In a new paper in Quarterly Journal of Royal Meteorological Society, we present a potential mechanism to explain the observed enhanced cloud formation over Western European forests, such as in Les Landes, France. This work was the MSc thesis project of Peter Bosman, the first author of the paper.
We have set up MicroHH to reproduce 17 July 2006. On this day, there was a clear difference in cloud cover between the forest of Les Landes and the surroundings visible in satellite images. We have run MicroHH twice more, once with 5% extra available energy added to the sensible heat flux, thus more heat added the system, and once with 5% added to the latent heat flux, thus more moisture added to the system.
Interestingly, an analysis of the cloud cover and liquid water path (the integrated amount of liquid water contained in the clouds) shows that adding more heat (label Warm) is far more efficient in creating more and wetter clouds than adding extra water (label Wet). While this finding is not new in itself, our analysis shows that this cannot be explained from changes in the relative humidity at the boundary layer top, an indicator that is often used. Instead, the explanation lies in the kinematic properties of the strongest plumes that rise from the surface. We show that their additional kinetic energy due to extra heat is the key driver of enhanced cloud formation when extra heat is added.
As forests tend to have higher sensible heat fluxes than their surroundings, we conjecture this to be a likely explanation for the enhanced cloud formation over the temperate forest in our case study.
Bosman, P.J.M., van Heerwaarden, C.C. and Teuling, A.J., 2019. Sensible heating as a potential mechanism for enhanced cloud formation over temperate forest. Quarterly Journal of the Royal Meteorological Society, https://doi.org/10.1002/qj.3441