After another busy start to Atlantic Hurricane Season in June, July is quick out the gate with the naming of Tropical Storm Elsa early Thursday morning. Elsa's forecast is particularly tricky given the unusually fast steering flow, potential land interactions down the road, and more general questions about its structure. We'll focus on the last point in this article, using Elsa's intricacies to run through an important tool to assess tropical cyclone structure when basic satellite pictures are not enough.
Visible and infrared (IR) satellite imagery is obviously quite powerful in letting us visualize the evolution of a storm in real time. But often, the deep convection we typically see near its center "covers up" some of the more intricate details of the storm's circulation and inner-core clouds/precipitation. In Elsa's case, we're particularly interested in the ability for convection to "wrap around" its circulation against the vertical wind shear in place. Doing so would allow Elsa to strengthen more quickly on its (fast) approach to the Lesser Antilles. Luckily, our ability to remotely sense the atmosphere with satellites stretches well beyond the visible and IR you're most used to! We just need to move toward a lower-frequency (higher-wavelength) part of the electromagnetic spectrum: Microwave. An example of a microwave image from Hurricane Gustav in 2008 is below.
Several satellites are equipped with sensors that measure emitted energy from the atmosphere with a frequency in the range of 20-200 GHz. This emitted energy can be quantified using the "brightness temperature", which is similar to how infrared satellite works - in that case, what is being shown is the temperature of the tops of clouds. The trick here is to multiple frequencies of radiation to sense! With this, microwave imagery can separately diagnose the lower and upper-level structure of a storm. It can "punch through" the cloud canopy you would see on visible/IR, and tell you if a storm has a closed eyewall, if its low-level and mid-level circulations are well-aligned, and more!
Above is a microwave image of Elsa from 5:33 PM EDT Thursday, using the 91-GHz frequency channel. This relatively high frequency gives us a better look at the middle and upper-level structure of clouds and precipitation. This is because it primarily receives radiation with low brightness temperatures, indicative of ice and upper-level cloud droplets associated with deep convection. In Elsa's case, this deep convection is skewed to one side of its circulation, unable to form a closed ring around the center due to moderate wind shear. Overnight, we'll look to see if future snapshots show the region of yellow and red colors is able to wrap around the north side of Elsa's circulation. This would be an indicator that Elsa is better able to defend itself against the wind shear, and to align its low-level and mid-level centers more effectively. This would allow it to strengthen after a day with very little intensity change.
Now, let's shift to the lower-frequency case, in this case 37-GHz, pictured above. This gives us the opposite picture of the higher-frequency image: Higher brightness temperatures stemming from liquid water in the lower (warmer) levels of the atmosphere. For this particular image, regions of cyan or magenta colors indicate more moisture, and we can similarly search for "rings" that would indicate a closed core. Again, this type of well-defined structure does not clearly show up for Elsa, which makes sense given its status as a fairly weak tropical storm for now.
Of course, there are plenty of subtleties associated with interpreting microwave imagery, and there are several different plots you can intercompare. But hopefully this makes it a little bit easier to see what we're looking for, as this is a very powerful tool to go "under the hood" and learn a lot about each storm in the absence of surface observations or aircraft reconnaissance data. Head to https://www.fnmoc.navy.mil/tcweb/cgi-bin/tc_home.cgi to check out the images yourself, which will update regularly after individual satellites pass over Elsa. Otherwise, don't hesitate to reach out if you'd like to discuss or learn more! Leave a comment here, find me on Twitter @JakeCarstens, or contact me at any of the other platforms on the "About" page on this website. Finally, if you have a particular topic you're interested in that you'd like me to write about, I'm happy to hear your suggestions. Thanks for reading!