Case Study of Dry Air Intrusions Behind an Extratropical Cyclone and Their Impacts on Boundary Layer Evolution

Planetary boundary layer (PBL) dynamics are critical for understanding atmospheric processes and for accurate forecasts of surface weather. The diurnal cycle and local surface properties strongly affect boundary layer characteristics. However, especially over the ocean, where the surface is less responsive to radiation changes, synoptic-scale weather systems can play a dominant role in shaping the PBL.

Dry intrusion (DI) is a free tropospheric flow that moves downward and equatorward along isentropic surfaces, which often mixes into the PBL. This airstream is fundamental to the structure of mid-latitude cyclones, spreading behind their cold front. DIs can enhance surface heat fluxes and the surface wind speed, impacting the height of PBL and the inversion strength. The DI airstream thus provides a means to delineate the understudied connection between the PBL evolution and the synoptic forcings.

This study examines the influence of DI entrainment on PBL structure, focusing on the Azores islands — a domain within a climatologically high DI occurrence with rare operational profile observations of the marine boundary layer. Using high-resolution ICON simulations, we analyze a winter case study in which a cold front is followed by a DI, tracking its impact on PBL properties over time. A Lagrangian approach allows us to diagnose and follow the trajectories of DI airmasses, providing insight into their evolution and spatial influence on atmospheric conditions.

Our findings reveal coherent temporal changes in PBL properties, such as the PBL height, inversion properties, moisture content, and spatial extent of these changes during the course of the studied event. There are distinct stages of PBL modification due to DI entrainment. Initially, DI influx leads to enhanced surface heat fluxes and significant PBL deepening. However, as the entrainment progresses, the PBL undergoes a transition, ultimately becoming shallower and more stable, with extremely low relative humidity values above it. This case study shows the interplay between DIs and surface fluxes that can both enhance and suppress PBL deepening, while the nature and timing of these transitions are topics for further research.

This study improves our understanding of DI-driven PBL evolution, highlighting the connection between large-scale weather systems and boundary-layer dynamics. A better representation of these processes in weather and climate models may improve forecasts in regions affected by DIs.