The Hurricane Hunter Satellites

Whenever a hurricane threatens the US, the Hurricane Hunter aircraft flown by NOAA (National Oceanic and Atmospheric Administration) and the US Air Force Reserve 53rd Weather Reconnaissance Squadron spring into action. Flying repeatedly through these powerful storms at a range of altitudes while deploying dropsondes in key locations, the Hurricane Hunters collect the most accurate data of the conditions inside a hurricane as they approach the American mainland. These data provide vital inputs for various weather models used to predict the future strength and track of a hurricane needed to plan any required response by government authorities.

A NOAA WP-3D Orion weather reconnaissance aircraft (NOAA)

While considered the gold standard for hurricane characterization, the Hurricane Hunter aircraft do have their limitations. The range of the various aircraft employed limit their coverage to regions near the American mainland. And during especially active periods of some hurricane seasons, available resources only allow the more threatening storms to be observed with targets farther away going unobserved thus affecting their forecasting. More importantly for other regions of the globe which also experience other types of destructive tropical cyclones like typhoons, Hurricane Hunter aircraft are not available at all, save for aircraft operated since 2009 by the Government Flying Service Hong Kong monitoring the South China Sea. More data are needed to help meteorologists which is where the Hurricane Hunter Satellites being developed by the Boston-based Tropical Weather Analytics, Inc. (TWA) come into play.

This map shows the tracks and strength of tropical cyclone across the globe over the past 150 years. Indicated are the approximate areas covered by American Hurricane Hunter aircraft the TWA’s Hurricane Hunter Satellites currently under development. Click on image to enlarge. (Adapted from NASA illustration)

A New Approach

The introduction of the first weather satellites in 1960 provided meteorologists with a new tool to observe weather globally, including regular monitoring of the development of tropical cyclones like hurricanes and typhoons for the first time in remote locations of the world’s oceans (see “The First Weather Satellite”). Over the following decades, improvements in the satellites as well as the remote sensing technologies employed by them have allowed scientists to develop new techniques to characterize these storms remotely.

A full-disk image of the Earth acquired from geosynchronous orbit by NASA’s ATS 3 on July 21, 1970 showing Hurricane Becky in the Gulf of Mexico. Click on image to enlarge. (NOAA)

From 1969 to 1984, American meteorologist Vernon Dvorak (National Environmental Satellite, Data, and Information Service – NOAA) developed a technique to estimate the strength of tropical cyclones based on the storm’s morphology as observed by weather satellites. In the decades to follow, the Dvorak Technique has been improved to provide ever more accurate assessments by incorporating more satellite data but, it has never approached the accuracy of characterizations afforded by Hurricane Hunter aircraft. Even in its latest incarnation, the automated Advanced Dvorak Technique can only estimate the central pressure of tropical cyclones to an accuracy of ±15 hPa (hectopascals – the equivalent of millibars frequently use in the US). Still more refined and less subjective techniques under development which incorporate a wider range of satellite remote sensing data, like SATCON being developed by Christopher Velden and Derrick Herndon (Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin – Madison), promise to improve the estimation of tropical cyclone central pressures to the ±6 hPa level leaving more room for improvement.

A new approach to determine the strength of tropical cyclones was formulated in 1994 by Kerry Emanuel (MIT and a member of the TWA Science Advisory Board) and Paul Joss (MIT and a now-retired cofounder of TWA). In a peer-reviewed paper published in 1988, Emanuel estimated the maximum theoretical strength of tropical cyclones by treating these storms as a thermodynamic system known to physicists as a Carnot Engine. Working for Visidyne, Inc. (the corporate antecedent of TWA), Emanuel and Joss found that the strength of a tropical cyclone could be determined using the Carnot Engine model if provided with accurate measurements of the cloud top altitudes in the region around the eye of these storms in combination with the cloud top temperatures, derived from satellite IR measurements, and sea surface temperatures, acquired from a range of sources (see “Using the Carnot Engine Model to Characterize Hurricanes from Orbit“).

Diagram of the RAMOS satellite based on the Russian Yacht universal space platform. Click on image to enlarge. (Khrunichev)

A proof of this concept was incorporated into the Fast Changing Events experiment of the RAMOS mission sponsored by the Missile Defense Agency (see “RAMOS: The Russian-American Observation Satellites”). Using a pair of coorbital satellites, the RAMOS satellites were designed to provide stereographic imagery which would allow the measurement of the absolute altitudes of clouds and other observed atmospheric features to an uncertainty of ±100 meters – about an order of magnitude more accurately than had been demonstrated in earlier satellite-based stereo experiments with clouds. Emanuel and Joss determined that this level of altitude measurement accuracy, in combination with other measurement uncertainties, was sufficient to determine the central pressure of a TC to an accuracy of about ±3.5 hPa (for more details, see “Hurricane Forecasts“). This represented a significant improvement over any other satellite-based method at the time and even today.

