A Case Study of Hurricane Katrina:Rapid Intensification in the Gulf of Mexico

Michael P. Erb

Introduction to Hurricane Katrina

• Eleventh Tropical Storm of 2005

• August 23-31, 2005

A. Tropical Depression 12

over southeastern

Bahamas

B. Initial landfall on tip of

Florida as a Category 1

Hurricane on August 25

C. Encounters warm water

and rapidly strengthens

D. Achieves maximum

strength of 902 mb as a

Category 5 Hurricane

E. Makes landfall near Buras-

Triumph, Louisiana as a

Category 3 Hurricane on

August 29

http://www.nhc.noaa.gov/pdf/TCR-AL122005_Katrina.pdf

• A 30 kt (34.5 mph) or more increase in maximum sustained winds over a 24-hour period

http://cimss.ssec.wisc.edu/tropic/archive/2005/storms/katrina/katrina.html

What is Rapid Intensification?

• In their paper, “Large-Scale Characteristics of Rapidly Intensifying Tropical Cyclones in the North Atlantic Basin,” John Kaplan and Mark DeMaria found the five factors most important to Rapid Intensification:

– previous 12-hour intensity change

– sea surface temperature

– low-level relative humidity

– vertical shear

– difference between the current intensity and maximum potential tropical

cyclone intensity

Factors Important to Rapid Intensification

• Analyze Hurricane Katrina

• Isolate the period during which Katrina underwent RI

• Calculate the five variables for each 6-hour period during this time

• Compare these values to the means calculated by Kaplan and DeMaria for both RI and non-RI Tropical Cyclones

My Project

http://www.nhc.noaa.gov/pdf/TCR-AL122005_Katrina.pdf

Best Track data for

Hurricane Katrina

"14090589.227.229 / 0000

"15090288.626.328 / 1800

"14590987.725.728 / 1200

"12593086.725.228 / 0600

"10094185.924.828 / 0000

"10094885.324.527 / 1800

hurricane10094284.724.427 / 1200

(kt)(mb)(°W)(°N)(UTC)Stage

Wind SpeedPressureLongitudeLatitudeDate/Time

Surface Wind and Pressure Tendencies

• Data sources:

– Global Forecast System (GFS) data

• 1 degree resolution of data; every 6 hours

• Acquired data for 0000, 0600, 1200, and 1800 UTC on August 28, 2005

• From this data I got the relative humidity (RH) at 700, 750, 800, and

850mb, and the zonal (east-west) and meridional (north-south)

component of wind at 200 and 850mb

– Sea Surface Temperature (SST) data from RSS website

• RSS uses Microwave Radiometer observations to record SST

• Daily SST observations at 0.25 resolution

• Used data for August 28, 2005

– NOAA’s “best track” plot for Katrina

• latitude (LAT), longitude (LON), and maximum sustained wind speed

(VMX) for every 6 hours

Data and Methodology

Used data to calculate my five variables at each of the four time periods.

• Previous 12-hour change in maximum wind speed (DVMX):

– Difference between the maximum wind speed at t=0 and the maximum

wind speed 12 hours earlier

"14090589.227.229 / 0000

"15090288.626.328 / 1800

"14590987.725.728 / 1200

"12593086.725.228 / 0600

"10094185.924.828 / 0000

"10094885.324.527 / 1800

hurricane10094284.724.427 / 1200

(kt)(mb)(°W)(°N)(UTC)Stage

Wind SpeedPressureLongitudeLatitudeDate/Time

Data and Methodology

• Vertical Shear (SHR):

– Subtracted the wind speed at 200mb from the one at 800mb for both the UGRD (east-west wind) and VGRD (north-south wind)

– Both were averaged from r = 200km to r = 800km

– Calculated magnitude of wind vector using:

SHR = (avgUGRD2 + avgVGRD2)1/2

Data and Methodology

• Observed Sea Surface Temperature (SST):

– Found the mean SST near the center of Katrina at all four times

– Radius = 50km

• 850–700-hPa relative humidity (RHLO):

– Averaged the RH at the 850, 800, 750, and 700-hPa levels for all points within 200 and 800 km of the center of Katrina

• Storm Potential Variable (POT):– Used to show, theoretically, how much the hurricane can intensify

– POT = MPI (maximum potential intensity) - VMX (maximum sustained winds)

Data and Methodology

75.8671.0067.8471.65RHLO (%)

7.8310.4120.7033.56POT (m s-1)

2.465.666.633.69SHR (m s-1)

30.5630.4930.4630.16SST (°C)

12.86123.15012.8610.000DVMX (m s-1)

1800 UTC1200 UTC0600 UTC0000 UTCVariable

7.340.347.6m s-1POT

-3.68.54.9m s-1SHR

4.365.469.7%RHLO

0.927.528.4°CSST

3.61.04.6m s-1DVMX

D =

RI – Non-RI

Non-RI

(N = 2462,

Ne

= 705)

RI

(N = 159,

Ne

= 92)

UnitsVariable

Comparison of Results

0

10

20

30

40

50

60

70

80

DVM X (m/s) SST (C) r=50km RHLO (%) Total-SHR (m/s) POT (m/s)

RI M ean

0000Z

0600Z

1200Z

1800Z

Non-RI M ean

Comparison of Results

• Four of the five variables had optimal values

• POT was most likely so low because Katrina was already such a powerful hurricane when it began its period of RI

• All in all, Hurricane Katrina seems to coincide with Kaplan and DeMaria’s conclusion

Conclusions

• Expand the scope of variables

• Analyze tropical cyclones that did not undergo RI

• By doing so, the relationship between RI and environmental forcingscan be defined more completely

Further Studies

• Xiaodong Hong, Simon W. Chang, Sethu Raman, Lynn K. Shay, and Richard Hodur, “The Interaction between Hurricane Opal (1995) and a Warm Core Ring in the Gulf of Mexico,” Monthly Weather Review 128 (May 2000): 1347-1365

• Simon W. Chang and Richard A. Anthes, “The Mutual Response of the Tropical Cyclone and the Ocean,”Journal of Physical Oceanography 9 (January 1979): 128-135

• John Kaplan and Mark DeMaria, “Large-Scale Characteristics of Rapidly Intensifying Tropical Cyclones in the North Atlantic Basin,” Weather and Forecasting 18, no. 6 (December 2003): 1093-1108

• H. E. Willoughby, J. A. Clos, and M.G. Shoreibah, “Concentric Eye Walls, Secondary Wind Maxima, and the Evolution of the Hurricane Vortex,” Monthly Weather Review 120, No. 6 (February 1982): 395-411

• National Hurricane Center, “Tropical Cyclone Report – Hurricane Katrina,” National Weather Service, http://www.nhc.noaa.gov/pdf/TCR-AL122005_Katrina.pdf

• NCDC, “National Model Archive & Distribution System,” NOAA, http://nomads.ncdc.noaa.gov/

• Remote Sensing Systems, “Daily Minimum SST,” RSS, http://www.ssmi.com/sst/microwave_oi_sst_browse.html

• Mark DeMaria and John Kaplan, “Sea Surface Temperature and the Maximum Intensity of Atlantic Tropical Cyclones,” Journal of Climate 7 (September 1994): 1324-1334

Sources