hurricane katrina_an environmental perspective

Hurricane Katrina: An Environmental PerspectiveAuthor(s): Ewen McCallum and Julian HemingReviewed work(s):Source: Philosophical Transactions: Mathematical, Physical and Engineering Sciences, Vol. 364,No. 1845, Extreme Natural Hazards (Aug. 15, 2006), pp. 2099-2115Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/25190316 .Accessed: 07/04/2012 20:48

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Phil Trans. R. Soc. A (2006) 364, 2099-2115

doi:10.1098/rsta.2006.1815 Published online 23 June 2006

Hurricane Katrina: an

environmental perspective

By Ewen McCallum* and Julian Heming

Met Office, FitzRoy Road, Exeter, Devon EXI 3PB, UK

On 29 August 2005, Hurricane Katrina hit the Gulf Coast of the USA to become one of the worst natural disasters in the country's history.

The forecasts and official warnings of the event issued by the US National Hurricane Center up to 60 h ahead were excellent and largely based on an 'ensemble' of model and statistical guidance. The Met Office Global Model is highlighted as one of the best

performers for Hurricane Katrina.

The active 2005 Atlantic hurricane season has fuelled the debate on the impact of climate change on tropical cyclones. Some recent publications have suggested that this

impact is already apparent, while others are more cautious. Inconsistencies remain

among many of the theoretical, modelling and observational studies.

Despite the excellent warnings, there was a tragic loss of life as a result of Hurricane Katrina which has led to political questions concerning complex socio-economic issues, the state of flood defences and how to coordinate the reaction to and mitigate the impact of such monumental natural hazards.

Keywords: Hurricane Katrina; Met Office; forecast

1. Introduction

Hurricane Katrina struck the Gulf Coast of the USA around dawn (local time) on 29 August 2005, wreaking havoc to coastal communities through a combination of wind, torrential rain and storm surge. It is probably the most expensive natural disaster in US history and one of its deadliest since 1928. Katrina was followed during this season by Category 5 hurricanes, Rita and Wilma. However, despite their significance in what was a record breaking hurricane season in the Atlantic basin, this paper will focus almost exclusively on the former well documented hurricane.

Vulnerability to hurricanes depends on two factors: event incidence and societal exposure. This paper will concentrate on the physical events and will

explore the environmental aspects of this storm within a historical perspective, from its formation as a tropical depression near the Bahamas on 23 August until it hit the Gulf Coast of the USA on 29 August. It will also highlight how the storm was handled by the various numerical weather prediction (NWP) models,

with particular focus on the Met Office Global Model. Comparisons will be made *

Author for correspondence ([email protected]).

One contribution of 20 to a Discussion Meeting Issue 'Extreme natural hazards'.

2099 ? 2006 The Royal Society

PHILOSOPHICAL TRANSACTIONS

-OF- ? THE ROYAL/A SOCIETY A JL

2100 E. McCallum and J. Heming

with the official forecasts leading up to the event from the National Hurricane Center (NHC) in Miami. Also, the usefulness of these warnings to the communities most affected will be considered.

2. Perspective

Hurricane Katrina (figure 1) was the third most intense hurricane (as measured

by central pressure) to hit the USA in recorded history (Knabb et al 2005). In the Atlantic basin, it briefly achieved the status of the fourth lowest central

pressure ever recorded (902 mbar), until a few weeks later when Hurricane Rita became the third most intense with a central pressure of 897 mbar. However, even Rita was upstaged in mid-October by Wilma which had the lowest recorded central pressure of 882 mbar in the Atlantic basin (see table 1). The 10 m storm

surge on 29 August caused by Katrina was the highest ever observed in North

America, but falls short of the all time record of 13 m which occurred in Bathurst

Bay (Australia) in 1899 (Whittingham 1958). This event resulted in tragic loss of life, but fortunately some of the original

estimates proved to be unfounded with the final death toll of around 1300. This was

less than the Okeechobee Hurricane in 1928 when the fatalities were estimated to

be around 2500. However, it will almost certainly be the costliest natural disaster in US history with total damage estimates of around US $ 75 billion (Knabb et al

2005) and parts of the city of New Orleans needed complete reconstruction.

Most media coverage focused on the flooding in New Orleans. This was

principally caused by the massive storm surge triggering breaches in the levees

protecting the city, which lies mostly below sea level (see figure 2). This was the

greatest disaster to hit the city since its foundation in 1718, although the city has

been brushed by hurricanes on average every 4 years with direct hits occurring every 13.4 years. In addition to the impact on New Orleans, considerable

destruction was also wrought on the Mississippi towns of Gulfport and Biloxi.

