SDSM 4.2 — A decision support tool for the
assessment of regional climate change impacts
User Manual
Robert L. Wilby
1,2
and Christian W. Dawson
3
August 2007
1
Department of Geography, Lancaster University, UK
2
Science Department, Environment Agency of England and Wales, UK
3
Department of Computer Science, Loughborough University, UK
© Wilby & Dawson, 2007 Page 2 of 94
Preface
General Circulation Models (GCMs) suggest that rising concentrations of greenhouse
gases will have significant implications for climate at global and regional scales. Less
certain is the extent to which meteorological processes at individual sites will be
affected. So–called “downscaling” techniques are used to bridge the spatial and
temporal resolution gaps between what climate modellers are currently able to provide
and what impact assessors require.
This manual describes a decision support tool for assessing local climate
change impacts using a robust statistical downscaling technique. SDSM 4.2
(Statistical DownScaling Model) facilitates the rapid development of multiple, low–
cost, single–site scenarios of daily surface weather variables under present and future
climate forcing. Additionally, the software performs ancillary tasks of data quality
control and transformation, predictor variable pre–screening, automatic model
calibration, basic diagnostic testing, statistical analyses and graphing of climate data.
This manual also describes the UKSDSM archive, a set of daily predictor
variables prepared for model calibration and downscaling at sites across the UK. The
archive contains variables describing atmospheric circulation, thickness, stability and
moisture content at several levels in the atmosphere, under climate conditions
observed between 1961 and 1990. Equivalent predictor variables are provided for four
GCM experiments of transient climate change between 1961 and 2099. Users seeking
to apply SDSM to regions outside the UK may obtain predictor variables online by
visiting: http://www.cics.uvic.ca/scenarios/index.cgi?Scenarios
Application of SDSM is illustrated with respect to the downscaling of daily
maximum temperature and precipitation scenarios fat a hypothetical location
(Blogsville) under present (1961–90) and future (2070–99) climate forcing.
© Wilby & Dawson, 2007 Page 3 of 94
Acknowledgements
SDSM Version 4.2 was supported by the Environment Agency of England and Wales
as part of the Thames Estuary 2100 project.
SDSM Version 3.1 was supported by the Environment Agency of England and Wales
as part of the Climate Impacts and Adaptation Research Programme. The UKSDSM
data archive was updated by Ian Harris (Climate Research Unit) and now includes
data kindly supplied by the UK Hadley Centre, CSIRO Atmospheric Research, and
the Canadian Centre for Climate Modelling and Analysis.
SDSM Version 2.2 was sponsored by the Environment Agency through the National
Centre for Risk Analysis and Options Appraisal.
SDSM Version 2.1 was supported by the Canadian Climate Impacts Scenarios (CCIS)
Group though the Climate Change Action Fund. Assistance in kind was provided by
A Consortium for the Application of Climate Impact Assessments (ACACIA) at the
National Centre for Atmospheric Research (NCAR). NCAR is sponsored by the U.S.
National Science Foundation.
NCEP Re–analysis data were provided by the NOAA-CIRES Climate Diagnostics
Center, Boulder, Colorado, USA, from their Web site at http://www.cdc.noaa.gov/
The Climate Impacts LINK Project is funded by the UK Department of the
Environment, Transport and the Regions (Contract Reference EPG 1/1/124).
© Wilby & Dawson, 2007 Page 4 of 94
CONTENTS
Page
i Preface 2
ii Acknowledgements 3
0 TECHNICAL INFORMATION 6
1 INTRODUCTION 7
1.1 Downscaling techniques 8
1.1.1 Dynamical 8
1.1.2 Weather typing 9
1.1.3 Stochastic weather generators 9
1.1.4 Transfer functions 9
1.2 Relative skill of statistical and dynamical downscaling techniques 10
1.3 Manual outline 11
2 OVERVIEW OF SDSM STRUCTURE AND UKSDSM ARCHIVE 12
2.1 Key functions of SDSM 13
2.1.1 Quality control and data transformation 14
2.1.2 Screening of downscaling predictor variables 14
2.1.3 Model calibration 14
2.1.4 Weather generator 14
2.1.5 Data analysis 15
2.1.6 Graphical analysis 15
2.1.7 Scenario generation 15
2.2 UKSDSM data archive 15
2.3 UKSDSM predictors 16
2.4 SDSM file protocols 17
2.5 Obtaining SDSM predictors online 19
3 GETTING STARTED 20
3.1 Settings 21
3.2 Advanced settings 22
4 QUALITY CONTROL AND DATA TRANSFORMATION 24
4.1 Quality control 24
4.2 Data transformation 25
5 SCREENING OF DOWNSCALING PREDICTOR VARIABLES 27
5.1 Setup 27
5.2 Temporal variations in predictor strength 28
5.3 Correlation matrix 29
5.4 Scatterplot 30
6 MODEL CALIBRATION 32
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6.1 File handling 32
6.2 Model type 33
6.3 Blogsville example 34
6.4 The *.PAR file 35
7 WEATHER GENERATOR 37
7.1 File handling 37
7.2 Ensemble size 38
7.3 Blogsville example 39
8 ANALYSIS OF OBSERVED AND DOWNSCALED DATA 41
8.1 Overview 41
8.2 Delta Statistics 44
8.3 The Statistics 45
9 FREQUENCY ANALYSIS 47
9.1 Setup 47
9.2 Diagnostics and plots 48
9.3 Extreme value analysis 50
10 SCENARIO GENERATION 55
10.1 Check settings 55
10.2 Setup 56
10.3 Blogsville example (temperature) 57
10.4 Blogsville example (precipitation) 60
11 GRAPHING MONTHLY STATISTICS 65
11.1 Line chart 65
11.2 Bar chart 66
11.3 Customizing charts 67
12 TIME SERIES ANALYSIS 69
12.1 Time series chart 69
12.2 Adjusting chart appearance 72
13 FINAL CAUTIONARY REMARKS 75
BIBLIOGRAPHY 76
APPENDIX 1: ENHANCEMENTS SINCE SDSM VERSION 3.1 84
APPENDIX 2: FREQUENTLY ASKED QUESTIONS 86
GLOSSARY 90
