Decoding a Vortex Data Message

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Now that you know about how U.S. Air Force and NOAA Hurricane Hunters operate, let's look at the major tools they have in their arsenal for collecting data. As an overall package, their instrumentation is called the Improved Weather Reconnaissance System (IWRS). Instruments mounted on the planes frequently collect flight-level data, which include air temperature, dew point, wind velocity, air pressure, and altitude of the aircraft (altitude is measured by radar (opens in a new window)). Onboard computers process flight-level data every second, but "complete" weather observations take 30 seconds. Moreover, the computers are tied to the aircraft's navigational system, allowing the flight meteorologist to determine the position (or location) of each observation. These data are also sent off the plane in real time in various coded formats. But, of course, meteorologists aren't just interested in observations at flight level. So, what tools do Hurricane Hunters have to detect weather conditions all the way down to the surface?

Dropwindsondes

Dropwindsondes (sometimes called "dropsondes" or just "sondes" for short) are instrument packages designed to be dropped from aircraft in order to take observations along their path to the surface. Dropsondes are very similar to the rawinsondes you learned about in your previous studies, but instead of ascending aboard a weather balloon, the descend toward the earth's surface. They have a long history of use in aircraft reconnaissance of tropical cyclones dating back to the 1950s. In the "old days," however, they couldn't be used to gather wind data in areas of clouds or rain. Therefore, forecasters at the National Hurricane Center "extrapolated" flight-level winds (700 mb) to the ocean surface. By "extrapolate" I mean that forecasters multiplied the maximum winds at flight level by a fraction between 0.80 and 0.90 to estimate the maximum surface winds (you will learn later in the course that the fastest winds in a hurricane typically blow at altitudes of several hundred meters above the sea surface).

This method ultimately proved to be fairly reliable, except for a few "misbehaved" storms. While scientific principles laid the groundwork for the extrapolation technique used by the National Hurricane Center, data collected by Global Positioning System (opens in a new window) (GPS)-based dropwindsondes beginning in 1997 proved that the scheme works pretty well most of the time. But without reservation, GPS-based dropwindsondes have improved the accuracy of estimating maximum surface winds in a hurricane (and model accuracy for predicting the path of tropical cyclones). If you're interested in learning more about the benefits of using GPS dropwindsondes, check out this research paper (opens in a new window).

Credit: (Left) Wikimedia Commons; (Right) NCAR

Hurricane Hunters routinely release dropwindsondes during their missions to penetrate the center of the a tropical cyclone, but the NOAA Gulfstream jet also releases them in the environment around the tropical cyclone to collect data about the surrounding environment. Immediately after a dropsonde gets released, a drogue parachute deploys, which stabilizes the sonde's descent by stopping it from tumbling, which is especially critical in the turbulent air motions within the eyewall. During descent, the in-situ sensors on the dropsonde (see image above) relay observations of pressure, temperature and relative humidity back to the aircraft via radio until the sonde splashes down into the ocean. These observations are processed by computers on board the aircraft as well as on the ground (computers can process real-time observations from multiple dropsondes simultaneously). For the record, on a typical mission, Hurricane Hunters may release 20-40 dropsondes, and in an average hurricane season, they can release well over 1,000 dropsondes on training and storm-reconnaissance missions. The data from dropsondes gets assimilated into some numerical weather prediction models, which improves forecast accuracy. 

While most dropsonde observations are in situ, technically the method for measuring wind speed using dropsondes qualifies as remote sensing. That might sound strange, but each sonde contains a full GPS, which allows satellites to remotely track its exact location. By tracking the changes in the sonde's location in time, computers calculate the wind speed by subtracting out the terminal fall speed and friction. Ultimately, dropsondes are often the best observation source for minimum surface pressure as well as for directly sampling low-level winds in the eyewall of a tropical cyclone. These data are immensely valuable for determining the intensity of a particular storm. Ongoing research continues to make dropsondes smaller (so-called "minisondes" are now available) and less expensive.

Uncrewed Aerial Vehicles (UAV)

Credit: NOAA

In recent years, NOAA has collaborated with private companies to develop "uncrewed aircraft systems" (UAS) or "uncrewed aerial vehicles" (UAVs), which they can deploy in addition to dropsondes during their flights. These flying drones (like the one pictured on the right) are capable of flying for more than one hour, can ascend and descend very rapidly, and have been designed to withstand the harsh conditions within a hurricane. They can fly as low as 50 feet above the ocean surface, and up to 15,000 feet above sea level, measuring pressure, temperature, humidity, and winds (among other variables) with their onboard instruments. These data are transmitted to the National Hurricane Center in real-time and get assimilated into some computer models, much like data from dropsondes. Since UAVs can stay in the air longer than a dropsonde, they're capable of covering more area within a tropical cyclone than a single dropsonde can.

Doppler Radar

While all Hurricane Hunter aircraft have radar onboard for helping the pilots navigate, the planes used by the NOAA Hunters also have radars aboard for data collection. Recall that Doppler Radar is an active remote sensor: It sends out a pulse of energy and measures what gets scattered back to it. The Gulfstream jet has two radars (one on the nose for help with navigation, and a Doppler radar on the tail), but the weather instrumentation aboard each NOAA WP-3D actually includes three radars (opens in a new window) (one on the nose for navigation, one on the lower fuselage, and a Doppler radar on the tail). In addition to giving insight into the precipitation occurring in the storm, recall from previous courses that Doppler radars have the capability of detecting wind velocities, which helps meteorologists observe the storm's wind field. For example, check out the side-by-side images below, collected from Tail Doppler Radar, showing reflectivity (left) and winds (right) at 2 kilometers in Hurricane Laura (2020). 

