Examination of Global Satellite- Based Tropical Cyclone Size Variations John A. Knaff,...
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Examination of Global Satellite-Based Tropical Cyclone Size Variations
John A. Knaff, NOAA/NESDIS/STAR, Fort Collins, Colorado,
Mark DeMaria, NOAA/NESDIS/STAR, Fort Collins, Colorado,
Scott Longmore, CIRA, Colorado State University, Fort Collins, Colorado
Charles R. Sampson, Naval Research Laboratory, Monterey, California
Motivation• Tropical cyclone (TC) size is an important factor for inferring
TC impacts and issuing warnings
• No global climatologies of tropical cyclone (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 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– 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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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
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
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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:
Decreasing trend, not statistically significant
NW Pacific:
Increasing trend, not statistically significant
Southern Hemisphere:
Decreasing trend, not statistically significant
Globe:
No trend.
12
N. Atlantic
Globe
Southern Hemisphere
NW Pacific
NE Pacific
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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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References:
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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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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What about Sandy 2012?
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25 Oct 05:45 UTC
27 Oct 17:45 UTC 29 Oct 23:45 UTC
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
Vmax:140ktLat: 26.3oNPC1: -1.51PC2: 1.15PC3: 2.06
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 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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Vmax:140ktLat: 17.2oNPC1: -1.52PC2: 0.87PC3: 1.39
V500: 9.17m/sR5 : 15.19o Lat
Vmax:155ktLat: 15.2oNPC1: -1.85PC2: 1.94PC3: 0.12
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
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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Vmax:130ktLat: 22.89oNPC1: -1.90PC2: 2.40PC3: -0.16
V500: 11.66 m/sR5 : 19.20o Lat
Vmax:125ktLat: 20.73oNPC1: -2.04PC2: 2.65PC3: 0.55
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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Vmax:125ktLat: 20.1oNPC1: -0.68PC2: -1.72PC3: -0.19
V500: 5.71 m/sR5 : 9.63o Lat
Vmax:125ktLat: 22.5oNPC1: -0.34PC2: -1.90PC3: -0.08
V500: 5.58 m/sR5 : 9.43o Lat
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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Vmax:115ktLat: 17.2oNPC1: -0.86PC2: -1.15PC3: -1.76
V500: 5.44m/sR5 : 9.20o Lat
Vmax:125ktLat: 14.1oNPC1: -1.28PC2: -1.04PC3: -0.46
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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Vmax:120ktLat: 19.6oNPC1: -1.73PC2: 2.17PC3: -0.38
V500: 10.53 m/sR5 : 17.37o Lat
Vmax:135ktLat: 17.0oNPC1: -1.85PC2: 1.94PC3: 0.12
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)
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