1.
Introduction
Damaging winds affected portions
of
South Florida during the early morning of
April 13, 2004. These winds occurred
behind a large area of stratiform precipitation associated with a mesoscale
convective system (MCS) that moved across the southeastern
Gulf of
Mexico during the evening of
April 12, 2004 (
Fig. 1). Wind speeds sustained between
30 and 50 mph with gusts reaching 76 mph were recorded on
Lake
Okeechobee.
These
winds produced a seiche effect on
Lake Okeechobee,
resulting in a 5.5 ft maximum water level differential between the north and
south sides of this shallow lake (
Fig. 8). In addition, damages from the high
winds were reported from communities situated along the lake shore. This paper
shows the event to be related to a wake low, a rare occurrence across
South
Florida.
2. Synoptic
& Mesoscale Environment
At 0000 UTC on April 13, 2004, a 1002-hPa surface low was
located over eastern Alabama. An
associated cold front stretched across the Gulf of Mexico
to the Yucatan Peninsula
and a stationary front was located across central Georgia
and South Carolina (Fig. 1).
Advection of
warm and moist air from the Caribbean was enhanced as an
850-hPa jet strengthened to 30-50 knots (15 to 26-m s-1) by early
evening. An outflow boundary from a previous MCS which tracked across South
Florida early on the morning of April 12th persisted
across the Florida Straits. This boundary exhibited pseudo-warm frontal
characteristics, and was lifting northward over South Florida
(white dashed line in Fig. 1). To the north of the boundary, a cool and stable boundary
layer was present over land.
A
high amplitude longwave mid/upper tropospheric trough was located over the
eastern United States.
The axis of this trough extended from the Great Lakes
southward to the central Gulf of Mexico while South
Florida was upstream of the ridge axis located near 65°W longitude. In addition, a 130-knot (67 m
s-1) polar jet and 60-knot (31 m s-1) subtropical jet
were splitting over the eastern Gulf of Mexico producing
significant upper level diffluence over South Florida.
This resulted in a Mesoscale Convective System (MCS) developing and moving
across the southeast Gulf of Mexico during the evening
of April 12, 2004 (Fig. 1).
3. Wake Low
Dynamics
An early model of the structure
and life cycle of mesoscale systems producing intense wake lows was proposed by
Fujita (1955). This model has been
examined and expanded upon by several researchers since. In general,
researchers have found wake low events to be associated with significant low
level warming and surface pressure falls of up to 5 hPa within an hour at
times. Quantitative evidence that subsidence warming can account for the
reduced pressures in wake lows were demonstrated in a numerical modeling study by
Johnson (2001). Johnson and Hamilton (1988) and Gallus (1996) proposed that a
wake low is formed by a descending rear inflow jet and that the warming due to
descent was maximized at the back edge of the precipitation area where
evaporative cooling was insufficient to offset adiabatic warming (Fig. 2).
This suggestion was furthered by Stumpf (1991)
who contended that stratiform precipitation regions can be dynamically
significant phenomena, generating rapidly descending inflow jets at their back
edges, capable of producing lower-tropospheric warming, intense low level
pressure gradients, and strong low level winds.
4. Wake Low
Observations
4.1
Vertical Wind/Temperature Profiles
Upper atmospheric wind and
temperature profiles were similar to other well documented cases where the environment
supported mesoscale convective systems (Maddox 1983), as well as case studies
from events in the Mississippi Valley
(Gaffin, 1999) and Oklahoma
(Hunter, 1989). The wind above 850-hPa was unidirectional from the southwest
and speeds increased with height. Near the surface, light easterly winds were
found to the north of the quasi-warm frontal boundary with southeast to south
winds in the warm sector. Rawinsonde observations from Key
West and Miami
revealed precipitable water values of 1.84 and 1.76 inches respectively. Observed
values of precipitable water prior to MCS development are typically in excess
of 1.4 inches (Maddox 1980).
