Large clouds dominate cloud coverage and solar reflectance

The representation of clouds in large scale numerical models has proven challenging, yet it is clear that uncertainty in cloud physical processes is a key driver of uncertainty in climate sensitivity and the sensitivity of clouds to atmospheric aerosol. Several papers have highlighted the importance of small clouds to the radiation budget, with implications for the treatment of clouds in models and the ability to detect clouds adequately with satellites. This work has tended to focus upon specific regimes (usually cumulus regimes), but bountiful satellite data exist to document cloud size globally. Here, we use a combination of satellite data from MODIS and in-situ data from aircraft, to address the question of what cloud size dominates the cloud cover and solar reflectance globally. We find that small clouds contribute most to the number distribution everywhere, but that the segment length or area distribution tends to be dominated by much larger clouds, even in regions dominated by small cumulus such as the trades and continental interiors. Indeed, over most of the Earth, the cloud length for which clouds smaller and bigger contribute equally to cloud cover is several hundred kilometers. This length increases when solar reflectance is considered. The results demonstrate that globally and over most regions the majority of cloud cover does not consist of small and broken clouds, but instead consists of extensive sheets. The distribution of cloud sizes is quite well simulated with The Met Office Unified Model.