How Skillfully can we Simulate Drivers of Aerosol Direct Climate Forcing at the Regional Scale?

Abstract:

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<span" roman"="Roman"" new="New">Assessing the ability of global and regional models to describe aerosol optical properties is essential to reducing uncertainty in aerosol direct radiative forcing in the contemporary climate and to improving confidence in future projections. Here we evaluate the skill of high-resolution simulations conducted using the Weather Research and Forecasting model with coupled chemistry (WRF-Chem) in capturing spatio-temporal variability of aerosol optical depth (AOD) and Ångström exponent (AE) by comparison with ground- and space- based remotely sensed observations. WRF-Chem is run over eastern North America at a resolution of 12 km for a year climatologically representative of typical meteorological and aerosol conditions. A small systematic positive bias in simulated AOD relative to observations is found (annual mean fractional bias MFB=0.17 and 0.39 when comparing with MODIS and AERONET respectively), whereas the spatial variability is well captured during most months. The spatial correlation of AOD shows a clear seasonal cycle with highest correlation during summer months (r=0.5-0.7) when the aerosol loading is large and more observations are available. AE is retrieved with higher uncertainty from the remote sensing observations, but the simulations present a negative bias (annual MFB =-0.10 for MODIS and -0.64 for AERONET), thus indicating the model is biased towards simulation of coarse mode aerosols, although the spatial correlation is 0.25-0.55 during most months. WRF-Chem also exhibits high skill in identifying areas of extreme and non-extreme aerosol loading, and its ability of correctly simulating the location and relative intensity of an extreme aerosol event (i.e. AOD>75^th percentile) varies between 30 and 70% during winter and summer months respectively. Current work is directed to quantify the need for and benefits of regional downscaling and use of high-resolution global models by direct comparison with analogous runs at coarser resolution (60 km).