When hurricane Katrina hit the US coast in August 2005, destruction and desolation followed in its wake.
21 April 2022
29 August 2005
By Jochen Kerkmann, Francois Parisot, Marc Jenner and Livia Briese (EUMETSAT)
Hurricane Katrina was the eleventh named storm and fifth hurricane of the 2005 Atlantic hurricane season. It was the costliest natural disaster, as well as one of the five deadliest hurricanes, in the history of the United States. Overall, at least 1,833 people died as a result of the hurricane and subsequent floods, and total property damage was estimated at $108 billion. (Source: Wikipedia )
Background and history
Hurricane Katrina was first classified as a Tropical Depression on 23 August 2005, when it was located between the Bahamas and eastern Cuba (see Track of hurricane Katrina, source: NOAA/CIMSS). Within two days it developed into a category 1 hurricane (on the Saffir-Simpson Hurricane Scale, i.e. mean wind speed between 115 and 147km/h), moving towards Florida where it made landfall on 25 August (Figure 1).
After crossing southern parts of Florida it moved into the Gulf of Mexico, first in a south-westerly, and then in a north-westerly, direction. Picking up energy from the warm waters of the Gulf, it quickly developed into a deadly category 5 hurricane (i.e. mean wind speed of more than 260km/h), which is the maximum intensity classification.
Unlike hurricane Isabel in September 2003 (also a category 5 hurricane), which degraded to a category 2 hurricane before making landfall in Northern Carolina and Virginia, hurricane Katrina maintained its full force making landfall over Grand Isle (Mississippi River Delta) at about 10:00 UTC on 29 August with winds reaching 250 km/h (category 4 hurricane).
The MODIS image from 17:00 UTC on 28 August shows the fully developed hurricane 17 hours before landfall (Figure 2). The image shows the very symmetric cloud structure of this huge hurricane and the relatively large eye, with the eyewall and the Central Dense Overcast (CDO) region.
It is well known that hurricanes require warm water, and, often, a threshold value of 26–28°C is mentioned in the literature as the minimum sea surface temperature needed for hurricane formation. However, the SST is very variable (e.g. it changes rapidly due to mixing processes) and it does not appropriately represent the heat content that is stored in the upper ocean.
A much more reliable dataset for the forecast of the intensity of hurricanes is altimeter data (as provided by Jason-1 and Topex/Poseidon, and, in future, by Jason-2, see Figures 3-6) .
Altimeter instruments measure the height of the ocean surface, or the height anomaly as compared to a reference height (Figure 3). The ocean height is mainly related to its internal thermal structure, i.e. it represents a vertically integrated measurement of the density, and, thus, the temperature of the ocean: the warmer the ocean, the higher the ocean surface.
Therefore, while sea surface temperatures (SST) derived from instruments such as AVHRR represent the temperature of the top (skin) layer of the ocean, from altimeter measurements one can derive information about the ocean temperature at deeper layers. An example is given below (Figure 4). It shows the depth (in metres) of the 26°C isotherm, which is a critical parameter for the forecast of hurricane formation/intensity, since it is much closer related to the heat stored in the upper layers of the ocean. The rule of thumb based on this parameter is: hurricane formation is possible when the sea temperature is above 26.5°C to at least a depth of 50m.
In the case of hurricane Katrina, a warm anticyclonic ring with an average depth of the 26°C isotherm of 90m was observed in the central part of the Gulf of Mexico (Figure 4). When hurricane Katrina reached this warm ring it rapidly intensified from a category 3 to a category 5 hurricane (as indicated by the coloured circles that mark the track of the hurricane). The ring of warm water is not visible in the SST field (Figure 5), because it is masked by the very thin, warm and stable upper layer formed in the Gulf of Mexico (with sea surface temperatures up to 32°C). This example demonstrates that altimeter-derived fields such as the depth of the 26°C isotherm or the Tropical Cyclone Heat Potential (not shown here) are essential for the forecast of the intensity of hurricanes. For further details, read GOM Surface Dynamics Reports from 29 August 2005, from G. Goni.
Note: The Sea Height Anomaly (SHA) field used is from the Naval Research Laboratory (NRL) based on Jason-1, GFO and Envisat. The SST field comes from TMI data and is produced by Remote Sensing Systems, which are sponsored by the NASA Earth Science REASoN (Research, Education and Applications Solution Network)) DISCOVER Project. The data is available at www.remss.com .
Colour-enhanced GOES IR image (29 Aug. 2005, 12:45 UTC, source: NOAA/CIMSS)