3. Overview of the key environmental factors concerning the life cycle of Katrina

Katrina's origins were a somewhat complex mix of a tropical wave, the remnants

of an earlier tropical depression and an upper tropospheric trough (Knabb et al

2005). These combined to form a tropical depression near the Bahamas on 23

August. In this region, sea temperatures were above 27 ?C, and there was

relatively weak vertical wind shear, high mid-level humidity and upper-level outflow. These conditions helped to organize the intense convection associated

with the disturbance and the storm started to gain strength. Katrina moved towards southern Florida to become a tropical storm (winds

of 39 mph) at 12:00 UTC on 24 August. It became the fifth hurricane of the 2005 Atlantic season on 25 August and made landfall as a Category 1 hurricane

on the Saffir-Simpson scale (Simpson 1974) at around 10:30 UTC close to

Aventura near Miami. There was substantial flooding with over 400 mm of rain

in the region. Interestingly, the eye of the hurricane passed over the NHC in

Miami; the anemograph trace showed a peak wind of approximately 80 mph

(figure 3). Katrina was then downgraded to a tropical storm as it lost some of

Phil Trans. R. Soc. A (2006)

Hurricane Katrina 2101

Figure 1. Hurricane Katrina at Category 5 strength at 20:45 UTC on 28 August 2005. (Image reproduced courtesy of the Naval Research Laboratory, Monterey, CA.)

Table 1. Most intense recorded hurricanes in the North Atlantic and at landfall over the USA as

measured by central pressure.

North Atlantic

rank hurricane year

pressure

(mbar)

USA landfall

rank hurricane year

pressure

(mbar)

1 Wilma 2005 882 2 Gilbert 1988 888 3 Labor day 1935 892 4 Rita 2005 897 5 Allen 1980 899 6 Katrina 2005 902

Labor day

Camille Katrina

Andrew

Indianola

Florida Keys

1935 1969 2005 1992 1886 1919

892 909 920 922 925 927

its low-level heat and moisture supply over land. However, this weakening was

only for a few hours. The storm regained hurricane status as it entered the Gulf

of Mexico, after taking a slightly south of westward track as it traversed

southern Florida.



Figure 2. Part of New Orleans following the flooding. (Image reproduced courtesy of FEMA.)

As the hurricane moved into the Gulf of Mexico, it became more intense over

the following 2 days and grew into a powerful Category 5 hurricane with

sustained winds of 175 mph and a central pressure of 902 mbar. As Katrina

moved northwestwards across the Gulf of Mexico, it crossed the loop current

(Hofmann & Worley 1986). This is an oceanic circulation which originates at the

Yucatan Channel and flows out through the Florida Strait via a clockwise loop, of varying magnitude, around the Gulf of Mexico. While the sea-surface

temperature, close to 30 ?C, was fairly constant along the track of the hurricane, the sea-surface height was above average (Scharroo et al. 2005). A greater depth of warm ocean (and consequent higher heat content) results in a higher than

average sea-surface height. The turbulent environment of hurricanes pulls water

from beneath the surface (upwelling) and if the water at depth is cool it can

result in the weakening of the system. However, if the water is warm at the

lower depths, then water being pulled to the surface is still warm and the

hurricane can increase in intensity if other atmospheric conditions are also

conducive to strengthening. When looking for these areas of deep warm water,

meteorologists track water that is at least 26 ?C. This deep, warm water is one

of the several critical factors to enable hurricanes to intensify and was

undoubtedly a contributory factor to the intensity of Hurricane Katrina.

Figure 4 shows Katrina's track across an area of positive sea-surface height

anomaly associated with the loop current and the rapid intensification of the

hurricane to Category 5 status.

Katrina weakened to a Category 4 hurricane on 29 August as it moved

towards the coast. This was probably due to a combination of several factors.

Katrina entrained dry air from continental USA, which was evident from

satellite imagery. This resulted in erosion of deep convection on the western side



Figure 3. Anemograph trace on 25 August as Hurricane Katrina passed over the NHC, Miami

(horizontal scale in knots). (Image reproduced courtesy of Bob Henson.)

of the hurricane. Increasing wind shear, slightly lower sea-surface temperatures and reduction in the thickness of the upper warm layer of the ocean may also

have contributed to the weakening, but further investigation would be necessary to confirm this. There were also changes to the internal structure of the eye of the

hurricane which could have hastened the weakening prior to landfall (Knabb et al 2005).

