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

摘要

A major continental-scale biomass burning smoke event from 28–30 June 2015,spanning central Canada through the eastern seaboard of the United States,resulted in unforecasted drops in daytime high surface temperatures on theorder of 2–5  °C in the upper Midwest. This event, with strong smokegradients and largely cloud-free conditions, provides a natural laboratoryto study how aerosol radiative effects may influence numerical weatherprediction (NWP) forecast outcomes. Here, we describe the nature of thissmoke event and evaluate the differences in observed near-surface airtemperatures between Bismarck (clear) and Grand Forks (overcast smoke), toevaluate to what degree solar radiation forcing from a smoke plumeintroduces daytime surface cooling, and how this affects model bias inforecasts and analyses. For this event, mid-visible (550 nm) smoke aerosoloptical thickness (AOT, ) reached values above 5. A direct surfacecooling efficiency of −1.5 °C per unit AOT (at 550 nm, ) was found. A further analysis of European Centre for Medium-RangeWeather Forecasts (ECMWF), National Centers for Environmental Prediction(NCEP), United Kingdom Meteorological Office (UKMO) near-surface airtemperature forecasts for up to 54 h as a function of ModerateResolution Imaging Spectroradiometer (MODIS) Dark Target AOT data acrossmore than 400 surface stations, also indicated the presence of the daytimeaerosol direct cooling effect, but suggested a smaller aerosol directsurface cooling efficiency with magnitude on the order of −0.25to −1.0 °C per unit . In addition, usingobservations from the surface stations, uncertainties in near-surface airtemperatures from ECMWF, NCEP, and UKMO model runs are estimated. This studyfurther suggests that significant daily changes in above 1, at which the smoke-aerosol-induced direct surface cooling effect could becomparable in magnitude with model uncertainties, are rare events on aglobal scale. Thus, incorporating a more realistic smoke aerosol field intonumerical models is currently less likely to significantly improve theaccuracy of near-surface air temperature forecasts. However, regions such aseastern China, eastern Russia, India, and portions of the Saharan and Taklamakandeserts, where significant daily changes in AOTs are more frequent, arelikely to benefit from including an accurate aerosol analysis into numericalweather forecasts.