Quantifying errors in surface ozone predictions associated with clouds over the CONUS: a WRF-Chem modeling study using satellite cloud retrievals

摘要

Clouds play a key role in radiation and hence O sub3/sub photochemistry by modulating photolysis rates and light-dependent emissions of biogenic volatile organic compounds (BVOCs). It is not well known, however, how much error in O sub3/sub predictions can be directly attributed to error in cloud predictions. This study applies the Weather Research and Forecasting with Chemistry (WRF-Chem) model at 12?km horizontal resolution with the Morrison microphysics and Grell 3-D cumulus parameterization to quantify uncertainties in summertime surface O sub3/sub predictions associated with cloudiness over the contiguous United States (CONUS). All model simulations are driven by reanalysis of atmospheric data and reinitialized every 2 days. In sensitivity simulations, cloud fields used for photochemistry are corrected based on satellite cloud retrievals. The results show that WRF-Chem predicts about 55?% of clouds in the right locations and generally underpredicts cloud optical depths. These errors in cloud predictions can lead to up to 60?ppb of overestimation in hourly surface O sub3/sub concentrations on some days. The average difference in summertime surface O sub3/sub concentrations derived from the modeled clouds and satellite clouds ranges from 1 to 5?ppb for maximum daily 8?h average O sub3/sub (MDA8 O sub3/sub) over the CONUS. This represents up to ～ 40?% of the total MDA8 O sub3/sub bias under cloudy conditions in the tested model version. Surface O sub3/sub concentrations are sensitive to cloud errors mainly through the calculation of photolysis rates (for ～ 80?%), and to a lesser extent to light-dependent BVOC emissions. The sensitivity of surface O sub3/sub concentrations to satellite-based cloud corrections is about 2 times larger in VOC-limited than NO subx/sub -limited regimes. Our results suggest that the benefits of accurate predictions of cloudiness would be significant in VOC-limited regions, which are typical of urban areas.