm may produce damaging winds at the surface. We
applied this research to the National Weather Service
Doppler radar (WSR-88D) at Shreveport to create a database
of storms which, we hoped, would enhance our ability to
issue more accurate severe thunderstorm warnings, and
better airport advisories for downbursts. The evaluation
of these data allowed us to determine preliminary severe
thunderstorm warning guidelines for downbursts (and associated
microbursts and macrobursts) using radial velocity convergence,
and to determine a climatology of downburst-producing
single cell thunderstorms.
In this paper,
you will find a conceptual model of a downburst-producing
single cell thunderstorm, a description of how the data
were gathered for this study, the results of the data,
including our initial recommendations for warnings based
on the data to this point, and a preliminary climatology
of downbursts. We plan to continue gathering data through
the summer of 1998 in order to refine these preliminary
results.
2. Conceptual model of a downburst-producing
single cell thunderstorm
Conceptual
models for downburst-producing single cell thunderstorms
were developed by Roberts and Wilson (1989) for low reflectivity,
moderate reflectivity, and high reflectivity downburst-producing
thunderstorms. We have composed a more general conceptual
model for downburst-producing single cell thunderstorms
based on WSR-88D cases we have examined. The type of downburst
depicted in this conceptual model and observed on the
Shreveport WSR-88D is usually referred to as a wet-microburst.
It is important to note that this conceptual model Fig. 1 applies
to single cell (or pulse type) thunderstorms only.
This conceptual model does not apply
to other types of thunderstorms, such as multicell squall
lines, bow echoes, or supercells.
It should be
noted that these downburst-producing single cell storms
can occur in the same environment, and at the same time,
as an organized squall line event. The event at Waskom,
Texas referred to earlier (Falk and Harrison 1996) occurred
in a single cell thunderstorm located ahead (or downwind)
of an advancing squall line.
Arrows in Fig. 1 depict storm scale wind flow within
our conceptual model of a downburst-producing single cell
thunderstorm. Storm top divergence will usually be detected
in the development stage of the single cell thunderstorm.
Since storm top divergence is associated with updraft
strength, it is not a good predictor of downburst potential,
although it may still be present as the downburst is occurring.
In other words, storm top divergence may, or may not,
be present in a downburst-producing single cell thunderstorm.
If storm top divergence is still strong while a downburst
is occurring, the storm will probably maintain its strength
longer than if storm top divergence is not present at
the time of the downburst. If storm top divergence is
not present as the downburst occurs, then the storm will
likely collapse (within 30 minutes) during the time of
the downburst.
Roberts and
Wilson (1989) showed that radial velocity convergence
(or, more simply, velocity convergence) may be a predictor
of potential microbursts from single cell storms. Doppler
radars, of course, measure velocities only along radial
lines. Velocity convergence is a term used to describe
situations in which, extending outward from the radar
along a radial, outbound velocities are adjacent to inbound
velocities. For convenience, the magnitude of the velocity
convergence is taken to be the sum of the absolute values
of the maximum outbound and inbound velocities.
We have chosen
to identify a term we call storm velocity convergence
(SVC) in our conceptual model. SVC is simply the velocity
convergence in the core of the storm just above the cloud
base, and it is usually best shown by the WSR-88D storm
relative motion (SRM) product. We believe SVC reflects
drier air entraining into the storm as evaporative cooling
and/or precipitation drag induce downward motion within
the storm, leading to a downburst at the surface. Roberts
and Wilson (1989) suggest the same causes for downbursts.
The cases we examined indicate that SVC is a good predictor
of a potential downburst when it is present in or near
the high reflectivity core of a single cell storm, somewhere
within the 5,000 to 11,000 ft AGL layer.
A low-level
divergence of winds will occur as the downburst contacts
the ground (Fujita 1981). This feature will be detected
by the WSR-88D only if the storm is within 20-30 miles
of the Doppler radar (due to beam height restrictions),
but is assumed to be present in storms at ranges beyond
30 miles, when SVC is detected. SVC extends the range
of detection of downburst-producing single cell thunderstorms
to about 90 miles from the radar. A radar depiction (not
shown) of SVC and low-level divergence in a microburst-producing
single cell thunderstorm was documented by Falk and Harrison
(1996).
