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

2. 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

3. 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, ...,]; temperatures and hydrological records [ NCEP dataset [

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

5. 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.

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

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

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

9. 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

10. 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

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

12. 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

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

14. 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),

15. 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),

16. 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]

17. 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

18. 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

19. 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

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

21. 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 ).
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

22. Thank you for your attention!