Meteorological modeling is performed retrospectively to simulate
the meteorological conditions (wind, temperature, vertical mixing,
etc.) from
past air pollution episodes. The numerical simulations are used
as input to the
photochemical model in the agency's
air quality modeling process. TCEQ currently uses MM5 
, the PSU/NCAR community
mesoscale meteorological model.
The Role of a Meteorological Model in Air Quality Modeling
In order for a
photochemical model to properly simulate pollutant formation,
it is crucial that the meteorological conditions be realistically
modeled. The TCEQ uses state-of-the-science, four-dimensional
computer models that incorporate atmospheric physical laws and
measured observations to predict weather conditions over time and
space. The meteorological model output is then used as input by the
photochemical model to predict pollutant concentrations.
How Does a Meteorological Model Work?
The basic structure of a meteorological model uses universal
laws of atmospheric physics as well as empirical relationships to
calculate wind flow, temperature, humidity, vertical air mixing,
and other parameters in time and space. The model predicts these
parameters in three-dimensional grid boxes stacked horizontally and
vertically at equidistant points over the domain(s) of
interest.
Example of Nested Modeling Domains
While the focus is on a metropolitan area, the meteorological
model needs forcing from large weather patterns to properly
simulate conditions at the local scale. This is accomplished by
running the model on multiple nested domains, from continental to
multi-county size. Each successively smaller domain receives
feedback from the larger domains to determine background flow and
boundary conditions, thus producing local atmospheric conditions
based on large-scale features.
During the model run, predicted parameters are blended with
observations through a process called Four-Dimensional Data
Assimilation (FDDA) or nudging. By assimilating ambient data, the
meteorological model is continuously nudged toward reality. The
resulting model output closely matches the monitored meteorological
values while producing meteorological parameters over the entire
domain that are valid according to atmospheric physics laws.
Processing Observed Meteorological Data
TCEQ staff acquire surface and upper air meteorological data
about the modeling domain from sources such as:
After the data are collected and quality assured for errors,
they are used for model validation or observational nudging.
Preparing Meteorological Model Outputs for the Photochemical
Model
After careful quality assurance of the output variables from the
meteorological model, they are then processed into a format that
can be input into the
photochemical model.
Major Meteorological Models
Following are major meteorological models used by the air
quality modeling community:
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