TC Lifecycle and Intensity Changes Part III: Dissipation / Transition Hurricane Katrina (2005) August 24-29 Tropical M. D. Eastin

Outline Dissipation Contributing Factors Extratropical Transition Definition and Statistics Common Large-Scale Factors Cyclone Phase Space Diagrams Tropical M. D. Eastin

TC Dissipation: Contributing Factors TC moves over a significant landmass: Loss of heat and moisture fluxes allow adiabatic cooling to dominate along inflow trajectories Increased surface friction drives additional inflow Net result is a decrease in deep convection required to maintain the warm core (considerable shallow convection continues) Warm core weakens and system fills over 12-48 hours At landfall, convection can temporarily increase due to enhanced friction and heat and moisture fluxes still occurring offshore Katrina (2005) 12h prior to landfall Katrina (2005) at landfall Katrina (2005) 12h after to landfall GOES IR imagery Tropical M. D. Eastin

Strong northerly shear TC Dissipation: Contributing Factors An increase in vertical wind shear: Moderate vertical shear can tilt the vortex, spreading the warm core column over a larger area, which increases the minimum surface pressure Strong vertical shear can “decouple” the low-level circulation from the upper-level warm core and its convection TS Chris (2006) Minimal Shear Strong northerly shear (12 hrs later) Tropical M. D. Eastin

Black arrow denotes the shear vector TC Dissipation: Contributing Factors An increase in vertical wind shear: Moderate vertical shear can significantly affect the convective structure Convection is often located down-shear-left (with respect to the shear vector) Hurricane Bonnie (1998) Shear Vector Black arrow denotes the shear vector Tropical M. D. Eastin

Saharan Air Layer (SAL) From Dunion and Velden (2004) TC Dissipation: Contributing Factors Joyce (2000) TC moves into or draws in dry air: The ingestion of dry mid-level air will induce local evaporative cooling, negative buoyancy, and cold/dry convective downdrafts with low θe. If a critical mass of such downdrafts reach low levels, the inflow layer θe will be considerably reduced and unable to recover before reaching the base of the eyewall cloud, preventing deep eyewall convection. 27 Sep Dry 60 kts Moist 28 Sep 27 Sep Dry 80 kts Saharan Air Layer (SAL) 29 Sep Moist 28 September SAL 28 Sep Joyce 60 kts GOES IR SSMI Water Vapor From Dunion and Velden (2004) Tropical M. D. Eastin

Ocean Mixed Layer Depth Intensity at Marked Locations TC Dissipation: Contributing Factors TC moves over cool SSTs or a Shallow Oceanic Mixed Layer: Strong surface winds in a TC boundary layer generate upwelling beneath the storm If the warm oceanic mixed layer is shallow, cold water will be mixed to the surface Colder water will reduce the sensible and latent fluxes, which will limit any increases in inflow θe and the potential for deep eyewall convection Hurricane Opal (1995) SST Pre-Storm C B A Ocean Mixed Layer Depth C B Intensity at Marked Locations A = 100 knots B = 130 knots C = 80 knots A Tropical M. D. Eastin

TC Dissipation: Contributing Factors Slow TC motion induces upwelling: Slow moving TCs generate a lot of upwelling in one location In some cases the warm ocean mixed layer can be completely eroded leaving SSTs < 26°C beneath the storm Note: Opal was not a slow moving TC Hurricane Opal (1995) SST Post-Storm Eye Warm Mixed Layer SSTs 26-30ºC Colder Deep Ocean SSTs < 26ºC Tropical M. D. Eastin

Extratropical Transition What is Extratropical Transition? A warm-core tropical cyclone moves north over colder water and structurally changes to a cold-core cyclone with distinct cross-storm asymmetries Statistics of Extratropical Transition: Hart and Evans (2001) 46% of all Atlantic TCs transitioned (1950-1996) 50% of landfalling TCs transition Transition is most common in October but occurs in all months Transition most often occurs between 35ºN and 45ºN Transition always involves interaction with a baroclinic cold-core trough Tropical M. D. Eastin

Extratropical Transition Indicators of Extratropical Transition: Acceleration of the TC into the Mid-latitudes Movement over cold SSTs Loss of organized convection in the inner core Loss of circulation in the upper-level outflow Acquisition of front-like characteristics Redistribution of precipitation to poleward or western side Spreading out of the surface wind field Asymmetry in temperature and moisture fields Intrusion of dry air into mid-levels of the storm Tropical M. D. Eastin

Extratropical Transition Hurricane Michael (2000) Extratropical Northward acceleration of Michael Tropical M. D. Eastin

Extratropical Transition Hurricane Michael (2000) A front begins to form Michael Outflow only to the north Evidence of a dry slot From Abraham et al. (2004) Tropical M. D. Eastin

Extratropical Transition Hurricane Michael (2000) Precipitation primarily north of the center Michael becomes extratropical From Abraham et al. (2004) Tropical M. D. Eastin

Extratropical Transition Hurricane Michael (2000) Estimated Center From Abraham et al. (2004) Tropical M. D. Eastin

Extratropical Transition Cyclone Phase Space: Developed by Bob Hart in 2003 Used to distinguish between a symmetric warm-core system (a TC) and an asymmetric cold core system (an extratropical cyclone) 900-600 mb Thickness Symmetry Checks for large synoptic-scale temperature gradients and fronts across the system’s track TCs are symmetric (low values near zero) Extratropical cyclones are asymmetric (large positive values) 900-600 mb Thermal Wind Checks for increasing or decreasing thickness with height to infer a warm core (increasing) or cold-core (decreasing) system TCs are warm-core (positive values) Extratropical cyclones are cold-core (negative values) http://moe.met.fsu.edu/cyclonephase/ Tropical M. D. Eastin

Extratropical Transition Hurricane Michael (2000) Tropical M. D. Eastin

TC Lifecycle and Intensity Changes Part III: Dissipation and Transition Summary Contributing Factors to Dissipation Movement over a significant land mass (physical processes) Moderate to strong vertical shear (physical processes) Dry Environment (physical processes) Cool SSTs and/or shallow oceanic mixed layer (physical processes) Extratropical Transition Definition Statistics of occurrence Indicators of Transition Cyclone Phase Space and assessing transition Tropical M. D. Eastin

References Abraham, J., J. W. Strapp, C. Fogerty, and M. Wolde, 2004: Extratropical transition of Hurricane Michael: An aircraft investigation Bull. Amer. Met. Soc., 85, 1323–1339 Black, M. L., J. F. Gamache, F. D. Marks Jr., C. E. Samsury, and H. E. Willoughby, 2002: Eastern Pacific Hurricanes Jimena of 1991 and Olivia of 1994: The effect of vertical shear on structure and intensity. Mon. Wea. Rev., 130, 2291–2312. Bosart, L. A., C. S. Velden, W. E. Bracken, J. Molinari, and P. G. Black, 2000: Environmental influences on the rapid intensification of Hurricane Opal (1995) over the Gulf of Mexico. Mon. Wea. Rev., 128, 322-352 Dunion, J. P., and C. S. Velden, 2004: The impact of the Saharan air layer on Atlantic tropical cyclone activity. Bull. Amer. Met. Soc., 75, 353-365. Hart, R. E., 2003: A cyclone phase space derived from thermal wind and thermal asymmetry. Mon. Wea. Rev. , 131, 585–616 Hart, R. E., and J. L. Evans, 2001: A climatology of the extratropical transition of Atlantic tropical cyclones. J. Climate, 14, 546–564. Tropical M. D. Eastin