Katrina eventually made landfall near Buras Louisiana as a Category 3

hurricane with a wind speed of 130 mph at 06:10 local time (11:10 UTC) on 29

August. It then crossed the Breton Sound making a second landfall near

Pearlington, Mississippi still as a Category 3 hurricane with 125 mph winds. The

hurricane quickly weakened as it left the warm waters of the Gulf, moving

steadily northeast across the USA dropping copious amounts of rain in its path. The final advisory from the NHC was issued at 15:00 UTC on 30 August and the remnants of Katrina were finally absorbed by a frontal boundary in northeastern

Canada on 31 August.

Phil. Trans. R. Soc. A (2006)


Figure 4. Sea-surface height anomaly (cm) on 28 August and the track of Hurricane Katrina along with the maximum wind speeds (mph). Figure reprinted from Scharroo et al (2005) with permission

from AGU.

4. Environmental impacts of Katrina

The three key elements most likely to cause serious environmental impacts from a major hurricane (Category 3, 4 or 5) like Katrina are the powerful

winds and embedded tornadoes (33 confirmed in this case), heavy rainfall

and lastly, but sometimes most importantly, the storm surge, which was up to 10 m in the case of Katrina. There is no doubt that the wind and rain

brought extensive damage to trees and property, particularly over coastal

areas of Mississippi, but what turned this event into such a major disaster was the record storm surge.

Figure 5 depicts the surge at various points around the coastline and

compares it with the last disastrous surge in the region caused by Hurricane

Camille in 1969. It can be seen that the peak surge of 10 m from Katrina

occurred at Clermont Harbor, Mississippi and was in general much greater than that caused by Camille and affected over 200 miles of coastline from

southeast Louisiana to the Florida panhandle. This was largely due to the

large size of the hurricane and the extremely damaging winds that were

observed in the northeastern quadrant of its circulation. The storm surge is

caused by the onshore rush of the water associated with the hurricane and is

largely due to the powerful wind and areal extent of those winds. The rise

in sea level caused by the extremely low pressure has a much smaller effect



Figure 5. Storm surge heights from Hurricane Camille (1969) and Hurricane Katrina along the Gulf coast. (Image reproduced courtesy of Bob Henson.)

(up to 1 m contribution). The storm surge is particularly damaging when it

coincides with high tide and is also complicated by the topography of the

coastline and the slope of the underlying shelf. Indeed, understanding the

impact of storm surges is a science in itself; there is a need to take into

account hydrological factors in the affected area, height of the land above or

below sea level and the quality of the defences put in place to protect a city from the sea. In this case, the long, gently sloping shelves and shallow water

of the Gulf coast contributed to a higher storm surge than would be observed

with steeper shelves. The sheer size of Katrina exacerbated the surge and of course the defences (levees) proved inadequate in this case.

There was an initial confusion as to when the levees of New Orleans broke, but the main breaches were due to the storm surge rather than flooding later

caused by heavy rain inland. The first reliable report of a breach of the levee

system to the public came from CNN at around 06:30 UTC on 29 August with a

report of a breach on 17th Canal Street, which connects into Lake

Pontchartrain. This was quickly followed by a total of three levee breaches,

resulting in 80% of the city being under water to a depth of 7-8 m. All reported breaches were to the levees protecting the city from this lake which lies to the

north of New Orleans (see map in figure 6).



Figure 6. New Orleans and coastal Mississippi. (Image reproduced courtesy of Google Inc.)

Clearly, what followed has been widely documented by the media,

emphasizing complex socio-economic and political factors which are clearly

beyond the scope of this paper and will need more time to be fully understood.

5. Forecasting aspects

The official forecast centre for hurricanes in the Atlantic is the NHC in Miami.

They issue 120 h tropical cyclone track and intensity forecasts four times per day for all tropical cyclones in the North Atlantic and eastern North Pacific east of

140? W. The forecast aids used by the centre range in complexity from simple statistical models to three-dimensional primitive equation models (De Maria

1997). NWP models have become increasingly skilful in the prediction of tropical

cyclone tracks in recent years. For example, figure 7 shows the mean track

forecast errors from the Met Office Global Model for all Northern Hemisphere

tropical cyclones since 1988, emphasizing steady improvement. The four NWP

models used by NHC for track prediction, which have produced the best

guidance in recent years, are the GFS (US global model), NOG APS (US Navy model), GFDL (high-resolution hurricane model) and the Met Office Global Model. Intensity forecasts show far less skill than track forecasts and the NHC uses a variety of forecast aids mainly based on statistical models.

