It's August 12 as I write this, and the Atlantic is starting to get going a bit with Fred, and another wave behind with a strong chance of development from NHC. This isn't unexpected - forecasters have talked at length over the past couple of weeks about an uptick of activity through August. After all, we're entering the climatological peak of Hurricane Season. So while I won't focus on Fred, Invest 95L, or future waves in this post, it's about that time to check hurricanes.gov regularly and make sure you have your hurricane plan prepared in case a storm heads your way.
Social media is a natural landing spot for rapidly-evolving information, and in tropical cyclone world, perhaps nothing fits the bill more than aircraft reconnaissance data. Hurricane Hunters from NOAA and the U.S. Air Force regularly fly into Atlantic storms that threaten land, and occasionally for research purposes. Loaded with instrumentation onboard the aircraft, as well as "dropsondes" that collect meteorological data as they descend to the surface, recon provides extremely useful, high-resolution data that goes a long way in improving tropical cyclone analysis and forecasting. Thanks to a certain Joint Typhoon Warning Center scientist (and former labmate of mine), we've been able to see a great deal of this info in real time for several years now.
So what exactly are we looking for during these recon missions? The answer to that question varies depending on what type of system we're looking at. Is it an Invest that still hasn't developed a closed circulation? A strengthening tropical storm on the way to hurricane intensity? A mature hurricane undergoing an eyewall replacement cycle? I'll use the full slate of Tropical Tidbits recon plots (shown above) to break down these different scenarios, and highlight some of the important features that sometimes slide under the radar compared to the winds. I'll annotate an example of each, and describe some of the things we look for in these varying intensity scenarios! Before that, I'll give a quick shoutout to incoming FSU student Alex Boreham (@cyclonicwx on Twitter) whose website, cyclonicwx.com, also plots recon data in real time, and even decodes the "Vortex Data Messages" that provide useful info on the inner-core structure of a storm. Let's dive in...
RECENT OBSERVATIONS (WINDS AND PRESSURE): On Tropical Tidbits, 10-minutes of zoomed-in wind/pressure data are shown at a time. Another plot shows the accumulated wind speed/direction data and the minimum pressure estimated by the aircraft each time it passes through the center. As you would expect, stronger winds and lower pressure corresponds to a more intense storm. Though the only direct measurements of surface wind/pressure are taken by dropsondes (more on that later), the aircraft records wind data at the level it's flying at, typically around 700 mb or 3 km above the surface. In addition, with the flight-level pressure and altitude, the surface pressure can be estimated by "extrapolation" using a formula for how pressure varies with height in the atmosphere. Where these observations come most in handy is gathering details about the center of the storm. Has the pressure lowered since the last time the plane passed through? Are the winds in the eyewall stronger? Has the storm "wobbled" in a slightly different direction? All of these questions are important to forecasts and local impacts. Let's discuss how to find the center using Fred as an example, since recon is often how we found the closed circulations that get storms named! Here, I'll focus on the winds, though I'll note that you will generally find a minimum in pressure near the same point.
Winds are plotted in the form of wind barbs: the direction that the barb is pointed indicates the direction wind is flowing from, and the extensions on their right denote the wind speed. Given that winds flow counterclockwise around a tropical cyclone (in the Northern Hemisphere), we should thus see wind barbs oriented the way that I've shown in the crude schematic above. This is exactly how you can find the center of a developing storm! As the aircraft flies through, it should pick up a sharp shift in wind direction. For example, if it's flying from west to east, it should detect a north-->south wind on the west side, and a south--> north wind on the east side. If the shift is not distinct in all directions, and the winds around the low do not form a circle, then it fails to meet the "closed circulation" criteria necessary to be classified as a tropical cyclone. The waves we'll see moving through the Atlantic begin as troughs, where the winds may turn in a counterclockwise direction somewhat, but generally fail to produce the west--->east needs to close off completely. Some aircraft missions will descend close to the surface in an attempt to detect exactly this! This is why Fred took a while to be named despite Potential Tropical Cyclone advisories - that closed circulation didn't emerge near the surface until Tuesday evening.
