Quantifying moisture perturbations leading to stacked lenticular clouds

Lenticular clouds of any significant depth usually display a layered structure in the vertical. Several potential mechanisms that may generate these layers have been suggested in the past, but there has been no definitive description of their formation, quantitative analysis of the potential mechanisms, or observations of the environments that these layers form within. Here, we investigate the source of the layering using high-resolution 2D numerical simulations of stacked lenticular clouds, based on an event of these clouds that occurred over the Pennines, England, in December 2011.

Model results show that the addition of layered moisture perturbations in the upstream model sounding leads to a similarly layered structure in the cloud mass. Relative humidity perturbations within these upstream layers only 0.5% drier than the surrounding environment are sufficient to generate striations of a similar size to those commonly observed in nature (creating an indentation of approximately 160m into the lateral edges of the cloud). Increasing the magnitude of the humidity perturbations increases the scale of the layering, with 400m indentations forced with a 1.5% relative humidity decrease in the drier layers.

These layers are generated without any associated layering of the static-stability profile. Further, the cloud lies within a smooth vertical displacement field, implying that the layers are not caused by variations in vertical ascent, and are instead solely due to the layered moisture field. These simulations confirm an earlier hypothesis by Richard Scorer as to the source of the layering, and apply a quantitative estimate as to the magnitude of the moisture perturbations required.