Wednesday, 5 June 2002
Arctic Stratus Cloud Properties and Radiaitve Forcing Derived From Ground-based Data Collected at Barrow, Alaska
A record of single-layer and overcast low-level Arctic stratus cloud properties has been generated using data collected at the Atmospheric Radiation Measurement site near Barrow, Alaska from May to September 2000. The record includes liquid-phase and liquid dominant mixed-phase Arctic stratus macrophysical, microphysical, and radiative properties, as well as surface radiation budget and cloud radiative forcing. The macrophyiscal properties consist of cloud fractions, base/top heights and temperatures, and cloud thickness derieved from a ground-based radar and lidar pair, and rawinsonde sounding. The microphysical properties include cloud liquid water path and content, and cloud-droplet effective radius and number concentration obtained from microwave radiometer brightness temperature measurements, and a newly developed parameterization. The radiative properties contain cloud optical depth, solar tranmission, and surface/cloud/top-of-atmosphere albedos derived from a newly developed parameterization and standard Epply precision spectral pyranometers. The shortwave, longwave, and net cloud radiative forcings at the surface are inferred from the measurements of standard Epply precision spectral pyranometers and pyrgeometers. There are approximately 300 hours and more than 3600 samples (5-min resolution) of single-layer and overcast low-level stratus during the study period. The 10-day averaged total and low cloud (Ztop < 3 km) occurrence fractions vary from 0.7 to 1.0 and from 0.4 to 0.8, with the mean values of 0.87 and 0.55, respectively. The 10-day averaged low cloud base heights are, in general, around 0.4 km, and top heights around 0.8 km, leading to 0.4 km of cloud thickness. The means of cloud-droplet effective radius and number concentrations are around 13 mm and 60 cm-3 during the summer - similar to the microphysical properties of middle latitudes marine stratus. During the spring season, the effective radius and number concentration are found to be 8 mm and 220 cm-3 - similar to the microphysical properties of middle latitudes continental stratus clouds. The solar transmission and cloud albedo are strongly linked with and affected by the snow/ice-covered surface. The net cloud radiative forcings have a cooling effect during the summer and warming effect during the spring and autumn seasons - suggest that Arctic stratus clouds may enhance the springtime melting and slow down the autumn freezing of the ice pack compared to no clouds.