On the link between the Amazon Forest Enhanced Vegetation Index and Shallow Cumulus Organization

During the dry season (boreal summer months) the Amazon region is affected by large-scale subsidence and experiences relatively stable meteorological conditions. Under these conditions the forest is frequently covered by shallow cumulus clouds fields, referred to here as Forest Cumulus (FCu). These clouds are shown to be sensitive to landcover and exhibit a high level of spatial organization.

In this study we use satellite data to perform a morphological classification of the FCu fields, based on large subsets of Amazon cloud spatial statistics. This information is then combined with satellite derived landcover and Enhanced Vegetation Index (EVI, commonly used as a measure for forest density and productivity) data to examine the link between FCu cloud field occurrence and EVI. Since meteorology and biomass burning can be considered as first order effects (i.e. can completely prevent cloud formation or induce frequent deep convection in certain areas) on cloud formation during the dry season, much effort is put into choosing regions with limited meteorological spatial variance and with low values of AOD (Aerosol Optical Depth), where FCu fields are likely to form.

Results from the years 2008-2011 show a clear positive linear relation between EVI (i.e. surface properties) and FCu field occurrence over forest landcover, implying a dynamical coupling between forest surface fluxes and the cloud organization above. These results are shown to be consistent for several spatial scales of surface EVI data. Over non-forest landcover the relationship between EVI and FCu occurrence is non-linear, showing a reduction of FCu for high EVI values. Additionally, we find that forest to non-forest transition zones display a superposition of the two different landcover dependencies.

This work should be considered as an initial step in understanding the dynamics of FCu cloud formation. Future work is planned to for an in depth investigation of these clouds and the physical mechanisms which control their formation.