• Jake

The Diurnal Cycle of Tropical Convection

We're approaching the end of June, and with that, our focus for Hurricane Season will gradually start to shift eastward from here. So far, the "Main Development Region" (sector of the Atlantic Ocean between Africa and the Lesser Antilles) has actually had a couple of solid waves occupy it, including one that was labeled an Invest earlier this month. No named storms have come from these waves yet, though, largely because of marginal water temperatures and strong vertical wind shear. But as the summer goes on, we'll start to see our fair share of Invests, named storms, and likely hurricanes in this part of the basin.

Tropical waves, like the African Easterly Waves that spawn most Atlantic hurricanes, typically emerge over water as disorganized clusters of thunderstorms with a fairly weak, broad, and open circulation. This is a necessary starting point for a tropical cyclone to form - we already have some vorticity (rotation) and moisture to work with. What we then look for is for these thunderstorms to better organize. This process gradually reduces pressure, tightens up rotation near the surface, and better protects the developing storm against negative environmental influences like dry air. Head to my previous article for more on the various roles convection plays in tropical cyclone formation and intensification!

But often, the coverage of thunderstorms within these waves undergoes its own ebbs and flows. This typically appears to takes place on an approximately daily time scale, as shown in the above video of the recent Central Atlantic wave labeled as an area of interest by the NHC. Indeed, many studies have uncovered a diurnal cycle of tropical oceanic convection - a pattern which repeats every 24 hours with the rising and setting of the sun. As you're tracking the tropics later this season, odds are you'll sometimes hear terms like "diurnal maximum" or "diurnal minimum" (DMAX or DMIN). I'll use the rest of this article to explain what that means, and dive into one of the handful of scientific theories for why this takes place!

Most studies agree that over the tropical oceans, thunderstorm activity picks up overnight, and fades during the day. This is in contrast to what we typically see over land, where enhanced daytime solar heating and circulation patterns like sea breezes kick up storms in the afternoon and early evening. So we often see these tropical waves looking their healthiest as we start to head into the nighttime in the US, when the waves have had a few hours without sunlight. This is the diurnal maximum, when greater thunderstorm coverage offers a greater opportunity for waves to organize, develop pressure falls and rotation, and perhaps become a tropical cyclone. While the process usually doesn't happen in one night, there are physical mechanisms at play which can greatly accelerate development, which I'll discuss shortly. This is also not to say that storms can't develop during the diurnal minimum! But typically, daytime hours are less favorable for thunderstorms to organize, and this DMIN period can serve to slow the tropical cyclone formation process until the overnight hours.

One of the more widely acknowledged theories for this cycle focuses on radiation and the role of clouds in influencing this. Given that tropical waves are a pre-existing area of thunderstorms, they have a large amount of deep clouds compared to their surrounding environments. These clouds are very efficient at trapping the radiation emitted below them from the Earth's surface and atmosphere ("longwave" radiation, because it has a higher wavelength and lower frequency than solar "shortwave" radiation). In contrast, the relatively cloud-free air away from the wave more readily allows this longwave radiation to escape to space, cooling the atmosphere there more effectively. This difference in heating across space then causes a circulation to form, shown in the (crude) schematic diagram below.

At upper levels of the atmosphere, air diverges from thunderstorms, because the rising motion within them encounters the tropopause, a "cap" separating our section of the atmosphere from the stratosphere above. To compensate for the strong cooling taking place outside the wave, air sinks and warms. When this moves toward the surface, it must diverge. This causes air at low levels to flow toward the existing cluster of thunderstorms, and when this converges together, it must rise upwards again. This circulation thus allows for thunderstorms to persist through the night, improving a system's odds of developing the closed circulation necessary to be considered a tropical cyclone. This "feedback" involving radiation from Earth still takes place during the day, but is muted by the stronger solar heating that takes place where there is less cloud coverage.

Of course, there are other ideas for why this cycle exists, so if you found this discussion interesting, I encourage you to check these ideas out! But the process I've just highlighted, and the role of radiation in general, is becoming an increasingly-studied component of both tropical cyclone formation and intensification. In fact, I study this in my current research, as many of the interactions that help thunderstorms to "self-organize" are also fundamental to hurricanes. If you've made it this far, thanks for reading! If you have any follow-up questions or comments, leave them here, find me on Twitter @JakeCarstens, or contact me on any of the other platforms seen on the "About" page of this site. Also, if you have any particular topics you want me to write about, I'm happy to take suggestions! Look for another article in a week or so, and until then, enjoy the relative quiet in the Atlantic.

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