Spatiotemporal variability of solar radiation introduced by clouds over Arctic sea ice
Abstract. The role of clouds in recent Arctic warming is not fully understood,
including their effects on the solar radiation and the surface
energy budget. To investigate relevant small-scale processes in detail, the intensive Physical feedbacks of Arctic planetary boundary layer, Sea ice, Cloud and AerosoL
(PASCAL) drifting ice floe station field campaign was conducted during early summer in the central arctic. During this campaign, the small-scale spatiotemporal
variability of global irradiance was observed for the first time on an
ice floe with a dense network of autonomous pyranometers. A total of 15 stations
were deployed covering an area of 0.83 km×1.59 km from
4–16 June 2017. This unique, open-access dataset is described here,
and an analysis of the spatiotemporal variability deduced from this
dataset is presented for different synoptic conditions. Based on
additional observations, five typical sky conditions were identified and
used to determine the values of the mean and variance of atmospheric
global transmittance for these conditions. Overcast conditions were
observed 39.6 % of the time predominantly during the first week, with
an overall mean transmittance of 0.47. The second most frequent
conditions corresponded to multilayer clouds (32.4 %), which
prevailed in particular during the second week, with a mean
transmittance of 0.43. Broken clouds had a mean transmittance of 0.61
and a frequency of occurrence of 22.1 %. Finally, the least frequent
sky conditions were thin clouds and cloudless conditions, which both
had a mean transmittance of 0.76 and occurrence frequencies of 3.5 %
and 2.4 %, respectively. For overcast conditions, lower global
irradiance was observed for stations closer to the ice edge, likely
attributable to the low surface albedo of dark open water and a
resulting reduction of multiple reflections between the surface and
cloud base. Using a wavelet-based multi-resolution analysis, power
spectra of the time series of atmospheric transmittance were compared
for single-station and spatially averaged observations and for
different sky conditions. It is shown that both the absolute magnitude
and the scale dependence of variability contains characteristic
features for the different sky conditions.