Another radar
depiction of a downburst-producing single cell thunderstorm
is shown in Color Plate
1. This thunderstorm was about 66 miles south-southwest
of the Shreveport WSR-88D, yet the WSR-88D was still able
to detect SVC in the high reflectivity core of this single
cell thunderstorm (due to the height AGL where SVC is
usually detected). This thunderstorm produced a downburst
of 70-80 mph which uprooted large trees.
3. Storm velocity
convergence database
Using the WSR-88D at Shreveport
(and one case from the Little Rock, Arkansas WSR-88D),
we recorded several parameters on storms that exhibited
storm velocity convergence (SVC). The criterion for a
storm to be included in the database was SVC of 32 kt
or greater in or near the storm core of a single cell
thunderstorm. Note that not all of the storms produced
severe surface winds (50 kt or more) or damage, since
these criteria were not required for a storm to be in
the database. We included storms that did not produce
strong winds in order to help establish the SVC threshold
for severe thunderstorm warnings. Using the SVC threshold
of 32 kt or more in a single cell thunderstorm, we were
able to gather information on 32 events during the summers
of 1996 and 1997 (all but one in Shreveport's county warning
area.).
To measure SVC, we used
the Doppler radar VR Shear function oriented along
a radial from the maximum inbound to the maximum
outbound wind in the convergent signature. SVC does not
require that the maximum wind pixels within the convergence
be gate-to-gate. They can be several pixels apart on the
WSR-88D velocity product. We mutiplied the VR Shear by
two to get an estimate of convergence (since VR Shear
is actually used to calculate rotational velocity, which
is an averaged number).
Convergence can also be
measured directly off the WSR-88D SRM display (or base
velocity), but one must remember that the displayed velocity
is the lowest value in the range of velocities of that
particular color. When using this method, it is best to
use the mid-range value of velocity for each indicated
color for an estimate of true convergence (VR Shear x
2 already takes this into account).
For each storm in the database,
we recorded the date of the storm, the location of the
storm, the range from the radar of the storm, the magnitude
of SVC, and the height of the maximum SVC. For each storm
that produced either a report of severe winds (50 kt or
greater) or wind damage, we also recorded the surface
wind speed and/or damage, and the lead time. Lead
time was defined as the time difference between
the initial detection of SVC of a least 50 kt, and the
first report of surface winds of at least 50 kt, or wind
damage, or radar detected divergent winds at the surface.
(It will be seen below that we found a good correlation
between SVC of this magnitude and surface winds of 50
kt or more.) We used real time reports of wind gusts or
damage whenever possible to get the best estimate of actual
time of occurrence.
4. Preliminary results
- Downburst warning guidelines and climatology
Severe
thunderstorm warning guidelines for downbursts
Based on the
evaluation of 32 events of single cell thunderstorms that
contained SVC in or near the high reflectivity core of
the storm, we made a preliminary determination of the
following warning guidelines for issuing a severe
thunderstorm warning for a downburst:
(1)
High reflectivity core of 50 dBZ to heights of 25,000
ft AGL, along with
(2)
Storm velocity convergence (SVC) of 50 kt somewhere in
the layer 5000 - 11,000 ft AGL in or near the high reflectivity
core.
Severe thunderstorm
warnings based on these guidelines should contain wording
about strong damaging winds.
These guidelines
produced an average lead time of 11.4 min between the
time SVC of at least 50 kt was detected and the severe
wind occurrence at the surface. Lead times ranged from
a low of zero min to a maximum of 18 min.
We found these
guidelines to be most reliable within 90 miles of the
WSR-88D. This is due to beam height restrictions and less
reliable velocity data at longer ranges. The detection
of SVC at ranges up to 90 miles from the radar is a significant
improvement over the previous low-level divergence detection
technique for downbursts, which is effective only to a
maximum range of 20-30 miles.
We observed
that several storms collapsed soon after SVC was detected.
Thus, the high reflectivity core of the storm may descend
rapidly after SVC occurs, so the guideline on the height
of the high reflectivity core is somewhat flexible. It
was also noted that single cell storms that contained
storm top divergence at the same time SVC was occurring
were longer lived than the storms that did not contain
strong storm top divergence. Thus, storms with strong
storm top divergence and SVC produced larger areas of
damaging winds (macrobursts).
