Dynamics of dry intrusion air streams and their relevance for extreme weather

Dry intrusion (DI) air streams typically comprise the cold and dry sector of extratropical cyclones. These air parcels descend slantwise from the midlatitude upper troposphere towards the surface in lower latitudes, where the airstreams typically fan out behind the cyclone’s trailing cold front. In this talk I will outline recent results based on a Lagrangian-based global climatology of DIs using ECMWF reanalysis data, allowing progress in understanding the intrusions occurrence frequencies, dynamical interactions and association with extreme weather.

Using representative case studies and longer-term climatologies we find that DI air starts its descent from the upper troposphere behind midlatitude troughs of Rossby waves, often associated with Rossby wave breaking. When interacting with cyclones in the storm tracks, they are associated with strong cold fronts in the cyclone southwest quadrant (in the northern hemisphere) and with marked anomalies in the lower troposphere. Namely, dry and cold anomalies prevail, which together with strong winds, enhance surface turbulent ocean heat fluxes and destabilize the boundary layer there.

The modification of the lower troposphere by DIs entails a strong association with a diverse set of weather extremes, from heavy precipitation, strong winds and extreme cold temperatures, to wildfires and dust storms. For example, DIs enhance (i) the Mistral wind in southern France and its impact on deep convection in the water column in the Gulf of Lion; (ii) large-scale Saharan dust storms and long-range dust transport to Europe and the Middle East; and (iii) wildfires in southeast Australia, combining dryness with extreme temperatures and strong winds. Using the Lagrangian approach and feature diagnostics we further find that the surface extremes (such as cold temperatures) associated with DIs may extend beyond the midlatitudes, into subtropical and tropical regions.