PNNL

Abstract

Cloud feedbacks play an important role in Arctic warming and sea ice loss. Cloud morphology, e.g., cloud size and spatial distributions, is among key factors that directly impact their radiative effects. In this work, we use two cases observed during the Cold-air Outbreaks (CAOs) in the Marine Boundary Layer Experiment (COMBLE) campaign to study evolution of cloud size distributions as an air mass is advected from the Arctic over a comparatively warm ocean and cloud mesoscale organization changes from rolls to cells. Cloud objects are identified from Moderate Resolution Imaging Spectroradiometer (MODIS) reflectance images through an object segmentation procedure. It is found that roll-to-cell transition is accompanied by a local minimum in wind shear and local maxima in cloud size and marine cold air outbreak index M. Regardless of distance from the ice edge, smaller clouds dominate the population number but not cloud cover. Cloud size distributions show bimodality in transition and cell regimes and thus cannot be described by single power law dependency. The mean cloud horizontal aspect ratio has weak fetch dependency and is around 2 in roll, transition, and cell regimes. Mean nearest neighbor distances between clouds of comparable areas normalized by equivalent cloud radius converge to a single value for all regimes and for all but the smallest clouds, suggesting that clouds of comparable sizes are separated by distance proportional to their sizes. The presented statistical results pave the way to examining cloud-environment and cloud-cloud interactions and evaluating model simulated cloud organizations during CAO events.