Meteorological Physical Background

The Spanish Plume is, by definition, an incident where warm air is lifted from the Spanish plateau ahead of an eastward-moving upper-level trough over the Bay of Biscay. In conventional upper-air and surface synoptic maps this incident can typically be illustrated as follows:

Upper-level flow within Spanish plume. Solid lines: upper level (500 or 300 hPa) geopotential height; blue solid line: the upper trough; yellow arrow: jet stream axis
Low-level flow within Spanish plume. Open red arrow: the warm conveyor belt; open blue arrow: the dry intrusion. Area within red solid line: a typical area of high ThetaE values at 850 hPa. Black solid lines: surface isobars. Classical surface fronts also shown

The strong heating of air near the surface over the Spanish plateau causes convergence and ascent, enhanced by the dynamical forcing of the eastwards moving upper-level trough. This leads to the formation of a thermal low over the Iberian Peninsula. The thermal low sets up a low-level cyclonic flow over the peninsula (Phase A), which includes the formation of a convergence line over the plateau. This induces the advection of cooler air and descending motion across the coastal areas of the Iberian Peninsula and western France from the Bay of Biscay and warm air advection over the interior of the peninsula. Therefore in the area east of the surface trough the advection of warm air enhances the ascents within the warm plume (Phase B), further accelerating decrease of surface pressure.

The approaching Cold Front has an important role in the further development of the Spanish Plume in two ways:

  1. As the front approaches the continent, the existing thermal gradient intensifies. This causes the strengthening of winds ahead of the upper-level trough and wind adjustment in the lower levels; consequently an intensification of warm air advection and ascent within the warm plume ahead of the surface trough takes place (Phase C).
  2. The possible over-running of cold air at upper levels from behind the front, and further destabilization of the already highly unstable air within the Spanish Plume, can trigger deep convection.

The development of the Spanish plume (Phase A, B and C); solid lines: surface flow, dashed lines: thermal advection at low levels, WA and CA warm and cold air advection maximum, respectively.

As thunderstorms develop along the plume, the ascending motion is often rapidly compressed into a relatively narrow band. This band quickly becomes the Cold Front, while the old Cold Frontal zone tends to lose its intensity. This is due to subsidence of air immediately behind the Plume. This feature can be seen as increasing gradient of isentropes along the Plume.

The example below illustrates this process: a Cold Front approaching from the west gradually loses its intensity (as seen by a steady and later decreasing gradient of isentropes), while in the area of the Spanish Plume the ThetaE gradient increases and the Cold Front actually seems to "jump" forward.

03 July 2005/18.00 UTC - Meteosat 8 IR10.8 image; green: equivalent thickness, magenta: Equivalent potential temperature 850 hPa
04 July 2005/00.00 UTC - Meteosat 8 IR10.8 image; green: equivalent thickness, magenta: Equivalent potential temperature 850 hPa
04 July 2005/06.00 UTC - Meteosat 8 IR10.8 image; green: equivalent thickness, magenta: Equivalent potential temperature 850 hPa

This process is similar to Non-Orographic Convergence Lines and Thickness Ridge Cloudiness, as they have rather similar life cycles: the line of convection ahead of the old front becomes the dominant feature, and the remnants of the old front are either absorbed into the developing convective cloud line or simply decay.

Stability (soundings)

The pronounced instability at low and middle levels is clearly seen in the soundings. Often there is a lid of warm air near 850 hPa level. This lid delays the initiation of surface-based convection. Consequently, Convective Available Potential Energy (CAPE) is stored until it is released either by thermal advection or ascending motion. The triggering is often explosive, and can result in severe thunderstorms.

Cooling aloft, ahead of the advancing upper-level trough, and heating from below due to warm advection and solar insolation further destabilizes the atmosphere.

The amount of potential instability increases remarkably when the low-level moist air is capped by significantly drier air at medium levels (400-600 hPa). Upper-level dry air intrusion from behind may support explosive convection once convection is initiated.

The wind often veers considerably with height, which means a moist south-southeasterly flow at low levels, and a steady or increasingly south-westerly flow at middle and upper levels.

27 June 2001/00.00 UTC - Radiosounding Uccle; The location of Uccle (06447) is shown in the Meteosat 7 IR image from 27 June 2001/00.00 UTC at the upper right corner of the sounding graph