Sensitivity Experiments of Direct Radiative Forcing Caused by Mineral Dust Simulated with a Chemical Transport Model

Abstract

Sensitivity experiments of direct radiative forcing (RF) caused by mineral dust (MD) for the optical and physical properties of MD, surface albedo, solar zenith angle, and cloud cover were conducted using the Streamer-based Radiative Transfer Model for ADEC Sciences (SRTMAS). The atmospheric and dust profiles were simulated with a chemical transport model at four locations: the sea off Japan, the desert in Tarim Basin, the Sahara Desert, and snow in Siberia. The optimum calculation condition to estimate RF was also tested for the effect of the number of streams used in SRTMAS on the accuracy of RF by MD and the influence of all aerosols other than MD on RF by MD or all aerosols. When several types of aerosols are contained in the atmosphere, the test results showed that the RF by MD should not be calculated from the difference in net radiation between MD-only case and an aerosol-free case, but the difference in net radiation between a case containing all aerosols and a case containing all aerosols except MD. The experiment results by this method confirmed that the sensitivity of instantaneous RF in the shortwave (SW) region at the top of the atmosphere (TOA) to the refractive index strongly depends on surface albedo. Namely, the effect of the difference in the MD model on instantaneous RF is significant over high albedo surfaces and is relatively small over the sea. This is because the multiple reflection between the atmosphere (dust) and surface enhances light absorption by dust particles over high albedo surfaces. Over desert surfaces, the instantaneous RF in SW at TOA produced both positive and negative values within the possible refractive index range of MD. The diurnally averaged RF in SW at TOA also produced both positive and negative values in the possible range of desert albedo. The effect of the spectral distribution of surface albedo on RFs in SW for the TOA, surface, and atmosphere was examined for various surfaces. It was found that for small dust particles with an effective radius of less than 0.6 μm, RFs by MD changed depending on the difference in surface type even if the broadband albedo was the same. This is due to the contrast of spectral distribution between albedo and the extinction coefficient of MD. The vertical positional relationship of cloud cover to dust layer was also very important for RF at TOA in all spectral regions over desert and sea surfaces. However, the effect of cloud cover was generally small over snow surface because cloud albedo was close to the underlying snow albedo.