an objective satellite-based tropical cyclone size climatology john a. knaff, noaa center for...

An objective satellite-based tropical cyclone size climatology

John A. Knaff, NOAA Center for Satellite Applications and Research

Fort Collins, Colorado

and

Scott Longmore, CIRA, Colorado State University


A little about RAMMBBackground

The Regional and Mesoscale Meteorology Branch (RAMMB) of NOAA/NESDIS conducts research on the use of satellite data to improve analysis, forecasts and warnings for regional and mesoscale meteorological events. RAMMB is co-located with the Cooperative Institute for Research in the Atmosphere (CIRA) at Colorado State University in Fort Collins, CO.

4 Federal Employees

~13FTE CSU Employees2

An objective satellite-based tropical cyclone size climatology

John A. Knaff, NOAA Center for Satellite Applications and Research


and

Scott Longmore, CIRA, Colorado State University


Motivation• Tropical cyclone (TC) size is an important factor for inferring TC impacts

and issuing warnings

• There are no global climatology of TC size

• Past Studies have concentrated on only two TC basins – Western North Pacific– North Atlantic

• Most past studies have made the use of two subjective measures of TC size (R34 and ROCI) that have known shortcomings those that have not suffer from short data records. – R34 – radius of 34-knot winds (radius of gale-force wind)– ROCI – radius of outer closed isobar

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Our Solution1. Relate features in the IR imagery and TC location to variables related to TC size variations over a

relatively long period of record (1995-2011, Atlantic and east Pacific)

1. IR PCs - Principle components (PCs) of the azimuthal mean IR brightness temperatures (Tb), radius = 2-602km @ 4km increments

• IR PCs have been used to estimate TC wind structure (Mueller et al. 2006, Kossin et al. 2007)

2. TC latitude• IR Brightness temperatures (Tbs) vary with latitude• TC size has been shown to increase with latitude (Merrill 1984)

3. V500 - Global analyses of the azimuthal mean tangential wind speeds at a fixed 500 km radius at 850 hPa (avoids frictional layer)

• V500 has been used to account for variations in TC circulation size to improve estimates of MSLP (Knaff and Zehr 2007)

• When speaking of rawindsonde composites of western North Pacific TCs… “Strong upward vertical motion exists inside about 4o (latitude) radius. From 4-6o there is moderate subsidence below 400 mb indicating the mean location of the moat region” – Frank (1977), where the moat region contains little or no deep convection.

• When speaking of the tangential flow in composites…“The scale of the circulation is very large. Even at 14o radius there is substantial mean radial flow and the strength of the outer tangential circulation is at least statistically correlated with central pressure” – Frank (1977)

4. R5, the radius of 5 kt tangential winds at 850 mb is linearly related to V500

2. Use global three-hourly records of infrared (IR) satellite data (~30 years) to create a climatology

1. Hurricane Satellite Archive (HURSAT v3.0; Knapp and Kossin 2007) (1978-2006)

2. CIRA/RAMMB TC IR image archive (Zehr and Knaff 2007) (2007-2011)5

Algorithm DevelopmentDevelopmental Data Details:

1. Six-hourly cases in the CIRA IR archive 1995-2011

2. N. Atlantic and E. Pacific

3. Six-hourly GFS/NCAR reanalysis (i.e., SHIPS database)

Predictant (TC circulation):• V500 (azimuthally averaged

tangential wind at 500km and 850 hPa)

Predictors:

• Storm Latitude (ᵩ)

• First three normalized Principle Components of the azimuthally averaged brightness temperature (Tb)PCs

The algorithm is based on multiple linear regression and explains 29% of the variance of V500

The regression equation is :

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The first 3 PCs explain 95% of the azimuthally averaged TbEOF patterns are shown to the left

TC Size Based upon TC Circulation (V500)

• We want to estimate TC size from TC circulation (i.e., V500)

• Here we define TC size as the radius of 5 kt positive tangential wind at 850 hPa that results from the TC circulation influencing a quiescent atmospheric environment and call it R5 with units of o latitude

• Use GFS/Reanalysis (1995-2011) climatology to estimate the decay of the TC vortex from 500 km to a radius where the tangential wind is 5 kt (R5 or the radius of background flow)

– The climatological radius of 5 kt tangential wind is 952 km– V500c = 5.05 m/s, V1000c = 2.25 m/s

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Algorithm Results

Despite explaining only 29% of the variance the algorithm seems able to discriminate TC size variations and separate the TC from it’s environment

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Small (<7.9o) Average Large (>13.9o)

