Atmospheric Modeling and Analysis Research
Coupled WRF-CMAQ Modeling System
While the role of long-lived greenhouse gases in modulating the Earth’s radiative budget has long been recognized, it is now widely acknowledged that the increased tropospheric loading of aerosols can also affect climate in multiple ways. Aerosols can provide a cooling effect by enhancing reflection of solar radiation, both directly (by scattering light in clear air) and indirectly (by increasing the reflectivity of clouds). On the other hand, organic aerosols and soot absorb radiation, thus warming the atmosphere.
Current estimates of aerosol radiative forcing are quite uncertain. The major sources of this uncertainty are related to the characterization of atmospheric loading of aerosols, of which the chemical composition and source attribution are highly variable both spatially and temporally. Unlike greenhouse gases, the aerosol radiative forcing is spatially heterogeneous and estimated to play a significant role in regional climate trends. The accurate regional characterization of the aerosol composition and size distribution is critical for estimating their optical and radiative properties and thus for quantifying their impacts on radiation budgets of the Earth-atmosphere system.
Traditionally, atmospheric chemistry-transport and meteorology models have been applied in an off-line paradigm. Using this approach, archived output describing the atmosphere’s dynamical state as simulated by the meteorology model is used to drive the transport and chemistry calculations of the atmospheric chemistry-transport model.
A modeling framework that facilitates coupled on-line calculations is desirable because it:
- provides consistent treatment of dynamical processes and reduces redundant calculations,
- provides the ability to couple dynamical and chemical calculations at finer time steps and thus facilitates consistent use of data,
- reduces the disk-storage requirements typically associated with off-line applications, and
- provides opportunities to represent and assess the potentially important radiative effects of pollutant loading on simulated dynamical features.
To address the needs of emerging assessments for air quality-climate interactions and for finer-scale air quality applications, EPA is developing a coupled atmospheric dynamics-chemistry model — the two-way coupled WRF-CMAQ modeling system.
In the prototype of this two-way coupled WRF-CMAQ modleing system, careful consideration has been given to its structural attributes, to ensure that it can evolve to address the increasingly complex problems facing EPA. The system design is flexible regarding the frequency of data communication between the two models, and can accommodate both coupled and uncoupled modeling paradigms. This approach also mitigates the need to maintain separate versions of the models for on-line and off-line modeling.
In the prototype coupled WRF-CMAQ system, the simulated aerosol composition and size distribution are used to estimate the optical properties of aerosols, which are then used in the WRF radiation calculations. Thus, the direct radiative effects of absorbing and scattering tropospheric aerosols estimated from spatially and temporally varying simulated aerosol distribution can be fed back to the WRF radiation calculations. This results in a “two-way” coupling between the atmospheric dynamical and chemical modeling components.
This extended capability provides unique opportunities to systematically investigate how atmospheric loading of radiatively important trace species affects the Earth’s radiation budget. Consequently, this modeling system is expected to play a critical role in EPA’s evolving research and regulatory applications exploring air quality-climate interactions.
Two sets of initial simulations have been conducted to test the evolving coupled WRF-CMAQ modeling system and to systematically assess the impacts of coupling and feedbacks.
Two key questions in application of the coupled modeling system for assessment of air quality-climate interactions are:
- Can aerosol radiative effects be detected in available measurements?
- Can such measurements be used to verify the directionality and magnitude of simulated effects?
The upper panels in the figure above demostrate the impact that aerosols estimated by CMAQ have on the meteorological models estimates of planetary boundary layer (PBL) height and downward shortwave radiation. The lower panel of the figure above is verification that the simulation, which includes these feedbacks, agrees better with the observed shortwave radiation.
- Pleim, J., J. Young, D. Wong, R. Gilliam, T. Otte, and R. Mathur, Two-Way Coupled Meteorology and Air Quality Modeling, Air Pollution Modeling and Its Application XIX, C. Borrego and A.I. Miranda (Eds.), 496-504, ISBN 978-1-4020-8452-2, Springer, The Netherlands, 2008
- Byun, D.W. and K.L. Schere (2006), Review of governing equations, computational algorithms, and other components of the models-3 Community Multiscale Air Quality (CMAQ) modeling system, Applied Mechanics Review, 59, 51-77