Atmospheric Deserts: Detection and Consequences

Abstract. We introduce the concept of atmospheric deserts (ADs), air masses that are advected away from hot and dry convective boundary layers in semi-arid or desert source regions. They can be expected to eliminate cloudiness, cause heat to build up in the target region, suppress thunderstorm formation in their centre and boost thunderstorm formation at their edges. A direct detection method, tracing the AD from source to target using Lagrangian trajectories is developed.

We illustrate this new concept of ADs with a case study in Europe from mid-June 2022. With the Lagrangian analysis tool (LAGRANTO) approximately 200 million trajectories are calculated, tracking the path of the air mass and the development of its properties as it progresses from North Africa towards and across Europe over the course of five days. k-means-clustering identifies four typical pathways that the trajectories follow. For one of the pathways, the air nearly conserves its well-mixed properties. Diabatic processes of radiative cooling, latent heating due to condensation, and cooling due to re-evaporation of precipitation, however, modify the air along the other pathways.

The case study demonstrates how ADs influence the weather in the target region. Thunderstorms are mainly absent in the centre of the AD, but erupt along a line parallel to its boundary. At this edge of the AD and the surface front, lifting occurs, causing the formation of thunderstorms. The AD does not reside directly above the local boundary layer for long enough to be the main cause for the heat wave affecting large parts of Europe, but may contribute to it. Subsidence heating of another air stream, was identified as one possible reason for the increased near-surface temperatures.