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

The canonical Spanish plume is a synoptic pattern associated with deep moist convective storms over western continental Europe and the UK. A large-amplitude trough or cut-off low in the jet stream extending to low latitudes produces a long fetch of southerly or southwesterly deep tropospheric flow. The near-surface air passes across the heated elevated terrain of the Iberian Peninsula and travels into western Europe. Thus, the preconvective environment is 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 review of 102 journal articles mentioning the Spanish plume paints an unevidenced, inconsistent, unclear, and inaccurate picture. Some articles correctly employ the original definition of the Spanish plume airstream as the elevated mixed layer; 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. We examine a Spanish plume case from 1–2 July 2015. We calculate air-parcel trajectories to determine four airstreams responsible for the unstable thermodynamic profile over the UK: (1) a near-surface continental airstream transporting hot, moist boundary-layer air from France; (2) a lid (850-hPa) airstream descended from the mid-troposphere over the eastern North Atlantic and western Iberian Peninsula, initially warm and dry, but gradually cooling and moistening and rising while traveling northward, forming a layer of convective inhibition; (3) a subtropical airstream of hot, dry air with steep lapse rates traveling poleward from North Africa and the Mediterranean then ascending; and (4) a middle-latitude upper-level trough airstream traveling eastward from the North Atlantic Ocean then ascending. These results challenge the canonical Spanish plume synoptic pattern in three ways. First, most air reaching the UK does not travel over the Iberian Peninsula, particularly the near-surface air from France. Second, the steep lapse rates were pre-existing from the subtropics rather than created when passing over the Iberian Peninsula. Third, the lid results from subsidence rather than surface heating over the Iberian Peninsula. Thus, the synoptic-scale pattern appears to have a larger control over the thermodynamics of the Spanish plume airstream than heating from the Iberian Plateau, suggesting that the canonical conceptual model for the Spanish plume requires revision.

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