The official NHC forecast is issued 3 h after the main forecast hours (00:00,06:00, 12:00 and 18:00 UTC). Any forecast aids which are available for input into this

forecast are termed as 'early'. However, the majority of numerical model outputs are not available until after this time and are referred to as 'late' models. To



Northern Hemisphere tropical cyclone forecast

positional errors

0"1-r. '"" .r"".' r""",""","r. .r.. ..-.T.......t..,...r.... .. T..,.,r........i.r.-.?.yin,,,? -?,,??. -,.?,,, ,,?,,?,.i?r?

1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

year

Figure 7. Met Office Global Model tropical cyclone track forecast errors. Each line represents a

forecast lead time in hours.

92?W 90?W lilil?

?8?W 86?W 84?W 82?W 80?W 78?W

Figure 8. 'Early' track forecasts from 18:00 UTC on 25 August 2005. (Image reproduced courtesy of

Jonathan Vigh, Colorado State University, CO.)



40?N

30?N

20?N

100?N 80?W

Figure 9. The probability that the track of Katrina passes with 120 km radius during the next 120 h using the ECMWF ensemble forecast tracks starting at 00:00 UTC on 26 August 2005. Blue tracks, ensemble

members; green track, control (unperturbed) forecast track; black track, operational forecast track.

(Image reproduced courtesy of ECMWF.)

45?N

40?N

35?N

30?N

25?N

20?N

Met Office

95?W 90?W 85?W 80?W 75?W

Figure 10. 'Early' track forecasts from 12:00 UTC on 26 August 2005. (Image reproduced courtesy of Jonathan Vigh, Colorado State University, CO.)



38?N

36?N

34?N

Met Office Global Model, US warning centre forecast tracks of Hurricane Katrina

96?W 93?W 90?W 87?W 84?W 81?W 78?W 75?W 72?W 69?W

UK 20050826 ? US 20050826

key to forecast tracks

(triangles denote analysed positions)

24 hourly real time observed positions S date/time of first symbol 12Z 24 Aug 2005

Figure 11. Met Office Global Model (red) and NHC (green) forecast tracks 12:00 UTC on

26 August 2005.

overcome this problem, the NHC interpolates forecast fields from the previous 'late'

model runs as input, so that the track predictions will be available in a timely manner along with the 'early' model output.

As Katrina approached landfall over southern Florida, models were predicting that it would regain strength over the Gulf of Mexico. However, there was

considerable uncertainty as to the track of the hurricane at this time. Figure 8 shows

the spread of forecast guidance tracks at 18:00 UTC on 25 August, which would

have been available to the NHC in real time. The acronyms in the figure represent various forms of guidance available. This includes statistical and climatological

guidance, as well as some of the numerical models mentioned previously. Figure 9

shows the spread of tracks in the ensemble forecast from 00:00 UTC on 26 August produced by the European Centre for Medium-Range Weather Forecasts

(ECMWF). These ensemble tracks represent the predictions from the same version

of the ECMWF model, but with small changes (perturbations) to the initial

representation of the hurricane. This kind of forecast provides a probabilistic

prediction of the track of the hurricane and gives a measure of the confidence in the

prediction, which in this case and at this point of time was relatively low.

Friday, 26 August was a crucial day in the forecast of Katrina. The 'early' forecasts for 12:00 UTC had divergent predictions of landfall from western

Louisiana to the Florida panhandle. However, the Met Office Global Model was

one of the first to indicate a more accurate path close to New Orleans. Figure 10



Met Office Global Model, US warning centre forecast tracks of Hurricane Katrina

T

96?W 93?W -*- UK 20050827 ?

key to forecast tracks

(triangles denote analysed positions)

81?W 78?W 75?W 72?W 69?W 90?W 87?W 84?W US 20050827

24 hourly real time observed positions S date/time of first symbol 00Z 24 Aug 2005

Figure 12. Met Office Global Model (red) and NHC (green) forecast tracks 00:00 UTC on