Unfortunately, the cancellation of the RAMOS program in 2004 put a hold on demonstrating this new approach. Emanuel, collaborating with Zhengzhao Luo (City College of New York) and other scientists, used observations of nine tropical cyclones from July 2006 to January 2007 acquired by NASA’s Earth Observation System A-Train constellation to provide an initial validation of the Carnot Engine model. With cloud top altitudes provided by the Cloud Profiler Radar (CPR) on CloudSat and cloud top temperature measurements derived from Aqua’s Moderate Resolution Imaging Spectroradiometer (MODIS) IR imagery, Luo and his team found that they could accurately determine the strength of tropical cyclones, especially those rated as Category 3 or stronger on the Saffir-Simpson scale which display a pronounced eye and spiral structure. Subsequent work by Emanuel and others has further validated the Carnot Engine model’s accuracy in characterizing the strength of tropical cyclones.

Here is a sample of A-Train constellation data of Hurricane Ileana in the east Pacific obtained on August 23, 2006 which was used by Luo et al. in their analysis. The upper panel is a MODIS IR image from Aqua with the ground track indicated for the CPR radar data from CloudSat, shown in the bottom panel. Click on image to enlarge. (Luo et al.)

Unfortunately, the only existing instruments capable of making cloud top altitude measurements with the required accuracy, like cloud radars or LIDAR, acquire data only along their ground tracks and it is unlikely that any given tropical cyclone’s eye would be observed during an overpass. It is only by happenstance that a handful of altitude profiles obtained during years of operation passed through the eye of a tropical cyclone to allow an analysis. Satellite-based stereographic imagery could provide accurate cloud top altitude measurements over wide swaths making the regular implementation of the Carnot Engine model practical.

Tropical Weather Analytics’ Hurricane Hunter Satellites

With the Carnot Engine technique developed by Emanuel and Joss validated, the scientists at Visidyne resurrected the idea of using satellite stereography employing a dedicated constellation of satellite pairs to measure cloud altitudes to ±100-meter accuracy. But before the project could proceed, there were a number of practical issues which needed to be addressed by a dedicated observation program. From 2014 to 2019, the scientists who would later become part of the TWA science team used high-resolution photography of tropical cyclones from the International Space Station to address critics’ legitimate concerns.

NASA astronaut Jack Fischer of ISS Expedition 52 shown taking photographs of Typhoon Noru inside the ISS Cupola during the CyMISS photography session on August 1, 2017. The fixed-mounted Nikon camera pointed out the window to his left was being used to take the photographs in support of CyMISS. (JSC-NASA)

Funded by a series of grants from CASIS (Center for the Advancement of Science in Space) which manages the ISS US National Laboratory, NASA’s Tropical Cyclone Experiment was performed by the future TWA science team in support of the CyMISS (Tropical Cyclone intensity Measurements from the ISS) to focus on two issues: Characterize the scene structure near the eye of tropical cyclone on a spatial scale of 100 meters per pixel or better and observe how that scene structure changes on timescales of up to a couple of hundred seconds. No other source of satellite imagery could fully address these issues simultaneously with the required spatial and temporal resolution. Using a special photography protocol developed by the Visidyne team in cooperation with the ISS ground support organization at NASA Marshal Space Flight Center and Johnson Space Center, about 80 tropical cyclones were observed during the five-year CyMISS project (see the Gallery for CyMISS 2014-2019 for a sample of images).

This is an anaglyphic 3D mosaic (left eye red, right eye blue) of Typhoon Noru as seen from the ISS at about 07:27 GMT on August 1, 2017. It was created by combining various parts the 240 images which have been processed to approximate a common overhead view covering an area of about 1,350 by 900 kilometers. Click on the image for more details. (A.J. LePage/Visidyne/JSC-NASA)

In addition to accumulating high-quality image sequences to be used for stereographic imaging software development, the project demonstrated that commercially available digital cameras had sufficient dynamic range to record cloud scene details down to the 100-meter scale or finer and that those cloud features do not change appreciably over the course of a couple of minutes, even in the dynamic environment of a tropical cyclone. With the algorithm being developed to address various practical issues of data collection and processing identified during the RAMOS era, not only can accurate measurements of the absolute cloud altitudes be made for use in a Carnot Engine model for tropical cyclones, but also the winds in all three dimensions are measured as a natural byproduct of the processing based on the apparent motions of those clouds as viewed by the two satellites – an outgrowth of the Wind Velocity Distribution experiment from the RAMOS program for which this author was the Principal Investigator (and, for full disclosure, is now the Chief Scientist of TWA as well as one of the company’s cofounders). It was quickly realized that 3D wind measurements, including unique determinations of vertical winds, to an accuracy of about one meter per second not only could be used to help in the forecasting of tropical cyclones, but had a range of other important meteorological applications including tracking the polar vortex to a previously unprecedented degree.

