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| Titel |
Identification and frequency of atmospheric circulation patterns causing spring frost in the northern French vineyards using the objective version of the Hess-Brezowsky classification |
| VerfasserIn |
H. Quénol, O. Planchon, L. Wahl |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250019142
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| Zusammenfassung |
| The possible impacts of climate change at small spatial scales are still very little known. The knowledge of the climate risks at small scales is yet essential for agricultural activities and productions like vine growing, because of their serious economic impacts. Because of their relatively high latitude, the vineyards of the northern Half of France are subjected to spring frost, which can cause serious damages e.g. in the Champagne area on April the 8th, 2003. A detailed study of the variability and frequency of spring frost events in four vineyards (Loire Valley, Champagne, Burgundy and Alsace) was carried out within the framework of the RICLIM-CNRS 2663 multidisciplinary Research Group “Climate Risks” and was supported by the research program TERVICLIM (ANR-JC07-194103) and by the MAIF Foundation (program about air-mass circulation dynamics and climate risks).
The northern Half of France is included in the Cfb type of climate (according to the Köppen’s classification) of northwestern and central Europe. However, the combined effects of the latitude, the continentality and the topography involve varied regional climates. Among the four studied wine-producing areas, the Loire Valley area is the warmest, the Champagne area is the coolest in summer, and eastern Burgundy and especially Alsace are the most subjected to the continentality effect (highest annual temperature oscillation and highest rainfall amounts in summer). Therefore, these areas are not equally subjected to the frost risk. Spring is a key season for the vine growing and during frost-producing weather patterns, northern France is subjected to a high spatial variability of temperature at regional and local scales. During the period 1960-2007, the number of spring frost day events was three times as high in Colmar (Alsace) than in Saumur (Loire Valley). Among the four wine-producing areas, Alsace records the most hard and frequent frosts in early spring, while the Champagne area records the latest frosts (until June). In the Loire Valley area, frost is rare as early as April. The combined effects of the continentality and the topographical features of the Upper Rhine Graben explain the hard frosts in early spring at Colmar, but also higher temperature at Colmar than at Reims from April. The Champagne area is the most exposed to frost-producing North-Westerly and Northerly atmospheric circulations in late spring (e.g. on May the 5th, 1996: minimum temperature of -1°C at Reims / Champagne and +3.8°C at Colmar).
The identification and frequency of atmospheric circulation patterns causing spring frost (daily minimum temperature below 0°C) and hard frost (daily minimum temperature below -5°C) were carried out using the objective computational version of the 29-type Hess and Brezowsky Grosswetterlagen system of classifying European synoptic regimes (James, 2007). Minimum temperature data were got from the Meteo-France database (Climathèque), for the spring months (March, April and May) and for the period 1960-2007, at the weather stations of Saumur (Loire Valley), Reims (Champagne), Dijon (Burgundy) and Colmar (Alsace).
More than 40% of the frost days occurring at all weather stations were associated with North-Westerly and Northerly circulation types, 27% with North-Easterly and Easterly circulation types and 16% with a main high or low pressure area over central Europe. More precisely, the cyclonic circulations involving a northerly flow over western Europe (15.6%) and Anticyclonic North-Easterly circulations (9,3%) are the most frequent circulations types associated with frost days. These circulation types bring air-masses favourable to radiation cooling, under clear sky and light wind, or cold air-masses from northern or eastern Europe causing advection cooling. The stations of eastern France can be subjected to frost events even during Westerly or Southerly circulations, while frost occurrence in the Saumur area requires a higher ratio of North-Easterly and Easterly circulations (10% at Colmar ; 37% at Saumur). Because of the location of Saumur in western France and relatively close to the Atlantic Ocean (about 140 km), frost-producing weather patterns are especially associated with atmospheric circulations and air-masses which cause widespread frost conditions in the whole northern Half of France (NE & E circulation types). The combined effects of the continentality and the topographical features of the Upper Rhine Graben (Alsace: Colmar) and Saône Graben (eastern Burgundy: Dijon) explain the high ratios of frost days associated with Westerly and Southerly circulations at the weather stations of Dijon and Colmar (respectively 57% and 40%). The shallow and low-level layer of cold air, which develop in the eastern France grabens during frost-producing weather patterns, can persist a few days after a change in circulation type. The relatively warm air associated with a cyclonic Southerly or Westerly circulation flows over the low-layer cold air. Therefore, frost can occur during several days with cyclonic Southerly or Westerly circulation types. The areas of Reims and Saumur are more directly exposed to the Southerly and Westerly circulations, therefore these two weather stations are quickly subjected to a milder weather.
The results about hard frost days occurring at least at one of the four reference weather stations show that 74% of these days are associated with Northerly and Easterly circulations, i.e. mostly anticyclonic weather patterns with cold air advections from northern or eastern Europe. The low frequency of Westerly and Southerly circulations (11%) confirms that these circulation types have not a cooling effect. Depending on the weather station, the ratio of hard frost days associated with Northern and Eastern circulation types is between 70 and 85%, while the ratio of hard frost days associated with Westerly and Southerly circulations is less than 10%. Hard frost days associated with Westerly and Southerly circulations are not observed at the weather station of Saumur, because of its geographical position and the lie of the lower Loire Basin area. Saumur is the first of the four reference station to be affected by the warm air-masses associated with the Westerly or Southerly circulations. Therefore, the low-layer cold air is immediately swept away. |
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