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 1/9/ th AMS Conference on Applied Climatology 2

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 TC latitude comes from TC best track data. 1/9/ th AMS Conference on Applied Climatology 3

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 1/9/ th AMS Conference on Applied Climatology 4

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 1/9/ th AMS Conference on Applied Climatology 5 Small ( 13.9 o ) Tropical Depression (Max<34 kt) Tropical Storm (34kt<Max≤64kt) Minor Hurricane/TC (64kt<Max≤95kt) Major Hurricane/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 o Latitude σ= 3.0 o Latitude

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 1/9/ th AMS Conference on Applied Climatology 6 Vmax:125kt Lat: o N PC1: PC2: 2.65 PC3: 0.55 V500: m/s R5 : o Lat Vmax:115kt Lat: o N PC1: 0.17 PC2: PC3: V500: 2.92 m/s R5 : 5.14 o 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) 1/9/ th AMS Conference on Applied Climatology 7 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) Frequency of Occurrence

Lifecycle of TC size Findings: 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 1/9/ th AMS Conference on Applied Climatology 8 North Atlantic NW Pacific S. Hemisphere NE Pacific Tropical Storms Minor TCs/Hurricanes Major TCs/Hurricanes Time of maximum intensity TC Size 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., sin=0).

Locations of Largest (25%) and Smallest (25%) TCs 1/9/ th AMS Conference on Applied Climatology 9 Minor TCs Hurricanes and max intensity Major TCs Hurricanes and 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 SmallestLargest

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). 1/9/ th AMS Conference on Applied Climatology 10 N. Atlantic NE Pacific NW Pacific Southern Hemisphere Southern Hemisphere 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 1.Low latitude TCs that move to the west and weaken slowly 2.TCs that weaken rapidly, and 3.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 Size Max Intensity 1/9/ th AMS Conference on Applied Climatology 11 Shrinking prior to maximum intensity (top 10%)

Inter-annual Trends in maximum TC Size ( ) 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 1/9/ th AMS Conference on Applied Climatology

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 1/9/ th AMS Conference on Applied Climatology 13

Questions

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, 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, – Characteristics based on best track, aircraft, and IR images. J. of Climate, 20, Frank, William M., 1977: The Structure and Energetics of the Tropical Cyclone I. Storm Structure. Mon. Wea. Rev., 105, 1119– /9/ th AMS Conference on Applied Climatology 15

Backup/question slides follow

Algorithm Development Developmental Data Details: 1.Six-hourly cases in the CIRA IR archive 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 () First three normalized Principle Components of the azimuthally averaged brightness temperature (T b )PCs The algorithm is based on multiple linear regression and explains 29% of the variance of V500 The regression equation is : 1/9/ th AMS Conference on Applied Climatology 17 The first 3 PCs explain 95% of the azimuthally averaged Tb EOF patterns are shown to the left

TC Size Based upon TC Circulation (V500) 1/9/ th AMS Conference on Applied Climatology 18

What about Sandy 2012? 1/9/ th AMS Conference on Applied Climatology Oct 05:45 UTC 27 Oct 17:45 UTC29 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) 1/9/ th AMS Conference on Applied Climatology 20 Vmax:130kt Lat: 27.3 o N PC1: PC2: 0.63 PC3: V500: m/s R5 : o Lat Vmax:140kt Lat: 26.3 o N PC1: PC2: 1.15 PC3: 2.06 V500: m/s R5 : o 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) 1/9/ th AMS Conference on Applied Climatology 21 Vmax:150kt Lat: 13.9 o N PC1: PC2: PC3: 0.67 V500: 5.90 m/s R5 : 9.93 o Lat Vmax:125kt Lat: 26.0 o N PC1: PC2: PC3: V500: 6.34 m/s R5 : o 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) 1/9/ th AMS Conference on Applied Climatology 22 Vmax:140kt Lat: 17.2 o N PC1: PC2: 0.87 PC3: 1.39 V500: 9.17m/s R5 : o Lat Vmax:155kt Lat: 15.2 o N PC1: PC2: 1.94 PC3: 0.12 V500: 8.82 m/s R5 : o 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) 1/9/ th AMS Conference on Applied Climatology 23 Vmax:115kt Lat: o N PC1: PC2: PC3: V500: 4.39 m/s R5 : 7.51 o Lat Vmax:115kt Lat: o N PC1: 0.17 PC2: PC3: V500: 2.92 m/s R5 : 5.14 o 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) 1/9/ th AMS Conference on Applied Climatology 24 Vmax:130kt Lat: o N PC1: PC2: 2.40 PC3: V500: m/s R5 : o Lat Vmax:125kt Lat: o N PC1: PC2: 2.65 PC3: 0.55 V500: m/s R5 : o 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) 1/9/ th AMS Conference on Applied Climatology 25 Vmax:125kt Lat: 20.1 o N PC1: PC2: PC3: V500: 5.71 m/s R5 : 9.63 o Lat Vmax:125kt Lat: 22.5 o N PC1: PC2: PC3: V500: 5.58 m/s R5 : 9.43 o 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) 1/9/ th AMS Conference on Applied Climatology 26 Vmax:115kt Lat: 17.2 o N PC1: PC2: PC3: V500: 5.44m/s R5 : 9.20 o Lat Vmax:125kt Lat: 14.1 o N PC1: PC2: PC3: V500: 5.92 m/s R5 : 9.96 o 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) 1/9/ th AMS Conference on Applied Climatology 27 Vmax:120kt Lat: 19.6 o N PC1: PC2: 2.17 PC3: V500: m/s R5 : o Lat Vmax:135kt Lat: 17.0 o N PC1: PC2: 1.94 PC3: 0.12 V500: m/s R5 : o 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) 1/9/ th AMS Conference on Applied Climatology 28 Vmax:130kt Lat: o N PC1: PC2: PC3: V500: 5.87 m/s R5 : 9.88 o Lat Vmax:115kt Lat: o N PC1: PC2: PC3: 0.87 V500: 3.75 m/s R5 : 6.48 o Lat

Our Proposed Solution 1.Use global three-hourly records of infrared (IR) satellite data (~30 years) i.Hurricane Satellite Archive (HURSAT v3.0; Knapp and Kossin 2007) ( ) ii.CIRA/RAMMB TC IR image archive (Zehr and Knaff 2007) ( ) 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 = 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 4 o (latitude) radius. From 4-6 o 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 14 o 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) 1/9/ th AMS Conference on Applied Climatology 29