Istria on the Internet - Geology & Meteorology

Major Wind Types

There are more than 45 identified localized winds in the Mediterranean region of which seven are of gale force strength (³ 34 knots):

Levante and Westerly (westerly winds) flow through the Strait of Gibraltar and are channeled between the Sierra Nevada Mountains in southern Spain and the Atlas Mountains in Morocco.

Mistral (northwest winds) flow into the Mediterranean Sea from the south coast of France and are channeled through the gaps between the Pyrenees Mountains, the Massif Central and the Alps.

Bora - Adriatic Sea (north-northeast winds) flow into the Adriatic Sea and are channeled through gaps within the Dinaric Alps.

Bora - Aegean Sea (north-northeast winds) and Etesian (northwest winds) flow into the Aegean Sea and are channeled through the Rhodope Mountains, the complex mountainous topography in Turkey and the Pindus Mountains in Greece.

Sirocco (southerly winds) flow into south central and southeastern Mediterranean Sea from the desert regions between Tunisia, Libya and Egypt. Siroccos are the only winds not channeled by mountainous topography.

Output - Wind Locations

The map above displays the typical source region (in red) during the onset of a gale-force event for all the identified wind types. Notice that for all wind types, the associated boxes have part of their respective borders along the coastline, indicating that the gale-force events originate from land.

A high probability of the onset of a gale-force event is when all of the following conditions are met:

wind speeds are of gale-force strength (³ 34 knots),
the region of gale-force winds is adjacent to the coastline,
the winds flow from the proper wind direction, e.g., for a Mistral, the wind flows from northwesterly to northerly.

The boxes in the map depict the general region of the gale-force winds during the initial outbreak of the gale-force event. The box usually expands well downwind of the original location. The full extent of the gale-force winds are as follows:

Mistrals can extend further south and east through the Strait of Sicily
Levantes can extend from the Balearic islands into the Gulf of Cadiz
Westerlys can extend from west of the Strait of Gibraltar into the Balearic Sea
Siroccos can extend from north Africa into the southern Mediterranean countries, including north Aegean and Adriatic Seas
Bora--Adriatic Sea can extend into the Ionian Sea and can cross into the Tyrennian Sea.
Bora--Aegean Sea and Etesians can extend south of the Aegean Sea and through most of the eastern Mediterranean Sea.

Climatology

Within the Mediterranean basin, the gale-force occurrences of the specified wind types have two distinct seasons: "winter" (October 1 through May 31) and "summer" (June 1 through September 30). A climatology of these wind events is displayed below and is divided between the two seasons.

The winter season is associated with more active weather and a complexity of ridges and troughs at the 500 mb level, as shown in 500mb_type flow patterns. The summer season is associated with relatively calm conditions both at the surface and the 500 mb level. The occurrences and strengths of these winds vary between winter and summer. A 14 month climatology of the winds depicts the seasonal differences.

Forecasters implementing MEDEX need to understand the following.

MEDEX predicts gale-force onset/continuation and cessation for 6 wind types during the winter season and 3 wind types during the summer season (shown below).
There is a transition period between each season (May-June and September-October). Users should be cautioned when forecasting during the transition periods for several of the wind types. Bora--Adriatic Sea and Bora--Aegean Sea occur primarily during the coldest part of the winter season while the Etesian occurs primarily during the hottest part of the summer season (August and September).

Climatology of Gale Force Wind Events - Collected from 14 Feb 1995 through 10 April 1996 [A gale-force wind event (occurrence) was expertly determined every 12 h using a variety of environmental data along with SSM/I surface wind speed data.]

Mediterranean Climatology

Special Sensor Microwave/Imager (SSM/I) Signatures

SSM/I ocean surface wind speed analysis is a powerful tool for gale force wind validation. The data interprets wave patterns over open-water regions of the world and converts them to 19.5 meter height wind speeds. These analyzed winds have accuracies of plus/minus 2ms^-1(Goodberlet et al., 1989).

The image above depicts gale force wind events (red regions) which occurred on 05 February 1996 for the named wind types. Land masses are in black and missing data are in gray. Missing data can occur due to noncoverage, moderate to heavy precipitation, or land contamination. (Note: The SSM/I examples provided for each of the winds are presented in the standard display with a white background only.) Gale-force winds (>17ms^-1) are within the red spectrum as noted above the color grid in the upper right corner of the display. Signatures of gale force wind events are available for each of the seven listed wind types.

SSM/I ocean surface wind speed products can be obtained from NESDIS

Gooderlet, M.A., Swift, C.T. & Wilkerson, J.C. (1989). Remote sensing of ocean surface winds with the special sensor microwave/imager. Journal of Geophysical Research, 94, 14547-14555.

Subjective 500 MB Flow Patterns

There are five major 500 mb flow patterns devised by the British (Meteorological Office, Air Ministry, 1962) which describe Mediterranean winter-time weather regimes. These patterns have been categorized as Type A, Type B, Type C, Type D and Type E, as illustrated below. With regard to MEDEX, an accurate assessment of the 500 mb flow pattern for predicting the onset or cessation of gale-force winds improves the accuracy of the forecast.

In MEDEX, the user inputs the 500 mb type by matching the 500 mb heights forecast chart with one of 5 types shown below. If there is no clear match, the user can select two types.

Ref: Meteorological Office, Air Ministry (1962). Weather in the Mediterranean, General Meteorology, Vol. 1. Her Majesty’s Stationery Office, London, 2nd ed., 373 pp.

Source:

Topography

Output - Wind Locations

Climatology