Environmental Engineering Reference
In-Depth Information
routine in WRF. The coupler (Wong et al., 2009) allows for flexibility in time
stepping between the two models; CMAQ can be called every WRF time step or
any user defined multiple. Additionally, simple switching of the buffer files to
disk files allows for identical uncoupled simulation (without feedback). Both
models use the same map projections, coordinate systems, and grid structures,
thereby ensuring consistent data use across both models. CMAQ has been modi-
fied to include meteorology-dependent processes: biogenic emissions, point source
plume-rise, and dry deposition velocity estimation, previously calculated upstream
of the CMAQ model. Simulated aerosol composition and size distributions are used
to estimate the optical properties of aerosols which are then used in the radiation
calculations in WRF. Thus, direct radiative effects of scattering and absorbing
aerosols in the troposphere estimated from the spatially and temporally varying
simulated aerosol distributions, can be fed-back to the WRF radiation calculations,
resulting in “2-way” coupling between the atmospheric dynamical and chemical
modeling components. Though both WRF and CMAQ are designed to run on
parallel computing environments, the details of domain decomposition, i.e., mapping
of sub-domains and processors, is quite different. The coupler is designed such that
these differences in the parallelization and coupling of the models, is transparent
to the user.
3. Results and Discussion
Pleim et al. (2008) previously analyzed the impact of different frequency of
coupling between the dynamical and chemical calculations in the coupled WRF-
CMAQ system and demonstrated that relatively large differences in simulated
instantaneous values compared to an off-line simulation.
Figure 2
presents an
illustration of the direct aerosol feedbacks simulated by the coupled WRF-CMAQ
modeling system over the eastern U.S. In general relatively high aerosol optical
depths are noted in regions of high surface and boundary-layer particulate matter
pollution. Aerosol direct radiative effects associated with scattering and absorption
of incoming radiation, result in a reduction of short-wave radiation reaching the
surface, which then translate to reduction in temperature at the surface as well as a
reduction in planetary boundary layer (PBL) height. For the moderate pollution
levels illustrated in
Fig. 2,
the noted impacts on short-wave reduction and sub-
sequent suppression in boundary-layer heights are relatively modest. As illustrated
in
Fig. 3, t
he inclusion of direct radiative effects of aerosol loading lead to slight
cooling and slightly better agreement with measured values. Application of the
modeling system to cases characterized by higher tropospheric PM
2.5
burden and
detailed comparisons with available measurements of short-wave radiation are
underway.
