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Atmospheric Modeling and Analysis Research

Planetary Boundary Layer Modeling for Meteorology and Air Quality

Air quality modeling systems are essential tools for air quality regulation and research.  These systems are based on Eulerian grid models for both meteorology and atmospheric chemistry and transport.  They are used for a range of scales from continental to urban. 

A key process in both meteorology and air quality models is the treatment of sub-grid-scale turbulent vertical transport and mixing of meteorological and chemical species.  The most turbulent part of the atmosphere is the planetary boundary layer (PBL), which extends from the ground up to around 1-3 kilometers during the day but is only tens or hundreds of meters deep at night.

Modeling the atmospheric boundary layer, particularly during convective conditions, has long been a major source of uncertainty in numerical modeling of meteorology and air quality.  Much of this difficulty stems from the large range of turbulent scales that are effective in the convective boundary layer (CBL). 

Both small-scale turbulence that is sub-grid-scale in most mesoscale grid models and large-scale turbulence extending to the depth of the convective boundary layer are important for vertical transport of atmospheric properties and chemical species.  Eddy diffusion schemes assume that all of the turbulence is sub-grid-scale and therefore cannot realistically simulate convective conditions. 

Simple nonlocal-closure planetary boundary layer (PBL) models, like the Blackadar convective model — a mainstay PBL option in the National Center for Atmospheric Research's mesoscale model (MM5) for many years — and the original Asymmetric Convective Model (ACM), also an option in MM5, represent large-scale transport driven by convective plumes but neglect small-scale, sub-grid-scale turbulent mixing.  A new version of the ACM (ACM2) has been developed that includes the nonlocal scheme of the original ACM combined with an eddy diffusion scheme.  Thus, the ACM2 can represent both the super-grid-scale and sub-grid-scale components of turbulent transport in the convective boundary layer. 

By testing the ACM2 in one-dimensional form and comparing it to large-eddy simulations and field data from the second and third Global Energy and Water Cycle Experiment (GEWEX) Atmospheric Boundary Layer Study, scientists have demonstrated the new scheme's accuracy in simulating PBL heights, profiles of fluxes and mean quantities, and surface-level values.  The ACM2 performs equally well for both meteorological parameters (e.g., potential temperature, moisture variables, and winds) and trace chemical concentrations. Its broader performance range gives the model an advantage over eddy diffusion models which include a nonlocal term in the form of a gradient adjustment.

The ACM2 is in the latest releases of the Weather Research and Forecast (WRF) model and Community Multiscale Air Quality (CMAQ) model and is being used extensively by air quality and research communities.  Comparisons to data from the Texas Air Quality Study II field experiment show good agreement with PBL heights derived from radar wind profilers and vertical profiles of both meteorological and chemical quantities measured by aircraft spirals.

The most direct measure of success for a PBL model for both meteorology and air quality is its ability to accurately simulate the vertical structure of both meteorological and chemical species.
The most direct measure of success for a PBL model for both meteorology and air quality is its ability to accurately simulate the vertical structure of meteorological and chemical species. The figure above shows an example of WRF and CMAQ profiles (both use the ACM2 scheme) compared to aircraft measurements. The top of the PBL mixed layer is well defined and modeled for both meteorology variables (Qv and Theta) and chemical variables (NOy). While such simultaneous measurements of vertical profiles of meteorology and chemistry are very rare, these limited results are encouraging.

 

Contacts: Jonathan Pleim

Related Publications:
  • Gilliam, R. C. and J. E. Pleim, 2009, Performance assessment of the Pleim-Xu LSM, Pleim surface-layer and ACM PBL physics in version 3.0 of WRF-ARW, submitted to J. Appl. Meteor. and Clim.
  • Pleim, J. E., 2007, A combined local and non-local closure model for the atmospheric boundary layer. Part 1: Model description and testing, J. Appl. Meteor. and Clim., 46, 1383-1395.
  • Pleim, J. E., 2007, A combined local and non-local closure model for the atmospheric boundary layer. Part 2: Application and evaluation in a mesoscale model, J. Appl. Meteor. and Clim., 46, 1396-1409.
  • Pleim, J. E., 2006, A simple, efficient solution of flux-profile relationships in the atmospheric surface layer, J. Appl. Meteor. and Clim., 45, 341-347.
  • Pleim, J. E., J. S. Chang, 1992.  A non-local closure model for vertical mixing in the convective boundary layer.  Atm. Env., 26A, 965-981.
  • Xiu, A., and J. E. Pleim, 2001: Development of a land surface model part I: Application in a mesoscale meteorology model. J. Appl. Meteor., 40, 192-209.
  • Pleim, J. E., and A. Xiu, 2003, Development of a land surface model. Part II: Data Assimilation.  J. Appl. Meteor., 42, 1811-1822.

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