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Invest 92L - Convective Bursts and the Hurricane Life Cycle

It's June 16 as I write this up, and all eyes are on the Gulf of Mexico, where Invest 92L looks set to creep northward and likely become Tropical Storm Claudette. While there are barriers to this becoming an intense storm, rain is the main story across much of the southeast, an area that's already seen plenty over the past couple of months.

As 92L gets going, odds are you'll see plenty of discussion about convective bursts (or CBs, for short). These can be identified on satellite imagery by the emergence and growth of regions with very cold infrared brightness temperatures (often less than -70°C). These indicate the tops of clouds from thunderstorms that stretch through the depth of the troposphere, like the example below from 2020's Hurricane Sally. "Organized deep convection" is part of the formal definition of a tropical cyclone, but CBs also play an integral role in both hurricane formation and intensification. Let's break down some reasons here, as we wait for CBs to start leaving their mark on 92L.

In any talk of tropical cyclone formation, vorticity (counterclockwise rotation, in our case) is going to come up. All of our hurricanes have some sort of vorticity to start with, whether that be from an African Easterly Wave, the Central American Gyre, a frontal system that moves over water, and so on. In 92L's case, that vorticity is fairly weak, but distributed over a broad region in the Bay of Campeche. Convective bursts have vorticity themselves, and help to organize and strengthen vorticity over a more coherent space. The rising motion within them takes a rotating column of air and stretches it out vertically, as shown in the schematic below from the University of Chicago. One burst is generally not enough to get a closed surface circulation, though! Persistent convection over one area helps a developing storm to organize and strengthen to the point of naming. That's why it's going to take 92L a bit longer to form - convection is underway, but it's too disorganized for now to let this evolution occur quickly.

Convective bursts near the center of circulation also generally help to intensify existing storms. The mechanism discussed above is still at play in this case, as well as the role of "latent heat". Condensation of water vapor into clouds releases heat into the surrounding atmosphere, some of which eventually sinks back down into the center of the storm (the eye of a mature hurricane). When this sinking takes place, the center becomes warmer, hence the label "warm core" that we assign to tropical cyclones. This warming process helps to lower the surface pressure in the center of the storm, strengthening the pressure gradient (difference in pressure between the center and its surrounding environment) that influences wind speed. It's unlikely that significant intensification will take place with 92L due to factors like dry air and wind shear, but convective bursts can also help to offset these somewhat.

Convection transports moisture up from the surface into the middle and upper troposphere. High mid-level humidity is frequently cited as an important ingredient for tropical cyclone formation, because that type of environment is more favorable for future convection. In this sense, convection can be thought of as having a "moisture memory" effect, where it moistens its surrounding environment, priming it for future bursts. That way, a developing storm can defend itself more readily against negative environmental influences like wind shear, even if that doesn't necessarily mean it will significantly strengthen.

So be on the lookout for this over the next day or two! Before a closed surface circulation becomes well-established, convective bursts can even help to relocate the center, which is important for effective model forecasts and impacts assessments. Given that the interaction between convection and tropical cyclones falls directly under my research, there's a good chance you'll hear a lot about it from me this season! Don't hesitate to reach out if you have any follow-up questions or comments - leave a comment here, find me on Twitter @JakeCarstens, or contact me at any of the other links on the "About" page on this site. Thanks for reading!

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