Transcript High-Resolution Simulation of Hurricane Bonnie (1998). Part II

High-Resolution Simulation of
Hurricane Bonnie (1998). Part II:
Water Budget
SCOTT A. BRAUN
J. Atmos. Sci., 63, 43-64
Introduction
• The total heat content of normal tropical air, if raised by
undilute ascent within cumulus towers, is insufficient to
generate a warm core capable of reducing the surface
pressure below 1000 mb (Riehl 1954; Palmen and Riehl
1957; Malkus and Riehl 1960; Kurihara 1975).
• Horizontal advection tended to transport drier air into the
core in the boundary layer and moist air from the eye to the
eyewall within the low-level outflow above the boundary
layer (Zhang et al. 2002).
• Few studies of the condensed water budget have been
conducted for hurricanes (Marks 1985; Marks and Houze
1987; Gamache et al. 1993).
• In this study, we compute budgets of both water vapor and
total condensed water from a high-resolution simulation of
Hurricane Bonnie (1998).
Simulation and analysis description
a. Simulation description
Coarse-resolution:
Started at 1200 UTC 22/08/1998 (36 hrs)
36 km: 91× 97
12 km: 160×160
High-resolution:
Started at 1800 UTC 22/08/1998 (30 hrs)
6 km: 225×225
2 km: 226×226
Vertical: 27 levels
8/23
8/24
•OBS
•MM5
TRMM dBZ at 2 km MSL
at 1800 UTC 22/08
TRMM dBZ at 1800 UTC 22/08
Simulated dBZ at 2 km MSL
valid 1200 UTC 23/08.
MM5 dBZ at 1200 UTC 23/08
>10 dBZ
contoured frequency by
altitude diagrams (CFADs;
Yuter and Houze 1995) of
reflectivity
40 m
6.8km
2.7km
12km
dBZ + w
(qcl+qci) + w
dBZ + Vr
tangential velocity
radial velocity
56ms-1
vertical velocity
qv
qcl + qci
qra, qsn, qgr
Budget formulation
qv is mixing ratio of water vapor;
qc is the mixing ratio of cloud liquid water and ice;
qp is the mixing ratio of rain, snow and graupel;
V’ is the storm-relative horizontal air motion;
w is the vertical air motion;
VT is the hydrometeor motion;
+ is source; - is sink;
C is the condensation and deposition;
E is the evaporation and sublimation;
B is the contribution from the
planetary boundary layer;
D is the turbulent diffusion term;
Z is the artificial source term associated with
setting negative mixing ratios to zero.
the azimuthally averaged horizontal advective flux is simply
that associated with radial transport
U and V are the Cartesian grid storm-relative horizontal velocities in the x and y directions;
u and v are the storm-relative radial and tangential winds,
the temporal and azimuthal mean:
h-1·(kg/m3)·[(kg/kg) · h-1]·h
=kg·m-3·h-1
the time-averaged and vertically integrated amount:
h-1·(kg/m3)·[(kg/kg) · h-1]·m·h
=kg·m-2·h-1
the time-averaged, volumetrically integrated amount:
(kg·m-3 ·h-1)·m3
=kg·h-1
Zx is artificial source terms associated with
setting negative mixing ratios (caused by errors
associated with the finite differencing of the
advective terms) to zero, that is, mass is added
to eliminate negative mixing ratios.
Budget results
a. Water vapor budget
condensation
horizontal flux divergence,
evaporation
vertical flux divergence,
(a) + (c)
(b) + (d)
divergence term
boundary layer source term
(a), (e), (f) interval:
2 g m-3 h-1
(b) and (d) interval:
20 g m-3 h-1
(c), (g), (h) interval:
0.5 g m-3 h-1
thin solid lines
show the zero contour
eyewall region (30-70 km)
outer region (70-200 km)
updraft
condensation occurring in updraft
much of the eyewall
condensation is associated
with hot towers.
The smaller contribution of
stronger updrafts is indicative
of the larger role of stratiform
precipitation processes outside
of the eyewall.
b. Condensed water budget
cloud sink
horizontal
flux
divergence
condensation
(total
source of cloud)
net source
vertical flux divergence
boundary layer source
cloud budget
(a) interval: 2 g m-3 h-1
(b) to (e) interval: 0.5 g m-3 h-1
(f) interval: 0.125 g m-3 h-1
thin solid lines show the zero contour
added water mass to offset
negative mixing ratios
source for rain
sink for rain
source for graupel
sink for graupel
cloud budget
(a) to (f) interval:
2 g m-3 h-1
thin solid lines show the
zero contour
source for snow
net microphysical source
sink for snow
horizontal
cloud sink flux divergence
precipitation
budget
precipitation fallout and
vertical flux divergence
added water mass to offset
negative mixing ratios
(a) to (c) interval:
2 g m-3 h-1
(d) interval:
0.5 g m-3 h-1
thin solid lines show the
zero contour
evaporation
condensation
(a) and (c) interval:
20 kg m-2 h-1
(b) interval:
5 kg m-2 h-1
total rain source
warm rain source
cold rain source
graupel source
precipitation fallout
6.8km
qv
c. Volume-integrated budgets
Zero/C ~ 12 %
Zero/C ~ 13 %
P/C ~ 65 %
d. The artificial water source
cloud liquid water
rain
snow
cloud ice
hydrometeors:
(a) shaded interval: 0.1 g m-3
(b) to (e) shaded interval: 0.5 g m-3
source terms:
(a) to (e) line interval: 0.5 g m-3 h-1
graupel
cloud water
graupel
rain
Conclusion
• A detailed water budget is performed using a high-resolution
simulation of Hurricane Bonnie (1998). The simulation
generally reproduces the track, intensity, and structure of the
storm, but overpredicts the precipitation as inferred from
comparison of model and TRMM radar reflectivities.
• The water vapor budget confirms that the ocean source of
vapor in the eyewall region is very small relative to the
condensation and inward transport of vapor, with the ocean
vapor source in the eyewall (0.7) being approximately 4% of
the inward vapor transport into the eyewall (16.8) region.
• In the eyewall, most of the condensation occurs within
convective towers while in the outer regions condensation
results from a mix of convective and stratiform precipitation
processes, with the stratiform component tending to dominate.
• Precipitation processes acting outside of the eyewall region
are not very dependent on the condensate mass produced
within and transported outward from the eyewall. Instead, the
precipitation derives from convection in outer rainbands and
the subsequent transition to stratiform precipitation processes.
Conclusion
• Although the artificial water mass source is very small at
any given grid point, its cumulative impact over large
areas and over time is more substantial, contributing an
amount of water that is equivalent to 15%–20% of the
total surface precipitation.
• This problem likely occurs in any MM5 simulation of
convective systems, but is probably much less a concern
for purely stratiform precipitation systems.