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High-Resolution Simulation of Hurricane Bonnie (1998). Part II: Water Budget. SCOTT A. BRAUN J. Atmos. Sci., 63, 43-64. Introduction. - PowerPoint PPT Presentation
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High-Resolution Simulation of Hurricane Bonnie (1998). Part
II: Water Budget
SCOTT A. BRAUNJ. Atmos. Sci., 63, 43-64
Introduction• The total heat content of normal tropical air, if raised by undi
lute 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 descriptiona. Simulation descriptionCoarse-resolution:
Started at 1200 UTC 22/08/1998 (36 hrs)36 km: 91× 9712 km: 160×160
High-resolution: Started at 1800 UTC 22/08/1998 (30 hrs) 6 km: 225×225 2 km: 226×226Vertical: 27 levels
8/23
8/24 •OBS•MM5
TRMM dBZ at 2 km MSL at 1800 UTC 22/08
Simulated dBZ at 2 km MSL valid 1200 UTC 23/08.
TRMM dBZ at 1800 UTC 22/08 MM5 dBZ at 1200 UTC 23/08
>10 dBZcontoured frequency by altitude diagrams (CFADs; Yuter and Houze 1995) of reflectivity
40 m 2.7km
6.8km
12km
dBZ + w
(qcl+qci) + w
dBZ + Vr
tangential velocity radial velocity
vertical velocity qv
qcl + qci qra, qsn, qgr
56ms-1
Budget formulationqv 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:
the time-averaged and vertically integrated amount:
the time-averaged, volumetrically integrated amount:
h-1·(kg/m3)·[(kg/kg) · h-1]·h=kg·m-3·h-1
h-1·(kg/m3)·[(kg/kg) · h-1]·m·h=kg·m-2·h-1
(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 resultsa. 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
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.
eyewall region (30-70 km) outer region (70-200 km)
b. Condensed water budget
cloud sink horizontal flux divergence
net source vertical flux divergence
boundary layer source added water mass to offset negative mixing ratios
condensation (total source of cloud)
(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
cloud budget
source for rain
source for graupel
source for snow
sink for rain
sink for graupel
sink for snow
net microphysical source horizontal flux divergence
precipitation fallout andvertical flux divergence
added water mass to offsetnegative mixing ratios
precipitation budget
cloud budget(a) to (f) interval: 2 g m-3 h-1
thin solid lines show the zero contour
(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
cloud sink
condensation evaporation
precipitation fallout
total rain source warm rain source
cold rain source graupel source
qv
6.8km
(a) and (c) interval: 20 kg m-2 h-1
(b) interval: 5 kg m-2 h-1
c. Volume-integrated budgets
P/C ~ 65 %
Zero/C ~ 13 %Zero/C ~ 12 %
d. The artificial water source
cloud liquid water
cloud ice
rain
snow
graupel
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
raincloud water
graupel
Conclusion • A detailed water budget is performed using a high-resolution simulation o
f 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.