Confidential manuscript submitted to Geophysical Research Letters

Supporting Information for“Local and non-local land surface influence in European heatwave
initial condition ensembles”
A. L. Merrifield1 , I. R. Simpson2 , K. A. McKinnon3 , S. Sippel1 , S.-P. Xie4 , C.
Deser2

1 ETH Zurich, Zurich, Switzerland
2 National Center for Atmospheric Research, Boulder, CO, USA
3 University of California Los Angeles, Los Angeles, CA USA
4 Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA USA

Contents
1. Text S1 to S2
2. Table S1
3. Figures S1 to S3

Text S1. Methodological Choices
The constrained circulation ensembles (CCEs) were constructed from 44 selectedcontrol (CTL) years: Years 2, 7, 12, 17, 22, 27, 32, 37, 42, 47, 52-84, 90. Each year between Year 52 and Year 84 were selected to obtain May and June soil moisture output
from the CTL restart files as land surface variables from Years 52 and 54-75 from theCTL were unavailable. For this reason, fewer CTL years (287 years) were averaged toobtain the soil moisture (SM) climatology than CTL years averaged for atmospheric fields(306 years). The CTL simulation is run with prescribed climatological sea surface temperature and sea ice, with climatologies calculated as the average of years 401-1399 of
the CESM Large Ensemble preindustrial control simulation [Kay et al., 2015]. Within
this idealized model framework, the only interactive component of the climate systemthat is coupled with the atmosphere and is varying in time is the land surface.
Sensitivity analyses were performed to (1) inform the decision to constrain circulation above approximately 300 hPa in the CCEtop and (2) determine a suitable num-

Corresponding author: Anna L. Merrifield, anna.merrifield@env.ethz.ch–1–

Confidential manuscript submitted to Geophysical Research Letters

Table 1.

Description of CESM1 simulation experiments used in this study.

Experiment

Abbreviation

Description

Control Simulation

CTL

306 year preindustrial control simulation, withprescribed climatological sea surface temperatureand sea ice from years 401 - 1399 of the CESM1
Large Ensemble preindustial control simulation.

Heatwave CaseYear 200

Year selected from CTL as a representative seasonally persistent European heatwave.

Constrained Cir-CCEtop

Ensemble of 44 CTL years, branched from Juneculation Ensemble,

1 and with Year 200 atmospheric circulation

Upper Atmosphere

imposed above 322 hPa.

Constrained Circula-CCEfull

Ensemble of 44 CTL years, branched from Junetion Ensemble, Full

1 and with Year 200 atmospheric circulation

Atmosphere

imposed over the full atmospheric column.

ber of CCE members. For the first methodological choice, constraint above approximately
700 and 500 hPa were also considered. The approximately 300 hPa constraint was chosen because it allowed for both the heatwave pattern to be imposed and the atmosphere
below the constraint to evolve in response to the SM anomalies present. For the second
methodological choice, ensembles of 23, 44, and 102 members were considered. Spreadin the CCEfull did not considerably increase between 44 and 102 members, so we considered 44 members sufficient to represent the heatwave intensity range.

Text S2. Hierarchical Partitioning
Hierarchical partitioning is a method of determining the independent and conjoint
contributions of predictors to goodness of fit models and is useful in that it quantifies
the relative contributions of correlated predictors [Hatt et al., 2004]. The idea is to identify these contributions through a systematic set of hierarchical regression models, where–2–

Confidential manuscript submitted to Geophysical Research Letters

CEU QH Anomaly ( [-] )

-2.
9%

-2.
4%

pe

-1.6

-1.6

pe

r1

r1%pCCEfull

r2 = 0.81

r2 = 0.88

Julyr2 = 0.87

r2 = 0.97

Augustr2 = 0.82

r2 = 0.98

0kg/m

2g/m

20k

er 1%pCCEtop
June0 kg

er 1/m 20 kg/m 2
-2.
8%
-3%

per1

per10k

0kg/mg/m

22

CEU SM Anomaly (kg/m2)

Figure 1.

