Linking Large-scale Circulation Descriptors To Precipitation Variability in the Northern French Alps

Antoine Blanc, Juliette Blanchet and Jean-Dominique Creutin

Institut des Géosciences de l’Environnement, Univ. Grenoble Alpes, CNRS, IRD, Grenoble INP, IGE, 38000 Grenoble, France

The Isère River downstream Grenoble, Northern French Alps

Context

The Isère River downstream Grenoble, Northern French Alps

The link between large-scale circulation (LSC) and precipitation is often investigated using:

Boé and Terray, 2008 Scherrer et al., 2016

Here, we investigate whether simple characteristics of LSC can also explain precipitation variability in the Northern French Alps.We refer to these characteristics as the atmospheric descriptors.

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Weather-pattern classifications

Modes of LSC variability

The Isère River downstream Grenoble, Northern French Alps

• Precipitation over the Isère River catchment downstream Grenoble in theNorthern French Alps. 9500 km², altitude from 200m to 4000m.

• Daily catchment precipitation obtained with the SPAZM precipitation data set(Gottardi etal., 2012)

• Period 1950-2017

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• The atmospheric descriptors are based on daily 500 hPageopotential height over Western Europe (blue rectangle)extracted from the ERA5 reanalysis (Hersbach et al., 2018).

• Two atmospheric descriptors: the Maximum Pressure Differenceand the singularity (Blanchet etal., 2018; Blanchet and Creutin.,2020).

Data & Region of study

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𝑴𝑷𝑫𝒌 = 𝒎𝒂𝒙𝒋(𝒛𝒋𝒌) - 𝒎𝒊𝒏𝒋(𝒛𝒋𝒌)

The MPD represents the range of geopotential heights (in meters) over Western Europe. Let zjk denotes the 500 hPa height of grid point sj and day tk. MPDk is defined as:

Min(Z500) = 5146 m

Max(Z500) = 5811 m

Example of December 13, 1981: MPD = 5811 – 5146 = 665 m

Maximum Pressure Difference (MPD)

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• The singularity of day tk is defined as the mean Teweles-Wobus score (Teweles & Wobus, 1954) between day tk and its 124 (0.5%)closest analog days within the period 1950-2017:

𝒔𝒊𝒏𝒈𝒌 =𝟏

𝑸

𝒌=𝟏

𝒌=𝑸

𝑻𝑾𝑺𝒌,(𝒒)

2007-03-01 and its 50th analog day. The two geopotential shapes resembleeach others: the geopotential shape of 2007-03-01 is not singular/is closely

reproduced in the climatology.

The singularity measures whether or not a shape of geopotential height field is closely reproduced in the 1950-2017 climatology.Low singularity => closely reproduced

1997-01-27 and its 50th analog day. The two geopotential shapes do not resembleeach others: the geopotential shape of 1997-01-27 is singular/is not closely

reproduced in the climatology.

Singularity

The TWS measures the similarity in shape betweengeopotential height fields, based on North-South andWest-East gradient.

The MPD and the singularity are compared to other atmospheric variables:

• The occurrence of the main atmospheric influences over Western Europe: Atlantic, Mediterranean, Northeast andAnticyclonic influences; based on the weather pattern classification of Garavaglia et al. (2010).

• NAO; constructed by the National Oceanic and Atmospheric Administration/Climate Prediction Center(https://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao.shtml).

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• The interannual singularity and the MPD are strongly related tothe occurrence of Atlantic circulations.

This result is interesting since the MPD and the singularity areconstructed independently on atmospheric circulations.

• NAO exhibits low correlation with MPD, even in winter.NAO does not drive the maximum pressure gradient in a domain

spreading from the Iberic Peninsula to Southern Great Britain.

Pearson correlation between the different atmospheric variables, at the interannual time-scale

Comparison with other atmospheric variables

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• Correlations between both MPD/singularity andseasonal precipitation are close to 0.7 in winterand autumn (50% of explained variance).

• MPD and seasonal precipitation are positivelycorrelated: seasons featuring a large (low) pressuredifference over Western Europe feature a large(low) seasonal precipitation in the NorthernFrench Alps.

• Singularity and seasonal precipitation are negativelycorrelated: seasons featuring a low (large)singularity of geopotential shapes over WesternEurope feature a large (low) seasonal precipitationin the Northern French Alps.

Correlation with seasonal precipitation in the Northern French Alps

Scatterplot of the MPD (first line) and the singularity (second line) against precipitation accumulation in the Isere River catchment, per season. Pearson correlation values are reported above each scatterplot.

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• When compared to the other atmospheric variables, the MPD and thesingularity feature among the largest correlation values with seasonalprecipitation.

• The singularity and the MPD explain precipitation variability as well asthe occurrence of Atlantic circulations, which is the best performingatmospheric influence.

• NAO poorly explains precipitation variability in the Northern FrenchAlps, whatever the season.

• Correlations are systematically lower in summer, when convectionalso contributes to precipitation accumulation.

Correlation with seasonal precipitation in the Northern French Alps

Interannual Pearson correlation between the atmospheric variables and seasonal precipitation accumulation over the Isère River

catchment. The dotted lines represent negative correlation values.

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• At the daily time-scale, neither the MPD nor the singularity ofgeopotential shapes nor the occurrence of Atlantic circulations(0 or 1 in this case) explain precipitation variability in theNorthern French Alps.

• However, most of the gain in correlation from 1 day to 1 yearis obtained after 10 days with correlation values remainingalmost stable from 10 days to 10 years.

• The atmospheric indices that poorly explain precipitationvariability at the interannual time-scale are also littleexplicative at shorter and longer time-scales

Correlation at different time-scales

Correlation between the atmospheric indices and precipitation accumulation in the Isere River catchment, per season. Correlation is represented for different smoothing lengths, at daily time-scale (left, logarithmic x-axis) and at yearly

time-scale (right, regular x-axis). The dotted lines represent negative correlation values.

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• A simple pressure difference over Western Europe as well as a singularity of geopotential shapes are well related to theoccurrence of Atlantic circulations.

• These atmospheric descriptors explain part of precipitation accumulation in the Northern French Alps (correlations up to 0.7) from10 days to 10 years, except during the summer.

• The periods featuring geopotential shapes that are closely reproduced in the climatology (low singularity), or that featurepronounced centers of action (large MPD) tend to be associated to large precipitation accumulation over the Northern FrenchAlps.

• NAO does not drive the pressure difference in a domain spreading from the Iberic Peninsula to Southern Great Britain, and itpoorly explains precipitation variability in the Northern French Alps.

These atmospheric descriptors are quite easy to implement and rely on a limited number of choices, in comparison to weatherpattern classifications for example (choice of the classification method, number of classes). Therefore, studying the temporal evolutionof such descriptors appears relevant for climate change studies.

Time

Conclusions

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Alps. Atmospheric Science Letters, 19 (3), e809.

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• Teweles, S., & Wobus, H. B. (1954). Verification of Prognostic Charts. Bulletin of the American Meteorological Society, 35 (10), 455-463.