Altered micro climate conditions increasing convective rainfall above cleared land in the transition zone to natural forest cover

One phenomenon, which I actually observed myself when I travelled to Yucatan in Mexico a few years ago, was that more clouds seemed to form above the deforested (pasture) land than above the natural forest adjacent to it. In this post, I want to explain this local climate effect and how this impacts the ground.

Building on what I have written about in earlier posts on the local climate effects of deforestation, we now know that temperatures tend to increase over deforested vs. natural forest land-cover. We also understand that there is much greater moisture recycling over forest area, due to higher rates of evapotranspiration, deep cumulus cloud formation and subsequent rainfall. The contrast that exists here, between the micro-climate characteristics of the two land-covers side-to-side, produces a sharp gradient in the landscape.

Especially the temperature differences are responsible for causing an effect often termed a “vegetation breeze”. These breezes enhance convection circulations above the deforested area on the transition to the forest, but draw moisture away and reduce cloud-cover over the forest itself. 

Knox et al (2010) used an analysis of past satellite imagery and forest-cover data to show the impact of the “vegetation breezes” on rain-bearing cloud formation. Findings generally concluded that, as expected, cloud cover is decreases in deforested areas far off from natural forests, but a particular increase of rain-clouds are shown on the non-forest side of the transition zone (within+/-10km). 

Cloud cover over Alta Floresta in Brazil, taken by  NASA satellite imagery, showing that cloud cover can be greater above the cleared land than the adjacent natural forest. (Source: click here)

The impact of this phenomenon on local and meso-sclae precipitation was further analysed in a large modelling study by Garcia-Carreras and Parker (2011). Varying heat-exchanges and amount of cleared/forested land-covers (PlanetEarth, www), they found a clear coupling between land heterogeneity and convective locally-generated rainfall. Rainfall increases 4-6 fold on a transition zone, compared to one continuous land-cover. Also – and this is quite important when thinking about the “edge effects” of deforestation – rainfall is suppressed over natural forest next to this transition zone to a cleared area. The degree of the suppression is related to the degree of the climatic gradient formed between the land-covers. 

Further thoughts on this:

When putting this phenomenon into the context of my previous posts on the wider climatic effects of deforestation in South America, we see that these small-scale effects may enhance those larger patterns. In particular, in the regions (e.g. the southern and north-eastern Amazon) where a drying signal is already threatening rainforest resilience, this further suppression of rainfall may quicken forest loss by inducing negative feedback. 

The findings of Garcia-Carreras and Parker (2011) also suggest that different deforestation practises and patterns (e.g. large clear-cut vs. selective or fish-bone deforestation) will change the impact of the suppression signal – which should be taken into account by forest management. 

The edge-effects (e.g. this impact on locally generated rainfall) impairs the "buffering effect" of rainforests to climatic variability, extending far into the natural forest. The highly fragmented land-cover seen in South America means that a lot of the remaining forest does no longer remain in stable condition, even if left untouched. Ewers and Banks-Leite (2013) back this up, by showing that about 12% of Atlantic Brazilian forest is impacted by such altered micro climate characteristics.

Increased convective rainfall over the cleared land can have different effects on the ground, ranging from benefiting farmers with greater moisture supply for crops to increasing surface runoff and erosion of soils.