A Satellite Perspective of Tornado-Producing Cells Associated with Hurricane Ivan September 15, 2004
Published: September 22, 2004
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.
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.
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.
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.
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.
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.