Re: Satellite Altimetry and the Intensification of Hurricane Katrina
- --- In ClimateArchiveDiscussion@yahoogroups.com, "Mike Neuman"
This study shows that it was the deep depths of the warm water cores
in the Gulf over which Katrina and Rita passed which most
significantly caused their rapid development into Category 5
hurricanes. It wasn't just the warm surface waters that causes the
rise in the intensity of the hurricanes.
--- In ClimateArchiveDiscussion@yahoogroups.com, Tim Jones
> Satellite Altimetry and the Intensification of Hurricane Katrina--- End forwarded message ---
> Eos, Vol. 86, No. 40,
> 4 October 2005
> PAGE 366
> Remotely sensed infrared images of Hur-
> ricane Katrina taken on 26, 27, and 28 August
> 2005 (Figure 1, left panels) show the aerial
> extent of the cloud cover and the central "eye"
> increasing as the storm that swamped areas of
> the U.S. Gulf Coast intensified. Computer ani-
> mations of such image sequences show fore-
> casters the tracks of storms and are a familiar
> staple of weather news. Less well known is the
> role that satellite altimetry plays both in fore-
> casting conditions that can intensify a tropical
> storm and in observing the storm conditions
> at the sea surface.
> Satellite altimeter data indicate that Katrina
> intensified over areas of anomalously high
> dynamic topography rather than areas of un-
> usually warm surface waters.Altimeter data
> from Katrina also for the first time observed
> the building of a storm surge.
> Radar altimeters on board several satellites
> measure wind speed, wave height, and sea
> level over small (2 to 5 km radius) overlap-
> ping patches sampled at a sequence of points
> along each satellite's ground track.The altim-
> eters use Ku-band radar that penetrates clouds
> and most rain, so that the dataare available in
> nearly all weather conditions.Profiles of mea-
> sured wind speed, wave height,and sea level
> along tracks passing through Katrina show the
> intensification of winds and waves associated
> with the growth of the storm and the wind-
> driven storm surge that hit the Gulf Coast (Fig-
> ure 1, right panels).
> Significant wave height (SWH) is defi ned as
> the peak-to-trough height of the largest third
> of the waves.Waveheights are largest at the
> point where each satellite track makes its clos-
> est approach to the eye of the storm, and they
> decrease symmetrically away from that point;
> the peak value of five m on 26 August had in-
> creased to 10 m on 27 August.
> Near-surface wind speed is inferred from the
> power returned to the satellite after the scat-
> tering of the radar signal at the ocean surface.
> The wind speed profi les show a slightly asym-
> metric distribution with maxima slightly south
> of the wave height maxima, and increasing
> from 17 to 25 m/s over the three days shown.
> The sea level measured by an altimeter is
> the combined effect of geoid undulations,
> tides, the ocean's response to meteorological
> forcing, and the dynamic ocean topography
> associated with currents in geostrophic bal-
> ance. The rightmost panels of Figure 1 show
> the residual sea level anomaly after removing
> the geoidal, tidal, and geostrophic signals as
> well as an inverse barometer response to at-
> mospheric pressure changes.The bottom right-
> most panel, from the Geosat Follow-On (GFO)
> altimeter, shows sea level windward of the eye
> rising toward the shoreline and reaching 90
> cm at the coast.This apparently is the wind-
> driven storm surge.To the authors' knowledge,
> this is the first observation of a storm surge by
> Popular press accounts [e.g., Kolbert,2005;
> Kristoff,2005] suggest that warm ocean surface
> waters intensified Katrina, but sea surface
> temperatures were around 30°C almost ev-
> erywhere along Katrina's path through the
> ocean (Figure 2a). If intensification was driven
> predominantly by sea surface temperature,
> Katrina would have strengthened gradually
> over time.
> Instead, Katrina intensified most rapidly
> when she was over areas of anomalously high
> dynamic topography, as measured by altim-
> eters (Figure 2b). Katrina intensified first over
> a warm-core eddy east of Florida as she grew
> from a tropical depression to a Category 1
> hurricane.Then, over the Loop Current and a
> warm-core ring in the Gulf of Mexico, she inten-
> sified from Category 1 to Category 5.
