An examination of the aerosol semi-direct effect for a polluted case of the ISDAC field campaign

The semi‐direct aerosol effect is examined with a mesoscale meteorological model for a polluted Arctic haze episode at Barrow, Alaska during the Indirect and Semi‐Direct Aerosol Campaign (ISDAC). Initialized with chemical and aerosol reanalysis fields from a global chemistry model, the WRF/Chem mesoscale model is used to simulate a polluted event over Alaska and its environs on 18–21 April 2008. It is shown that the atmosphere is sensitive to changes in the black carbon concentration, even though it comprises just a small fraction (less than one percent) of the total aerosol mass. Comparisons with a baseline run (which does not include aerosol radiative effects) show that in regions where black carbon is concentrated, the semi‐direct effect heats the lower troposphere by about 0.15 K. Additional sensitivity tests reveal that the heating is more uniform and higher in magnitude by up to 0.1 K when the initial concentration of black carbon is doubled, and a reduction in heating occurs when black carbon is reduced to zero. At Barrow, atmospheric warming is sensitive to variations in the black carbon concentration, and heating generally occurs above 0.5 km altitude where black carbon is located. A more stably stratified lower troposphere due to the warming aloft and surface cooling from the aerosol direct effect leads to a redistribution and reduction in the cloud optical thickness and liquid water content. This cloud reduction decreases the amount of downward surface longwave radiation and further lowers the surface temperature at Barrow. Key Points Atmosphere is sensitive to changes in black carbon concentration At Barrow, heating due to black carbon is localized in both altitude and time A more stably stratified lower atmosphere reduces cloud amount