Surface and tethered‐balloon observations of actinic flux: Effects of arctic stratus, surface albedo, and solar zenith angle

Abstract

As part of the First ISCCP Regional Experiment (FIRE III) Arctic Cloud Experiment actinic flux measurements were made above the Arctic Sea ice during May 1998. The actinic flux, which is also referred to as the 4π radiative flux, is the relevant radiative parameter needed to determine photodissociation rates. It is shown that for a plane‐parallel cloud the change in the net irradiance as a function of the optical depth is proportional to the magnitude of the actinic flux. Continuous actinic flux measurements were made just above the snow‐covered ice surface by a UV‐A and a visible 4π radiometer (wavelengths ∼365 and ∼550 nm, respectively). In addition, vertical profiles of the actinic flux through low arctic stratus clouds were obtained by means of a visible 4π radiometer suspended under a tethered balloon. The cloud thermodynamic and microphysical structure was determined from observations made by the National Center for Atmospheric Research C‐130 aircraft. In addition, the phase and liquid water path of the cloud was assessed from microwave radiometer, lidar, and radar data. During clear‐sky conditions the diurnal variation of the magnitude of actinic flux was controlled mainly by Rayleigh scattering and surface reflection. Above a stratus 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. 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 increased significantly with height. The vertical profiles that were observed in arctic stratus differed from those measured in Atlantic stratocumulus; in the latter the actinic flux was found to increase gradually from cloud base to cloud top. A delta‐Eddington model is utilized to illustrate that the exact shape of the vertical profile is very sensitive to the solar zenith angle. During the arctic experiments the solar zenith angle was generally much larger than during the observations in Atlantic stratocumulus.