Sensitivity of shortwave radiative flux density, forcing, and heating rate to the aerosol vertical profile

[1] The effect of the aerosol vertical distribution on the solar radiation profiles for idealized and measured profiles of extinction and single-scattering albedo (SSA) during the May 2003 Atmospheric Radiation Measurement Aerosol Intensive Observation Period (AIOP) is investigated using the rapid radiative transfer model shortwave code. Calculated profiles of downwelling and upwelling solar flux density during the AIOP are compared with the measurements from solar broadband radiometers aboard a profiling research aircraft. The profiles of aerosol extinction, SSA, and water vapor obtained from the aircraft that carried the radiometers serve as the model inputs. The uplooking radiometers were mounted on a stabilized platform that kept the radiometers parallel with respect to Earth's horizontal plane. The results indicate that the vertical shape of the aerosol extinction profiles has very little impact on the clear-sky direct radiative forcing at the top of atmosphere and surface but is important for forcing profiles of partially absorbing aerosol. The vertical distributions of absorption profiles drastically influence the forcing and heating rate profiles. Using aircraft data from 19 AIOP profiles over the southern Great Plains, we are able to achieve broadband downwelling solar flux density closure within 0.8% (bias difference) or 1.8% (RMS difference), well within the expected measurement uncertainty of 1%–3%. The poorer agreement in upwelling flux density (bias −3.7%, RMS 10%) is attributed to the use of inaccurate surface albedo data. The accurate, vertically resolved aerosol extinction data play an important role in tightening solar radiative flux density closure. This study also suggests that aircraft solar radiative flux density measurements from a stabilized platform have the potential to determine solar heating rate profiles. These measurement-based heating rate profiles provide useful data for heating rate closure studies and indirect estimates of single-scattering albedo assumed in radiative transfer calculations.

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