RELATIONSHIP BETWEEN SOLAR NET RADIATIVE FLUXES AT THE TOP OF THE ATMOSPHERE AND AT THE SURFACE

Previous work has discussed the existence of a linear relationship between the net solar radiative flux densities at the surface and at the top of the atmosphere (TOA) that can be exploited for inferring the net surface radiation directly from the satellite observed net radiation. In physical terms the net solar flux at the surface can be estimated from the difference between the satellite-inferred net flux at TOA and total solar absorption in the atmosphere. This paper presents model calculations of the influence on solar absorption of water vapor, solar zenith angle, cloud-top altitude, and cloud optical thickness. The model results indicate a somewhat complex relation between the solar net fluxes at the surface and at the top of the atmosphere. It is pointed out that cloud altitude and optical depth have a large impact on solar atmospheric absorption; high clouds decrease solar absorption by the atmosphere whereas low clouds increase it. This difference between solar atmospheric absorption for low and high clouds increases with cloud optical depth. An intriguing result is that changes of total atmospheric absorption with cloud-top height are nearly completely compensated by corresponding changes in the net flux at the top of the atmosphere, thus leaving the surface solar net flux constant. Furthermore, this paper provides a very simple parameterization for estimating the clear-sky solar atmospheric absorption as a function of solar zenith angle and the vertically integrated water vapor content of the atmosphere.