27 August 2005.

shows 'early' track guidance for this time (including interpolated 'late' model

guidance from the previous run) together with the track from the Met Office

Global Model. The 72 h forecast error for landfall over New Orleans was just 59 km. Figure 11 shows the 12:00 UTC 26 August Met Office forecast track and

the observed track alongside the more conservative forecast from the NHC based on a consensus of model forecasts. The Met Office forecast track was clearly more

accurate, although it should be pointed out that the NHC did not have this run of

the Met Office model and other 'late' model guidance available to their

predictions at this time. By the time the NHC issued their guidance at 00:00

UTC on 27 August, they had a very accurate prediction of the track, although

slightly slow near landfall, while the Met Office model forecast track was slightly to the west of the observed track (see figure 12). By this time, the consensus of

model forecast tracks had converged on the New Orleans area. The spread of the

ECMWF ensemble forecast members also reduced markedly between 00:00 UTC on 26 August (figure 9) and 12:00 UTC on 26 August (figure 13). The former showed a strike probability of approximately 20% over New Orleans. This had

risen to approximately 60% in the forecast issued just 12 hours later. All forecast

guidance produced by the NHC from 60 h before landfall onwards gave excellent

warning to the areas worst affected.

Post-storm analysis by the NHC has indicated that track forecasts from the

Met Office model were the best guidance available to the NHC for the whole

lifetime of Hurricane Katrina at lead times up to 48 h and the second best at 72 h



100?W 80?W

Figure 13. The probability that the track of Katrina passes within 120 km radius during the next 120 h using the ECMWF ensemble forecast tracks starting at 12:00 UTC on 26 August 2005. Blue

tracks, ensemble forecast members; green track, control (unpeturbed) forecast track; black track,

operational forecast track. (Image reproduced courtesy of ECMWF.)

(Knabb et al 2005). Provisional figures for the whole Atlantic hurricane season

for 2005 indicate that the Met Office Global Model and the GFDL regional model

provided the best dynamic guidance to the NHC.

6. Warnings

The official warnings that were disseminated by the NHC reflected the accurate

and consistent model guidance in the 60 h prior to the landfall of Katrina. This was then cascaded down to local centres who interpreted it in a variety of ways.

Perhaps the most dramatic warning was issued by the National Weather Service, New Orleans on 28 August under the heading of 'Devastating damage expected'. The body of the message said 'Most of the area will be uninhabitable for weeks,

perhaps longer. At least one-half of well-constructed homes will have roof and

wall failure. All gabled roofs will fail leaving those homes severely damaged or

destroyed. Power outages will last for weeks as most power poles will be down

and transformers destroyed. Water shortages will make human suffering incredible by modern standards.' Although the impact of wind damage in New

Orleans was not as severe as it could have been, some aspects of this dramatically worded message bear marked similarities with what actually occurred.

There is no doubt that the forecasts and warnings were accurate up to 60 h

prior to landfall and most people took the advice of New Orleans officials to leave

the city. However, the hurricane exposed complex societal issues in America's

Deep South and some people lacked the resources to leave. Some had also

witnessed less severe hurricanes in the past and for what ever personal reason

(e.g. friends and family, fear of looters) decided to ride out the hurricane.



Early media coverage of the impact of Katrina was upbeat, largely because

reporters were stationed in the highest parts of the city, although some reports did talk of catastrophic flooding by late hours on Monday 29 August. Unfortunately, the federal response did not match the gravity of the situation until much later in the week and of course the political fallout was massive. There is no doubt that Katrina will act as a wake-up call for all planners and

agencies around the world that have to cope with natural disasters. However, questions also need to be asked about people's perception of warnings and we

may need to look at innovative ways to reiterate warnings and messages to help people act on them. There is a danger that a population can become immune to

repeated severe weather warnings and perhaps the worst-case situations call for extra measures, special warning products and more explicit depiction of risk.

However, it is worth noting that 80% of the population of about a million people acted on the excellent warning and were able to evacuate safely.

The almost total evacuation of Houston and other towns in Texas and Louisiana in advance of Hurricane Rita a few weeks later was in all probability a

reaction to the events that many had witnessed in the media concerning Katrina.

7. Climate change

Every severe weather event is usually accompanied by much speculation in the media about the impact of man-made climate change and Hurricane Katrina (followed in

the same season by Category 5 hurricanes, Rita and Wilma) was no exception. As far as tropical cyclones are concerned, recent literature provides a mixed

signal on the possible impact of climate change.

(i) Trenberth (2005) suggests that a trend in observed tropical cyclone activity is hard to prove owing to the large natural variability of cyclone

activity compared to the sample size. He suggests that the intensity and

rainfall from cyclones are probably increasing due to human-induced

changes in the environment, but that the effect on cyclone numbers

remains unclear.

