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Upon finishing this page, you should be familiar with the operations of the Air Force Hurricane Hunters program. Specifically, you should be able to identify their general flight area and flight range, and identify the data contained in the lines of the main coded report that they transmit -- the Vortex Data Message (VDM).
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In this section, we're going to focus on the activities of the Air Force Hurricane Hunters, but did you know that Hurricane Hunters are not the only aviators that contribute to weather analysis and forecasting? As you've learned, the data collected by radiosondes aboard weather balloons contribute to the constant pressure analyses that you're accustomed to (at 500 mb or 300 mb, for example). But, the data captured by instruments on weather balloons is also supplemented by in-situ observations taken by commercial jets. Recall that the standard height of the 300-mb surface is 9,000 meters -- roughly 30,000 feet, which is a representative altitude where commercial aircraft often cruise.
How can we tell what data is contributed by commercial aircraft? Check out the DiFax 300-mb analysis from 00Z on December 11, 2003 from the National Centers for Environmental Prediction (below). DiFax maps are being used less and less these days, but they were once the standard format for common weather analyses and forecasts.
On the sample DiFax 300-mb analysis above, the station models designated by circles represent radiosonde observations aboard weather balloons. These observations are supplemented by observations from commercial aircraft (square station models colored red for emphasis). Aircraft routinely measure temperature, wind direction, wind speed, and altitude expressed in hundreds of feet (check out a template for the upper-air station model corresponding to an observation taken by commercial aircraft). For example, the aircraft observation over the Gulf of Mexico (lower right) indicates a westerly wind (approximately 270 degrees) blowing at 45 knots and a temperature of minus 38 degrees Celsius. The aircraft was cruising at an altitude of 33,000 ft.
Aircraft observations over coastal Atlantic and Pacific waters also appear on 00Z and 12Z DiFax analyses. Of course, aircraft take observations at other times, too. Indeed, the Aircraft Meteorological Data Relay (AMDAR), and the Aircraft Communication Addressing and Reporting System (ACARS) in the U.S. continuously collect digital communications from commercial aircraft. Not only do AMDAR and ACARS observations show up on DiFax analyses, but they're also incorporated into the initialization of some numerical weather prediction models.
But, observations from commercial aircraft are not enough to fully cover the tropics, obviously. To compensate, meteorologists incorporate satellite-derived winds (wind speeds and directions estimated by satellite at specified altitudes), which we'll cover later in this lesson. Some of these satellite-based observations, however, can be seen in the DiFax analysis above by station models designated by a star (highlighted in green for emphasis). You should also note that the indicated wind speed and wind direction represent the only weather data on station models based on satellite measurements.
To get back to the topic at hand (aircraft observations over the tropics), I point out that commercial and private aircraft prudently fly around big storms. However, a group of intrepid aviators in the U.S. Air Force Reserve, popularly known as "Hurricane Hunters," are available to fly reconnaissance missions into tropical cyclones whenever they develop. During the off-season, they also fly into fierce winter storms that rage along the Atlantic and Pacific Coasts. By the way, NOAA has its own Hurricane Hunters, and I'll talk more about them later in this lesson. To see what the Hurricane Hunters are up to on any given day, check out the Tropical Cyclone Plan of the Day.
Stationed at Keesler Air Force Base in Biloxi, Mississippi, the Hurricane Hunters formally belong to the 53rd Weather Reconnaissance Squadron. During hurricane season, the squadron is ready to spring into action at any sign of a tropical cyclone developing in the region spanning approximately from the mid-Atlantic Ocean (longitude 55 degrees West) to the Caribbean Sea and the Gulf of Mexico. Hurricane Hunters also fly reconnaissance into tropical cyclones over the central and eastern Pacific Ocean, particularly those that might pose a threat to Hawaii or mainland North America. Hurricane Hunters rely on the durable WC-130-J aircraft (from the class of WC-130 aircraft) equipped with an arsenal of weather instruments to monitor tropical cyclones. As an overall package, this instrumentation is called the Improved Weather Reconnaissance System (IWRS).
Flying into the storm
When Hurricane Hunters enter a tropical cyclone, they typically fly an alpha pattern. After flying the first diagonal across the storm (usually at least 105 nautical miles (120 statute miles) on either side of the center), executing a successful alpha pattern amounts to simply making a series of left-hand turns. In this way, WC-130 never flies directly into the teeth of the wind (remember that northern hemispheric low-pressure systems have a counterclockwise circulation). Avoiding the strong direct headwinds allows the aircraft to save fuel and fly longer missions. Moreover, the aircraft collects data in all four quadrants of the storm after making only two passes through the center. The aircraft passes through the center about every two hours and continues the pattern until the next WC-130 is ready to take its place if NHC wants fixes on the storm every six hours and "round-the-clock" surveillance. If NHC wants fixes on the storm less frequently (every 12 or 24 hours, for example), then there's no immediate replacement aircraft when the mission is complete (each mission lasts roughly eight hours, on average).
