Effects of clouds, surface albedo and ozone variations on ultraviolet radiation climatology at Storm Peak Laboratory

Depletion of stratospheric ozone increases the ultraviolet (UV) exposure risk to biological organisms and ecological systems. Analysis of long-term observations indicate that there has been a significant reduction in ozone column amount for the upper- and mid-latitudes of the northern hemisphere, and the effects of this trend are more critical at high altitudes due to the reduced atmospheric path extinction. The roles of cloud reflection and multiple scattering between clouds and the surface are also enhanced in alpine environments. A new high-altitude measurement site for ultraviolet and solar radiation research and monitoring has been established at the Desert Research Institute's Storm Peak Laboratory (SPL). The SPL is an atmospheric research laboratory located at 3210 meters (10,525 ft) elevation, within the alpine zone of the Routt National Forest in north-central Colorado. The radiation monitoring system at SPL provides continuous measurements of diffuse and direct fluxes in 15 UV, visible and near-infrared channels using shadowband filter radiometers. Simultaneous in-situ sampling of ozone column amount, water vapor path, surface UV albedo and cloud physical parameters provides information on the local environment which is used in evaluation of UV radiative transfer models. Measurements at the site have indicated that multiple reflection between snow and broken orographic cumulus increases instantaneous fluxes to more than 130% of clear-sky values. The time-integrated exposure to UV flux both within and beneath orographic cloud can produce a significant biological risk. Results of modeling and measurement studies of spectral transmittance and scattering will be presented, demonstrating the influence of varying cloud types, surface conditions and ozone profiles on UV microclimate in alpine terrain. Applications of this research to satellite remote sensing of UV flux and ozone will also be discussed.