The Spanish Plume Elevated Mixed Layer: A Review of Its Use and Misuse within the Scientific Literature

The Spanish plume is a synoptic pattern associated with deep moist convective storms in western and central Europe. A large-amplitude trough or cutoff low in the jet stream extending to low latitudes produces a long fetch of southerly or southwesterly flow in the lower troposphere across the Iberian Peninsula and into 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. A literature review of 102 peer-reviewed journal articles mentioning the Spanish plume is performed (of which 84 have only
passing mentions). Some articles 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. The origin of the airstream is variously described as the Iberian Peninsula, northern Africa, or both, often unevidenced. Some air in so-called Spanish plumes does not even cross Spain. Descriptions of convective storms in Spanish plume synoptic patterns also are largely unevidenced, with release of instability attributed to various synoptic-scale and mesoscale processes. This review reveals these and other issues with the literature on the Spanish plume, painting a sometimes unevidenced, inconsistent, unclear, and inaccurate picture. The goals are to recommend proper usage of the term Spanish plume and articulate future
research questions, specifically related to quantifying interactions with terrain through diurnal sensible heat fluxes and orographic flow modification to produce favorable environments for convective storms.

SIGNIFICANCE STATEMENT: The Spanish plume is a weather pattern favoring intense thunderstorms in the United Kingdom and northwest Europe. It is traditionally associated with a long fetch of southerly or southwesterly winds that transport air heated by the high Spanish Plateau aloft overtop near-surface air. However, some articles use this term incorrectly. Other articles make unevidenced claims or claims that are inconsistent internally or with other literature. This review aims to 1) standardize terminology so that the Spanish plume term is used correctly and 2) improve understanding through recommending research related to how the Spanish Plateau influences thunderstorm development. Consistent understanding and usage of scientific terms is important for clear and concise communication with other scientists, as well as the general public.