An explanation of the importance of the polar vortex as a driver for winter weather. Click on diagram to enlarge. (NOAA)

In 2016, Tropical Weather Analytics, Inc. (TWA) was founded with the goal of launching a constellation of pairs of miniaturized satellites using off-the-shelf technology to acquire the unique, wide field-of-view (FOV) image sequences required for precision stereography (see “Tropical Weather Analytics Launched to Provide the Most Accurate Hurricane Measurements from Space”). After many iterations of design studies, it was found that pairs of 12U CubeSats with a mass of about 20 kilograms each could meet the requirements. A constellation of up to five satellite pairs operating in different orbital planes will provide observations with sufficient frequency to meet the needs of a range of customers. In order to minimize the size, cost and complexity of the satellites and their instruments, the initial generation of Hurricane Hunter Satellites will use off-the-shelf, space-rated cameras operating at visible wavelengths limiting their operations to daylight hours.

A CubeSat of the type that will be employed for TWA’s Hurricane Hunter Satellite constellation. (Nanoavionics)

Each Hurricane Hunter Satellite coorbital pair will be placed into a 500-kilometer, high inclination orbit to allow stereographic imaging of almost any point on the globe during daylight hours. In order to maintain the desired 300-kilometer separation between each satellite pair chosen to optimize stereo imagery (as well as counter the effects of orbit decay), each Hurricane Hunter Satellite will be fitted with a small propulsion system. For their primary instrument, each satellite will carry an array of 2D visible-band cameras with nadir image scale of around 100 meters per pixel to yield a total FOV of about 30° by 120°, with the long axis oriented perpendicular to the satellites’ ground track. With the FOVs of the satellite pair directed to the midpoint on the ground between the subsatellite points during an observation session, sequences of overlapping stereo images will be acquired up to about 1,000 kilometers from the ground track. The current design goal is that each satellite pair will be capable of making stereo observations over a swath about 2,000 kilometers wide and 2,000 kilometers long and do so at least 12 times each day. The actual numbers will depend on the results of engineer trade studies currently underway based on the cameras selected, satellite downlink capabilities, power requirements and other considerations.

A schematic representation of a pair of TWA’s Hurricane Hunter Satellites making stereo observations of a hurricane. Click on image to enlarge. (TWA)

Data Products

Commercial satellite imagery is hardly new but, the Hurricane Hunter Satellite does offer unique capabilities. Many of the commercial providers of satellite imagery have concentrated on very high-resolution data products covering areas just a couple of tens of kilometers across with image scales of meters per pixel or less. But this is analogous to observing the surface of a basketball with a microscope. For meteorological studies, a wider view of the basketball’s surface is needed. The wide-field-of-view, 100-meter-class images from the Hurricane Hunter Satellites provide a broader, regional view of the clouds and surface below for meteorology and other potential applications. Here is a summary of some of the data products TWA anticipates providing:

Cloud Cover Imagery

Operating in a stereo imagery mode, the Hurricane Hunter Satellites will allow the creation of 2D and 3D cloud cover images for weather analysts. The goal is to produce such images of targeted regions covering 2,000 kilometers on a side with a nominal 100-meter pixel footprint and the line-of-sight geolocated to better than 100 meters with respect to the reference geoid. The closest competitor for this type of data product is the visible band imagery from the Visible Infrared Imaging Radiometer Suite (VIIRS) carried by NOAA’s Suomi NPP and the new Joint Polar Satellite System (JPSS) satellites like NOAA-20 which yields image swath widths of 3,060 kilometers and a pixel footprint of 375 meters at nadir. The Hurricane Hunter Satellite imagery has almost four times that resolution.

This mosaic shows Super Typhoon Atsani as viewed from the ISS on August 19, 2015 in support of CyMISS. Covering an area of 2000 by 1500 km at a scale of 500 m/pixel, this image approximates the cloud coverage image expected from the Hurricane Hunter Satellites (which will be a third again as wide with up to 5 times the resolution) Click on the image to view a full resolution version. (A.J. LePage/TWA/NASA-JSC)

Polar orbiting satellites using the older Advanced Very-High-Resolution Radiometer (AVHRR) like NOAA’s earlier generation POES satellites or Europe’s MetOp constellation has a swath width of 2,500 kilometers with a pixel footprint of 1,100 meters – an order of magnitude coarser than the Hurricane Hunter Satellites. With NOAA’s and Europe’s polar orbiting weather satellites clustered in planes which cross the equator at about 7:30 AM or at 1:40 PM, their imagery is likewise clustered in the early morning or early afternoon for any given location during daylight hours. With up to five satellite pairs in the Hurricane Hunter Satellite constellation distributed across a range of crossing times throughout the day, any given location on the globe can be observed up to 2½ times more frequently. And the 3D imagery from TWA will be unique.

