erica mcgrath-spangler dept. of atmospheric science colorado state university cheas may 14, 2007
DESCRIPTION
Implementation of a boundary layer heat flux parameterization into the Regional Atmospheric Modeling System. Erica McGrath-Spangler Dept. of Atmospheric Science Colorado State University ChEAS May 14, 2007. Acknowledgements: Scott Denning, Kathy Corbin, Ian Baker. Overview. Motivation - PowerPoint PPT PresentationTRANSCRIPT
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Implementation of a boundary layer heat flux parameterization into the Regional Atmospheric Modeling
System
Erica McGrath-Spangler
Dept. of Atmospheric Science
Colorado State University
ChEAS May 14, 2007
Acknowledgements: Scott Denning, Kathy Corbin, Ian Baker
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ChEAS meeting: May 14, 2007
Overview
• Motivation
• Parameterization
• Experiment Setup
• Results
• Conclusions
• Future Work
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ChEAS meeting: May 14, 2007
Motivation
• A 20% error in Zi produces a 20% error in CO2 tendency
• Zi is very difficult to determine accurately in mesoscale models because of the coarse resolution
€
dCO2
dt∝NEE
ZiZi is the depth of the PBL
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ChEAS meeting: May 14, 2007
SAM model Courtesy Tak Yamaguchi
QuickTime™ and aYUV420 codec decompressor
are needed to see this picture.
White = pos buoyantRed = neg buoyant
Large-Eddy Simulation: Morning Mixed-Layer Development
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ChEAS meeting: May 14, 2007
Mesoscale Models
• Mesoscale models can’t resolve overshooting thermals because of grid spacing
• Process is not currently parameterized in RAMS
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ChEAS meeting: May 14, 2007
Mixing at the top of the PBL
• At the top of the boundary layer, the Richardson number is very large ( )
• Since the mixing coefficient is inversely proportional to the Richardson number, the mixing is ~ 0 within the capping inversion
• Very difficult to initiate growth of the boundary layer
• RAMS does not include any process to initiate mixing
€
dθv
dt
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ChEAS meeting: May 14, 2007
Closure Assumption
• Heat flux at the boundary layer top is negatively proportional to the surface heat flux
• Mixes warm, dry free tropospheric air into the PBL and cool, moist boundary layer air into the capping inversion
€
w'θv' | zi = −α w'θv' | s
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ChEAS meeting: May 14, 2007
€
∂θ∂t
=α w'θv' | s
Δz
€
∂rv∂t
=qvM
ρ dryΔz
• Also mix the three wind components, TKE, and CO2 concentration
• The tendencies from entrainment mixing are the quantities themselves times the mass flux divided by density and the layer thickness
€
M =ρα w'θv' | s
Δθv
Units of kg m-2 s-1
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ChEAS meeting: May 14, 2007
RAMS setup
• RAMS version 5.04 modified to BRAMS version 2.0
• 42 vertical levels starting at 15m and vertically stretched by ~1.1 up to 6600m
• Includes a shallow convection parameterization
• Use Mellor and Yamada (1982) closure option for vertical diffusion
• Smagorinsky (1963) used for horizontal diffusion
• Coupled to SiB version 3
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ChEAS meeting: May 14, 2007
Idealized simulation
• Cyclic lateral boundary conditions– No weather systems can be horizontally advected into
the system
• Initialized horizontally homogeneously from a dry sounding
• Homogeneous surface– Flat topography at sea level– Vegetation is C3 broadleaf and needleleaf trees– Loam soil type– FPAR = 0.8– LAI = 4.0
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ChEAS meeting: May 14, 2007
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ChEAS meeting: May 14, 2007
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ChEAS meeting: May 14, 2007
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ChEAS meeting: May 14, 2007
Pot Temp_ML vs alpha
299
299.5
300
300.5
301
301.5
0 0.05 0.1 0.15 0.2 0.25 0.3
alpha
Temperature (K)
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ChEAS meeting: May 14, 2007
PBL height vs alpha
1840
1860
1880
1900
1920
1940
1960
1980
2000
2020
2040
0 0.05 0.1 0.15 0.2 0.25 0.3
alpha
Height (m)
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ChEAS meeting: May 14, 2007
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ChEAS meeting: May 14, 2007
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ChEAS meeting: May 14, 2007
Conclusions
• In nature, overshooting thermals warm, dry, and deepen the PBL
• Mesoscale models don’t include overshooting thermals
• I’ve introduced a parameterization into RAMS that accounts for this process
• Hope to be able to better simulate Zi and CO2 concentrations
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ChEAS meeting: May 14, 2007
Future Work
• Compare mesoscale simulations to an LES run of RAMS and to observations– Both with and without the parameterization
included
• Parameterization also affects surface temperature and dew point that are observed
• Assimilate those variables in order to better determine a value for the tunable parameter
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ChEAS meeting: May 14, 2007
Thanks
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ChEAS meeting: May 14, 2007
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ChEAS meeting: May 14, 2007
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ChEAS meeting: May 14, 2007
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ChEAS meeting: May 14, 2007
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ChEAS meeting: May 14, 2007
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ChEAS meeting: May 14, 2007
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ChEAS meeting: May 14, 2007
Mixing ratio_ML vs alpha
4.5
4.7
4.9
5.1
5.3
5.5
5.7
5.9
6.1
6.3
0 0.05 0.1 0.15 0.2 0.25 0.3
alpha
Mixing ratio (g/kg)