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Danny’s Leg Up: A Convectively Coupled Kelvin Wave (CCKW)

By: Bob Henson 11:38 PM GMT on August 19, 2015

Tropical Storm Danny might not be making the scene were it not for the help of a subtle but important atmospheric feature, called a convectively coupled Kelvin wave (CCKW), that’s contributed to a temporary break from hurricane-hostile El Niño conditions. Danny has changed little over the last few hours: as of 5:00 pm EDT Wednesday, peak sustained winds remained near 50 mph, and the latest National Hurricane Center outlook continues to bring Danny to hurricane status, though not until Friday. Tonight will be a good test of whether Danny can rebuild a solid convective core after its disruption from dry air and Saharan dust over the last 24 hours. For a more complete look at Danny and today’s other tropical activity, see our post from earlier this afternoon and the post from WU blogger Steve Gregory.

What’s a CCKW?
CCKWs are huge impulses, spanning thousands of miles, that move from west to east through the stratosphere, typically rolling along at about 30 to 40 mph. CCKWs are centered on the equator, with their effects progressively weaker as you move toward the subtropics. Like a giant chimney, each CCKW has a broad zone of rising air at its heart, tilted toward the west as you move up. The resulting circulation (see Figure 1) favors the development of showers and thunderstorms ahead of the CCKW, as low-level air converges. The resulting storms are then supported by upper-level divergence toward the center of the CCKW, plus low-level westerlies near the equator that can enhance cyclonic spin. An eastward-moving CCKW can intersect the train of westward-moving waves rolling through the Atlantic, giving one or more of them a boost that can help them consolidate into tropical cyclones.

Figure 1. Schematic cross section through a convectively coupled Kelvin wave (CCKW). Image credit: Michael Ventrice.

Global weather prediction models such as the GFS and ECMWF can now latch onto CCKWs and preserve them as they make their way around the globe. CCKWs are nondispersive waves, meaning they tend to maintain their structure and can survive long enough to make several trips around the world over the course of several weeks. This gives them a noteworthy predictive value, but it takes some time and training to analyze CCKWs, which are too subtle to be seen with the naked eye on satellite images. One of the most useful tools is a Hovmöller diagram (see Figures 2 and 4 below), a time-versus-longitude plot often used to analyze areas where outgoing infrared radiation is consistently strong or weak. A CCKW will often foster and suppress enough clouds along its path to appear in a Hovmöller diagram, where its signal can be separated from the noise of day-to-day storminess. Sometimes a CCKW will overtake an active phase of the slower-moving Madden-Julian Oscillation, providing a twofold boost to showers and thunderstorms at that location before the CCKW moves on.

“It can be hard to see CCKWs for sure in these [diagrams], as these waves often destructively interfere from time to time with other equatorial waves and/or standing oscillations,” Ventrice told me. “Thus, you'll see a clear eastward propagation over some location, a break, then continuation of the signal over another location to the east at a later time.”

Figure 2. A Hovmöller diagram showing CCKW activity in July-August 2006 as traced by reductions in outgoing infrared radiation (encircled bands), averaged by longitude (bottom axis) over the main development region of the Atlantic (7.5°N to 12.5°N). Negative values of longitude are °W; positive values are °E. The CCKWs occur in the context of other, stronger features, so it can take careful analysis to find them. The green “D” near the center of the image indicates where and when Tropical Storm Debby became a named storm, supported by a CCKW passing from upper left to lower right through the “D.” Image credit: Michael Ventrice.

WSI scientist Michael Ventrice unraveled some of the links between CCKWs and hurricane formation several years ago, as part of his doctoral work at the University at Albany, State University of New York. He and colleagues at WSI now consult the CCKW regularly, and Ventrice has helped NHC develop code that allows the center to analyze CCKWs on a daily basis. “It seems to have really caught on among forecasters in the last three years,” Ventrice told me. For example, a forthcoming paper in Monthly Weather Review by Carl Schreck (North Carolina State University) surveys CCKWs around the globe and their role in tropical cyclogenesis.

