Eye Through the Hurricane
As its 235-km-per-hour winds passed the Bahamas, Frances was heading for landfall on the Florida coast some time on Saturday, and three quarters of a million Americans are in the process of evacuating their homes. To wait and watch for Frances might be suicidal for human beings, but space-based observers such as Envisat observe its passage without danger.
"Because of Envisat's multi-sensor capability, we can slice right through the hurricane with just a single satellite," explained José Achache, ESA Director of Earth Observation Programmes.
"Effectively Frances is taken apart for meteorologists to study. The data returned by Envisat includes cloud structure and height at the top of the hurricane, wind and wave fields at the bottom, sea surface temperature and even sea height anomalies indicative of upper ocean thermal conditions that influence its intensity."
Important processes occur at a range of altitudes and locations throughout a hurricane - basically a large powerful storm centred around a zone of extreme low pressure.
Envisat carries both optical and radar instruments, enabling researchers to observe high-atmosphere cloud structure and pressure in the visible and infrared spectrum, while at around the same time using radar backscatter to measure roughness of the sea surface and so derive the wind fields just over it.
Those winds converging on the low-pressure eye of the storm are what ultimately determine the spiralling cloud patterns that are characteristic of a hurricane.
Florida-based scientists have begun to take advantage of this unique single-spacecraft combination of instruments - the Medium Resolution Imaging Spectrometer (MERIS) and Advanced Synthetic Aperture Radar (ASAR) - as hurricane season gets into full swing.
The University of Miami's Centre for Southeastern Tropical Advanced Remote Sensing (CSTARS) ground station has an agreement to acquire ASAR and MERIS data direct from Envisat, with ERS-2 wind scatterometer data set to follow in the near future. Their access to Envisat data has come just as the second hurricane in less than a month is heading towards the Florida coast.
"With MERIS and ASAR, Envisat can image both the ocean and atmosphere pretty much simultaneously, which is a very useful capability during hurricane season," said Hans Graber, Professor of Applied Marine Physics at the University of Miami and Co-Director of CSTARS.
"Specifically in terms of Frances, the eye of the hurricane seems to be rolling a lot right now from the top of the clouds, looking quite unstable, the information from an ASAR image should help localise its size and position on the ocean," Graber said. "And wind fields around the eye wall can be derived from ASAR data - right now all we have to go on are measurements from the hurricane hunter planes that fly right through the storm."
Simultaneous MERIS and ASAR acquisitions are planned for Friday by CSTARS, even as the storm comes closer to predicted landfall the following morning.
"Our current activity is along the lines of a shakedown - we're investigating how this can be used," added Graber. "Our final goal is to get this working on an operational basis during hurricane season. We have a deal to use radar data from the Canadian Space Agency, and also have access to other satellite resources for high temporal coverage of the affected region.
"The potential is there to extract a large amount of useful information which can help the US National Hurricane Center increase the accuracy of their hurricane predictions and reduce danger to the public."
Another instrument aboard Envisat is being used to take the temperature of Frances, both down at the surface of the ocean and at the heights of its towering clouds.
"We produced a combined AATSR sea surface temperature and cloud top temperature image, which shows the sea surface temperature to be as high as 29ºC in the area," remarked Carsten Brockmann of Brockmann Consult, a German company processing both MERIS and AATSR hurricane imagery. "This two-sensor combination gives meteorologists a lot of information to help them understand the dynamics of the hurricane and better predict its development."
AATSR information can be correlated with MERIS data cloud height and development to gain a good estimate of the hurricane's precipitation potential, and improve understanding of how this relates to its overall intensity. Condensation of water vapour releases latent heat, which warms the vicinity of the hurricane eye. This in turn evaporates more surface water and feeds the heat engine powering the hurricane.
What hidden depths fuel the storm?
Oceanic features, such as warm core rings, eddies, and the Gulf Stream, represent a source of enhanced heat fluxes to the atmosphere that may cause the strengthening of tropical cyclones, such as hurricanes.
Several hurricanes have intensified when their tracks pass over eddies or other masses of warm water with high TCHP values.
For example, in 1995 Hurricane Opal suddenly intensified in the Gulf of Mexico after passing over a warm ring with TCHP values of up to six times the threshold to sustain a tropical cyclone.
Previously, researchers used sea surface temperature alone to estimate the role of the upper ocean thermal conditions on hurricane intensification. The problem with this is that the sea surface temperature measured by AATSR or comparable satellite instruments may not by themselves show these warm upper ocean features, particularly during summer months in tropical regions.
Tropical cyclone wind forces easily erode this thin upper layer by mixing the upper waters to depths that may go down to 100 meters, giving the tropical cyclones the potential to absorb ocean thermal energy, if conditions are appropriate. Now, estimates of TCHP based on satellite observations of sea surface temperature and sea surface height can detect these features.
"These water features are critical for identifying regions of high TCHP values that may potentially contribute to the intensification of a hurricane", Goni explained. "These regions of high TCHP values provide the hurricanes with the opportunity to absorb much more thermal energy if overall conditions are right. My research is taking advantage of the fact that these warm water masses cause an upward elevation in ocean height of up to 30 cm. Such sea height anomalies can then be mapped with space-based radar altimeter data."
The US Naval Research Laboratory (NRL) combines Envisat RA-2 data with data from similar radar altimeters aboard the Jason-1 and GFO satellites to enhance overall accuracy and spatial and temporal coverage, forming the source for altimetry products which, in turn, form the basis for NOAA/AOML-produced maps of tropical cyclone heat potential depicting the upper ocean thermal conditions, shown here overlaid against Hurricane Frances' track so far.
"At this time I use this product only for research purposes, providing an enhanced understanding of the life of a hurricane. However, analogous products are being produced and used operationally for forecasting by the National Hurricane Center", Goni concluded.
Altimetry-based wind speed and wave height products are also distributed by the French firm Collecte Localisation Satellites (CLS), and can reveal sea surface features related to the presence of hurricanes.