Relationships between CEU Soil Moisture and Sensible Heat Flux Fraction, indica-tive of the terrestrial segment of the land surface feedback on SAT [Dirmeyer , 2011], for June(blue), July (red), and August (yellow) in the CCEtop (dots) and CCEfull (stars). R2 values foreach month and ensemble combination are given in the legend in the upper right. Least-squares
regression coefficients accompany lines of best fit.

individual predictors are assigned high independent contribution if they explain variability in the target variable independently from the other predictors. Conversely, joint variability is the fraction of variability that can be explained by several variables, and thusthat part of explained variability cannot be assigned to an individual variable. Hierarchical partitioning systematically uses all hierarchies of variables and results in an exact linear partitioning. Joint effects are split among the respective contributing variables
and added proportionately.

Text S3. Observational Consistency
We also consider observational estimates of root zone soil moisture from the Global
Land Evaporation Amsterdam Model version 3.2a (GLEAM v3.2a [Martens et al., 2017]).

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Confidential manuscript submitted to Geophysical Research Letters

A histogram of GLEAM May soil moisture (OBS; CEU-averaged and normalized withrespect to 1980-2017 climatology) is shown in Fig. S3, superposed on normalized CESM1May CEU soil moisture used as initial conditions for the CCEs. For the CCE, a distribution of the 44 members is shown. For OBS, 38 years of observational estimates are used.
Both the CCE and OBS soil moisture distributions have a longer dry than wet side tail.
Central European soils were approximately 1.2 σ drier than average prior to the 2003
European heatwave.

ReferencesDirmeyer, P. A. (2011), The terrestrial segment of soil moisture–climate coupling,Geophys. Res. Lett., 38, L16,702, doi:10.1029/2011GL048268.
Hatt, B. E., T. D. Fletcher, C. J. Walsh, and S. L. Taylor (2004), The Influenceof Urban Density and Drainage Infrastructure on the Concentrations and Loads
of Pollutants in Small Streams, Environmental Management, 34, 112–124, doi:
10.1007/s00267-004-0221-8.
Kay, J. E., C. Deser, A. Phillips, A. Mai, C. Hannay, G. Strand, J. Arblaster,S. Bates, G. Danabasoglu, J. Edwards, M. Holland, P. Kushner, J. F. Lamarque, D. Lawrence, K. Lindsay, A. Middleton, E. Munoz, R. Neale, K. Oleson,
L. Polvani, and M. Vertenstein (2015), The Community Earth System Model(CESM) Large Ensemble Project: A community resource for studying climatechange in the presence of internal climate variability, Bull. Amer. Met. Soc., 96,1333–1349, doi:10.1175/BAMS-D-13-00255.1.
Martens, B., D. G. Miralles, H. Lievens, R. van der Schalie, R. A. M. de Jeu,
D. Fernández-Prieto, H. E. Beck, W. A. Dorigo, and N. E. C. Verhoest (2017),
GLEAM v3: satellite-based land evaporation androot-zone soil moisture, Geoscientific Model Development, 10, 1903–1925, doi:
110.5194/gmd-10-1903-2017.

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Confidential manuscript submitted to Geophysical Research Letters

Std. Dev. (˚C)a) SAT Ensemble spread, CTL
June

July

August10.5i.

ii.

iii.

0b) Independent Contribution to SAT spread, CTL
JulyAugustCRE

June

ii.

iii.

iv.

v.

vi.

vii.

viii.

ix.

SMω700

i.

0%

20%

40%

60%

80%

100%

Percent Variance Explained

Figure 2.

a) Ensemble standard deviation (i-iii) of CTL SAT (◦ C) in the CEU (box) for June,

July, and August respectively. Dotted contours show orographic features above 600 m at 200 m
intervals. b) The independent contribution to CTL SAT spread of processes related to heatwave
intensity. Panels i-iii show the June through August independent contribution of radiation, represented by the cloud radiative effect (CRE). Panels iv-vi show the contribution of subsidence,
represented by vertical velocity at 700 hPa (ω700 ), and panels vii-ix show the contribution of theland surface feedback, represented by root zone soil moisture (SM).

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Number of Years

Confidential manuscript submitted to Geophysical Research Letters

Normalized May CEU Soil Moisture (σSM)
Figure 3.

Normalized May CEU SM (σSM ) for the CCEs (gray bars) and observational esti-mates (OBS; red bars). The OBS value in 2003, prior to the European heatwave, is shown as adashed red line.

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