Mechanisms of Banner Cloud Formation

Banner cloud are clouds in the lee of steep mountains or sharp ridges. Their formation has previously been hypothesized as due to three different mechanisms: (i) vertical uplift in a lee vortex, (ii) adiabatic expansion along quasi-horizontal trajectories (the so-called Bernoulli-effect), (iii) a mixing cloud, i.e. condensation through mixing of two unsaturated air masses.

In the present work, these hypotheses are tested and quantitatively evaluated against each other by means of Large Eddy Simulation. The model setup is chosen such as to represent idealized, but prototypical conditions for banner cloud formation. It is found that the lee vortex mechanism is very likely to be the most relevant explanation for banner cloud formation. Key aspect is a pronounced windward-leeward asymmetry in the Lagrangian vertical uplift with a plume of large positive values in the immediate lee of the mountain; this allows the region in the lee to tap moister air from closer to the bottom surface. By comparison, the horizontal pressure perturbation is more than two orders of magnitude smaller than the pressure drop along a trajectory in the rising branch of the lee vortex; the ``Bernoulli mechanism'' is, therefore, very unlikely to be a primary mechanism. Banner clouds are unlikely to be ``mixing clouds'' in the strict sense of their definition. However, turbulent mixing may lead to small, but non-negligible moistening of parcels along time mean trajectories; although not of primary importance, the latter may be considered as a modifying factor to existing banner clouds.