14th WRF Workshop
24 June 2013
How radiative processes can influence
tropical cyclones
Robert Fovell and Yizhe Peggy Bu
University of California, Los Angeles
rfovell@ucla.edu
Thanks to:
Greg Thompson, NCAR/DTC; Ligia Bernardet and Mrinal Biswas, DTC;
Brad Ferrier, NCEP; Hui Su and Longtao Wu, JPL; Kristen Corbosiero, U at Albany
1
Theme
• Focus is on cloud-radiative forcing (CRF)
• CRF…
• Is not clear-sky or “background” radiative forcing
• Is modulation of clear-sky forcing by hydrometeors
• CRF consists of…
• Longwave (LW) cooling at cloud tops, at all times
• Shortwave (SW) warming at cloud tops, during day
• LW warming within clouds
• CRF depends on microphysics parameterizations (MP)
• Interaction with radiation is particle size-dependent, thus speciesdependent
• MPs should share size information to the radiation scheme… and
one of them finally does (thanks to Greg Thompson)
2
Background & motivation
3
Semi-idealized experiment
very small part of domain shown
• Real-data WRF-ARW
@ 3 or 4 km for 72 h
• Uniform SST
• Single sounding
• No initial flow
• NO LAND
• 7 MPs
• One initial condition
4
Fovell et al. (2009)
Fovell et al. (2010)
Semi-idealized experiment
very small part of domain shown
• MPs yielded
different…
• …amounts of various
hydrometeors
• …diabatic heating
patterns
• …symmetric wind
structures
• …asymmetry patterns
• …motions
• …intensities
5
Fovell et al. (2009)
Fovell et al. (2010)
Influence of CRF
CRF-on
CRF-off
6
Fovell et al. (2010)
Troposphere-averaged vertical
velocity
CRF-on
CRF-off
150 km
Fovell et al. (2010)
with ARW, curved Earth
CRF caused storms to be
wider, less asymmetric,
weaker
7
Semi-idealized HWRF experiments
Single sounding, uniform SST, no initial flow, no land
8
HWRF experimental design
• HWRF rev. 6557 (28 March 2013)
• Greg Thompson’s microphysics scheme
• RRTMG radiation package
•
•
•
•
connected
2012 operational configuration (3 telescoping domains)
Thompson/RRTMG CRF-on vs. CRF-off
Operational HWRF* is Ferrier + GFDL
Focus on structure
9
no land
* For 2013 and earlier seasons
HWRF simulation strategy
for semi-idealized experiments
Vortex-following
1 full diurnal cycle
HWRF model physics called every time step
10
HWRF results when “clouds are cloudy”
Control run: Thompson/RRTMG with CRF-on
Averaged over 1 diurnal cycle
11
200 km
100 km
Houze (2010)
400 km
Lower troposphere vertical velocity
(sfc-500 mb)
Expected structural
asymmetry owing to
beta shear
Thompson/RRTMG
12
18 km
Thompson/RRTMG structure
354 km
Radial (shaded) and tangential velocity (m/s)
Temporally and azimuthally averaged
13
Thompson/RRTMG structure
Condensation (shaded) and net radiative forcing (K/h)
14
Thompson/RRTMG structure
net cooling
~ 7 K/day
C
W
net warming
~ 1 K/day
Condensation (shaded) and net radiative forcing (K/h)
Net radiation = LW + SW and includes background (clear-sky) forcing
Radiation contour interval differs for positive and negative values
15
C
W
LW only
W
SW only
16
Clear-sky radiative forcing
(unperturbed by convection)
SW
LW
Averaged over annulus
of 350 km radius and
one diurnal cycle
17
Clear-sky radiative forcing
(after TC development; clouds still transparent)
LW
SW
SW
LW
Averaged over annulus
of 350 km radius and
one diurnal cycle
18
From a CRF-off simulation
Total radiative forcing
(after including CRF)
LW
SW
LW
SW
SW
LW
Averaged over annulus
of 350 km radius and
one diurnal cycle
19
CRF-on vs. CRF-off
CRF-on
CRF-off
SW
SW
SW
20
Averaged over 350 km radius
“A difference is a difference only if it makes a difference.”
