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An objective satellite-based tropical cyclone size climatology

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 Fort Collins, Colorado. A little about RAMMB. Background.

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An objective satellite-based tropical cyclone size climatology

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  1. 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 Fort Collins, Colorado

  2. A little about RAMMB Background 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 Employees

  3. 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 Fort Collins, Colorado

  4. 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

  5. Our Solution • 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) • 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) • TC latitude • IR Brightness temperatures (Tbs) vary with latitude • TC size has been shown to increase with latitude (Merrill 1984) • 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) • R5, the radius of 5 kt tangential winds at 850 mb is linearly related to V500 • Use global three-hourly records of infrared (IR) satellite data (~30 years) to create a climatology • Hurricane Satellite Archive (HURSAT v3.0; Knapp and Kossin 2007) (1978-2006) • CIRA/RAMMB TC IR image archive (Zehr and Knaff 2007) (2007-2011)

  6. Algorithm Development The first 3 PCs explain 95% of the azimuthally averaged Tb EOF patterns are shown to the left The algorithm is based on multiple linear regression and explains 29% of the variance of V500 The regression equation is : Developmental Data Details: • Six-hourly cases in the CIRA IR archive 1995-2011 • N. Atlantic and E. Pacific • 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

  7. 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

  8. Small (<7.9o) Average Large (>13.9o) Algorithm Results N=18686 N=38667 N=4133 Tropical Depression (Max<34 kt) • R5: • Mean =10.9 oLatitude • σ = 3.0 oLatitude Despite explaining only 29% of the variance the algorithm seems able to discriminate TC size variations and separate the TC from it’s environment N=9045 N=39741 N=10127 Tropical Storm (34kt<Max≤64kt) N=1783 N=16394 N=9361 Minor Hurricane/TC (64kt<Max≤95kt) N=426 N=6412 N=6369 Major Hurricane/TC (Max≥95kt) That is, variations in the environment contribute to the regression’s scatter Tb

  9. 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 Vmax:115kt Lat: 15.60oN PC1: 0.17 PC2: -2.36 PC3: -0.87 Vmax:125kt Lat: 20.73oN PC1: -2.04 PC2: 2.65 PC3: 0.55 V500: 2.92 m/s R5 : 5.14oLat V500: 11.90 m/s R5 : 19.58oLat

  10. 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) Vmax:130kt Lat: 27.3oN PC1: -1.97 PC2: 0.63 PC3: -0.93 Vmax:140kt Lat: 26.3oN PC1: -1.51 PC2: 1.15 PC3: 2.06 V500: 11.32 m/s R5 : 18.64oLat V500: 10.67 m/s R5 : 17.60oLat

  11. 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) Vmax:125kt Lat: 26.0oN PC1: -0.48 PC2: -1.74 PC3: -0.79 Vmax:150kt Lat: 13.9oN PC1: -0.89 PC2: -0.84 PC3: 0.67 V500: 5.90 m/s R5 : 9.93oLat V500: 6.34 m/s R5 : 10.64oLat

  12. 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) Vmax:130kt Lat: 22.89oN PC1: -1.90 PC2: 2.40 PC3: -0.16 Vmax:125kt Lat: 20.73oN PC1: -2.04 PC2: 2.65 PC3: 0.55 V500: 11.90 m/s R5 : 19.58oLat V500: 11.66 m/s R5 : 19.20oLat

  13. 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) Vmax:125kt Lat: 22.5oN PC1: -0.34 PC2: -1.90 PC3: -0.08 Vmax:125kt Lat: 20.1oN PC1: -0.68 PC2: -1.72 PC3: -0.19 V500: 5.71 m/s R5 : 9.63oLat V500: 5.58 m/s R5 : 9.43oLat

  14. Basin Specific TC size Distributions North Atlantic Tropical Storms (34 kt ≤ Vmax ≤ 63 kt) Minor TCs/Hurricanes/Typhoons (64 kt ≤ Vmax ≤ 95 kt) Major TCs/Hurricanes/Typhoons (Vmax > 95 kt) Eastern North Pacific • 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 Western North Pacific Frequency of Occurrence North Indian Ocean Southern Hemisphere TC Size (R5)