FLIGHT LEVEL PRESSURE/WIND TIME SERIES: About 2 hours worth of data appear on the same plot here, allowing us to see the details of flight-level winds and extrapolated pressure across multiple passes through the eye and eyewall. The center of most storms will appear as a quick drop in pressure and winds, surrounded by peaks in wind speed on either side as the aircraft passes from one side of the eyewall to the other. Flight-level winds are not always indicative of what's happening below, but in general, you can expect the surface winds to be slightly weaker than what you see on this plot. This part is simple enough, but let's dive into a couple of subtle details that can reveal important aspects of storm structure (examples below).
a) VORTEX TILT: Wind shear causes a tropical cyclone vortex to tilt somewhat, so the center of circulation at one altitude may be different from that at another! One way you can see this is by noting an offset between the minimum pressure recorded by the aircraft, and the minimum wind speed near the same time. The minimum wind speed corresponds to the shift in winds at flight level, indicating the center of circulation up there. In contrast, the minimum in pressure (since that value is extrapolated down to the surface) is more suggestive of the center near the surface. Based on the direction of this offset, and the direction the plane is flying in, this can give you an idea of what direction the storm is tilted, thus informing you about the wind shear. In the example of a strong hurricane, this may be a sign that some weakening could take place, or for a weaker storm/invest, a sign that intensification is likely slow to take place, if at all.
b) SECONDARY EYEWALLS: The likely subject of a future article, secondary eyewalls frequently form in mature hurricanes, farther away from the center than the original eyewall. These are the precursor to so-called "eyewall replacement cycles" (EWRCs). The most common result of an EWRC is a weakening of the storm's maximum winds, but an expansion of the overall wind field, which can increase wind and storm surge risks farther away from the center. Satellite and radar imagery can help us discover secondary eyewalls in some cases, but aircraft wind is often quite powerful! Here, look for 2 distinct peaks in wind speed on one side of the storm's center, with weaker winds in between. Over time, a successful EWRC will lead to the outermost wind speed peak contracting inward and overtaking the original.
SFMR SURFACE WINDS: In addition to measuring winds at flight level, aircraft come equipped with an instrument called a "Stepped Frequency Microwave Radiometer" (example below). By detecting the microwave radiation emitted by the ocean surface, SFMR is able to estimate both wind speed and rainfall rate below the aircraft. This can provide more detailed information about storm intensity compared to the estimation of surface wind speed based on the flight-level wind. However, SFMR has a few quirks where caution should be exercised. First, data can often be contaminated in parts of the storm with particularly intense rainfall. These values are often flagged, and will appear as "Suspect SFMR" on Tropical Tidbits. Second, data become less reliable near coastlines due to the "shoaling" of ocean waves in progressively shallower water. Finally, ongoing research suggests that in the strongest hurricanes, SFMR may produce values of wind speed that are too high. Notably, this was cited as a reason why Hurricane Iota's peak intensity was slightly downgraded in postseason analysis released earlier this year. But otherwise, there's no gimmicks here - higher values suggest a stronger storm.
TEMPERATURE AND DEWPOINT TIME SERIES: Given the attention that pressure and wind preferentially receive as the most common measures of intensity, these can slide under the radar. But much can be learned about the health and intensification of a storm from just a couple of subtle details here! When the aircraft reaches the center (pressure and wind minimum, as discussed earlier), you'll often notice a sudden change in the temperature and dewpoint as well. For example, you may see a quick divergence of the two lines, where the temperature increases and the dewpoint decreases. This makes sense when you think about the eye of a mature hurricane being fairly dry and clear... more on that when we talk about dropsondes next. The spike of warmth is arguably most notable here - a tropical cyclone is a "warm core" system, meaning its center is warmer than the surrounding environment, especially in the middle and upper levels! Something to watch for as a flight progresses: Is that spike of warmth getting warmer with each pass through the center? If so, that's a sign of a strengthening storm, as warming implies decreases in pressure, which cause winds to intensify. As for what causes this warming, we'll get to that in the next (and final) section...
