Dust direct radiative effects on the earth-atmosphere system over east Asia: Early spring cooling and late spring warming

Focusing on three dust storms occurring in spring 2001, we developed a detailed aerosol parameterization scheme and integrated it in a radiative transfer model to characterize possible impacts of solar altitude angle on dust direct radiative effects over China desert regions and the North Pacific, using actual daily solar altitude angles. Increasing solar altitude angle from early spring (or winter) to late spring (or summer) leads to increase of positive clear sky radiative forcing, and decrease of negative radiative forcing due to dust aerosols at the top of the atmosphere. Because solar altitude angle increases from early to late spring, dust-clear sky radiative forcing may change from negative to positive at the top of atmosphere, showing a change from cooling to heating of the earth-atmosphere system over high-albedo deserts and nearby regions. Over low-albedo ocean negative clear sky radiative forcing by dust may decrease, suggesting a change from strong to weak cooling on the earth-atmosphere system. The impacts of solar altitude angle on cloudy sky radiative forcing due to dust are similar to those of clear sky. Impacts of low cloud on dust radiative forcing are the same as increasing surface albedo. This causes the transition of dust cooling effects into heating effects over deserts to occur earlier, and causes decrease of negative radiative forcing over the ocean and even cause a change from weak negative radiative forcing to weak positive forcing over local areas. Even in the same East Asian desert regions and nearby areas, the strength and sign of the radiative forcings depend on storm dates and thus solar altitude angle. The nearer to early spring (or winter) a dust storm occurs, the easier it leads to negative radiative forcing at the top of atmosphere, which indicates cooling effects on the earth-atmosphere system. In contrast, the nearer to late spring (or summer) a dust storm occurs, the easier it leads to positive radiative forcing at the top of atmosphere, showing heating effects. Over East Asian deserts and nearby regions, dust layers may be regarded as cooling sources in early spring (winter) and warming sources in late spring (summer).