Subgridscale spatial variability of cloud water in large-scale numerical models is currently poorly parameterized. However, it is imperative that these parameterizations are improved because of the important effects that such variability can have upon cloud radiative properties and precipitation formation. It is therefore necessary to characterize and to better understand the mechanisms responsible for producing cloud spatial variability.

Liquid water path spatial variability in boundary layer cloud around the NE Pacific subtropical high is investigated using a year (August 2000-August 2001) of 1km resolution data from MODIS (MOderate Resolution Imaging Spectroradiometer). Variability is characterised using spectral analysis and probability distribution functions on scales from 1-256 km. It is shown that a few key parameters are sufficient to prove a good description of the cloud variability. Maps detailing the seasonal variation of the cloud inhomogeneity are presented. NCEP reanalysis data and observed sea-surface temperatures are used to find links between large-scale atmospheric forcings and elements of the cloud liquid water path variability. Finally, application of the analysis to the problem of subgridscale cloud parameterization in large-scale numerical models is discussed.