(ii) Emanuel (2005) finds that the power dissipated by western North Pacific and North Atlantic cyclones has more than doubled since the 1970s. He relates the

growth to an increase in sea temperatures, but finds that these increases

cannot explain the entire rise in the power of the cyclones. Emanuel concludes

that future warming may lead to cyclones becoming more powerful.

(iii) Webster et al. (2005) conclude that in recent decades the frequency of tropical

cyclones has not changed significantly, but the proportion reaching

Categories 4 and 5 has doubled over the last 30 years.

(iv) Other authors have questioned whether there is evidence for any upward trend in hurricane destruction (Pielke 2005) and the methods of data analysis used in the aforementioned studies (Landsea 2005). Indeed, the publication of

Emanuel (2005) and Webster et al. (2005) has stimulated a robust debate on

the quality and consistency of the historical database of tropical cyclone

intensity and has triggered moves for a re-analysis of the data using modern

techniques.



An alternative approach is to study how tropical cyclones may change in climate

models forced with increased levels of greenhouse gases. However, there is large

uncertainty in the results of these models. Sugi et al (2002) and McDonald et al

(2005) both found fewer tropical cyclones in the future simulations. There are large

regional variations in the sign of the changes in both the models and the results of

these studies are of the opposite sign in some regions. Some models suggest a clear

shift towards increased intensity (e.g. Knutson k Tuleya 2004; McDonald et al

2005) or a decrease in frequency and intensity, but an increase in precipitation

(Hasegawa k Emori in revision), while others show no change in intensity (Sugi et al 2002). Ensembles of experiments which sample modelling uncertainty and

natural variability and long integrations are needed to quantify these effects.

There remains a marked discrepancy between the relatively small magnitude of tropical cyclone intensity increase projected by numerical modelling studies, such as Knutson k Tuleya (2004), and the much larger magnitude of change observed by Emanuel (2005) and Webster et al (2005), which has yet to be reconciled (Pielke et al 2005).

In the Atlantic, the situation is complicated further by the presence of the

Atlantic multi-decadal oscillation, which regulates hurricane activity on the

time-scale of several decades. The Atlantic has been in an active phase of this

cycle since 1995 and the 2005 hurricane season fits in with this pattern of

increased hurricane activity (Goldenberg et al. 2001; Landsea 2005). It is wise to avoid associating individual events such as Katrina with the

possible impacts of climate change as it is probable that this hurricane would

have occurred irrespective of any recent increase in greenhouse gases. It is

impossible to say whether the peak intensity was affected by recently observed

trends of global warming. Discussion of the possible impacts of climate change on

tropical cyclones needs to be related to trends in activity on a global scale over a

period of several decades although intense hurricanes such as Katrina, Rita and

Wilma will add some impatience to that debate.

8. Conclusion

Hurricane Katrina, which hit the Gulf coast to the east of New Orleans on 29

August 2005 was one of the worst natural disasters in US history and one that was

subject to the full glare of the modern media with its 24 h news channels and the chatter of the Internet. It was the third most intense hurricane (in terms of central

pressure) to hit the USA in recorded history and its record 10 m storm surge caused

catastrophic flooding in low-lying coastal areas of the Gulf, most notably in the city of New Orleans which suffered its greatest disaster since its foundation in 1718.

The forecasts and official warnings of the event up to 60 h ahead were excellent and largely based on an 'ensemble' of NWP models, which have improved the

accuracy of tropical cyclone forecasting enormously over the last 20 years. This is

largely due to:

? increasing diversity, accuracy and coverage of observations, especially satellite

data, ?

better assimilation techniques that can extract increasing amounts of useful information from satellites,



? better NWP models, including higher resolution and more accurate treatment

of physical processes, such as clouds and precipitation, and ? more powerful computers that make it possible to cope with the increasing

data, run higher resolution models and allow those models to run fast enough in real time to be useful to forecasters.

The Met Office Global Model was highlighted in the paper and performed particularly well, being one of the first to predict the correct location of landfall

of Hurricane Katrina over the Gulf coast. Work is continuing on improvements to numerical models and observing systems, which should further improve the

prediction of tropical cyclones in the future.

Katrina was a wake-up call, in particular to tropical cyclone-prone areas of the

globe at or below sea level. Despite the excellent warnings, this was a major

catastrophe for the USA which has fuelled a political debate on the state of flood

defences and what in general should the global society do to coordinate and help

mitigate the impact of such massive natural hazards.

The authors wish to thank Bob Riddaway of ECMWF for his very helpful comments on an earlier

draft of this paper and to Bob Henson of UCAR, Boulder, CO for some of the pictures used in this

paper.

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