Credit: AOML

Keep in mind that the range of the ground-based system of radars along the East Coast of the United States (and the Caribbean Islands) is limited and only captures hurricanes that are relatively close to land (opens in a new window), making radar data from the NOAA Hurricane Hunters indispensable as an operational forecasting and research tool. Furthermore, data from these airborne Doppler radars are assimilated into some operational forecasting models. In case you want to look at data from current or past hurricanes and tropical storms, the Hurricane Research Division provides an archive of their radar data (opens in a new window), but note that radar data is not available for every storm.

As useful as Tail Doppler Radar data is for analyzing the wind field within a tropical cyclone, it doesn't actually detect the surface wind field (neither does land-based Doppler radar when a storm is close to land, for that matter), which is obviously of great interest to forecasters. In the "good old days", to get a feel for the surface winds, the flight meteorologist applied what could be considered an aviator's version of the Beaufort Wind Scale (opens in a new window). Instead of observing canvas sails in the wind (as Sir Francis Beaufort did), the flight meteorologist estimated wind speeds by the "look" of the sea. Indeed, the appearance of white caps, foam, sea spray, patches of green foam, or streaks in ocean foam offers clues that allow an experienced flight meteorologist to gauge the speed (and direction) of surface winds. A major shortcoming of this approach was that sometimes the weather officer just couldn't see the sea surface (obscured by heavy rain, clouds, darkness, etc.). Furthermore, this approach is somewhat subjective; even when the weather officer could see the ocean surface, its appearance could vary based on the altitude of the flight.

Other Tools for Measuring Surface Winds

Estimating surface winds within a tropical cyclone is a challenging problem, but we aren't just limited to what the flight meteorologist can see on the ocean surface. The development of remote sensing instruments aboard Hurricane Hunter flights has helped forecasters more objectively estimate surface winds. One such instrument is a passive remote sensor called the Stepped Frequency Microwave Radiometer (opens in a new window) (SFMR), which has been in operation since 2008. The underlying principle that the SFMR employs is that the bulk radiative properties of a substance depend on the "nature" of the substance (size, shape, exposed surface area, etc.). By changing the nature of a substance, its radiative behavior changes, too. If that seems odd to you, think about the difference between fog and a glass of water. Both consist of liquid water, yet you can see right through a glass of water, while fog obscures your vision because of the different ways that light scatters off the fog droplets.

Likewise, the nature of a substance can impact the emission of radiation, which serves as the basis for the SFMR's ability to detect surface wind speeds. You may not realize it, but the sea emits some natural microwave radiation (everything does, actually), but these emissions from the sea are not very large. In microwave-cooking terms, for example, you couldn't cook anything using the microwave radiation emitted by the ocean, but I assure you that natural microwave emissions from the sea are detectable by airborne radiometers like the SFMR.

Credit: NOAA

A relatively smooth ocean (winds are relatively light) emits a certain amount of microwave radiation. But, winds blowing over the ocean change the nature of the surface (and thus, its radiative properties). As wind speed increases, patches and streaks of sea foam (essentially, bubbles) start to cover the ocean surface, and it turns out that these patches and streaks of sea foam emit more microwave energy than a smooth, "foamless" sea. The bottom line here is that the SFMR can infer surface wind speeds by detecting increases in microwave emissions from a foamy sea. And, the coverage of sea foam is a function of wind speed (the faster the wind speed, the foamier the sea).

Of course, it's raining to beat the band outside of the eye of a hurricane (particularly in the eyewall), and raindrops certainly would attenuate microwave emissions from the sea (by "attenuate," I mean that raindrops absorb microwave energy from the sea and thus limit the intensity of the energy reaching the SMFR). But the SFMR measures microwave emissions at six different frequencies between 4.6 and 7.2 Gigahertz (hence, the term "stepped frequency"). At any rate, scientists account for the absorption and scattering properties by raindrops at each frequency. By "stepping" through each frequency, scientists can correct for the attenuation of microwave emissions by rain. In the process of correcting for this attenuation, the rainfall-rate can be recovered, yielding bonus data from the SFMR.

I should point out, however, that research has indicated some inconsistencies with surface wind measurements from the SFMR. Wind estimates in very intense hurricanes seem to have a high bias (estimates are too fast). Furthermore, the depth of the ocean in shallow areas near land as well as sea-surface temperatures can impact the behavior of the sea surface and microwave emissions. So, while the SFMR continues to operate on Hurricane-Hunter flights, research to calibrate these measurements is ongoing, and forecasters at the National Hurricane Center question their reliability (and sometimes the data are not even made public). NOAA's Hurricane Hunters are currently experimenting with next generation instruments, such as the Rain, Ocean, Atmosphere Radar System (opens in a new window) (ROARS), which may be able to replace the SFMR for surface wind retrieval from Hurricane Hunter flights, but unlike the SFMR, ROARS is an active remote sensor (it's another specialized radar mounted on the plane). 

What ultimately happens to all the data that Hurricane Hunters collect on their flights? It gets transmitted (in various coded formats) to the National Hurricane Center. Perhaps the most commonly used coded message is the Vortex Data Message (VDM), which focuses on conditions near the core of the storm. These messages contain a wealth of data about the current strength and demeanor of the storm, so we're going to look at them in-depth in the next section. In the meantime, the Explore Further section below contains some links for tracking data from Hurricane Hunters in real time. Check it out, if you're interested.