Abstract
Abstract. This study implemented first, second and glaciation aerosol indirect effects (AIE) on resolved clouds in the two-way coupled Weather Research and Forecasting Community Multiscale Air Quality (WRF–CMAQ) modeling system by including parameterizations for both cloud drop and ice number concentrations on the basis of CMAQ-predicted aerosol distributions and WRF meteorological conditions. The performance of the newly developed WRF–CMAQ model, with alternate Community Atmospheric Model (CAM) and Rapid Radiative Transfer Model for GCMs (RRTMG) radiation schemes, was evaluated with observations from the Clouds and the See http://ceres.larc.nasa.gov/. Earth's Radiant Energy System (CERES) satellite and surface monitoring networks (AQS, IMPROVE, CASTNET, STN, and PRISM) over the continental US (CONUS) (12 km resolution) and eastern Texas (4 km resolution) during August and September of 2006. The results at the Air Quality System (AQS) surface sites show that in August, the normalized mean bias (NMB) values for PM2.5 over the eastern US (EUS) and the western US (WUS) are 5.3% (−0.1%) and 0.4% (−5.2%) for WRF–CMAQ/CAM (WRF–CMAQ/RRTMG), respectively. The evaluation of PM2.5 chemical composition reveals that in August, WRF–CMAQ/CAM (WRF–CMAQ/RRTMG) consistently underestimated the observed SO42- by −23.0% (−27.7%), −12.5% (−18.9%) and −7.9% (−14.8%) over the EUS at the Clean Air Status Trends Network (CASTNET), Interagency Monitoring of Protected Visual Environments (IMPROVE) and Speciated Trends Network (STN) sites, respectively. Both configurations (WRF–CMAQ/CAM, WRF–CMAQ/RRTMG) overestimated the observed mean organic carbon (OC), elemental carbon (EC) and and total carbon (TC) concentrations over the EUS in August at the IMPROVE sites. Both configurations generally underestimated the cloud field (shortwave cloud forcing, SWCF) over the CONUS in August due to the fact that the AIE on the subgrid convective clouds was not considered when the model simulations were run at the 12 km resolution. This is in agreement with the fact that both configurations captured SWCF and longwave cloud forcing (LWCF) very well for the 4 km simulation over eastern Texas, when all clouds were resolved by the finer resolution domain. The simulations of WRF–CMAQ/CAM and WRF–CMAQ/RRTMG show dramatic improvements for SWCF, LWCF, cloud optical depth (COD), cloud fractions and precipitation over the ocean relative to those of WRF default cases in August. The model performance in September is similar to that in August, except for a greater overestimation of PM2.5 due to the overestimations of SO42-, NH4+, NO3-, and TC over the EUS, less underestimation of clouds (SWCF) over the land areas due to the lower SWCF values, and fewer convective clouds in September. This work shows that inclusion of indirect aerosol effect treatments in WRF–CMAQ represents a significant advancement and milestone in air quality modeling and the development of integrated emissions control strategies for air quality management and climate change mitigation.
Highlights
Atmospheric emissions resulting from consumption of fossil fuels by human activities contribute to climate change and degrade air quality
As summarized by Lohmann and Feichter (2005) and the IPCC (2007), other aerosol indirect effects may include the semi-direct effect, which refers to an evaporation of cloud droplets caused by the absorption of solar radiation by soot, and the thermodynamic effect that refers to a delay in the onset of freezing by the smaller cloud droplets, causing supercooled clouds to extend to colder temperatures
To estimate the first and second indirect aerosol forcing, the cloud droplet number concentrations are diagnosed from the activation of CMAQ-predicted aerosol particles using a aerosol activation scheme for multiple externally mixed lognormal modes, with each mode composed of uniform internal mixtures of soluble and insoluble material developed by Abdul-Razzak and Ghan (2002, 2000)
Summary
Atmospheric emissions resulting from consumption of fossil fuels by human activities contribute to climate change and degrade air quality. The second AIE is based on the idea that decreasing the mean droplet size in the presence of enhanced aerosols decreases the cloud precipitation efficiency, producing clouds with a larger liquid water content and a longer lifetime (cloud lifetime effect), and its recognition is commonly attributed to Albrecht (1989). The glaciation AIE is based on the idea that increases in IN because of enhanced aerosols (dust, organic carbon, black carbon and sulfate) result in more frequent glaciation of a supercooled liquid water cloud due to the difference in vapor pressure over ice and water and an increase in the amount of precipitation via the ice phase, leading to a decrease in cloud cover and a shorter cloud lifetime (IPCC, 2007; Lohmann, 2002). The IPCC (2007) concludes that increasing concentrations of the long-lived greenhouse gases have led to a combined radiative forcing of +2.63 [±0.26] W m−2, and the total direct aerosol radiative forcing is estimated to be −0.5 [±0.4] W m−2, with a medium to low level of scientific understanding, while the radiative forcing due to the cloud albedo effect ( referred to as first indirect) is estimated to be −0.7 [−1.1, +0.4] W m−2, with a low level of scientific understanding
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