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A major continental-scale biomass burning smoke event from 28–30 June 2015,\udspanning central Canada through the eastern seaboard of the United States,\udresulted in unforecasted drops in daytime high surface temperatures on the\udorder of 2–5  °C in the upper Midwest. This event, with strong smoke\udgradients and largely cloud-free conditions, provides a natural laboratory\udto study how aerosol radiative effects may influence numerical weather\udprediction (NWP) forecast outcomes. Here, we describe the nature of this\udsmoke event and evaluate the differences in observed near-surface air\udtemperatures between Bismarck (clear) and Grand Forks (overcast smoke), to\udevaluate to what degree solar radiation forcing from a smoke plume\udintroduces daytime surface cooling, and how this affects model bias in\udforecasts and analyses. For this event, mid-visible (550 nm) smoke aerosol\udoptical thickness (AOT, τ) reached values above 5. A direct surface\udcooling efficiency of −1.5 °C per unit AOT (at 550 nm, τ550) was found. A further analysis of European Centre for Medium-Range\udWeather Forecasts (ECMWF), National Centers for Environmental Prediction\ud(NCEP), United Kingdom Meteorological Office (UKMO) near-surface air\udtemperature forecasts for up to 54 h as a function of Moderate\udResolution Imaging Spectroradiometer (MODIS) Dark Target AOT data across\udmore than 400 surface stations, also indicated the presence of the daytime\udaerosol direct cooling effect, but suggested a smaller aerosol direct\udsurface cooling efficiency with magnitude on the order of −0.25\udto −1.0 °C per unit τ550. In addition, using\udobservations from the surface stations, uncertainties in near-surface air\udtemperatures from ECMWF, NCEP, and UKMO model runs are estimated. This study\udfurther suggests that significant daily changes in τ550 above 1, at which the smoke-aerosol-induced direct surface cooling effect could be\udcomparable in magnitude with model uncertainties, are rare events on a\udglobal scale. Thus, incorporating a more realistic smoke aerosol field into\udnumerical models is currently less likely to significantly improve the\udaccuracy of near-surface air temperature forecasts. However, regions such as\udeastern China, eastern Russia, India, and portions of the Saharan and Taklamakan\uddeserts, where significant daily changes in AOTs are more frequent, are\udlikely to benefit from including an accurate aerosol analysis into numerical\udweather forecasts

atmospheric chemistry and physics

An evaluation of the impact of aerosol particles on weather forecasts from a biomass burning aerosol event over the Midwestern United States: observational-based analysis of surface temperature