Inland Deforestation Organizes Convective Extremes.

Large-scale deforestation is rampant in the Amazon region and has been shown to induce decreases in mean precipitation over deforested regions. Yet, the impact of deforestation on precipitation extremes remains largely unexplored. Here we analyse lightning flash data over Rondônia, Brazil, and show that extended deforested areas give rise to a clear spatio-temporal organization of intense thunderstorms. When eliminating the effect of the large-scale atmospheric flow, we find that during early afternoon, initial lightning clusters form along forest edges, whereas they later migrate towards the centre of the deforested region— resulting in intense lightning. We replicate this marked behaviour by using an idealized cloud resolving model with prescribed dry soil moisture patch sizes comparable to the mesoscale deforestation scales, O(100 km), found in Rondônia. By tracking cold pools and associated moist convection in the model data, we show that it is convective cold pools merged with thermal breezes that mediate an inter- action between the forest edge and the centre of the deforested region. Our results suggest that specific deforestation patterns can increase the risk of extreme precipitation by amplifying cold pool propensity to self-organize thunderstorms. Our results imply that city and land use planning should factor in associated emergence of extreme events — potentially leading to flash flooding despite seemingly benign synoptic conditions.