Soundings
derived from Local Analysis and Prediction System (LAPS) objective analyses
across Lake Okeechobee indicated warming by as much as 2
to 3°C in the layer between 700 and 800-hPa from 0700 to 0800 UTC (Fig. 3). A
similar degree of cooling was depicted between 450 and 650-hPa in the same time
period. Additionally, between 0800 and 0900 UTC, the LAPS soundings show
descent of the anomalously warm air to the 850-hPa to surface layer, especially
near the lake where the atmosphere lacked an elsewhere substantial temperature
inversion in the boundary layer.
Note that over
mainland South Florida, LAPS analyses are typically
quite reliable due to the increased number of mesonet, aircraft, and NWS
observations that are readily available, particularly within the last few years
(Etherton et al., 2004; Etherton and Santos,
2004). Although aircraft data are far less frequent late at night, between 0600 and 0900 UTC some flights (approaching or departing
any of the three international airports in close proximity to each other on the
southeast Florida coast) report upper air data and they are processed by the
LAPS program. Most of the flights departing to or coming in from the west fly
between the Fort Myers to Lake Okeechobee area at an altitude of around 15-25
kft. So even though LAPS had limited aircraft data some were still available,
enough to detect the warming at the transition between the low to mid levels. LAPS
ingests high resolution satellite data which may have helped the analysis detect
the low to mid level warming/cooling signatures also. In short, LAPS sounding
analysis supports the observation of subsidence induced low level adiabatic
warming as will be further shown in sections 4.2 through 4.4.
4.2 Satellite
Imagery
Subtle hints of
subsiding air could be found both in the GOES-12 infrared and water vapor
satellite imagery. Infrared satellite showed cloud tops warming 10°C in the 30
minutes between 0845 UTC and 0915 UTC (top panels Fig. 4) when the strongest
surge of low level winds (sections 4.3 and 4.4) was observed, and the corresponding
water vapor imagery suggests mid-tropospheric drying was taking place from Lake
Okeechobee southwest to the Gulf of Mexico (bottom panels Fig. 4).
4.3 High-Resolution Radar Data
From a radar
perspective, a series of distinctive events can be identified, two of which are
presented in
Fig. 5. These events were accompanied by significant pressure
falls, strong winds, low level warming, and/or a combination of these, all
signatures accompanying wake low events (see
Fig. 3 and Fig.
6a,
b).
Between 0400
and 1000 UTC, a large area of stratiform rain covered much of
South Florida
with its trailing edge remaining from
Marco
Island to
Lake
Okeechobee. The initial wind surge event happened shortly before
0500 UTC (
Fig. 6a).
Figure 5 (top panels) shows the 0.5 degree base
reflectivity and velocity images. Notice a west southwest rear inflow jet with
maximum radial velocities in excess of 40 knots (20 m s
-1) (white circles)
while a rear inflow notch signature is present just southwest of Lake
Okeechobee on the trailing edge of the stratiform precipitation shield.
About 4 hours
later, the most significant peak wind associated with the event occurred (
Fig.
6a). A band of high velocities was observed along the trailing edge of the
stratiform precipitation shield around 0918 UTC. Radial velocities from the
Miami
radar exceeded 50 knots (26 m s
-1) and are shaded yellow and light
blue in
Fig. 5 (white circles in bottom panels). Maximum radial velocities of
up to 106 knots (55 m s
-1) were also detected. The radar beam height
of the 0.5° tilt was around 6,000 feet at the distance of the sharp
reflectivity gradient. Looking at the orientation of the wind maximum and the
zero isodop, these features represent a rear inflow jet from the southwest which
is supported by
Fig. 3, the LAPS sounding analysis.
An
interesting feature with this final event is the magnitude of the observed
velocities by Doppler radar. It appears the radar is seeing contribution to the
along radial component from a descending and slanting rear inflow jet that is
turning out of the southeast at lower levels as it descends due to the lower pressures to the
northwest and higher pressures to the southeast (next section). This is
evidenced by the superimposed surface observations in Fig. 7 where a 6.1 hPa
pressure gradient is noticed between Naples
and Miami.
