Properties and Characteristics of Atmospheric Deserts over Europe

Abstract. Atmospheric deserts are air masses that are advected from the deep, hot, dry boundary layer of arid or desert source regions. We track air masses travelling from North Africa across Europe continuously during the period between May 2022 and April 2024. The Lagrangian analysis tool LAGRANTO is used to calculate 120 h-long trajectories at every hour and a spatial resolution of 5 km in the horizontal and 10 hPa in the vertical. Atmospheric deserts occur in up to 60 % of the time in parts of Europe. They can occur everywhere in Europe and can cover up to 55 % of the area in the domain 30° W to 60° E and 37° N to 73° N. Three typical, synoptic-scale patterns occurring during atmospheric desert events are identified from a cluster analysis. The patterns show a very zonal flow, a ridge over western Europe and a deepening trough over north-western Europe, respectively, leading to eastward or north-eastward advection of air from the source region. Typically, atmospheric deserts persist for about one day on average, slightly longer close to the source region, but 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 be more than nine days in summer in the Mediterranean. Atmospheric desert air frequently resides between the local boundary layer height and the troposphere, and therefore modifies the temperature profile throughout the free troposphere. The atmospheric desert air rarely enters the local boundary layer, and if it does, it happens at over high orography and in the warm season. In some regions, atmospheric deserts frequently form a lid on top of the local boundary layer, but it only persists for less than two and a half days on average – too short to cause a heat wave. Two main groups of air streams were found from a subjective investigation of trajectory clusters and analysis of average changes along the trajectories. One group consists of three air streams that ascend strongly. Two of the three air streams in this group become even warmer and dryer than they were in the source region through condensation and evaporation. The third, and least frequent air stream in this group cools and dries, likely due to mixing with another air mass, especially over continental Europe in the cold season. The three air streams in the other group remain at medium altitudes, and two of them cool, either due to radiative or evaporative cooling or mixing. The third air stream in this group is the previously known ‘elevated mixed layer’, which almost conserves its thermodynamic properties.