Overlap statistics of cumuliform boundary-layer clouds are studied using large-eddy simulations at high resolutions. The cloud overlap is found to be highly inefficient, due to the typical irregularity of cumuliform clouds over a wide range of scales. The detection of such inefficient overlap is enabled in this study by i) applying fine enough discretizations and ii) by limiting the analysis to exclusively cumuliform boundary-layer cloud fields. It is argued that these two factors explain the differences with some previous studies on cloud overlap. In contrast, good agreement exists with previously reported observations of cloud overlap as derived from lidar measurements of liquid water clouds at small cloud covers. Various candidate functional forms are fitted to the results, suggesting that an inverse linear function is most successful in reproducing the observed behavior. The sensitivity of cloud overlap to various aspects is assessed, reporting a minimal or non-systematic dependence on discretization and vertical wind-shear, as opposed to a strong case-dependence, the latter probably reflecting differences in the cloud size distribution. Finally, calculations with an offline radiation scheme suggest that accounting for the inefficient overlap in cumuliform cloud fields in a general circulation model can change the top-of-atmosphere short-wave cloud radiative forcing by -20 to -40 W m^-2, depending on vertical discretization. This corresponds to about 50 to 100 % of the typical values in areas of persistent shallow cumulus, respectively.

Quicklooks from all data available from the Raman lidar ‘Caeli’ at Cabauw are now available through this link. These pages are publicly accessible. You can find information about the lidar system here, and also quicklooks from data collected during campaigns. In addition, you can see quicklooks from other related lidar instruments in Bilthoven and Cabauw.

Over much more than 100 years the Human Observer has made cloud cover determinations at many places on earth while just looking at the sky. These data are a valuable source of information about trends in cloudiness on long time scales. In the last 20 years a number of instruments have been developed to determine cloud cover. The question we asked ourselves was which of these instruments and techniques would best be able to replace the Observer.

Geert Lenderink and Erik van Meijgaard have recently published a new study on the relation between temperature, atmospheric moisture and summertime precipitation extremes in observations and model results. They elaborate on their earlier findings reported in Nature Geoscience from measurements taken at De Bilt that extreme hourly precipitation rates increase faster with temperature than can be expected from basic physics: the famous Clausius-Clapeyron relation. They found a 14% increase per degree, which is twice the rate expected from Clausius-Clapeyron.