TropicalDepression

(Max<34 kt)

TropicalStorm

(34kt<Max≤64kt)

MinorHurricane/TC(64kt<Max≤95kt)

MajorHurricane/TC

(Max≥95kt)

Tb

That is, variations in the environment contribute to the regression’s scatter

N=18686 N=38667 N=4133

N=9045 N=39741 N=10127

N=1783 N=16394 N=9361

N=426 N=6412 N=6369

R5:Mean =10.9 oLatitude σ = 3.0 oLatitude

Major (>95kt) TC Examples:

HAGUPIT (2008, 09/23 12:30)

Rank 2/266 (0.8%)West Pacific

Rank 2/738 (0.3%) Globally

FELICIA (1997, 07/19 06:00)

Rank 158/158 (100.0%)East Pacific

Rank 737/738 ( 99.9%) Globally

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Vmax:125ktLat: 20.73oNPC1: -2.04PC2: 2.65PC3: 0.55

V500: 11.90 m/sR5 : 19.58o Lat

Vmax:115ktLat: 15.60oNPC1: 0.17PC2: -2.36PC3: -0.87

V500: 2.92 m/sR5 : 5.14o Lat

Large Atlantic Examples:

Katrina (2005, 08/28 17:45)

Rank 1/90, 8/738 (1.1% globally)

Opal (1995, 10/4, 11:45)

Rank 3/90, 31/738 (4.2% globally)

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Vmax:130ktLat: 27.3oNPC1: -1.97PC2: 0.63PC3: -0.93

V500: 10.67 m/sR5 : 17.60o Lat


V500: 11.32 m/sR5 : 18.64o Lat

Small Atlantic Examples:

Felix (2007, 9/3, 2:45)

Rank 89/90, 667/738 (90.4 % globally)

Charley (2004, 8/13, 17:45)

Rank 82/90, 639/738 (86.6% globally)

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Vmax:150ktLat: 13.9oNPC1: -0.89PC2: -0.84PC3: 0.67

V500: 5.90 m/sR5 : 9.93o Lat

Vmax:125ktLat: 26.0oNPC1: -0.48PC2: -1.74PC3: -0.79

V500: 6.34 m/sR5 : 10.64o Lat

Large W. Pac Examples:

HAGUPIT (2008, 09/23 12:30)

Rank 2/266, 2/738 (0.3% globally)

KROSA (2007, 10/5, 18:30)

Rank 4/266, 6/738 (0.8% globally)

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V500: 11.66 m/sR5 : 19.20o Lat


V500: 11.90 m/sR5 : 19.58o Lat

Small W. Pacific Examples:

MEARI (2004, 9/24, 5:25)

Rank 259/266, 681/738 (92.3% globally)

LONGWANG (2005, 9/28, 11:25)

Rank 260/266, 686/738(93.0% globally)

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V500: 5.71 m/sR5 : 9.63o Lat


V500: 5.58 m/sR5 : 9.43o Lat

Basin Specific TC size Distributions

• Tropical Storms

(34 kt ≤ Vmax ≤ 63 kt)

• Minor TCs/Hurricanes/Typhoons

(64 kt ≤ Vmax ≤ 95 kt)

• Major TCs/Hurricanes/Typhoons

(Vmax > 95 kt)

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North Atlantic

Eastern North Pacific

Western North Pacific

North Indian Ocean

Southern Hemisphere

Findings:

• TCs become larger as they intensify• East Pacific has the smallest TCs• The West Pacific has the largest

size distributions• Atlantic has the largest ranges of

TC size

TC Size (R5)

Fre

quen

cy o

f O

ccur

renc

e

Lifecycle of TC sizeFindings: • Initial size determines/is related

to the size at later times• TCs grow during intensification • Lifecycle of TC size appears

basin dependent• Atlantic TCs continue to grow

after peak intensity • After peak intensity TCs

generally become smaller, particularly in the E. Pacific and S. Hemisphere.

• Atlantic TCs grow the most on average

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North AtlanticNW PacificS. HemisphereNE Pacific

Tropical Storms

Minor TCs/Hurricanes

Major TCs/Hurricanes

Time of maximum intensity

TC

Siz

e

Figure shows composite averages of of TC size (R5) based on the timing of maximum intensity (dashed line at time = 0 h). Averages are shown for TCs tropical storms (top), minor TCs (middle), and major TCs (bottom).

Panels on the left are of R5, and on the right are R5 calculated without the latitude contribution (i.e., sinᵩ=0).

