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Tropical Cyclone Structural Changes in
Response to Ambient Moisture
Variations
Qinghong Zhang and Yue Ying Dept. of Atmospheric and Oceanic Sciences,
Peking University, China
qzhang@pku.edu.cn
Rapid Change Phenomena in Tropical Cyclones Nov. 7 th 2012 Kaikou， Hainan Island
TC structural parameters
(Holland and Merrill 1984)
Azimutha
l wind spee
d (m
s ‐1 )
Radius (°)
Intensity Change
Strength Change
Size Change
Peak (strong or weak)
Spread (large or small )
Correlation between intensity and size
r=0.28
Merrill 1984 Intensity
Size
1957~1977 Atlantic Hurricanes
Occurrence (%) of intensity and size of Northwest Pacific TCs during 2001~2011
To forecasters: Stop focusing only on intensity ! TC structural change: multi‐variable evolution
Environmental factors controlling: TC intensity: SST, vorticity, vertical shear, ... TC size: surface LH SH flux, moisture, ...
Size
Intensity “phase diagram”
evolution path
What types of evolution paths are there?
angular momentum conserved
radial advection of angular momentum
tangential wind tendency
tangential wind budget
Size
Intensity
Intensification
Size
Intensity
Size‐growth
What types of evolution paths are there?
• extra angular momentum generated
Idealized experiments (Wang 2009, Hill and Lackmann 2009) moisture’s role during structural evolution.
Q1: Does moisture do matter with TC structure evolution from observation?
Q2: What if TC is in vertically sheared environment?
• Internal dynamics: VRW, rainband structure, eyewall replacement cycles. • Environmental factors: SST anomalies, dry air intrusion, vertical wind shear,
interaction with other weather systems.
Q: What controls TC structural evolution?
Intensifying TCs are associated with more ambient moisture Hendricks et al. 2010
2001~2011 intensity and structure western North Pacific
• JTWC Best Track intensity: Max wind speed
size: radius of 34 knot
• Moisture: TRMM, SSM/I microwave Precipitable Water
Intensity & size change • DEF: change in ±6h
“weakening” 12hΔVmax<‐3 m s ‐1
“contracting” 12hΔSize<‐20 km
“growing” 12hΔSize>20 km
“intensifying” 12hΔVmax>3 m s ‐1
“growing & intensifying”: More environment moisture. “contracting & weakening”: More dry air instrun from north
average contracting growing
intensifying
weakening
Moisture Anomaly with TC intensity & Size Change In the western North Pacific Ocean 2001‐2011
Both positive & negative Anomaly in moisture: Angular Momentum conserved
Average
Moisture Anomaly with TC intensity & Size Change
contracting growing
intensifying
weakening
Summary‐1
TC size change, as well as TC intensity change may related to ambient moisture, in the western North Pacific.
• Growing and intensifying TCs, comparing with contracting and weakening ones, displayed more ambient moisture content, their moisture fields are more axi‐symmetric.
• Grow‐weaken & contract‐intensify TCs, both negative and positive y moisture abnormally exist
Q1: Does moisture do matter with TC structure evolution from observation?
simulation of TC Talim 2005
• WRF model real‐case run. • 4km grid spacing, 26 levels.
• explicit convection simulation with WSM6 microphysics scheme.
• initialized with NCEP final analyses data.
• bogus Rankine vortex according to JTWC best track data, spun up for 24 h before interested period.
Simulated evolution path
horizontal wind speed at z=1km (m s ‐1 )
a
b
c
d
a (t=0h)
b (t=18h)
size‐growth
Simulated evolution path
horizontal wind speed at z=1km (m s ‐1 )
a
b
c
d
b (t=18h)
c (t=24h)
inner‐core decay
Simulated evolution path
horizontal wind speed at z=1km (m s ‐1 )
a
b
c
d
c (t=24h)
d (t=36h)
outer‐core intensification
Sensitivity experiment 1
• CTRL • Q‐, subtracting 2g kg ‐1 QVAPOR (dry environment)
• Q+, adding 2g kg ‐1 QVAPOR (moist environment)
modification applied at t=0h within 300~600km annular region
moisture’s impact on structural evolution path
• Dry environment: faster intensification small size
• Moist environment: slow intensification rate size‐growth a
b
c
d
b c
d
b
c
d
moisture’s impact on secondary circulation
1. Latent heating changes secondary circulation (Sawyer‐Eliassen equations)
Q‐
Q+
W Vr
2. Vr determines inward advection of angular momentum, thus the evolution path
inner‐core intensification
Size‐growth
Conceptual model of TC life cycle
Dry Environment:
large Vr convection advected inward...
...and spin‐up the inner‐core
“intensification”
Conceptual model of TC life cycle
Moist environment:
more updraft small Vr
convection forms a ring...
...and becomes outer rainbands “size‐growth”
Asymmetric rainbands
• vertically sheared environment
homogeneous environment
with easterly vertical wind shear
‐v r η due to rainband induced flow
Does the location of moist air matter?
‐v r η > 0 upstream side of rainband
‐v r η < 0 downstream side of rainband
Reimer and Montgomery 2011
Sensitivity experiment 2
• Q‐, subtracting 2g kg ‐1 QVAPOR • QN‐, subtracting 2g kg ‐1 QVAPOR from the northern (upstream) sector
• QS‐, subtracting 2g kg ‐1 QVAPOR from the southern (downstream) sector
upstream
downstream
Does the location of moist air matter?
QN‐ produces similar result to Q‐
TC is more sensitive to moisture in the North (from upstream side of rainbands).
Moist air from the North travels faster into TC inner core.
Backward trajectories from t=36h at z=1km level seeded along r=100km radius.
time (h)
Conceptual model in sheared environment
Moist/dry air feeding into TC, leading to different structure.
Air from downstream side does not influence TC much.
Summary‐2 • Moist (dry) environment favors TC size‐growth (inner core
intensification).
• In sheared environment, TC outer rainbands are sensitive to their upstream moisture supply. Both the amount and the location of moisture is important in this case.
Moist
Dry
Q2: What if TC is in vertically sheared environment?
Thank You!
CI 56 518 GI
CS 174
92 GW
SI 525
852 330GS
330SW 133CW
Size change
Intensity change
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