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
[email protected] - 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
[email protected] - 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)
[email protected] - 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
[email protected] - 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
[email protected] - 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
[email protected] - 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
◆
[email protected] - 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
[email protected] - 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 !)
[email protected] - 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!
◆
[email protected] - 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.)
[email protected] - 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
[email protected] - 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)
[email protected] - 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
[email protected] - 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
[email protected] - 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
[email protected] - 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)
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Intensity PDFs
PDF of all 6-hourly 10-m
wind speed hits
[email protected] - 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
[email protected] - 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
[email protected] - 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
[email protected] - 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