We also looked
for single cell thunderstorms that produced severe downburst
winds, but did not contain SVC. We found
none! However, this is not to say that all downburst-producing
single cell thunderstorms contain SVC. Further evaluation
of this will be made after more data are acquired.
It was noted
as the data were being gathered that storms producing
severe downburst winds had reflectivity cores of 50 dBZ
to at least 25,000 ft AGL. Although this feature was not
checked in the earlier part of the database, during the
latter half of the study all of the severe downburst events
had these high reflectivity cores. The WSR-88D mid-level
LRM product can be used to quickly determine which storms
have high reflectivity cores, and thus should be further
examined for the SVC signature. It should be noted that
the magnitude and height of the high reflectivity core
(50 dBZ to at least 25,000 ft AGL) may vary somewhat over
other parts of the country, and should be adjusted for
local climatology of downburst storms.
As noted in
the above guidelines, the best layer to detect SVC appears
to be 5000 - 11,000 ft AGL. SVC detected above 11,000
ft AGL did not correlate well with damaging winds at the
surface, and SVC was usually not detected at all below
5000 ft AGL. However, there were three events that occurred
close enough to the WSR-88D (within 32 miles) to see the
low-level divergent signature. This feature is thought
to be detectable only below 2500 ft AGL.
Out of the
32 events recorded, SVC ranged from a low of 32 kt (our
lower end to be included in database) to a high of 110
kt. Figure 2 shows magnitude of SVC versus range
of the storm from the WSR-88D for the 32 events. The figure
also shows which SVC events produced severe weather. In
the database, 50% of the events had SVC of at least 50
kt. Of these events, 81% produced severe weather (75%
produced severe downburst winds and 6% produced severe
hail). The severe hail report was somewhat of an anomaly
since only one event out of 32 produced severe hail. Of
the events that contained SVC under 50 kt (the other 50%),
13% produced severe downburst winds. These two events
were at ranges beyond 90 miles from the WSR-88D.
It can be seen
in Fig. 2 that SVC of 50 kt or greater appears
to be a reliable guideline to issue a severe thunderstorm
warning for downbursts, at least in the area of our study.
The SVC must occur in or near the high reflectivity core
of a single cell thunderstorm. As previously mentioned,
the convergence "couplet" does not have to be gate-to-gate,
but can be separated by a nominal distance, depending
on the diameter of the storm.
Radar interpretation
of SVC should not be confused with convergence that occurs
along outflow boundaries, along squall lines, near the
leading edge of bow echo storms, or in supercells. These
types of thunderstorms usually occur in stronger vertically
sheared environments than single cell storms, and are
more "organized" than single cell storms. The radar meteorologist
must have a firm grasp of the physical processes that
are taking place within thunderstorms to properly interpret
the radar display. Thus it is important to be knowledgable
of the conceptual models (or storm structure) of various
types of storms (Falk 1997, Ray 1986, Doswell 1985).
We hope in
the future to correlate the magnitude of SVC with the
magnitude of surface winds, but at this point, we will
limit our discussion to saying 50 kt or more of SVC in
the 5000 - 11,000 ft AGL layer correlated well with surface
winds of 50 kt or greater.
Local airport
advisory guidelines for downbursts
Local
airport advisories are issued for many airports
by National Weather Service offices. Downburst-producing
single cell storms are one of the most dangerous weather
phenomena for aircraft. Local airport advisory criteria
suggest that the thunderstorm be within 5 mi of the airport
sometime during the valid time of the advisory. Wind speed
criteria (35 kt or more) for local airport advisories
are lower than that for severe thunderstorms warnings,
thus the guidelines for issuing a local airport advisory
for a downburst should have lower thresholds.
Preliminary
indications from the database are that a local
airport advisory should be issued for a downburst-producing
single cell thunderstorm based on the following guidelines:
(1)
High reflectivity core of 45 dBZ to heights of 25,000
ft AGL, along with
(2)
Storm velocity convergence (SVC) of 35 kt somewhere in
the layer 5000 - 11,000 ft AGL in or near the high reflectivity
core.
The local airport
advisory should specifically mention either the word "downburst"
or the word "microburst" because these words convey a
message to the air traffic controllers and pilots that
this thunderstorm poses a significant threat to aircraft
operations.
