Tale Of Two Downbursts

Introduction
Lake Michigan sailors have had a challenging summer. In mid-July, competitors in the Chicago-Mac were treated to a rare type of downburst known as heat burst (or dry downburst) near Milwaukee late on Saturday night (click here for the summary). Three weeks later, sailors at the T-10 North American Championship near Chicago had an encounter with a different kind of downburst, one that blasted the fleet with hurricane-force winds. These downbursts, while similar in some ways, are very different in others. The downburst at the T-10 Nationals offers an opportunity to introduce a pair of relatively unknown Doppler weather radar products – storm heights (echo tops) and vertically integrated liquid (VIL) – that can help shed light on the evolution of an approaching thunderstorm.

Updrafts and Downdrafts
During their mature phase, all thunderstorms have an updraft and downdraft (Figure 1). The updraft provides the storm’s energy by delivering warm, moist air to the upper levels of the storm. The speed of an updraft – a measure of the storm’s strength – is determined by the temperature difference between the warmer air inside the updraft and the cooler air outside it. The larger the temperature difference, the faster the updraft.

Figure 1: Mature of thunderstorm life cycle.

The storm’s downdraft is the descending flow of rain cooled air. Downdrafts form when hydrometeors, precipitation particles such as water droplets, ice crystals, hail, etc., become too heavy to be suspended by the storm’s updraft, or when the updraft weakens. As the hydrometeors fall, the melting of ice crystals and the evaporation of water droplets serve to cool the air under the storm. In contrast to an updraft, the air within a downdraft is colder and therefore denser than the surrounding air. The speed of a downdraft is largely determined by the temperature difference between air within the downdraft and the surrounding air. The greater the difference, the faster the air falls.

Downbursts
Downbursts are downdrafts that produce strong straight-line winds at the surface. (All downbursts are downdrafts, but not all downdrafts are downbursts.) Downbursts are further subdivided in microbursts and macrobursts depending on the size of the area affected and the duration of the peak winds.

Downbursts come in two varieties — wet or dry. Wet downbursts are the most common, contain cool air, and are accompanied by rain. As the name suggests, dry downbursts are rain-free and produce warm dry winds. Both varieties form from a mass of cool air in the middle levels of a strong thunderstorm. Moisture and temperature characteristics of the air beneath the storm determine what type of downburst reaches the surface — environments with high relative humidity result in wet downbursts and low relativity promote the dry variety.

The T-10 North American Championship Thunderstorm Forecast
At 7:00 am on Thursday morning, a warm front stretching east from a low near northern Iowa was draped across the southern end of Lake Michigan (Figure 2). The warm front was expected to move north as the low progressed east. Southern Lake Michigan would lie in the low’s warm sector, the triangular region between its warm front and cold front. Warm sectors have a well-deserved reputation for promoting thunderstorm development. In recognition of this potential, the Storm Prediction Center (SPC) placed most of southern Lake Michigan under a Slight risk on its Day One Convective Outlook (Figure 3), a product that forecasts the potential for severe thunderstorms.

Figure 2: Surface analysis at 7:00 am on August 3, 2017.

Figure 3: SPC Day One Convective Outlook for August 3, 2017.

The SPC’s Thunderstorm Outlooks (Figures 4 to 6) indicated the highest probability of thunderstorm development near the T-10 North American Championship (40%) was the four-hour period beginning at 3:00 pm. The nearshore marine forecast issued by the Chicago office of the National Weather Service mentioned “showers and thunderstorms likely” on Thursday afternoon.

Figure 4: SPC Thunderstorm Outlook from 7:00 am to 11:00 am on August 3, 2017.
Figure 6: SPC Thunderstorm Outlook from 3:00 pm to 7:00 pm on August 3, 2017.
Figure 5: SPC Thunderstorm Outlook from 11:00 am to 3:00 pm on August 3, 2017.

A Downburst Reaches the T-10 Fleet
The T-10 Championship was hosted by the Chicago Corinthian Yacht Club. The course for the regatta was located a couple of miles offshore and approximately two nautical miles north of the Harrison-Dever crib. Two automated weather stations are located near the course. CHII2 is located on the Harrison-Dever crib and station 45174 is located nearly 10 nautical miles north-northwest of the course.

At 12:40 pm (1740Z), about an hour before the downburst hit the fleet, the storm was 18 nm west-southwest of the course and appeared small and unimpressive on the NWS Chicago radar image (click here). (Click here for a primer on Doppler Weather Radar). The storm was moving east at less than 20 knots. Over the next hour (click here for radar loop), the storm steadily grew larger, and more importantly, dramatically increased in height from 30,000 feet at 12:40 pm to approximately 47,000 feet as it reached Lake Michigan around 1:30 pm (Figure 7).

