Jonathan Pleim

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A New Combined Local and Non-Local Boundary Layer
Model ACM2

• Jonathan Pleim
• NOAA/ARL
• USEPA/ORD
• RTP, NC

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Outline

• Model development
• 1-D testing and evalution
• Large Eddy Simulation (LES)
• GABLS Experiment (CASES99)
• WRF testing and evaluation
• Surface stats
• PBL Heights
• Vertical Profiles
• Preliminary CMAQ testing

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Purpose

• Develop a simple PBL model that
• Produces realistic fluxes and profiles in PBL
• Accurate PBL height simulations
• Equally applicable to both meteorology and
  chemistry models
• Appropriate for all stability conditions without
  discontinuities
• Computationally efficient

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Background

• Local flux-gradient proportionality (i.e. Eddy
  diffusion) is not appropriate for Convective
  Boundary Layers
• Upward heat flux penetrates to 80 of h while
  potential temperature gradients are very small
  through most of the PBL
• Eddies in CBL are larger that vertical grid
  spacing (local closure is not appropriate)
• Two common alternative approaches

1. Gradient adjustment term
Deardorff 1966,Troen and Mahrt 1986, Holstlag
and Boville 1993, Noh et al. 2003
2. Transilent or non-local closure
Stull 1984, Blackadar 1976, Pleim and Chang 1992
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Asymmetric Convective Model (ACM)

• Modification of the Blackadar convective scheme
• Simple Transilient model with very sparse,
  efficient semi-implicit matrix solver
• Rapid upward transport by convectively buoyant
  plumes
• Gradual downward transport by compensatory
  subsidence
• Part of the PX-LSM in MM5

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ACM
ACM2
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ACM2

• Added eddy diffusion to ACM
• Allows local mixing at all levels
• More realistic profiles in lower layers
• Gradual transition from stable to unstable
• Full column integration (PBL and FT together)

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ACM Model equations
Where Ci is mass mixing ratio at center of Layer
i Ki1/2 is eddy diffusivity at top of layer i
m2s-1 Mu is upward convective mixing rate
s-1 Mdi is downward mixing rate from layer i to
layer i-1 s-1
h is top of PBL
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Non-local Mixing rates
Convective mixing rate derived by conservation of
buoyancy flux
Buoyancy flux at top of first layer defined by
eddy diffusion
Where
Thus, Convective mixing rate is a function of Kz
but not a function of potential temperature
gradient
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ACM2 Model equations
Mixing rate, defined by Kz, is partitioned into
local and non-local components
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Partitioning of local and non-local components
The Question is How to define fconv? Clearly
it should increase with increasing instability,
but what is its upper limit for free convection?
and what should be its stability
function? First a sensitivity study for
convective conditions
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1-D experiments Variations in partitioning of
local and non-local transport
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Normalized heat flux and potential temperature
profiles
LES (Stevens 2000)
ACM2
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Non-local partitioning

• These tests suggest that the upper limit of fconv
  should be about 50
• An expression for fconv can be derived from
  gradient adjustment models (e.g. Holstlag and
  Boville 1993) at top of surface layer

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Non-local fraction (fconv) as function of
stability
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LES experiment low heat flux (Q 0.05 K m
s-1), weak cap
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LES experiment low heat flux (Q 0.05 K m
s-1), strong cap
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LES experiment high heat flux (Q 0.24 K m
s-1), strong cap
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Comparison of ACM2, ACM1, and EDDYNL (Holtslag
and Boville 1993) compared to the 05WC LES
experiment.
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Profiles of sensible heat flux. Local,
non-local, and total sensible heat flux compared
to LES
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The second GABLS model intercomparison

• Multi-day Intercomparison of 23 PBL models for
  CASES99 field study
• Given initial profiles
• Tg time series
• Constant geostrophic wind
• Large scale subsidence
• 2.5 of potential evaporation
• P, zo

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GABLS T-2m Intercomparison
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GABLS Experiment Simulation of CASES99October
22-24, 1999
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GABLS Profile intercomparison
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CASES99 profiles
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Near surface Temperature profile from CASES99
main tower
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WRF

• ARW
• NOAH LSM
• 12 km resolution
• FDDA
• PBL comparisons
• ACM2
• MYJ
• YSU
• August 2006

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PBL Heights from TexAQS II

• PBL heights derived from 10 radar wind profilers
  in Texas area by Jim Wilczak and Laura Bianco
  NOAA/ESRL
• Observations and models averaged by hour of the
  day for August 1-31, 2006

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Average PBL ht for Aug 2006
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Average PBL ht for Aug 2006
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Ozonesonde 2006
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Beltsville - Aug 28, 18Z
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Houston 8/31, 18Z
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Houston windspeed and O3
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Narragansett Aug 2, 17Z
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Narragansett Ozone, CO
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Huntsville Aug 28, 18Z
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Huntsville Aug 29, 18Z
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Huntsville Ozone, CO
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Valparaiso Aug 31, 19Z
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Egbert Aug 29, 19Z
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Egbert WS and O3
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CMAQ

• Configuration
• Domain 199 x 205 x 34 _at_ 12 km res
• V4.5 CB4 AE3
• ACM2 vs Eddy
• Preliminary evaluation
• Ground level statistics (AMET)
• Ozonesondes from ICARTT 2004

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O3 Vertical Profiles for urban and rural grid
cells
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CO
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NOx
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Conclusions

• ACM2 is a combination of local and non-local
  closure techniques
• Similar capabilities to eddy diffusion w/
  counter-gradient adjustment but more readily
  applicable to any quantity (e.g chemistry)
• ACM2 produces more realistic superadiabatic
  surface layer than ACM1
• LES and 1-D tests show accurate simulation of
  vertical profiles and PBL heights
• MM5 and WRF tests show good ground level
  performance and accurate PBL heights

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More Conclusions

• CMAQ model comparisons show shallower and more
  well mixed profiles with ACM2 than with eddy
  diffusion
• Ozonesonde comparisons generally showed q and qv
  profiles similar to YSU but deeper PBL structure
  than MYJ
• ACM2 is uniquely suited for both meteorology and
  chemistry modeling

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Acknowledgements

• Rob Gilliam (NOAAASMD)
• Tanya Otte (NOAA/ASMD)
• Lara Reynolds (CSC)
• Jim Wilczak (NOAA/ESRL)
• Laura Bianco (NOAA/ESRL)
• Keith Ayotte (Australia)