Ventrice’s own research includes a set of three CCKW-related papers published in Monthly Weather Review in April 2012, July 2012, and June 2013. Ventrice’s dissertation is online, including an analysis of how a CCKW assisted in the formation of Tropical Storm Debby in 2006. There’s much more about CCKWs, including daily analyses, on Ventrice’s personal website. He also discussed CCKWs as part of a guest post on this blog last year.

How could a CCKW help give Danny a kick-start?
I was intrigued on Tuesday when a tweet from @MJVentrice credited a weak CCKW with assisting in the formation of the tropical depression that became Danny. According to Ventrice, this CCKW formed near the International Date Line around August 8, then amplified as it moved into the Atlantic basin. It’s now centered over the heart of the main development region, the swath of deep tropics in the north Atlantic that plays host to many of each year’s tropical cyclones. Over the last several days, this CCKW has likely enhanced low-level moisture and convection on its forward flank, in the eastern Atlantic, which would have helped nourish the easterly wave that became Danny as it came off the African coast. Only a minority of such waves make it to tropical-storm status; Ventrice believes that CCKWs are one of the key factors that can make or break such a system.

Figure 3. Tropical Storm Danny is now on the western edge of an eastward-moving CCKW, as indicated by this composite of Atlantic rain rates (shaded), departures from average wind at the 200-millibar height (vectors), and velocity potential anomalies at the 200-millibar height (contours). The velocity potential anomalies are related to upper-level divergence (blue contours) and convergence (red contours), with divergence favoring upward motion. For the latest version of this image, see Michael Ventrice’s website. Image credit: Michael Ventrice.

As Danny and the CCKW move in opposite directions, Danny is finding itself left behind. “The environment behind the CCKW can stay favorable for a couple of days, where you have enhanced low-level spinning, reduced vertical wind shear, and enhanced outflow,” Ventrice told me. (Carl Schreck’s new paper also finds that tropical cyclogenesis is favored for several days behind the crest of a CCKW.) However, Danny appears to be already feeling a lack of support, with the peak of the CCKW having long passed it by. “This may be the reason why we are seeing convection decouple from Danny this afternoon [Wednesday], as the low-level circulation associated with Danny is becoming exposed,” he said.

Fairly soon, Danny will encounter the more suppressed environment well behind the CCKW, together with the likelihood of higher wind shear and drier air favored by the ongoing El Niño regime. “I’ve seen a lot of cases where a mature tropical cyclone gets run over by the suppression behind a CCKW,” said Ventrice. “Can the hurricane create its own environment to protect itself from this?” One thing is for sure: the more latitude Danny gains, the less it will be influenced--for better or worse--by this CCKW. Meanwhile, it’s possible that the CCKW that gave Danny a boost will go on during the next week to favor development of another one or two easterly waves now over Africa, as suggested by some long-range model runs. After that, said Ventrice, “it doesn’t look like there’ll be much in the way of CCKW activity over the next two weeks.”

On Thursday morning, we’ll have a complete update on Danny, as well as Invest 93C in the Central Pacific and Typhoon Goni and Super Typhoon Atsani in the Northwest Pacific. Later on Thursday, watch for our summary of July’s global climate. In the meantime, here's a fascinating infrared loop of Atsani's core, featuring 2-km-resolution imagery from the Himiwari-8 satellite, courtesy of the University of Wisconsin/CIMSS. Jeff Masters is offline most of this week but will be fully back on board next week.

Bob Henson

Figure 4. A Hovmöller plot for May-September 2015 from the Cooperative Institute of Climate and Satellites-North Carolina (CICS-NC) showing variations in 200-mb zonal (west-to-east) wind speed by longitude over time. After August 17, the plot draws on long-range predictions from the NOAA Climate Forecast System model. Tropical cyclones over the Atlantic and Pacific are shown as lettered hurricane symbols. CCKWs are encircled in blue. A weak CCKW at lower right may have played a role in the development of Tropical Depression 11 (“E”) in the Northeast Pacific as well as Danny (“F”) in the Atlantic. Regularly updated plots are available from the Cooperative Institute of Climate and Satellites–North Carolina. Image credit:


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