Grantee Research Project Results
The Effects of Clouds and Aerosols on the Wavelength Dependence of Ultraviolet and Visible Solar Radiation at the Earth's SurfaceEPA Grant Number: U914964
Title: The Effects of Clouds and Aerosols on the Wavelength Dependence of Ultraviolet and Visible Solar Radiation at the Earth's Surface
Investigators: Erlick, Caynelisa N.
Institution: University of Chicago
EPA Project Officer: Hahn, Intaek
Project Period: January 1, 1996 through September 24, 1997
Project Amount: $102,000
RFA: STAR Graduate Fellowships (1996) RFA Text | Recipients Lists
Research Category: Academic Fellowships , Air Quality and Air Toxics , Fellowship - Atmospheric Sciences
The objective of this research project is to develop a model to help describe the absorption and to predict with accuracy (by including the effects of fractional cloud cover and multiple cloud layers) the amount of ultraviolet radiation reaching the surface of the earth, given current ozone trends. The explanation for the anomalous solar absorption in clouds may lie in the representation of molecules, cloud drops, and aerosol particles in cloud layers and in the treatment of multiple scattering within cloud layers in current solar radiative transfer models. The inclusion of absorbing molecules and aerosol particles in the interstitial air between cloud drops and inside the cloud drops, combined with a more exact treatment of the way that the radiation bounces in the cloud layers and encounters these absorbing species, should account for most of the discrepancy.
I will use column ozone and tropospheric reflectivity data from the total ozone mapping spectrometer (TOMS) aboard the satellite Nimbus-7, which was in operation from November 1, 1978, to May 31, 1993. I also will use ground-based radiation measurements from a network of Robertson-Berger meters, which measures ultraviolet radiation with a spectral response function close to the spectral sensitivity of Caucasion skin to sunburn. First, I will examine how the ratio of measured cloudy-sky flux to calculated clear-sky flux at the ground varies with TOMS reflectivity. Given that the TOMS reflectivity is the combined reflectivity of the troposphere and the ground, it is expected that the higher the reflectivity, the lower the ratio of cloudy-sky to clear-sky flux. I will use the new model to extrapolate drop-number densities from the TOMS reflectivities. The calculated reflectivities for different standard drop-size distributions will be compared with TOMS reflectivity measurements at the TOMS wavelengths to find a match. After a match is found, the corresponding number density of cloud drops can be used to calculate the magnitudes of the scattering parameters in the cloud layer at all of the other wavelengths. In turn, these data can be used to determine the magnitude of absorption occurring in the cloud and the total ultraviolet flux reaching the ground. These results will be compared with ground-based Robertson-Berger meter data. I also will investigate the composition of the cloud drops. If a given site is known to be polluted, a standard vertical profile of aerosol particles will be added to the model, and the results will be compared with ground-based measurements. Futhermore, the procedures above should reveal whether the combined effects of pure-water cloud drops, gas-phase ozone molecules, dry-aerosol particles, and cloud drops containing dissolved and undissolved aerosol particles and ozone molecules can explain the magnitude of solar absorption in clouds. Lastly, I will investigate multiple cloud layers, allowing for horizontal inhomogeneities in the cloud layers that will require expanding the model from one to two dimensions. This should add to the explanation of the absorption in clouds and allow the model to agree more closely with ground-based measurements on a daily basis.