> These dynamic topography highs are a proxy
> for the vertically integrated heat content within
> the water column.The depth of the warm water
> pool, and not merely the temperature at the
> surface, provides the reservoir of energy to
> intensify a storm [Shay et al.,2000]. Later, Hur-
> ricane Rita also intensified to Category 5 over
> the same deep warm pool in the Gulf. However,
> Rita weakened afterwards when passing over
> an area of low dynamic topography near the
> Texas coast.
> Since the dynamic topography changes only
> slowly over weeks, altimeter data collected
> long in advance of a hurricane can be used to
> forecast the potential for intensifi cation [Goni
> and Trinanes,2005].
> The observed increase in intensity of hur-
> ricanes over the last decades has been corre-
> lated with increasing sea surface temperatures
> [Emanuel,2005;Trenberth, 2005].The example
> from Katrina suggests that any increase in
> hurricane intensity may be more directly cor-
> related with variations in the thickness of the
> warm layer.
> Radar altimeter data are from the NASA/
> Centre Nationale d'Études Spatiales TOPEX
> and Jason 1 altimeters, the European Space
> Agency ERS-2 and Envisat altimeters, and
> the U.S. Navy GFO altimeter. Data were
> assimilated and analyzed through the Radar
> Altimeter Database System (RADS).The U.S.
> National Oceanic and Atmospheric Adminis-
> tration's (NOAA) Geostationary Operational
> Environmental Satellite GOES 12 provided
> the infrared images of clouds, and the in-
> frared sensors on NOAA Polar Operational
> Environmental Satellites (POES) were used
> to construct the sea surface temperature
> field.Thanks to Eelco Doornbos (Delft Uni-
> versity of Technology,The Netherlands) for
> computing ERS-2 orbits. Remko Scharroo was
> supported by NASA grant NRA-03-OES-05.The
> views expressed here are solely the opinions
> of the authors and do not constitute a state-
> ment of policy, decision, or position on behalf
> of NOAA or the U.S. Government.
> Emanuel, K. (2005), Increasing destructiveness of
> tropical cyclones over the past 30 years,Nature,
> Goni, G., and J.Trinanes (2005), Gulf of Mexico
> surface dynamics reports, NOAA Atl. Oceanogr.
> and Meteorol. Lab., Miami, Fla., 29 Aug. (Available
> at http://www.aoml.noaa.gov/phod/altimetry/
> Kolbert, E. (2005), Storm warnings,New Yorker,
> 19 Sept.
> Kristoff, N. (2005),The storm next time,N. Y.Times,
> 11 Sept.
> Trenberth, K. (2005), Uncertainty in hurricanes and
> global warming,Science, 308, 1753-1754.
> Shay, L. K., G. J. Goni, and P. G. Black (2000), Effects of
> a warm oceanic feature on Hurricane Opal, Mon.
> Weath. Rev., 128,1366-1383.
> ---REMKO SCHARROO, Altimetrics LLC, Cornish,
> N. H.; and WALTER H. F. SMITH and JOHN L. LILLIBRIDGE,
> NOAA Laboratory for Satellite Altimetry, Silver Spring,
> For additional information, contact R. Scharroo;
> E-mail: remko@a...
> See pdf.
> Fig. 1.The left column shows a comparison of GOES
> 12 infrared images and altimeter data collected
> by (top) Jason 1 and TOPEX, (middle) Envisat and
> ERS-2, and (bottom) GFO during near-coincident
> overflights of Hurricane Katrina on 26, 27, and
> 28 August 2005.The images were
> taken within 20 minutes of the altimeter
> passes.The three columns on the right show the
> altimeter measurements of wave height, wind
> speed, and sea level anomaly, respectively, as a
> function of latitude along the altimeter tracks
> shown on the infrared images.
> Fig. 2.The location and intensity of Katrina at
> intervals of six hours (circles indicate data
> from National Hurricane Center advisories) show
> two intensification events. (a) Intensification
> is not correlated with sea surface temperature
> (from POES high-resolution infrared data). (b) In
> contrast, the intensifications correlate well
> with highs in the ocean dynamic topography (from
> Jason 1,TOPEX, Envisat, and GFO sea surface
> height data).The Loop Current can be seen
> entering the Gulf south of Cuba and exiting south
> of Florida; the warm-core ring (WCR) is the
> prominent high shedding from the Loop Current in
> the center of the Gulf.The crosshair symbols on
> the storm tracks show the storm position at the
> times of the three rows of panels in Figure 1.
> Eos, Vol. 86, No. 40, 4 October 2005
> Posted by Tim