Simulating the effects of regional forest cover on mid-latitude boundary-layer clouds

It has been observed repeatedly that the presence of forests at regional scale can affect cloud formation and precipitation, but the direction and extent of this effect is highly variable. It primarily depends on the differences in albedo and roughness length with the surrounding areas, leading to contrasted water and energy balances, and also depends on the water availability and the possibility for secondary atmospheric circulations. In contrast with Amazonia where cloud enhancement has been reported over deforested areas, recent satellite observations have shown that temperate European forests promote summertime cloud formation. This is notably the case of the Landes forest in South-West France, where the destruction of 40% of the pine trees by storm Klaus in 2009 produced a sharp drop in the positive cloudiness anomaly.

To better understand these effects, the atmospheric Meso-NH model coupled with the SURFEX platform has been used at 500 m horizontal resolution at the scale of the region with advanced physical parametrizations. Two complementary study cases were selected, with significantly different soil moisture conditions. Both present summer situations of cloud cover over the Landes forest in contrast to its surroundings. After a validation step using satellite observations, near surface measurements and radiosoundings, simulated fields were compared with pre-Klaus and post-Klaus surface conditions.

In both cases, deforestation tends to decrease cloud top height or reduce the occurrence of the highest clouds. The surface flux partitioning appears as a key factor: the wet case shows a significant decrease in cloud cover, while the dry one shows a slight increase in cloud lifetime. This is in agreement with previous research using satellite data, showing that deforestation generally decreases cloud cover in temperate regions, while having a somewhat contrasting role. The atmospheric budgets of wind velocity components, TKE, temperature and air moisture have been analysed to understand the physical processes at play. A further study with a larger number of simulated case studies and a variety of meteorological situations will allow us to move towards a climatological approach.