Velocity Potential (VP200) and Subseasonal Forecasting
We've reached the end of July, and are quickly approaching the peak of Atlantic Hurricane Season. Things have been quiet since Elsa became extratropical on July 9, a stark contrast to 2020 where 5 tropical cyclones, including 2 US-landfalling hurricanes (Hanna/Isaias), formed between July 9-31. This year is closer to the norm, however, given the climatological challenges that systems face in June/July. These include relatively cool ocean temperatures in the "Main Development Region" (MDR, area in the eastern Atlantic between the Lesser Antilles and Africa) and strong vertical wind shear over the Caribbean Sea.
These 3 weeks of relative calm have given many tropical forecasters a chance to look a bit farther into the future. This "subseasonal" forecasting approach goes beyond what traditional numerical weather prediction models (GFS, ECMWF, etc.) can reliably simulate, instead focusing on the large-scale background state to examine if environments will be favorable for tropical cyclones to develop. Climatology tells us that water temperatures in the MDR will become more favorable in August, giving African Easterly Waves a better chance to develop into tropical storms. That part is simple enough, but odds are you've seen an image recently like the one below, courtesy of Dr. Mike Ventrice (mikeventrice.weebly.com, or @MJVentrice on Twitter... but if you're reading my article, there's a good shot you already follow him). Plots like this have become a particularly powerful tool for forecasting hurricane activity 2-3 weeks out, depicting the "Velocity Potential" (VP200, as it's often calculated at a pressure level of 200 mb).
The arrows on this plot are a good place to set the stage. Much like how we can split vectors up into "x" and "y" components in a math/physics class, or wind into "east-west" and "north-south" components, we can also split the wind up into rotating and non-rotating parts. The non-rotating part, known as the "irrotational" wind, is what the vectors here show, and what the Velocity Potential describes! This variable thus answers a pretty simple question: "Is air converging or diverging in the upper atmosphere?" And this is especially powerful because what happens up there can tell us quite a bit about what happens below! Let's break that down with a quick animation I made for the Weather Analysis and Forecasting class I taught in the Spring, shown below.
This is based on the idea that the 200 mb level, where Velocity Potential is usually calculated, is near the tropopause, a "cap" separating the troposphere where our weather mostly takes place from the stratosphere above. Air cannot punch through the surface, and except for particularly strong convection, it doesn't punch through the tropopause either. Therefore, divergent upper-level air implies rising motion below, and convergent upper-level air implies sinking motion below! The former is associated with widespread convection (more favorable for tropical cyclones), while the latter inhibits convection (less favorable for tropical cyclones). With that in mind, let's bring it back to the original VP200 plot, and introduce some annotations to help break it down.
What you'll always see in these plots is the anomaly of VP200 (how much it differs from the average value), but the idea is the same. As mentioned in the annotation on the lower right side of the plot, these cells of diverging (rising) and converging (sinking) air typically move eastward over time. There are two main types of disturbances that cause this, which are a fundamental part of subseasonal hurricane forecasting: The Madden-Julian Oscillation (MJO) and convectively-coupled Kelvin Waves (CCKWs), which each deserve separate articles (and entire PhD dissertations). This explains why forecasters have pointed to the middle of August as a time for the Atlantic basin to "wake up", so to speak - as the cell of rising motion moves into the Atlantic, expect convection to become more widespread and easterly waves to start looking healthier when they emerge from the African coast.
This isn't necessarily to say that the center of these cells is most favorable for tropical cyclones, though, as strong divergence aloft can introduce strong vertical wind shear! For example, we could expect the most activity to take place at the back end of one of these cells - an environment still favorable for rising motion, somewhat weaker vertical shear, and an often-moister environment for waves to traverse through after convection has already been fairly widespread for several days. In addition, this doesn't allow us to point to specific storm formations, tracks, intensities, etc. quite yet. But with this tool, subseasonal forecasting has gained plenty of skill in identifying the times when we might expect business to pick up.
Thanks for reading! I've been taking advantage of a quiet 3 weeks to step back from the tropics a bit and refresh, so once things get going again, follow along for more frequent posts. If you have any questions or comments, drop them here, find me on Twitter @JakeCarstens, or contact me via any of the other methods on the "About" page of this website. In addition, if you have any topics you'd like to see an article about, I'm happy to take any suggestions. Take care in the meantime, and enjoy every day we can squeeze out without a storm in the Atlantic!