Analysis of Southern European Cold Spells via a Statistical Mechanics Approach Miriam D’Errico miriam.derrico@lsce.ipsl.fr Davide Faranda (LSCE) Pascal Yiou (LSCE) Cesare Nardini (SPEC - IRAMIS )

Objective We investigate winter cold spells in the Southern Europe region as they have a great impact on agriculture, transportation and society. It is unclear whether anthropogenic forcing would increase or decrease their occurrence and change their dynamics. Due to the scarcity of these events, climate simulations are required to answer this research question. In this talk we question whether : Different cold spells detected in the NCEP reanalysis share similar dynamical characteristics Their average dynamic can be reproduced in PLASIM model Perche queste variabili termodinamica dinamica(circolazione)fisica

PART 1 PART 1: Cold spells in NCEP Reanalysis dataset Definition: A winter cold spell is a marked and unusual cold weather characterized by a sharp and significant drop of air temperatures leading to extremely low values, possibly associated to heavy snowfalls. Sources: newspapers and periodicals [La Repubblica; Le Parisien, …,]; websites [meteo-net.it, www.3bmeteo.com, ...,]; temperatures and hydrological records [www.evalmet.it]; NCEP dataset [www.esrl.noaa.gov];

Cold spell dates Using documentary research method we have found 32 cold spell events over the past decades 1948-2018 04/01/1954 t 1950 1960 1970 1980 1990 2000 2010 2020

Cold spell variables Our analysis includes the following variables: Sea-level pressure (SLP) Geopotential height at 500 hPa (HGT500) Temperature at 850 hPa (T850) Snow depth We consider this as a minimal set of variables that define a cold spell in term of circulation, thermodynamic and precipitation.

Mean winter (DJF) conditions 1948-2018 Cold spell in Southern Europe NCEP reanalysis Mean winter (DJF) conditions 1948-2018 Cold spell 2017- 01- 05 Anomaly (C) (C) T 850 SNOW DEPTH

Mean winter (DJF) conditions 1948-2018 Cold spell in Southern Europe NCEP reanalysis Mean winter (DJF) conditions 1948-2018 Average cold spells Anomaly (C) (C) T 850 (kg/m^2) (kg/m^2) SNOW DEPTH Different cold spells have similar atmospheric circulation patterns

Mean winter (DJF) conditions 1948-2018 Cold spell in Southern Europe NCEP reanalysis Mean winter (DJF) conditions 1948-2018 Average cold spells Anomaly (hPa) (hPa) SLP (m) (m) HGT 500 Different cold spells have similar atmospheric circulation patterns

Average cold spell dynamics NCEP reanalysis The dynamic pattern consists of a strengthening ridge across the Western Europe: intense northerly winds advecte cold polar air to Western Europe a large upper low / pool of low geopotential heights is placed across the South-Eastern Europe

Cold spell in Southern Europe NCEP reanalysis From the animation: similar dynamical evolution in cold spells over several days. In order to quantify the degree of similarities among cold spells in time: We fix a starting date for each event We compute the pairwise correlation between several atmospheric fields as SLP , T850 , Z500 recorded during these events at different time lags. We construct a matrix of those pairwise correlations and average them We analyse for how long these correlations are significantly non zero

Correlations coefficient Correlation matrix for SLP fields NCEP reanalysis date Correlations coefficient

Mean of correlation matrix Monte Carlo significance testing showed the non-attendance of the pick at 0 lag The coefficients of the correlation matrix are significantly nonzero for the first 10 lags of the events

PART2: Simulations of cold spells with AGCM at low resolution Cold spell simulations PART2: Simulations of cold spells with AGCM at low resolution

PLASIM PLASIM simulations The simulations are performed with the last version (17) of The Planet Simulator, a coupled system of climate components for Earth developed as a model of intermediate complexity [Fraedrich, 2012] . PLASIM is a reduced complexity AGCM, with the 3-D primitive-equation atmosphere model PUMA at its core [Fraedrich et al., 2005, Lunkeit et al. , 2007] Ragone et al. [2017] used PLASIM to force heat waves conditions via large deviations (Atmospheric General Circulation Model),

PLASIM PLASIM simulations The simulations are performed with the last version (17) of The Planet Simulator, a coupled system of climate components for Earth developed as a model of intermediate complexity [Fraedrich, 2012] . PLASIM is a reduced complexity AGCM, with the 3-D primitive-equation atmosphere model PUMA at its core [Fraedrich et al., 2005, Lunkeit et al. , 2007] Ragone et al. [2017] used PLASIM to force heat waves conditions via large deviations We use the T42 resolution 10 level We perform 400 years of reference simulation (time running ~15 days on 2CPU) To select the cold spells in PLASIM simulation we used 32 Analogues of averaged cold spells of NCEP v (Atmospheric General Circulation Model),

PLASIM simulations Analogues Our analysis is based on analogues of atmospheric circulation Analogues are selected in the 400 years of simulations The selection is made on the basis of minimizing the Euclidean distance of daily SLP over Southern Europe region [Yiou et al. 2013]

Mean of correlation matrix PLASIM-NCEP Mean of correlation matrix NCEP PLASIM NCEP : correlation among the anomalies of the 32 cold spells PLASIM : correlation among the anomalies of the 32 best analogues of the averaged observed cold spell

Cold spell simulations PLASIM In order to validate if the precursors of cold spells in PLASIM are similar to those observed in the reanalysis… …we made perturbed ensemble PLASIM seasonal runs in winter conditions… … starting from the run that produces the best analogue of the (average) observed cold spells

Cold spell simulations PLASIM The following analysis are made over a restricted area including only the region where the effects of the cold spell are maxima (7.5W to 20E , 35N to 47.5N) The mean and the anomalies of the 32 cold spell events over the new region are averaged for each lag days for PLASIM and NCEP reanalysis

Anomaly of cold spells PLASIM - NCEP (C) T 850 (kg/m^2) SNOW DEPTH

Summary and outlook Detection and analysis of winter cold spells in the Southern Europe region Analogues are used to find cold spells in PLASIM simulations with similar features to those found in NCEP reanalysis (over 1948-2018 ). PLASIM reproduces the dynamics of cold spells including the precursors Outlook: Under different but fixed emission scenarios (RCP8.5), we will determine the effect of climate change on the characteristics of cold spells

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