A global analysis of dry intrusion temperature anomalies

Dry intrusions (DIs) are a peculiar dynamical feature. They are synoptic scale descending air streams, located behind cold fronts of midlatitude cyclones. They are strongly linked to extreme weather events such as cold spells, extreme winds, dust storms and extreme wildfires. In these extreme events, strong negative temperature anomalies are induced by the arrival of DIs, somewhat surprising considering the strong subsidence of the air.

In this work we focus on the dynamical processes leading to the formation of the temperature anomalies. To this end, a global Lagrangian database of DIs and a temperature anomaly decomposition along the trajectories is used to highlight the contribution of adiabatic, advective and diabatic processes. Although DIs with cold temperature anomalies are frequently discussed due to their association with extreme events, our results show that both positive and negative anomalies are found at the end of the DI descent.  Globally, DIs with a positive temperature anomaly are as frequent as DIs with negative temperature anomalies.

Most DIs start from a negative temperature anomaly and the air is adiabatically heated as it descends. The heating is counterbalanced by adiabatic cold horizontal advection. The diabatic contribution is also negative along the DI descent. However, in the last 24 hours of the descent, the diabatic contribution increases substantially only in the negative temperature anomaly trajectories. Change in diabatic cooling occurs as the DI air reaches the boundary layer and is associated with an increase in humidity in the airmass. This diabatic contribution is the main difference between the positive and negative anomalies seen at the end of the DI descent.

We present a comprehensive look on the dynamical processes that affect the formation of temperature anomalies near the surface from descending air streams. We include global results to first understand DI climatology, as well as individual case studies demonstrating the uniqueness of each case.