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 nowledgements: Scott Denning, Kathy Corbin, Ian Ba
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Implementation of a boundary layer heat flux parameterization into the Regional Atmospheric Modeling…
ChEAS meeting: May 14, 2007 Motivation A 20% error in Z i produces a 20% error in CO 2 tendency Z i is very difficult to determine accurately in mesoscale models because of the coarse resolution Z i is the depth of the PBL
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Implementation of a boundary layer heat flux parameterization into the Regional Atmospheric Modeling
System
Erica McGrath-SpanglerDept. of Atmospheric Science
Colorado State UniversityChEAS May 14, 2007
Acknowledgements: Scott Denning, Kathy Corbin, Ian Baker
ChEAS meeting: May 14, 2007
Overview
• Motivation• Parameterization• Experiment Setup• Results• Conclusions• Future Work
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
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dCO2
dt∝ NEE
ZiZi is the depth of the PBL
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
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
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
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dθvdt
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
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w'θv' | zi = −α w'θv' | s
ChEAS meeting: May 14, 2007
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∂θ∂t
= α w'θv' | sΔz
€
∂rv∂t
= qvMρ 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
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M = ρα w'θv' | sΔθv
Units of kg m-2 s-1
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