– Darrell Huff, How to Lie With Statistics
21
200 km
100 km
CRF-on
CRF-off
400 km
Lower troposphere vertical velocity
(sfc-500 mb)
22
Thompson/RRTMG
icloud = 0 in namelist.input
Note 34-kt wind radius
> 70% larger with
CRF-on
34-kt wind radii
23
Radial velocity (colored) &
Tangential velocity (contoured, m/s)
Thompson/RRTMG
CRF-on
Thompson/RRTMG
CRF-off
354 km
24
20 m/s tangential wind contour highlighted
Radial velocity (colored) &
Tangential velocity (contoured, m/s)
Thompson/RRTMG
CRF-on
Thompson/RRTMG
CRF-off
Difference field
CRF-on storm:
wider eye, stronger
outflow, broader wind
field
25
Total condensate (colored) &
Net radiative forcing (contoured, K/h)
C
W
Thompson/RRTMG
CRF-on
W
Thompson/RRTMG
CRF-off
has clear-sky forcing only
354 km
26
Net radiation = LW + SW and includes background (clear-sky) forcing
Radiation contour interval differs for positive and negative values
Total condensate (colored) &
Net radiative forcing (contoured, K/h)
C
W
W
Thompson/RRTMG
CRF-on
Thompson/RRTMG
CRF-off
Difference field
CRF-on storm:
wider, thicker anvil
Eye size difference
27
Microphysics diabatic forcing (colored) &
qe (contoured, m/s)
Thompson/RRTMG
CRF-on
Thompson/RRTMG
CRF-off
240 km
28
Microphysics forcing in Domain 3
340K qe contour highlighted
Microphysics diabatic forcing (colored) &
qe (contoured, m/s)
Thompson/RRTMG
CRF-on
Thompson/RRTMG
CRF-off
CRF-on storm:
evidence of greater
convective activity
beyond eyewall
Difference field
29
Extra heating
Eye size difference
Microphysics forcing in Domain 3
340K qe contour highlighted
Analysis
• TCs with CRF active have:
•
•
•
•
Wider eyes
Broader tangential wind fields
More radially extensive diabatic heating
Stronger, deeper upper-level outflow (stronger inflow, too)
• Comparable results from WRF-ARW versions of this
experiment (not shown)
30
The physics of CRF: how and why
Axisymmetric simulations with George Bryan’s CM1
Moist and dry versions
31
CM1 experimental design
•
•
•
•
•
•
•
Axisymmetric framework (f-plane 20˚N)
5 km resolution
Thompson microphysics*
Rotunno-Emanuel moist-neutral sounding
Goddard radiation
Averaging period: 3-4 diurnal cycles
Moist and dry experiments
32
*Not identical to HWRF version
Radial velocity (colored) &
Tangential velocity (contoured, m/s)
CRF-on
CRF-on storm:
wider eye, stronger
outflow, broader wind
field
CRF-off
300 km
Stronger, deeper outflow
Difference field
33
Eye size difference
Radial velocity (colored) &
Tangential velocity (contoured, m/s)
CRF-on
34-kt wind radius
considerably larger with
CRF-on
CRF-off
Tangential wind at
lowest model level
Difference field
34
C
W
Total condensate (colored) &
Net radiative forcing (contoured, K/h)
CRF-on
CRF-off
CRF-on storm:
evidence of broader
convective activity
Difference field
35
CRF-fixed
CRF forcing actively
encourages storm
expansion
36
CRF < 0
Warming appears to
be more significant
factor
CRF > 0
CRF-fixed
37
400 km
Dry model radial wind response (colored)
to applied radiative forcing
(contoured, K/h)
Radial velocity difference (colored) &
CRF-clear sky radiative forcing difference
(contoured, K/h)
CRF-enhanced outflow
carries hydrometeors outward,
positive feedback
38
CRF < 0
Warming appears to
be more significant
factor (again)
CRF > 0
39
Dry model vertical velocity response
(colored) to applied radiative forcing
difference (contoured, K/h)
0.0075 m/s
~ 650 m per day
CRF < 0
CRF > 0
40
Dry model vertical velocity response
(colored) to applied radiative forcing
(contoured, K/h)
CRF < 0
CRF > 0
Warming appears to
be more significant
factor (again)
Weak but broad and persistent
lifting leads to more convective
activity
41
Cloud-radiative forcing (in-cloud)
encourages stronger outflow and
deep, broad ascent
…leading to more radially extensive
convective activity…
…resulting in a broader wind field
42
A CM1 microphysics diabatic
heating difference field (CRF-on – CRF-off)
Extra heating owing to CRF
400 km
Eye width
difference
43
Diabatic forcing from moist model
CRF-on
Dry model response
44
400 km
Diabatic forcing from moist model
CRF-on
widens, broadens
Dry model response
45
400 km
Diabatic forcing from moist model
CRF-on
Dry model response
46
400 km
CRF-related cooling and warming in HWRF
47
48
Warming appears to
be more significant
factor (again)
49
Operational HWRF
deep clouds are not
cloudy
(almost no CRF)
Operational physics called every time step
50
“[A]ll models are wrong; the practical question is how wrong do they have to
be to not be useful.”
– George E. P. Box
51
Schematic model - I
52
Schematic model - II
53
Schematic model - III
54
Schematic model - IV
55
Summary
• Cloud-radiative forcing can have a significant impact on storm
structure
• Directly by encouraging stronger outflow, broad ascent
• Indirectly by fostering enhanced convective activity
• Quickly -- much faster than one might guess
•
•
•
•
Operational HWRF physics produces little to no CRF response
Is direct validation of in-cloud LW forcing even possible?
Thanks Jimy and Wei for the invitation
Supported by the NOAA Hurricane Forecast Improvement
Program (HFIP)
56
[end]
57
C
W
W
CRF-on
CRF > 0
C
CRF-off
CRF < 0
Total condensate (colored) &
Net radiative forcing (contoured, K/h)
Thompson/RRTMG
58
CRF-on
CRF-off
RRTMG (CRF-on)
GFDL (no CRF)
[as in operational]
Lower troposphere vertical velocity
(sfc-500 mb)
Ferrier-based
59
Thompson/RRTMG
CRF-on
Ferrier/RRTMG
CRF-on
Thompson/RRTMG
CRF-off
Ferrier/GFDL
No CRF
60
CRF-on
CRF-off
Thompson
Total condensate (colored) &
Net radiative forcing (contoured, K/h)
354 km
Ferrier
61
CRF-on
CRF-off
Thompson
Radial velocity (colored) &
Tangential velocity (contoured, m/s)
Ferrier
62
CRF-on
CRF-off
Thompson
Ferrier
Microphysics diabatic forcing (colored) &
qe (contoured, m/s)
240 km
63
SW
Radiative forcing (K/day)
averaged 0 ≤ R ≤ 354 km
in domain 2
LW
One full diurnal cycle
T = Thompson
F = Ferrier
64