I should note here that the range of reconnaissance aircraft varies from 2,200 to 3,600 miles (the range depends, in part, on flight altitude). Thus, newly forming tropical cyclones over the eastern and central Atlantic Ocean are, for all practical purposes, out of range for reconnaissance aircraft. In its place, tropical forecasters rely on remote sensing from satellites to assess the intensity and structure of storms (more to come later in this lesson).
Special weather instruments mounted on the WC-130 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). 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 (we'll explore one in just a moment).
For most of the missions flown into hurricanes, the standard flight level is 700 mb (recall that the standard 700-mb height is 3,000 meters). When forecasters at the National Hurricane Center spot a suspicious cluster of tropical showers and thunderstorms on satellite imagery, Hurricane Hunters will fly a Low-level Investigative Mission at 500 or 1500 feet above the sea surface. At such altitudes, wind data can reveal a closed, low-level circulation that allows forecasters to upgrade the system to a tropical depression. As the depression develops into a tropical storm, Hurricane Hunters typically increase the flight level to 850 mb (recall that the standard 850-mb height is 1,500 meters). As the tropical cyclone further intensifies, Hurricane Hunters increase their flight level to 700 mb (the conventional maximum flight level inside hurricanes). I should note here that Hurricane Hunters fly at higher altitudes on other missions (such as reconnaissance in winter storms).
Vortex Data Messages (VDMs)
Observations from the eye are transmitted to the National Hurricane Center using several specifically coded messages. Perhaps the most commonly used is the Vortex Data Message (VDM), which focuses on conditions near the core of the storm. To see an example of a VDM along with basic descriptions of some of the main blocks of data, check out the annotated VDM below. This particular VDM tabulated the vital signs of Hurricane Otto on the morning of November 24, 2016.
Coded observations in the Vortex Data Message allow the National Hurricane Center to assess the current strength and demeanor of the storm, which, in turn, help to increase the accuracy of their forecasts. Note in the annotated VDM above that much of the information relates to the characteristics of the center of the storm, the maximum winds observed while flying inbound, and the maximum winds observed while flying outbound. However, this annotated VDM really doesn't tell you the specifics of how to translate each item (units, what specific codes mean, etc.). Not to worry, though. We'll cover those important details in the next section. During hurricane seasons over the Atlantic and eastern Pacific, you can access the current Vortex Data Message at the National Hurricane Center. There's also a link for an archive of reconnaissance messages, just in case you're interested.
Vortex Data Messages aren't the only coded messages disseminated by Hurricane Hunters, however. For more on other coded messages from the Hurricane Hunters, check out the Explore Further section below.
The Use of 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 (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.
Once Hurricane Hunters release dropwindsondes in the eyewall of hurricanes (where the surface winds are strongest), the free-falling sonde deploys a drogue parachute immediately, which stabilizes the sonde's descent by stopping it from tumbling in the turbulent air motions within the eyewall. During descent, the in-situ sensors on the dropsonde (see image below) 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, in an average hurricane season, Hurricane Hunters release approximately 1,000 to 1,500 dropsondes on training and storm-reconnaissance missions.
Technically, the method for measuring wind speed using dropwindsondes qualifies as remote sensing. No, I haven't lost all my marbles. 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.
Wind speed measurements aren't the only remote sensing done by Hurricane Hunters, however. They also use radar to locate the eye and eyewall as well as other tools for measuring surface wind speed, among other things (more later). In the meantime, on the next page, we'll explore the details of how to translate a Vortex Data Message -- perhaps the most widely-used coded message relayed by the Hurricane Hunters.
Explore Further...
Other Coded Messages
In order to extract relevant information from a Vortex Data Message, you need to be able to decode it, but VDMs are not the only coded messages transmitted by Hurricane Hunters. For starters, all of the dropwindonde observations that you learned about above are transmitted in code. Furthermore, Air Force Hurricane Hunters also transmit coded reports called RECCO observations. By way of background, each reconnaissance flight generates many of these "spot reports", which, as a general rule, convey meteorological conditions at a single position inside the storm or in the vicinity of the storm. These spot reports can be intriguing because they sometimes correspond to positions where maximum winds are observed.
If you're into interpreting raw data from dropwindsondes or following RECCO observations, you can get both in real-time from this Web page at the National Hurricane Center. However, you'll need this guide for decoding RECCO observations and this guide for decoding dropsonde (and other reconnaissance) observations in order to make use of the data.