4.4 Surface
Observations
Three distinct pressure falls and
wind speed maxima occurred as the back edge of the precipitation shield passed
over
Lake Okeechobee. The first observed wind gust of 22
m s
-1 was preceded by a pressure fall of around 5 hPa in one hour
(
Fig. 6a). The second and third events were close together and consisted of a
wind gust of 25 m s
-1 around 0745 UTC and a more intense gust reaching
34 m s
-1 near 0915 UTC (
Fig. 6a). Pressure falls of up to 5 hPa were
observed in the hour preceding the observed peak gusts.
The well mixed
marine layer near Lake Okeechobee and the southwest Florida
coast (similar temperature and wind behaviors to those in Fig. 6a were observed
in Naples on the southwest Florida
coast) allowed the high wind speeds to be registered in those areas. The effect
on inland locations was primarily a rise in temperature, since the presence of
a strong surface-based inversion prevented the strongest winds from reaching
the surface. The temperature at Palmdale (Fig. 7) climbed from 69°F (21°C) at
0600 UTC to 76°F (24°C) at 0900 UTC, with nearly 50% of that rise occurring
between 0800 and 0900 UTC (Fig. 6b). Although not of the same magnitude, a
similar temperature trend was observed in Moore Haven. Notice that this warming
was not only at the surface but throughout the low levels as illustrated
earlier in section 4.1, Fig. 3. Additionally, this degree of warming was not
observed prior to the first peak wind around 0500 UTC across Lake
Okeechobee although Fig. 6b suggests a warming trend had begun to
take place.
This suggests adiabatic warming with the descending
rear inflow jet had been taking place throughout the early morning hours on the
trailing edge of the light precipitation shield that over the course of several
hours led to the formation of the wake low west of the Lake Okeechobee region
and the formation of an intense low level pressure gradient (see Figs. 7 and 10). All these factors together combined to result in stronger wind surges near 0800 UTC and 0900 UTC.
4.5 Wind
Damage Reports
The strong winds inflicted damage to communities
near Lake Okeechobee as well as along the Gulf of Mexico. The winds severely damaged several mobile homes
along the south shore of the lake, and blew out a store window in Belle Glade (Fig. 7). A 50-knot wind gust at the Fort Myers Southwest Florida International Airport pushed a Boeing 737 aircraft into a jetway bridge.
Roofs, carports, and other structures were damaged in Fort Myers, Moore Haven, Belle Glade, and Pahokee (Fig. 7).
4.6 Lake
Okeechobee Water Level
Water level rises and falls were recorded by the
U.S. Army Corps of Engineers gages on Lake
Okeechobee in response to the high winds, and corresponded
closely with the time of greatest wind speeds. Figure 8 shows that the lake
level was fairly uniform near 14.5 feet until rapidly rising between 0400 and 0500
UTC (23:00 to 00:00 EST) when the average north end gages initially peaked at 15.9
feet. A second peak of 17.6 feet occurred around 0900 UTC (04:00 EST). A lake level minimum was measured at the south end
gages around 1100 UTC (06:00 EST)
when the average level fell to 12.1 feet. These lake responses correlate well
with the observed maximum wind surges discussed in the previous sections. The
multiple peaks noticed beyond these times in the north end gage are related to
the seiche oscillating effect of the water level which lasted for several hours
after the wind surges as it gradually damped out. The south end gage stopped
reporting shortly after 1200 UTC.
4.7 Numerical Modeling Data
The 0000 UTC
run of the workstation ETA, run locally at the National Weather Service Miami
office, depicted an area of deep layer subsidence along the trailing edge of a cluster
of rain and thunderstorms. The forecast of 700 hPa vertical motion valid
at 0800 UTC showed maximum downward motion of 7
to 9 µb s-1 (Fig. 9).
