Europe’s Elevated Mixed Layer: New Insights into the Spanish Plume

The Spanish plume is a synoptic pattern associated with deep moist convective storms over western continental Europe and the United Kingdom. A large-amplitude trough or cut-off low in the jet stream extending to low latitudes produces a long fetch of southerly or southwesterly flow in the lower troposphere across the heated high terrain of the Iberian Peninsula and into western Europe. The preconvective environment is traditionally characterized by an elevated mixed layer of hot dry air with steep lapse rates (i.e., the Spanish plume airstream) overtop a warm surface layer and capping inversion, resembling the loaded-gun convective sounding in the central United States.

A previous literature review of 102 peer-reviewed journal articles mentioning the Spanish plume paints an unevidenced, inconsistent, unclear, and inaccurate picture. For example, some authors correctly employ the original definition of the Spanish plume airstream as the dry elevated mixed layer, whereas others incorrectly apply the term to the surface (sometimes humid) airstream. Confusion extends to the origin of the airstream, which has been variously described as the Iberian Peninsula, northern Africa, or both, often unevidenced. Some air in so-called Spanish plumes does not even cross Spain.

In this presentation, we examine a 3-day long Spanish plume synoptic pattern and analyze the conditions that created the environment favorable for two episodes of convective storms within that period. In particular, the origin of the elevated mixed layer during this period challenges our preconceived notions of what an elevated mixed layer is. Specifically, some air parcels ending up within the elevated mixed layer do not necessarily pass across the Iberian Peninsula and, even those that end up in the warm capping inversion, are not heated by the surface. Model sensitivity experiments with changing the sensible heat fluxes and the terrain height further reveal the sensitivities of the airflow that creates a favorable synoptic environment for convective storms.

Finally, we discuss the relevance of our findings to those of the North American elevated mixed layer, recommending future research related to quantifying interactions with terrain through diurnal sensible heat fluxes and orographic flow modification to produce favorable environments for convective storms.