implementation of a boundary layer heat flux parameterization into the regional atmospheric...

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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

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

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

w'θv' | zi = −α w'θv' | s

ChEAS meeting: May 14, 2007

∂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

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

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

ChEAS meeting: May 14, 2007

ChEAS meeting: May 14, 2007

ChEAS meeting: May 14, 2007

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 0.3alpha

Temperature (K)

ChEAS meeting: May 14, 2007

PBL height vs alpha

18401860188019001920194019601980200020202040

0 0.05 0.1 0.15 0.2 0.25 0.3alpha

Height (m)

ChEAS meeting: May 14, 2007

ChEAS meeting: May 14, 2007

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

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

ChEAS meeting: May 14, 2007

Thanks

ChEAS meeting: May 14, 2007

ChEAS meeting: May 14, 2007

ChEAS meeting: May 14, 2007

ChEAS meeting: May 14, 2007

ChEAS meeting: May 14, 2007

ChEAS meeting: May 14, 2007

ChEAS meeting: May 14, 2007

Mixing ratio_ML vs alpha

4.54.74.95.15.35.55.75.96.16.3

0 0.05 0.1 0.15 0.2 0.25 0.3alpha

Mixing ratio (g/kg)

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