INTERPRETATION
DISCUSSION
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Regional and Mesoscale Meteorology Team Cooperative Institute for Research in the Atmosphere (CIRA) Colorado State University Fort Collins, Colorado |
September 22, 2004
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Landfalling tropical cyclones pose a number of serious threats, including high wind, storm surge, flooding rainfall, and tornadoes. During the unusually active tropical season in 2004, Hurricane Ivan produced over 100 tornadoes in the US resulting in 8 deaths. Evacuations in the vicinity of the landfall location no doubt saved many lives, but tornadoes often occur hundreds of kilometers from the landfall area, and forecasting their exact location hours ahead of time is impossible. As a result, forecasters must issue tornado warnings in the traditional "nowcasting" timeframe, i.e., minutes ahead of time.
Tornadoes associated with landfalling tropical cyclones typically occur in very high shear, low instability situations, and therefore their radar characteristics differ greatly from tornado-producing classic supercells. Relatively fewer large-ice particles result in lower reflectivity values, and the horizontal extent of the tornado-producing cells is often small. A small hook-shaped appendage can sometimes be found with these cells, and their rotation can occasionally be detected by the radar's radial velocity output. Nonetheless, the subtle radar signatures with these tornadic cells often result in a large false-alarm-ratio, so having additional data sources should aid in the warning decision-making process.
On Wednesday, September 15, Hurricane Ivan was approaching the Gulf
coast of Florida and Alabama, and rainbands in its northeast quadrant produced
29 tornadoes which caused all 8 deaths. The GOES-12
Infrared Loop on 15 September shows Ivan's approach, and convective
bursts in the Florida panhandle and southwest Georgia are evident based
on rapidly cooling cloud tops. As will be shown below, almost every tornado
occurs with one of these bursts of convection.
GOES-12 Infrared Loop |
In the absence of high-level cirrus clouds, visible imagery can be used
to identify more precisely the locations of each of these convective bursts.
The 1-km-resolution GOES-12 Visible Loop
shows many overshooting tops emerging from the lower deck of clouds. They
are especially evident in the late afternoon because their associated shadows
become more pronounced with the low sun angle. These overshooting tops
can be more easily identified by viewing the visible loop (above). It should
be noted that GOES-12 was in "Rapid Scan Operation" (RSO), meaning 8 images
per hour, compared to the usual 4, were available over the eastern US.
Without this improved temporal resolution, these individual overshooting
tops would be more difficult to identify.
GOES-12 Visible Loop |
In order to examine a possible relationship between convective bursts
and tornado reports, we will look more closely at several different times.
The first three tornado reports occurred around 2045 UTC: one in Gulf County,
FL, and two in Early County, GA. The 2040
UTC Visible Image shows the approximate location of the tornadoes (at
the tips of the red pointing arrows) and the associated overshooting tops.
There is a slight northward shift of the overshooting top relative to the
tornado report location; this is due to parallax. The map on these images
is drawn relative to the ground. Storm tops over 10,000 ft. MSL in the
midlatitudes result in noticable parallax and the map should actually be
shifted northward around 7 km so the tornado locations correctly fall underneath
the overshooting tops.
The 2040 UTC Infrared Image shows
the cloud top associated with the Gulf Co., FL, storm has a brightness
temperature of -69 C (the coldest in the area), and between 2025-2040 UTC,
this cloud top cooled from -57 C to -69 C. The brightness temperatures
with the Early Co., GA, storm are warmer, but they represent a local minimum
in the immediate area. It should be mentioned that the Gulf Co. storm moved
northwestward and an associated tornado struck Panama City Beach, resulting
in damage to a commercial area and 1 fatality.
2040 UTC Visible Image |
2040 UTC Infrared Image |
Next, we will look at two tornado reports from Calhoun county in GA from 2120-2130 UTC. The 2132 UTC visible image shows the approximate location of a tornado report in southwestern Calhoun County at 2130 UTC. Notice the overshooting top evident just to the north of the report; again, parallax correction places the overshooting top directly over the location of the tornado. The 2132 UTC infrared imageshows a cloud top with a brightness temperature of -51 C, another local minimum in the area. This cloud top continued to cool with time (see the IR loop above), reaching a minimum at 2155 UTC.
At 2125 UTC, another tornado was reported in Panama City, FL. However,
in this case, neither the visible nor the infrared imagery showed a well-defined
overshooting top or cold cloud top. The radar data (below) shows convection
in the area but the precise location of rotation is difficult to pinpoint.
2132 UTC Visible Image |
2132 UTC Infrared Image |
The final tornado we will examine occurred at 0223 UTC near Blountstown
in Calhoun Co., Florida. Four fatalities were reported, making this the
single deadliest tornado of the day. Since this occurred well after dark,
only the 0225 UTC infrared image
will be shown. A convective burst was observed around 0100 UTC southwest
of Tallahassee near the coast; cloud tops continued to cool and this convective
complex moved (interestingly) northeastward. The tornado report occurred
underneath -70 C cloud tops at 0223 UTC, shortly before the cloud tops
began to warm.
0225 UTC Infrared Image |
Radar loops from KEVX (Eglin AFB, FL panhandle) and KTLH (Tallahassee,
FL) are provided below. Both base reflectivity and radial velocity are
shown. In the KEVX reflectivity loop, step forward to 2039 UTC (approximately
the same time of the initial Gulf Co. tornado report). Between 2039-2104
UTC, five distinct cells, each with hook-like signatures, can be seen approaching
the Florida coast (Bay, Walton, and Okaloosa counties). Notice also that
velocity couplets can be found with several of these cells. However, only
the southeastern most of these cells was associated with a tornado report.
Recall from the visible loop that only
this southeasternmost cell had a distinct overshooting top. This single
example suggests that looking for satellite features such as overshooting
tops can improve the warning false-alarm ratio. Finally, it is quite possible
that these other cells were also producing waterspouts and/or tornadoes,
but they may not have been reported; a large number of residents had already
evacuated from the this portion of the Florida panhandle.
KEVX Base Reflectivity |
KTLH Base Reflectivity |
KEVX Radial Velocity |
KTLH Radial Velocity |