Preprints
https://doi.org/10.5194/egusphere-2024-2396
https://doi.org/10.5194/egusphere-2024-2396
20 Aug 2024
 | 20 Aug 2024

Mechanisms of surface solar irradiance variability under broken clouds

Wouter Mol and Chiel van Heerwaarden

Abstract. Surface solar irradiance variability is present under all broken clouds, but the patterns, magnitude of variability, and mechanisms behind it vary greatly with cloud type. In this study, we performed numerical experiments to understand which main mechanisms drive surface solar irradiance extremes across a diverse set of cloud conditions based on observations. The results show that we can capture the essence in four mechanisms. We find that for optically thin (τ < 6) and clouds, scattering in the forward direction (forward escape) is the dominant mechanism. In cloud types such as altocumulus, it is able to produce irradiance enhancements of up to 50 % of clear-sky values due to small gaps in the cloud field. For flat, optically thick clouds (τ > 6) like stratus, downward escape becomes the dominant mechanism, and the irradiance extremes are found underneath the cloud edge or gaps. Albedo has a significant effect under optically thick cloud cover, contributing 10 to 60 % of the total irradiance enhancement for low (0.2) to high (0.8) albedo. For deep convective clouds, side escape is the dominant mechanism enhancing domain-averaged diffuse irradiance. This effect has a large area of influence, extending over 20 km from the sunlit side of the cloud. Extreme irradiance enhancement, however, comes from downward escape and forward escape just underneath the cloud edge on the sunlit side, not from side escape. These results provide a framework for understanding the vast diversity and complexity found in surface solar irradiance and cloudiness. A next step is to apply this analysis to multi-layered cloud fields and non-isolated deep convective clouds.

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Wouter Mol and Chiel van Heerwaarden

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comments and recommendations about manuscript by Mol and Heerwaarden', Anonymous Referee #1, 24 Sep 2024
  • RC2: 'Comment on egusphere-2024-2396', Philipp Gregor, 15 Oct 2024
Wouter Mol and Chiel van Heerwaarden

Data sets

Supplementary data for "Mechanism of surface solar irradiance variability under broken cloud cover" Wouter Mol and Chiel van Heerwaarden https://doi.org/10.5281/zenodo.11503610

Wouter Mol and Chiel van Heerwaarden

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Short summary
Sunlight varies often and quickly under broken cloud cover, and every cloud field creates a unique pattern of sunlight on the surface below. These variations affect many processes in the Earth system, from photosynthesis and chemistry, to cloud formation itself. The exact way in which cloud particles interact with sunlight is complex and expensive to calculate. We demonstrate a simplified framework which explains how sunlight changes for potentially any cloud field.