Properties and characteristics of atmospheric deserts over Europe: a first statistical analysis

Abstract

Atmospheric deserts are air masses that are advected from the deep, hot, dry boundary layer of arid or desert source regions. They frequently occur across Europe, where they can modify the temperature profile throughout the free troposphere. To investigate their occurrence, characteristics, and development, we track air masses travelling from North Africa across Europe continuously during the period from May 2022 to April 2024. Using the Lagrangian analysis tool LAGRANTO together with ERA5, we calculate 120 h-long trajectories at every full hour with a spatial resolution of 5 km in the horizontal and 10 hPa in the vertical. Cluster analysis is used to identify typical synoptic patterns that occur during atmospheric desert events, as well as to cluster the trajectories and find typical air streams. We find that Atmospheric deserts occur in parts of Europe in up to 60 % of the time. They can cover up to 54 % of the area and 72 % of the land area in the domain 30° W to 60° E and 37 to 73° N. On average, atmospheric deserts persist for about one day, with slightly longer durations near the source region, however the duration and extent vary considerably with the seasons. While the 90th percentile of the duration is between one and two days for most of the domain and most of the seasons, it can exceed nine days in summer in the Mediterranean. Atmospheric desert air frequently resides above the local boundary layer and extends through much of the free troposphere. Intrusions of atmospheric desert air into the local boundary layer are rare and occur mainly over high orography and during the warm season. In some regions, atmospheric deserts frequently reside directly on top of the local boundary layer, forming a so-called lid. The lid, however, only persists for less than two and a half days on average-too short to cause a heat wave. Four typical, synoptic-scale patterns are identified. They feature either a trough extending into the source region, or a ridge extending from the source to the target region, which lead to an eastward or north-eastward advection of air from the source region. Five typical air streams are present. One reaches the target region after a strong ascent with even higher potential temperature and lower moisture than it had in the source region due to condensation and evaporation. Three air streams experience a decrease in potential temperature, either due to mixing, or evaporative or radiative cooling. The last air stream behaves like an “elevated mixed layer”, as it almost conserves its thermodynamic properties.