Surface and Tethered-Balloon Observations of Actinic Flux: Effects of Arctic stratus, Surface Albedo and Solar Zenith Angle

As part of the FIRE III (First ISCCP Regional Experiment) Arctic Cloud Experiment actinic flux measurements were made above the Arctic Sea ice during May 1998. FIRE III was designed to address questions concerning clouds, radiation and chemistry in the Arctic sea ice region. The actinic flux, which is also referred to as the 4p-radiative flux, is the relevant radiative parameter needed to determine photodissociation rates. Moreover, it is discussed that the actinic flux may be used to determine vertical absorption profiles of the net irradiance, provided that the single scattering albedo is known. The diurnal cycle of UV-A (wavelength about 365 nm) and visible (wavelength about 550 nm) actinic fluxes during clear and cloudy conditions was measured by two 4p- radiometers installed just above the ice surface. In addition, vertical profiles of the visible actinic flux through low arctic stratus clouds were observed by means of a tethered balloon. The cloud thermodynamic and microphysical structure was assessed from observations made by the NCAR C-130 aircraft. The liquid water path was retrieved by a microwave radiometer. During clear skies the diurnal variation of the actinic flux was controlled mainly by Rayleigh scattering. Above the cloud layer the actinic flux was found to be almost the same as during clear sky conditions. This could be attributed to the fact that the effective albedo of the arctic sea ice and the cloud is only slightly higher than the ground albedo alone. The observed vertical actinic flux profiles in arctic stratus are discussed and compared with similar measurements made in Atlantic stratocumulus. In the arctic stratus clouds the actinic flux was found to be nearly constant with height, except in a shallow layer near the cloud top where the actinic flux significantly increased with height. The role of the solar zenith angle and ground albedo on in-cloud actinic flux profiles is discussed. It is concluded that the observed strong increase of the actinic flux in the upper part of the arctic stratus layer is a typical feature associated with large solar zenith angles