Increasing storm height is an indication of a strengthening updraft and an intensifying thunderstorm. Particularly interesting was the dramatic increase in storm height from 32,000 to 47,000 feet in just nine minutes beginning at 1:23 pm (1823Z). The storm reached Lake Michigan at approximately 1:30 pm (1830Z). The base reflectivity image at 1:32 pm (1832Z) (Figure 8) suggested the storm also contained a sizeable load of hail based on the large area of dark red shading indicating values greater than 50 dBZ. Although there are no observations to support this conclusion, a weak on-shore lake breeze front may have contributed to the intensification of the storm as it approached the lake.

Note: The observation times on the following graphs are in UTC. Five hours must be subtracted to convert to Central Daylight Saving Time. A short primer on meteorological timekeeping can be found here.

Figure 7: Radar derived storm height (Echo tops).

Figure 8: Base reflectivity at 1:23 pm (1823Z) on August 3, 2017.

The fleet was between races at 1:40 pm (1840Z) when the rapidly transitioning storm reached the course and produced a sudden downburst. The nearby race committee measured a hurricane-force gust of 67 knots. Ron Kallen and the crew of M*A*S*H were slightly south of the storm and caught the full brunt of the blast. As they were running downwind waiting for the wind and waves to subside, the mast suddenly broke in four places (Figure 9). Fortunately, none of the crew were injured.

Figure 9: M*A*S*H

As is often the case when downbursts encounter racing fleets, wind observations varied significantly across the relatively small course. Even the nearby automated weather stations (CHII2 and 45174) didn’t capture the full fury of the storm. The peak gust at CHII2 (two nm south) was only 24.2 knots (click here for graph) – well below the 67 knots observed by the race committee.  Station 45174 (10 nm north) observed a mere 13.4 knot gust (click here for graph), although the same storm produced a gust of 24.6 knots nearly 30 minutes later.

Doppler Weather Radar Observes the Downburst
The downburst may have avoided detection by CHII2 and 45174, but it couldn’t hide from the Chicago NWS Doppler Weather Station. The base reflectivity image at 1:42 pm (1842Z) (Figure 10) shows the location of the storm, M*A*S*H, and CHII2.

The base velocity image at the same time (Figure 11) shows the distinctive velocity pattern of a downburst—wind flowing toward and away from the radar station from a common area. Using the Doppler shift, velocity imagery displays the motion of targets, such as raindrops, associated with the thunderstorm. The movement of these targets can be used to infer the speed and direction of the wind relative to the radar station (the Chicago NWS radar station is southwest of the T-10 course).

Figure 10: Base reflectivity at 1:42 pm (1842Z) on August 3, 2017.

Figure 11: Base velocity at 1:42 pm (1842Z) on August 3, 2017.

The velocity scale (left side of the image) is in knots, with negative values (green shading) representing wind blowing toward the station (inbound) and positive values showing (red shading) wind blowing away from the station (outbound) (click here for an annotated image). Due primarily to the curvature of the Earth, the altitude of a radar beam steadily increases as distance from the station increases. While the signature of the downburst was captured by the station, the speed of the downburst at the surface was not observed because the radar beam was nearly 2,200 feet above the surface. (Click here for a base reflectivity and base velocity radar loop.)

In addition to velocity imagery, radar stations measure the height of a storm – echo tops (ET) – and the amount of water contained in the storm – vertically integrated liquid (VIL). (These relatively unknown products are available on radar apps such as RadarScope.)  Between the radar scans at 1:37 pm (1837Z) and 1:42 pm (1842Z), the storm’s height decreased from 48,000 to 43,000 feet (Figure 12). (Click here for a base reflectivity and echo top radar loop.)

Figure 12: Radar derived storm height (echo tops) and vertically integrated liquid (VIL).

During this same period, VIL (amount of water in the storm) suddenly decreased from 79.7 to 70.0 kg/m2. (Figure 12). Five minutes later, VIL further dropped to 61.6 kg/m2. Taken together, these radar-derived observations suggest the storm’s updraft rapidly weakened (reduced storm height) and was no longer able to suspend its load of raindrops and hail in the upper regions of the storm. Rapidly falling raindrops and hail, along with the contribution of evaporative cooling, resulted in the downburst. (Click here for a base reflectivity and vertically integrated liquid radar loop.)

Conclusion
Considering the proximity of the hurricane-force downburst to the 30-boat Tartan 10 fleet, it surprising there wasn’t more damage. However, for the crew of M*A*S*H, it was a case of déjà vu. During the Chicago-Mac less than three weeks earlier, strong northerlies and steep waves following the passage of a cold front caused M*A*S*H to lose her mast.

Coping with severe weather is part of the challenge of racing sailboats. Skippers must prepare their boats, train their crew, and maintain a watchful eye for approaching storms.


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