The diurnally-averaged aerosol direct radiative effect and the use of the daytime-mean and insolation-weighted-mean solar zenith angles

Abstract

This study examines the application of the daytime-mean and insolation-weighted-mean solar zenith angles to calculate the diurnally-averaged aerosol direct radiative effect (DRE), DREDA, for observed clear-sky conditions at the Atmospheric Radiation Measurement Program (ARM) Southern Great Plains (SGP) and Tropical Western Pacific (TWP) sites. As compared with the reference calculation, the DREDA are too strong (i.e., more negative) using daytime-mean solar zenith angles (DTMSZA) and are too weak (i.e., less negative) using insolation-weighted-mean solar zenith angles (IWMSZA). The biases in the DREDA for DTMSZA (IWMSZA) are about −0.8 (0.7) W m−2 at the TOA and about −1.5 (0.9) W m−2 at the surface. Noting the mean TOA DREDA of −2.85 W m−2 and the surface DREDA of −7.09 W m−2 at SGP, the relative biases for DTMSZA (IWMSZA) are 26% (−24%) for the TOA DREDA, and 17% (−11%) for the surface DREDA. At TWP, the mean TOA DREDA is −2.81 W m−2 and the surface DREDA is −11.84 W m−2, which corresponds to relative biases for DTMSZA (IWMSZA) of 29% (−27%) for the TOA DREDA and 15% (−9%) for the surface DREDA. By introducing an adjusted insolation-weighted-mean solar zenith angle, the mean biases in the DREDA at the TOA becomes −0.09 W m−2, and the biases in the DREDA at the surface are about −0.41 W m−2. Also presented are the effects of using the mean solar zenith angles on the shortwave (SW) upward fluxes at the TOA and SW downward fluxes at the surface in both clear-sky and pristine-clear-sky (i.e., gases only) conditions.