The magnitude
and height of the high reflectivity core will vary somewhat
over other parts of the country due to local climatology
of downburst-producing storms. It appears that SVC of
35 kt is sufficient to produce 35 kt surface winds. Certainly
the guidelines for severe thunderstorm warnings are sufficient
for a local airport advisory to be issued.
Climatology
of downbursts
From the database
we were able to make some preliminary observations of
the climatology of downburst-producing single cell thunderstorms.
Downbursts occurred in the summer season from early June
through the end of September, and were much more common
than we previously suspected.
Although we
recorded only one severe downburst in 1996, we recorded
13 in 1997. We also recorded 18 SVC events in 1997 that
likely produced downburst winds, but were not severe.
This brings the total of downbursts in 1997 (both severe
and non-severe) to 31. This wide difference in number
from one year to the next is likely due to the fact that
we were not as aware of the radar signatures of downbursts
in 1996 as we were in 1997. Of course, it could also be
argued that 1997 was a "good" year for downbursts. We
were also more diligent in 1997 in getting ground truth
wind reports from storms we suspected of producing downburst
winds. At any rate, it is apparent that downbursts can
be common in the summer months, and may be the most prevalent
severe weather phenomena during the summer (at least in
the south central and southeast U.S.).
Almost all
of the downbursts occurred during the mid-afternoon to
early evening hours during the hottest time of the day.
However, the single cell thunderstorm that exhibited the
highest SVC (110 kt) occurred around 1330 UTC (830 AM
CDT)! This thunderstorm produced a macroburst which uprooted
trees and downed power lines near Hope, Arkansas. Any
single cell storm that has a high reflectivity core should
be monitored for SVC at any time of the day during the
summer.
We noted that
on a given day several storms could produce downbursts,
so they can and do occur in "clusters" on favorable days.
But there were also some days when only one storm (out
of many storms) produced a severe downburst, such as the
storm mentioned in the preceeding paragraph. The severe
downburst at Waskom, Texas (Falk and Harrison 1996) was
another such isolated event.
Additional
events will help us further determine a climatology of
downbursts.
5. Conclusion
A database of downburst-producing
single cell thunderstorms was collected during the summers
of 1996 and 1997. By analyzing these data, a conceptual
model of a downburst-producing single cell storm was developed,
based on previous single cell microburst research and
on the radar signatures we have seen.
Information was gathered
on 32 storms that exhibited a signature we called storm
velocity convergence (SVC), and from these data, guidelines
were developed for issuing severe thunderstorm warnings
and local airport advisories for downbursts. We also were
able to develop a preliminary climatology of downburst-producing
single cell thunderstorms.
As we continue to gather
additional events through the summer of 1998, we will
review and refine our guidelines for warnings and climatology
of downbursts.
6. References
Doswell III, C.
A., 1985: The operational meteorology of convective weather,
Volume II: Storm Scale Analysis. NOAA Technical Memorandum
ERL ESG-15, NOAA National Severe Storms Lab, Norman, OK.
Eilts, M. D., J.
T. Johnson, E. D. Mitchell, R. J. Lynn, P. Spencer, S.
Cobb, and T. M. Smith, 1996: Damaging downburst prediction
and detection algorithm for the WSR-88D. 18th
Conference on Severe Local Storms, American Meteorological
Society, 541-545.
Falk, K. W., and
M. L. Harrison, 1996: Waskom, Texas, single cell microburst
detected on National Weather Service Doppler radar - 23
July 1996. Technical Attachment SR/SSD 96-43, National
Weather Service, Southern Region, Fort Worth, TX.
Falk, K. W. 1997:
Techniques for issuing severe thunderstorm and tornado
warnings with the WSR-88D Doppler radar. NOAA Technical
Memorandum NWS SR-185, NOAA National Weather Service,
Southern Region, Fort Worth, TX.
Fujita, T. T., 1981:
Tornadoes and downbursts in the context of generalized
planetary scales. Journal of the Atmospheric Sciences,
38, 1511-1534.
Ray, P. S., 1986:
Mesoscale Meteorology and Forecasting, American
Meteorological Society, 793 pp.
Roberts, R. D.,
and J. W. Wilson, 1989: A proposed microburst nowcasting
procedure using Single-Doppler radar. Journal of Applied
Meteorology, 28, 285-303. |