Quantifying and Modelling the Effect of Cloud Shadows on the Surface Irradiance at Tropical and Midlatitude Forests

Boundary-Layer Meteorology. 2018. Vol. 166. No. 2. P. 165–198.

Kivalov S., Fitzjarrald D.

Cloud shadows lead to alternating light and dark periods at the surface, with
the most abrupt changes occurring in the presence of low-level forced cumulus clouds.
We examine multiyear irradiance time series observed at a research tower in a midlatitude
mixed deciduous forest (Harvard Forest, Massachusetts, USA: 42.53◦N, 72.17◦W) and one
made at a similar tower in a tropical rain forest (Tapajós National Forest, Pará, Brazil:
2.86◦S, 54.96◦W). We link the durations of these periods statistically to conventional mete-
orological reports of sky type and cloud height at the two forests and present a method
to synthesize the surface irradiance time series from sky-type information. Four classes of
events describing distinct sequential irradiance changes at the transition from cloud shadow
and direct sunlight are identified: sharp-to-sharp, slow-to-slow, sharp-to-slow, and slow-to-
sharp. Lognormal and the Weibull statistical distributions distinguish among cloudy-sky
types. Observers’ qualitative reports of ‘scattered’ and ‘broken’ clouds are quantitatively
distinguished by a threshold value of the ratio of mean clear to cloudy period durations.
Generated synthetic time series based on these statistics adequately simulate the temporal
“radiative forcing” linked to sky type. Our results offer a quantitative way to connect the
conventional meteorological sky type to the time series of irradiance experienced at the
surface.

Keywords: Cloud shadows at the surface, Solar irradiance, Synthetic irradiance time series