docTropical cyclone intensity errors associated with lack of two
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Tropical cyclone intensity errors associated with lack of two
Tropical cyclone intensity errors associated with lack of two-way ocean coupling in high-resolution global simulations Colin M. Zarzycki National Center for Atmospheric Research Advance Study Program (ASP)/Climate and Global Dynamics (CGD) 32nd AMS Conference on Hurricanes and Tropical Meteorology April 18th, 2016 zarzycki@ucar.edu - 32nd AMS Conference on Hurricanes and Tropical Meteorology, San Juan, PR, USA, April 2016 Tropical cyclone “cold wakes” Typhoon Ioke (2006) http://www.remss.com/storm-watch • Tropical cyclones cool ocean surface during passage • Negative feedback on TC intensity • SST α TC intensity • Larger “cold wake” for stronger TCs zarzycki@ucar.edu - 32nd AMS Conference on Hurricanes and Tropical Meteorology, San Juan, PR, USA, April 2016 TCs in global climate models NCAR/DoE Community Atmosphere Model 25km 100km 1980-2002 simulated NATL trajectories IBTrACS 1980-2002 observed NATL trajectories Zarzycki and Jablonowski, (2014, JAMES) zarzycki@ucar.edu - 32nd AMS Conference on Hurricanes and Tropical Meteorology, San Juan, PR, USA, April 2016 Prescribed SSTs • High-resolution climate models generally run with prescribed SSTs! Small et al., (2014, JAMES) CESM coupled SST bias (Walsh et al., 2015, BAMS) • Ocean = (Heat * ∞) • No SST response to TCs TCs: coupled SSTs • Why? • Expensive at high resolution • Potential SST biases “kill” TCs TCs: prescribed SSTs • QUESTION: Does this bias high-res climate TCs? Hannay et al., 2015 zarzycki@ucar.edu - 32nd AMS Conference on Hurricanes and Tropical Meteorology, San Juan, PR, USA, April 2016 Oceans in GCMs: a hierarchy of complexity fixed SST Infinite heat reservoir thermodynamic slab Prescribed OHT empirical mixing slab Relaxation to reference state 1D mixing 3D dynamic ocean Weak relaxation to reference state + vertical advection + horizontal advection Increasing ocean complexity Fig. courtesy Brian Medeiros, NCAR zarzycki@ucar.edu - 32nd AMS Conference on Hurricanes and Tropical Meteorology, San Juan, PR, USA, April 2016 Oceans in GCMs: a hierarchy of complexity fixed SST Infinite heat reservoir thermodynamic slab Prescribed OHT empirical mixing slab Relaxation to reference state 1D mixing 3D dynamic ocean Weak relaxation to reference state + vertical advection + horizontal advection Increasing ocean complexity Fig. courtesy Brian Medeiros, NCAR zarzycki@ucar.edu - 32nd AMS Conference on Hurricanes and Tropical Meteorology, San Juan, PR, USA, April 2016 Modeling the ocean: simply @SST 1 = (SSTclim @t ⌧ Newtonian relaxation to climatology SST ) + 1 Fnet ⇢o c p h Sensible, latent, longwave/ shortwave fluxes (traditional slab ocean) Xcool Rcool ✓ SST Tdeep To ◆✓ ho h ◆ zarzycki@ucar.edu - 32nd AMS Conference on Hurricanes and Tropical Meteorology, San Juan, PR, USA, April 2016 Is a traditional slab ocean enough? Surface fluxes Vincent et al., (2012, JGR), Fig. 9 Advection Vertical mixing Increasing TC intensity Total zarzycki@ucar.edu - 32nd AMS Conference on Hurricanes and Tropical Meteorology, San Juan, PR, USA, April 2016 The simplest cold wake parameterization™? @SST 1 = (SSTclim @t ⌧ SST ) + Sensible, latent, Newtonian longwave/ relaxation to shortwave climatology fluxes (traditional slab ocean) 1 Fnet ⇢o c p h Xcool Rcool ✓ SST Surface Empirical stress cooling weighting rate function Tdeep To ◆✓ ho h ◆ Scaling based on mixed-layer depth (h !, forcing ") Scaling based on deep water ΔT (stratification !, forcing !) zarzycki@ucar.edu - 32nd AMS Conference on Hurricanes and Tropical Meteorology, San Juan, PR, USA, April 2016 The simplest cold wake parameterization™? @SST 1 = (SSTclim @t ⌧ SST ) + LARGE-SCALE RESTORATION 1 Fnet ⇢o c p h SURFACE FLUXES Xcool Rcool ✓ SST Tdeep To ◆✓ ho h