Additionally, the development of the wake low led to
the creation of an intense low level pressure gradient analyzed by LAPS. The analysis
showed the coupling of lower pressure west of the lake and a
meso high to the south where debris from the MCS that moved across the
southeast Gulf of Mexico the evening of the 12
th was producing light
to moderate rainfall and more dominant evaporational cooling at the low levels.
Figure 10 shows the 925 hPa height analysis from the 1100 UTC LAPS analysis
depicting this well. We believe this contributed to windy conditions remaining
for a few hours after the occurrence of the high wind events around 0500, 0800,
and 0900 UTC (
Fig. 6a).
5. Operational Considerations
Challenges
remain in our quest to better observe and predict the strength of wake low wind
events in an operational environment. Most critically, many of the observational
datasets were not available to the forecaster rapidly enough to provide insight
to the degree found in this examination. LAPS analysis latencies were typically
around 1 hour, while many of the mesonet surface observations were made
available every 30 minutes. With the advent of the Advanced Weather Interactive
Processing System (AWIPS) Operational Build 4.2, LAPS latency is now less than
30 minutes. The authors believe this will help monitor potential events as the
one described here in a more timely manner. The mesonet sites on Lake
Okeechobee record peak wind gust data, but only sustained winds
were available to forecasters at the time. This has also changed since the
event occurred and real time gust data is now available to the forecasters from
those sites. Official NWS sites interrogated to acquire instantaneous data
during the event didn't capture the magnitude of the situation due to the fact
that a significant inversion prevented the strongest winds from reaching the
surface at most of these land based locations. Information available to
meteorologists in assessing potential for wake low damaging wind events has
been greatly enhanced over the last few years, and will no doubt continue to
improve dramatically. Monitoring and timely assimilation of these observational
and forecast datasets can help us recognize and provide accurate and timely
warnings for damaging wake low events.
Cross sectional
analysis of the velocity data at the time could have also helped reveal the
unfolding event and descending jet. This could help gain some lead time on the
event assuming it is properly attributed to the conceptual model of a wake low.
This is perhaps the most important lesson learned from this event together with
a reminder of the importance of monitoring closely all available data.
6. Discussion and Conclusions
The passage of
the trailing edges of the stratiform precipitation shield and associated band
of high wind velocities, as depicted by the lowest elevation angle radar scan
(
Fig. 5), coincided with the times of peak wind gust, short-duration pressure
minima, and low level warming (
Figs. 3 and
6a,
b). These were observed in official
NWS and mesonet surface data from
Lake Okeechobee and
surrounding areas.
Both
direct and indirect evidence of subsidence was found in the observational and
numerical model datasets. Infrared satellite data showed a narrow band of
significant cloud top warming while the water vapor imagery indicated
mid-tropospheric drying around the time of the second and third wind surges
around 0800 and 0900 UTC. LAPS soundings revealed a descending warm layer
during the two hours preceding the 76 mph (34 m s
-1) wind gust
observed on
Lake Okeechobee. The 0000 UTC NWS Miami
workstation ETA model predicted a band of strong downward vertical motion which
closely matched the observed event.
The
LAPS analyses (
Figs. 3 and
10) depicted mid-tropospheric cooling, lower
tropospheric warming, and strong low level pressure gradients occurring while
radar detected no precipitation size hydrometeors around 6,000 feet on the
trailing edge of the precipitation shield. This lends credibility to the idea
that evaporative cooling was the dominant dynamic mechanism in the mid-levels,
while adiabatic warming was occurring in the low-levels. Surface observations showed
temperature increases of 3 to 4°C around the time of the highest observed peak
wind and even up to 10°C during the 5 to 6 hours period preceding the highest
observed peak wind. This suggests that the evaporative cooling was indeed
insufficient to offset adiabatic warming, as was found in Johnson and Hamilton
(1988).
These data
support the contention that this high wind event was the result of a wake low,
a rare event across
South Florida. While the presence of
wake lows may be apparent in real-time, the magnitude of such events can be
difficult to assess until later.
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