Cloud Altitude Maps

Along with the cloud coverage imagery, cloud altitude maps of the 2,000-kilometer square region recorded will be produced. These maps will have a spatial footprint as small as 300 to 500 meters, altitude uncertainties of ±100 meters with a typical geolocation accuracy of 100 meters or better. Combined with other sources of data, these cloud altitude maps will provide vital inputs for weather forecasting models.

One notable competitor for this data product is cloud altitude maps produced using data from NOAA’s current generation of GOES weather satellites. While these data cover wide areas, the typical ground footprint of available cloud top altitude data products is about 10 kilometers with altitude accuracies of no better than 500 meters (and typically much larger at higher altitudes). Higher altitude accuracy is possible with LIDARs such as the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) carried on CALIPSO sponsored by NASA and the French space agency, CNES. But these data are limited to measuring cloud heights only along the satellite’s ground track. The Hurricane Hunter Satellites offer a unique combination of spatial footprint, altitude accuracy and wide area coverage to provide vital input data for weather models. These fine-scale data for key parts of the globe promise to provide a significant improvement in weather forecasting models.

Wind Data

As mentioned earlier, another important set of measurements to be provided by TWA using Hurricane Hunter Satellite data are 3D wind data including unique measurements of vertical winds which are important for characterizing convective weather phenomena like powerful thunderstorms. Like the cloud altitude maps, these data will have a spatial footprint as small as 300 to 500 meters, altitude uncertainties of ±100 meters with a typical geolocation accuracy of 100 meters or better. Unlike other sources of wind data, the 2,000-kilometer square region being observed by the Hurricane Hunter Satellites will provide hundreds of thousands 3D wind data points with an accuracy of one meter per second or better – many orders of magnitude more data than is provided by existing ground, air or space-based instruments.

A sample Derived Motion Winds data product from NOAA’s GOES-East satellite. The winds are divided into three broad altitude bins: yellow for low-level (<3 km), cyan for mid-level (3 to 7 km), and red for high-level (7 to 14 km). Click on image to enlarge. (NOAA)

The closest existing analog to TWA’s wind data product is NOAA’s Derived Motion Winds generated using multispectral imagery from the Advanced Baseline Imager (ABI) carried by the newest generation of GOES satellites. Like TWA’s approach, the Derived Motion Winds uses the apparent motion of cloud features over time to determine the wind speeds. However, these data processing algorithms assume only horizontal cloud motion and have a comparatively coarse ground footprint of no better than ten kilometers with a wind velocity accuracy of 7.5 meters per second. With the cloud altitudes inferred from sounding data, the altitude uncertainties range from two kilometers near the surface to as much as 7 kilometers near the top of the troposphere. As with the cloud altitude maps, the fine-scale, 3D wind data to be provided by the Hurricane Hunter Satellites for key parts of the globe promises to provide a significant improvement in weather forecasting models.

Tropical Cyclone Characterization

Finally, the data product which inspired the Hurricane Hunter Satellite concept will be an accurate characterization of the strength of tropical cyclone using the Carnot engine model. This information, along with high resolution cloud coverage imagery, cloud altitude maps, and especially 3D wind measurements will serve as vital inputs in the future for models used to predict the track and strength of tropical cyclones. TWA’s Hurricane Hunter Satellites promise to add a new dimension to weather forecasting in general.

More information on TWA and its Hurricane Hunter Satellites can be found on the TWA website.

General References

Kerry A. Emanuel, “The Maximum Intensity of Hurricanes”, Journal of Atmospheric Science, Vol. 45, No. 7, pp. 1143-1155, April 1988 [Paper]

Paul Joss, “The Cyclone Intensity Measurements from the ISS (CyMISS)”, Space Station Research Explorer, NASA, September 2014 [Web page]

Zhengzhao Luo, Graeme L. Stephens, Kerry A. Emanuel, Deborah G. Vane, Natalie D. Tourville, and John M. Haynes, “On the Use of CloudSat and MODIS Data for Estimating Hurricane Intensity”, IEEE Geoscience and Remote Sensing Letters, Vol. 5, No. 1, pp. 13-16, January 2008 [Paper]

Christopher S. Velden and Derrick Herndon, “A Consensus Approach for Estimating Tropical Cyclone Intensity from Meteorological Satellites: SATCON”, Weather and Forecasting, Vol. 35, No. 4, pp. 1645-1662, August 2020 [Paper]

Vernon F. Dvorak, “Tropical Cyclone Intensity Analysis and Forecasting from Satellite Imagery”, Monthly Weather Review, Vol. 103, No. 5, pp. 420-430, May 1975 [Paper]

Vernon F. Dvorak, “Tropical Cyclone Intensity Analysis Using Satellite Data”, NOAA Technical Report NESDID 11, September 1984 [Report]