NRL Monterey, Marine Meteorology Division
The mistral is a strong, cold northwesterly wind system that blows from Southern France into the Gulf of Lions (See locater map below). Although strongest in the Gulf of Lions, with sustained winds often exceeding 40 kt, and gusts sometimes to 100 kt, its effects are often felt past Sicily into the eastern portion of the Mediterranean basin. Wave heights associated with the mistral are commonly 15-20 ft (4.5-6.0 m) and have sometimes reached a maximum height about 30 ft (9 m). The mistral is most common in winter and in spring with the strongest episodes tending to occur in the transition between those two seasons. Duration of effects with winds exceeding 30 kt for over 65 hours have been reported at some locations.
A favored sequence for mistral development is passage of a cold front into the Gulf in advance of a short wave trough at upper levels. The upper- level trough produces southwesterly flow aloft and positive vorticity advection over the position of the cold front in the Gulf. This pattern leads to cyclogenesis in the Gulf of Genoa. As the upper level trough advances so that northerly winds aloft are in phase with the northerly surface flow over Southern France, strong mistral winds occur (Fett, et al, 1981).
Mistral winds have been well documented in literature, as in the previous reference, with satellite visible and infrared data, and conventional weather reports. With the advent of Special Sensor Microwave Imager (SSM/I) data in the late 1980's it was possible to directly measure wind speed effects and changes in total integrated water vapor associated with a mistral. This study shows examples of infrared and SSM/I imagery obtained during such an event.
|Locater Map for Mistral|
|Fig. 1. Meteo 7 Infrared Image. 29 Mar 2001 1930Z|
|Fig. 1 is a Meteosat infrared image of the European area. A cloud vortex
is evident in this image, having just moved southward into the Northern
Mediterranean over the islands of Corsica and Sardinia. The vortex
follows a frontal band, seen running from southwest to northeast in the
lower right portion of the image. The Rhone River valley extends
northward along 5łE from the region between Montpellier and Marseilles,
France (See locater map). Characteristically, as shown in the Meteosat
image above, this valley is clear of clouds during mistral events.
Clearing is due to sinking effects as cold, dry air, is advected southward
around the west edge of the cyclonic circulation associated with the cloud
It would be anticipated that strong mistral winds should be commencing at about this time in the northern portion of the Gulf of Lions. However, convective cloud forms in the Bay of Biscay, caused by convective instability associated with an upper level cold trough, indicate that this upper level trough has not yet passed Marseilles, so that upper level flow is not yet aligned with the northerly low-level flow to maximize mistral conditions across the Mediterranean Basin.
|Fig. 2 SSM/I Wind Image. 29 Mar 2001 1941Z||Fig. 3. SSM/I Integrated Water Vapor Image. 29 Mar 2001 2044Z|
|Fig. 2 reveals direct evidence of the strong winds just entering the Gulf
of Lions south of Marseilles. A protected area of weaker winds extends
southeast of the rugged coastal mountain area (Costa Brava) of northern
Spain. High wind speeds indicated in the Bay of Biscay are probably
erroneous, since this is the convective area of cumulus congestus or
cumulonimbus shown in the IR data of Fig. 1. In this region rain can
cause faulty SSM/I wind retrievals.|
Fig. 3 is SSM/I water vapor data close to the time of the IR and SSM/I wind images. No significant drying of the lower atmosphere in the region of the mistral winds is apparent at this time. This is indicative of early mistral development, before upper level winds are in phase with lower level winds.
|Fig. 4. Meteo 7 Infrared Image. 30 Mar 2001 1200Z|
|Fig. 4 is a Meteosat infrared view of the area about 16 hours later. The vortex is well developed at this time and appears as a tight cloud vortex. Cold streaks from upper level cloudiness west of the vortex suggest that the upper level trough has now moved past Marseilles, so that upper level winds are in phase with lower level northerly winds crossing the Mediterranean.|
|Fig 5. SSM/I Wind Image. 30 Mar 2001 2034Z||Fig. 6. SSM/I Water Vapor Image. 30 Mar 2001 2034Z|
|Fig. 5, the SSM/I wind image 8 hours later, reveals that a narrow band of
strong winds with speeds in excess of 35 kt extends across the entire
Mediterranean from France to North Africa and even beyond, past Southern
The corresponding SSM/I water vapor image, Fig. 6, shows a narrow tongue of very dry air overlapping the strong mistral wind region. Although the strong winds create very moist conditions near the surface of the sea, the northerly flow is sinking and divergent, which reinforces the low level drying tendency.
1. SSM/I wind data can be used to monitor the initiation and further development of mistral winds.
2. Strongest mistral wind conditions can be anticipated when upper level northerly winds are in phase with lower level northerly wind flow.
3. The presence of a cloud vortex over the Gulf of Genoa area, as seen in satellite imagery during winter and spring, is a certain sign of mistral development. Lacking such a signal one should not expect mistral conditions.
4. SSM/I water vapor data can also be used to detect mistral conditions. Strongest mistrals tend to exist when very dry water vapor conditions overlap the SSM/I strong wind indications. Lacking such correspondence, mistral conditions can be expected to be much weaker or non- existent.
Fett, R. W. et al., 1981, Navy Tactical Applications Guide (NTAG), Vol. 3, North Atlantic and Mediterranean, Naval Research Laboratory, Monterey, CA, pp200.
Author: Bob Fett
Last Updated: Mon Dec 9 10:42:50 2002
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