DROPSONDES: Low-level recon missions generally launch dropsondes downward from a pressure level of about 700 mb. Their instruments are able to measure temperature, wind, pressure, and humidity, similar to the upward-moving radiosondes attached to weather balloons. With this similarity in mind, dropsonde data is plotted on a "skew-T log-P" diagram, with pressure on the y-axis and lines of constant temperature sloping up and to the right. The main two lines on the plot - the temperature (red, right) and dewpoint (green, left). Wind barbs are located on the right-hand side of the plot, corresponding to the pressure level that the wind is recorded at as the dropsonde descends. For simplicity, numerical values of these variables are listed in tables on Tropical Tidbits, so this could be a good way to get used to wind barbs if that's something you aren't familiar with! For low-level missions, I'll split our "what to watch for" into 2 regions: The center, and basically anywhere else, with a preference toward the eyewall.
a) THE CENTER: Dropsondes are our best way to assess minimum surface pressure, because they are often the only way this can be directly sampled for a storm over water. The aircraft outputs an "extrapolated" sea level pressure from its flight level, but this is usually off by a small margin. So this is the first big thing to look for in an "eye" dropsonde - how low is the pressure? In its purest form, because tropical cyclones have approximately calm winds in their centers, the dropsonde will record this surface pressure while measuring roughly zero wind. This is difficult to achieve in reality - it's hard to pinpoint the exact location when you're dropping something from 10,000 feet above the surface! So a correction that usually gets applied is as follows: Take the pressure that the dropsonde records, and subtract 1 mb from that for every 10 knots of wind recorded at the surface. For example, if the dropsonde records a pressure of 997 mb with a 12 knot wind, the actual minimum pressure is likely closer to 996 mb.
Above the surface, the temperature and dewpoint profiles become the main focus. A mature tropical cyclone will have drier air in its eye than in the surrounding environment, so above the lowest 1 km of the atmosphere, it's a sign of a healthy storm to see the red and green lines well separated. Another notable feature common in hurricanes is a sudden divergence of these profiles - as you move upwards on the plot, the temperature curve abruptly slopes to the right, and the dewpoint to the left. This is known as a "subsidence inversion", which indicates sinking air in the eye. This is why there's an eye in the first place - sinking motion there suppresses cloud coverage, leading to clear skies compared to the surrounding environment!
b) AWAY FROM THE CENTER: Outside of the eye, dry air (the separation between the temperature and dewpoint profiles) is less favorable for a strengthening storm. Here, we generally look for these profiles to be well-aligned, indicating plenty of moisture existing throughout the column of the atmosphere that the dropsonde falls through (the temperature and dewpoint values are nearly equal). Deploying these instruments in the eyewall can also capture some particularly intense winds! These can inform us to an extent on overall intensity, but dropsondes measure wind gusts more than they do sustained winds that the Saffir-Simpson scale is based on. On local scales, these strong winds just above the surface have the potential to "mix down" via convection, which can contribute to local wind gusts stronger than the estimated maximum sustained wind of the storm.
Of course, every storm has its own unique quirks, so there's a good chance you won't see a recon image that matches up exactly with one of the annotated plots here. That's okay! Hopefully, some of the meteorological background behind these explanations can help piece things together the next time you track a mission. This article is by no means exhaustive - there's plenty more you can learn about a storm from aircraft recon! But before the plots start to pop up more on your social media feeds with stronger storms likely to come, I hope you've found this to be a good primer. Like always, if you have any questions, comments, follow-up points, and so on, don't hesitate to reach out! Comment here, find me on Twitter @JakeCarstens, or contact me through any of the other methods on the "About" page of this site. I've gotten a couple of suggestions on future articles to write, which is awesome, but I'm always happy to take more of those as well. That's all for now. Until next time, fingers crossed that these next few weeks don't give us too much to worry about!