Localised and widespread heavy rainfall cause different impacts and are increasingly recognised to be associated with different atmospheric regimes. Here we develop a simple model of a convective ensemble based on a spectrum of entraining plumes. The model relates both rainfall intensity and mean rainfall over the ensemble to the large-scale thermodynamic environment. According to this spectral plume model, widespread heavy rainfall occurs when the free-troposphere is relatively moist, whereas localised heavy rainfall occurs under a drier and more unstable troposphere. These results are confirmed using 20 years of weather radar observations and reanalysis data near Darwin, Australia. Further, using the connection between the convective environment and the entrainment distribution within the plume spectrum model, a bulk estimate of convective entrainment is diagnosed from the observational data. This estimate suggests that entrainment decreases with increasing boundary layer depth, but is largely independent of the size or organisation of convective rain cells. The analysis framework developed here is a useful tool for both diagnosing processes and evaluating high-resolution models.
Atmospheric deserts are air masses originating in hot, dry, and deep convective boundary layers over arid and/or elevated regions and are advected across cooler, moister regions while possibly being modified by diabatic processes and differential advection. They are frequently advected over Europe and can modify temperature profiles throughout the free troposphere, with implications for convective potential. Atmospheric deserts generalise elevated mixed layers, which are known to influence near-surface temperatures and the formation of thunderstorms, but it remains unknown how frequently atmospheric deserts co-occur with strong positive temperature anomalies and lightning. We also investigate if atmospheric deserts frequently co-occur with dust events, since the source region for European atmospheric deserts is also the main source region for dust events. Atmospheric desert air is tracked during the study period from May 2022 to April 2024, using a direct detection method based on Lagrangian trajectories initiated at a very high spatio-temporal resolution. Positive temperature anomaly events, thunderstorms, and dust events are identified from reanalysis datasets and ground based lightning observation data. Conditional probabilities and corresponding standardised log-odds ratios provide insight into whether the mentioned extreme events occur more frequently in the presence of atmospheric deserts. The probability of events with strong positive temperature anomalies, thunderstorms, or dust anomalies is enhanced in the presence of atmospheric deserts in almost the entire domain. In most of the domain, strong positive temperature anomaly events are more likely given the presence of atmospheric desert air, but it could not be confirmed that the limitation of boundary layer growth by the capping stable layer was the main driver. Lightning probability is clearly enhanced along the atmospheric desert edges, and, in contrast to expectations, also in the atmospheric desert centres. In western Europe, up to 50 % of occurrences of atmospheric desert air are accompanied by unusually high amounts of dust. Dust events co-occur almost exclusively with atmospheric deserts.
Stratosphere–troposphere exchange (STE) is a fundamental process in which ozone-rich stratospheric air enters the troposphere, modulating tropospheric ozone distributions and, under favorable conditions, influencing surface air quality. This study investigates an atypical STE event over North America from 6 to 14 March 2016, which coincided with a severe surface ozone pollution episode in Mexico City. Analyses using ERA5 reanalysis, potential vorticity (PV) diagnostics, ozone tracers, Lagrangian trajectories, and isentropic methods attribute the event to anticyclonic Rossby wave breaking, a PV cut-off low, and a persistent tropopause fold. Deep intrusions of high-PV air penetrated the troposphere, with maximum downward transport occurring approximately 1 d before the peak in surface ozone (O3) mixing ratios. Equatorward wave amplification facilitated coherent isentropic transport, enabling efficient descent of ozone-rich stratospheric air over the elevated basins of central Mexico. Backward trajectory analysis confirmed stratospheric origins, while isentropic advection quantified quasi-horizontal transport along 320–340 K surfaces. Tropopause folding, intensified by the subtropical jet and the elevated terrain of central Mexico, resulted in enhanced O3 mixing ratios of 8 × 10−8 kg kg−1 at 750–700 hPa, preconditioning background ozone levels prior to surface exceedance rather than serving as a direct surface source. The analysis identifies recurrent tropopause folds preceding high-ozone episodes, highlighting the persistent role of STE as a dynamical preconditioning mechanism for regional air-quality extremes. These results indicate that the location of cities in elevated tropical basins, in combination with Rossby wave dynamics and quasi-horizontal isentropic transport pathways, determines the depth, persistence, and significance of STE.
Marine heatwaves and coral bleaching pose significant threats to the Great Barrier Reef (GBR), particularly during austral summer when atmospheric conditions can significantly enhance ocean surface warming. One such condition is the development of doldrums, characterised by weak winds and reduced cloud cover, which promote increased solar heating of the ocean surface. When these conditions persist, this buildup of heat can develop into marine heatwaves contributing to widespread coral bleaching. Despite these impacts, the formation and persistence of the doldrums remain poorly understood. Here, we identify doldrums events over the GBR through a K-means cluster analysis and investigate the tropical and extratropical interactions associated with their onset and maintenance. Expanding upon previous links between doldrums formation and Rossby waves, the cyclonic anomaly within the Rossby wave packet is found to disrupt the typical trade wind easterlies over the GBR through anomalous westerlies over eastern Australia. If the cyclonic anomaly propagates east quickly, the doldrums tend to cease; if the anomaly stalls, the doldrums tend to persist. These stalled Rossby wave packets are associated with repeated cyclonic Rossby wave breaking over Australia, which weakens the zonal flow and enhances blocking near New Zealand. Concurrently, favourable conditions are needed in the tropics to establish and maintain the doldrums. During longer doldrums, the slower-moving Madden–Julian Oscillation and Equatorial Rossby wave more frequently align their suppressed phases over the GBR, reinforcing anomalous westerlies to further counter the trade easterlies. Finally, Lagrangian trajectory analysis reveals that dry descending air within the doldrums typically originates in the mid-latitudes. Here, the positioning of the cyclonic anomaly or cut-off low forces the parcels to sweep across southern Australia over the dry interior before reaching the GBR. Together, these results highlight the tropical-extratropical interactions that promote the doldrums persistence over the GBR and favours the development of both marine and terrestrial heatwaves.
The Mediterranean is a major extratropical cyclone hotspot and heavily impacted by climate change. The aim of this study is to investigate the impact of future climate change with respect to pre-industrial conditions on extreme cyclones (EXCs) which induce extreme wind, precipitation and compounding precipitation-wind events. Using a regional climate model simulation, we show that the mean cyclone frequency is reduced by roughly a third in the Mediterranean by the end of the 21st century under the representative concentration pathway RCP8.5. Precipitation extremes occur 6 h before the pressure minimum, while the wind extreme happens during the minimum itself. For precipitation EXCs, future projections show increased precipitation during their most intense phase in the western Mediterranean (WMED), whereas precipitation from these cyclones remains similar in the eastern Mediterranean (EMED). Moreover, precipitation EXCs in the EMED are shifted southward, whereas the latitude of precipitation EXCs in the WMED remains unchanged in the future. Wind speed EXCs become more intense in both the WMED and EMED in the future under RCP8.5. The reason for this intensification is that wind speed EXCs in the future are located on the left exit of the jet streak, the latter also being intensified in the future. The future change of compounding precipitation and wind speed cyclones is similar to the individual precipitation and wind speed EXCs, with the exception that wind speed of compounding EXCs is reduced in the EMED. Thus, we find that despite a general reduction of cyclones in the Mediterranean, precipitation and wind speed EXCs intensify in the future in some areas, which implies strong socio-economic consequences for the Mediterranean region.