Locations of Largest (25%) and Smallest (25%) TCs at peak intensity

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Minor TCsHurricanes and Typhoons@ max intensity

Major TCsHurricanes and Typhoons@ max intensity

Findings:

The majority of the smallest TCs have occurred in the eastern North Pacific or at low latitudes.

Small minor TCs occasionally occur in the subtropics

Large major and minor TCs have occurred in regions where baroclinic environments more often exist

Smallest Largest

Seasonality of TC size

Findings:

Atlantic:

A slight preference for large (small) major hurricanes in August (September); small major TCs in September and July

NE Pacific:

Preference for small (large) major hurricanes early (late) in the season

Preference for small minor hurricanes in July and August.

NW Pacific:

Small minor and major typhoons occur more often before and after the peak activity (non-monsoonal).

Southern Hemisphere:

Larger percentage of small minor and major TCs tend to occur before and after the seasonal peak (non-monsoonal).

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N. Atlantic N. Atlantic

NE Pacific NE Pacific

NW Pacific NW Pacific

SouthernHemisphere

SouthernHemisphere

Minor TCs Major TCs

TC Growth

Findings:

Maximum TC growth occurs with 1. TCs that recurve at

higher latitude and slowly weaken

Minimum growth is associated with 2. Low latitude TCs that

move to the west and weaken slowly

3. TCs that weaken rapidly, and

4. TCs that are in weak steering/constantly changing environments

Post-peak Intensity

Weakening and Growing (top 10%)

Maximum intensity and maximum size are simultaneous

Max SizeMax Intensity

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Shrinking prior to maximum intensity(top 10%)

Inter-annual Trends in maximum TC Size (1981-2011)

Findings:

No significant trends in TC size

Atlantic:

Decreasing trend, not statistically significant

NE Pacific:


NW Pacific:

Increasing trend, not statistically significant

Southern Hemisphere:


Globe:

No trend.

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N. Atlantic

Globe

Southern Hemisphere

NW Pacific

NE Pacific

Recent Developments

• This work has been accepted for publication Knaff, J. A., S. P. Longmore, and D. A. Molenar, 2013: An objective satellite-based tropical cyclone size climatology. J. Climate, in press.

http://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-13-00096.1

• We have started making these TC size estimates in real-time

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1920

km

2560 km

Summary

• An objective TC size algorithm is developed based on TC latitude and patterns in IR imagery

• The algorithm is used to conduct a basic climatology of global TC size

• Some important findings are– The smallest TCs primarily occur in the eastern North Pacific and at lower

latitude– Initial TC size is an important factor for determining the TC size at later times– The most intense Atlantic TCs tend to grow after their peak intensity. In other

basins this is not the case.– Maximum (minimum) growth is associated with recurvature and regions of

enhanced baroclinicity (eastward movement and rapid weakening).– Large (small) TCs have a tendency to develop when and in regions where

the environmental relative vorticity is enhanced (suppressed)– There are no long-term trends in TC size

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Additional Reading

Knapp, K. R., and J. P. Kossin (2007), New global tropical cyclone data from ISCCP B1 geostationary satellite observations. J. of Appl. Remote Sens., 1, 013505.

Merrill, R. T., 1984: A comparison of large and small tropical cyclones. Mon. Wea. Rev., 112, 1408–1418.

Mueller, K.J., M. DeMaria, J.A. Knaff, J.P. Kossin, and T.H. Vonder Haar 2006: Objective estimation of tropical cyclone wind structure from infrared satellite data. Wea Forecasting, 21, 990–1005.

Kossin, J.P., J.A. Knaff, H.I. Berger, D.C. Herndon, T.A. Cram, C.S. Velden, R.J. Murnane, and J.D. Hawkins, 2007: Estimating hurricane wind structure in the absence of aircraft reconnaissance. Wea. Forecasting, 22:1, 89–101.

Knaff, J.A., and R.M. Zehr, 2007: Reexamination of Tropical Cyclone Wind-Pressure Relationships. Wea Forecasting, 22:1, 71–88.

Zehr, R.M., and J.A. Knaff, 2007: Atlantic major hurricanes, 1995-2005 – Characteristics based on best track, aircraft, and IR images. J. of Climate, 20, 5865-5888.

Frank, William M., 1977: The Structure and Energetics of the Tropical Cyclone I. Storm Structure. Mon. Wea. Rev., 105, 1119–1135.

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Backup/question slidesfollow

What about Sandy 2012?