VERT. MIXING/ UPWELLING Looking for “cheap” and constrained method of assessing impact of SSTAs in the mean sense! ◆ zarzycki@ucar.edu - 32nd AMS Conference on Hurricanes and Tropical Meteorology, San Juan, PR, USA, April 2016 Experimental design • Variable-resolution global CAM-SE • ~25 km grid spacing over NATL/NPAC • 66 ensemble members • 25 FIXEDSST, 25 SLAB, 16 THERMO (no mixing term) • June-December (7 month season per member) • 1980-2000 avg. climo forcing (SSTs, aerosols, solar, etc.) zarzycki@ucar.edu - 32nd AMS Conference on Hurricanes and Tropical Meteorology, San Juan, PR, USA, April 2016 Simulated TC trajectories FIXEDSST 25 yrs THERMO 16 yrs SLAB 25 yrs zarzycki@ucar.edu - 32nd AMS Conference on Hurricanes and Tropical Meteorology, San Juan, PR, USA, April 2016 What do cold wakes look like? ~6 week movie Sept.-Oct. SLAB ensemble member #3 10-m wind contoured rainbow (bottom), SST anomaly red/blue (left) zarzycki@ucar.edu - 32nd AMS Conference on Hurricanes and Tropical Meteorology, San Juan, PR, USA, April 2016 Can we match obs? ALL STORMS This study Dare and McBride (2011, MWR), Fig. 5 All members, avg. Lagrangian SST anomaly over 1° area centered on TC MSLP zarzycki@ucar.edu - 32nd AMS Conference on Hurricanes and Tropical Meteorology, San Juan, PR, USA, April 2016 Can we match obs? ALL STORMS This study U10 > 32 m/s (HURR) Slow-moving (V0 < 5 m/s) This study Dare and McBride (2011, MWR), Fig. 5 All members, avg. Lagrangian SST anomaly over 1° area centered on TC MSLP Observed slowmoving hurricanes (by S-S cat) from Lloyd and Vecchi (2011, Jclim), Fig. 3 zarzycki@ucar.edu - 32nd AMS Conference on Hurricanes and Tropical Meteorology, San Juan, PR, USA, April 2016 Can we match obs? SLAB (WITH MIXING) ONLY! This study Mei and Pasquero (2013, JClim), Fig. 14 zarzycki@ucar.edu - 32nd AMS Conference on Hurricanes and Tropical Meteorology, San Juan, PR, USA, April 2016 Large-scale climate response? NATL MDR SSTA (SLAB) Ensemble mean Taylor climatology EPAC MDR SSTA (SLAB) WPAC MDR SSTA (SLAB) zarzycki@ucar.edu - 32nd AMS Conference on Hurricanes and Tropical Meteorology, San Juan, PR, USA, April 2016 Intensity PDFs PDF of all 6-hourly 10-m wind speed hits zarzycki@ucar.edu - 32nd AMS Conference on Hurricanes and Tropical Meteorology, San Juan, PR, USA, April 2016 Intensity PDFs PDF of all 6-hourly 10-m wind speed hits zarzycki@ucar.edu - 32nd AMS Conference on Hurricanes and Tropical Meteorology, San Juan, PR, USA, April 2016 Intensity response as a function of SSTA SLAB (WITH MIXING) ONLY! hPa reduction percoupling. 1°C SSTA F IG . 9. Reduction in sea level pressure associated with 3-7 two-way coupling versus one-way Reductio SLP (DPSL) as a function of both ambient (background) SST and SSTA is shown in (a.) with mean DPS zarzycki@ucar.edu - 32nd AMS Conference on Hurricanes and Tropical Meteorology, San Juan, PR, USA, April 2016 Intensity statistics 10-m wind (m/s) FIXEDSST THERMO Mean 28.2 27.7 Highest 5% 48.2 47.0 Highest 1% 55.0 54.8 Highest 0.1% 60.7 59.8 Max 66.5 63.9 TS Category Category Category Category Category 1 2 3 4 5 FIXEDSST 423 319 149 209 150 20 64% decrease! SLAB 26.6 42.8 49.1 55.5 59.8 SLAB 445 340 160 183 60 1 zarzycki@ucar.edu - 32nd AMS Conference on Hurricanes and Tropical Meteorology, San Juan, PR, USA, April 2016 Summary • Impact of TC cold wake isolated via observationallyconstrained slab ocean w/ empirical mixing/upwelling • Thermodynamic-only slab not sufficient • CAM (~25km) capable of producing intense TCs (Cat 3+) … therefore realistic cold wakes • If GCM resolves intense TCs (~Cat 3+) without two-way coupling, high-end TCs ~10-20mph (4-8m/s) too strong! • Disproportionate impact in tail of intensity distribution • Potential stakeholder influence when applying discrete intensity classifications (ex: Saffir-Simpson) • High-res GCMs will push further into this space over coming years
