Detection and consequences of atmospheric deserts: insights from a case study

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 and the application of the detection method with a case study in Europe from mid-June 2022. With the Lagrangian analysis tool LAGRANTO, approximately 45 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 5 d. 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. In this case in June 2022, thunderstorms were mainly absent in the centre of the AD but broke out along a line parallel to its boundary. At this edge of the AD and the surface front, lifting occurred, causing the formation of thunderstorms. The AD did not reside directly above the local boundary layer for long enough to be the main cause of the high near-surface temperatures in large parts of Europe but may have contributed to it. Subsidence heating of another airstream was identified as one possible reason for the increased near-surface temperatures. This case supports the assumption that ADs co-occur with thunderstorms at their edges and with increased near-surface temperatures in their centres and gives some insights into the responsible processes.