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25 Oct 05:45 UTC

27 Oct 17:45 UTC 29 Oct 23:45 UTC

Operational best track

Large E. Pacific Examples:

HERNAN (2002, 9/1 12:00)

Rank 9/158, 239/738 (32.4% globally)

RICK (2009, 10/18, 06:00)

Rank 15/158, 290/738 (39.3% globally)

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V500: 9.17m/sR5 : 15.19o Lat


V500: 8.82 m/sR5 : 14.63o Lat

Small E. Pacific Examples:

FELICIA (1997, 07/19 06:00)

Rank 158/158, 737/738 (99.9% globally)

(KENNETH 2005, 9/18, 12:00)

Rank 146/158, 720/738 (97.6% globally)

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Vmax:115ktLat: -14.2oNPC1: -0.41PC2: -1.19PC3: -1.38

V500: 4.39 m/sR5 : 7.51o Lat

Vmax:115ktLat: 15.60oNPC1: 0.17PC2: -2.36PC3: -0.87

V500: 2.92 m/sR5 : 5.14o Lat

Our Proposed Solution

To relate patterns in infrared (IR) satellite images of TCs and storm latitude to a measure of TC size

• TC size estimates come from global analyses of the azimuthally averaged tangential flow at 850 hPa and 500 km (V500) radius that is scaled to provide a size estimate

• IR TC images come from both HURSAT v3.0 and the CIRA IR TC image archive and span 1978-2011

• TC latitude comes from TC best track data.

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Algorithm Development

• Create a multiple linear regression that estimates TC circulation (V500, based on GFS) based on– 1-dimensional IR principle components ( first 3)– Storm latitude

• Estimates TC size by scaling TC circulation to a radius where the TCs influence vanishes at 850 hPa (R5) using a climatological vortex decay rate

• The algorithm explains 29% of the V500 variance

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Small W. Pacific Examples:

KUJIRA (2009, 05/04, 18:30)

Rank 262/266, 690/738 (93.5% globally)NESAT (2005, 6/03, 17:25)

Rank 256/266, 665/738 (90.1% globally)

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V500: 5.44m/sR5 : 9.20o Lat


V500: 5.92 m/sR5 : 9.96o Lat

Large S. Hemisphere Examples:

GAEL (2009, 2/7, 00:00)

Rank 17/204, 44/738 (6.0% globally)

YASI (2011, 2/2, 06:14)

Rank 26/204, 81/738 (11.0% globally)

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V500: 10.53 m/sR5 : 17.37o Lat


V500: 10.16 m/sR5 : 16.78o Lat

Small S. Hemisphere Examples:

BERTIE (2005, 11/23 00:00)

Rank 203/204, 734/738 (99.5% globally)

GELANE (2010, 2/19, 12:00)

Rank 193/204, 672/738 (91.1% globally)

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Vmax:130ktLat: -17.3oNPC1: -1.06PC2: -1.39PC3: -0.61

V500: 5.87 m/sR5 : 9.88o Lat

Vmax:115ktLat: -12.5oNPC1: -0.19PC2: -1.92PC3: 0.87

V500: 3.75 m/sR5 : 6.48o Lat

Our Proposed Solution1. Use global three-hourly records of infrared (IR) satellite data (~30 years)

i. Hurricane Satellite Archive (HURSAT v3.0; Knapp and Kossin 2007) (1978-2006)

ii. CIRA/RAMMB TC IR image archive (Zehr and Knaff 2007) (2007-2011)

2. Relate features in the IR imagery and TC location to variables related to TC size variations

i. IR PCs - Principle components (PCs) of the azimuthal mean IR brightness temperatures (Tb), radius = 2-602km @ 4km increments

• IR PCs have been used to estimate TC wind structure (Mueller et al. 2006, Kossin et al. 2007)

ii. TC latitude• IR Brightness temperatures (Tbs) vary with latitude• TC size has been shown to increase with latitude (Merrill 1984)

iii. V500 - Global analyses of the azimuthal mean tangential wind speeds at a fixed 500 km radius at 850 hPa (avoids frictional layer)

• V500 has been used to account for variations in TC circulation size to improve estimates of MSLP (Knaff and Zehr 2007)

• When speaking of rawindsonde composites of western North Pacific TCs… “Strong upward vertical motion exists inside about 4o (latitude) radius. From 4-6o there is moderate subsidence below 400 mb indicating the mean location of the moat region” – Frank (1977), where the moat region contains little or no deep convection.

• “The scale of the circulation is very large. Even at 14o radius there is substantial mean radial flow and the strength of the outer tangential circulation is at least statistically correlated with central pressure” – Frank (1977)

32

an objective satellite-based tropical cyclone size climatology john a. knaff, noaa center for...

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