Relative humidity as a proxy for cloud formation over heterogeneous land surfaces

Relative humidity as a proxy for cloud formation over heterogeneous land surfaces.

The influence of heterogeneously forced land surfaces on the potential formation of convective clouds is investigated by performing numerical experiments using a large eddy simulation model (LES). The land surface in the model was divided into two patches that had the same sum of latent and sensible heat flux, but different Bowen ratios in order to simulate heterogeneous land surfaces. For heterogeneities in the meso-gamma scale (2-20 km) sensitivity analyses were carried out on the heterogeneity amplitude (Bowen ratio difference between contrasting areas).

A study of the atmospheric boundary layer (ABL) and entrainment structure showed that all entrainment events are localized over patches with higher Bowen ratios (warm patches) than their surroundings (cold patches). Here, the local fluxes are up to five times as large as under homogeneous conditions. The heterogeneity-induced strong thermals can further penetrate the characteristic inversion at the ABL top, thereby reaching lower absolute temperatures than under homogeneous conditions. As the moisture is also located over the warm patch, higher relative humidities than over the cold patches are found here. These values also exceed values found over homogeneous land surfaces, which is an indication that land surface heterogeneities facilitate cloud formation.

The analyses of the thermodynamic area-averaged vertical profiles indicate that, depending on the heterogeneity amplitude, the total entrainment is larger than under homogeneous conditions. This allows for deeper ABLs, which is beneficial for cloud formation. A statistical decomposition of heat transport in turbulent and mesoscale contributions showed that entrainment enhancement is directly related to mesoscale motions, as the enhancement is enclosed in the mesoscale component. Finally, spectral analysis of vertical wind, potential temperature and specific humidity in the entrainment zone indicated that temporal growth of the spatial length scales is accelerated by the presence of heterogeneity.