  15. North Atlantic NW Pacific S. Hemisphere NE Pacific Tropical Storms 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 Minor TCs/Hurricanes TC Size Major TCs/Hurricanes 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). Time of maximum intensity

  16. Locations of Largest (25%) and Smallest (25%) TCs at peak 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 Minor TCs Hurricanes and Typhoons @ max intensity Major TCs Hurricanes and Typhoons @ max intensity

  17. Major TCs Minor TCs Seasonality of TC size N. Atlantic N. Atlantic 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). NE Pacific NE Pacific NW Pacific NW Pacific Southern Hemisphere Southern Hemisphere

  18. Max Size Max Intensity TC Growth Findings: Maximum TC growth occurs with TCs that recurve at higher latitude and slowly weaken Minimum growth is associated with Low latitude TCs that move to the west and weaken slowly TCs that weaken rapidly, and TCs that are in weak steering/constantly changing environments Post-peak Intensity Weakening and Growing (top 10%) Shrinking prior to maximum intensity (top 10%) Maximum intensity and maximum size are simultaneous

  19. Inter-annual Trends in maximum TC Size (1981-2011) N. Atlantic 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. NE Pacific NW Pacific Southern Hemisphere Globe

  20. 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 1920 km 2560 km

  21. 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

  22. 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. VonderHaar 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.

  23. Backup/question slidesfollow

  24. What about Sandy 2012? Operational best track 25 Oct 05:45 UTC 27 Oct 17:45 UTC 29 Oct 23:45 UTC

  25. 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) Vmax:155kt Lat: 15.2oN PC1: -1.85 PC2: 1.94 PC3: 0.12 Vmax:140kt Lat: 17.2oN PC1: -1.52 PC2: 0.87 PC3: 1.39 V500: 9.17m/s R5 : 15.19oLat V500: 8.82 m/s R5 : 14.63oLat

  26. 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) Vmax:115kt Lat: -14.2oN PC1: -0.41 PC2: -1.19 PC3: -1.38 Vmax:115kt Lat: 15.60oN PC1: 0.17 PC2: -2.36 PC3: -0.87 V500: 2.92 m/s R5 : 5.14oLat V500: 4.39 m/s R5 : 7.51oLat

  27. 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.

  28. 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

  29. 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) Vmax:125kt Lat: 14.1oN PC1: -1.28 PC2: -1.04 PC3: -0.46 Vmax:115kt Lat: 17.2oN PC1: -0.86 PC2: -1.15 PC3: -1.76 V500: 5.44m/s R5 : 9.20oLat V500: 5.92 m/s R5 : 9.96oLat

  30. 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) Vmax:135kt Lat: 17.0oN PC1: -1.85 PC2: 1.94 PC3: 0.12 Vmax:120kt Lat: 19.6oN PC1: -1.73 PC2: 2.17 PC3: -0.38 V500: 10.53 m/s R5 : 17.37oLat V500: 10.16 m/s R5 : 16.78oLat

  31. 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) Vmax:130kt Lat: -17.3oN PC1: -1.06 PC2: -1.39 PC3: -0.61 Vmax:115kt Lat: -12.5oN PC1: -0.19 PC2: -1.92 PC3: 0.87 V500: 3.75 m/s R5 : 6.48oLat V500: 5.87 m/s R5 : 9.88oLat

  32. Our Proposed Solution • Use global three-hourly records of infrared (IR) satellite data (~30 years) • Hurricane Satellite Archive (HURSAT v3.0; Knapp and Kossin 2007) (1978-2006) • CIRA/RAMMB TC IR image archive (Zehr and Knaff 2007) (2007-2011) • Relate features in the IR imagery and TC location to variables related to TC size variations • 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) • TC latitude • IR Brightness temperatures (Tbs) vary with latitude • TC size has been shown to increase with latitude (Merrill 1984) • 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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