Linking climate change modelling to impacts studies: recent advances in downscaling techniques for hydrological extremes

Transcription

1 Linking climate change modelling to impacts studies: recent advances in downscaling techniques for hydrological extremes Dr Hayley Fowler, Newcastle University, UK CMOS-AMS Congress 2012, Montreal, Canada 30 th May 2012

2 Overview 1. Downscaling - the research gaps, opportunities and recommendations for best practice 2. Limitations in downscaling extreme rainfall 3. High resolution regional climate modelling the CONVEX project Initial results

3 Downscaling - the research gaps, opportunities and recommendations for best practice

4 Types Dynamical Downscaling RCMs Statistical Downscaling Perfect Prognosis relationship between predictor and predictand Bias Correction Weather Generators (use CFs) e.g. UKCP09 NO CLEAR BEST METHOD

5 Assessing model ability in future prediction/projection? reproduction of regional/global mean climate? low frequency variability? Inter-annual variability? Large-scale atmospheric modes? Extremes? Spatial characteristics? Others...

6 Research Gaps Variability on decadal and longer time scales (nonstationarities in climate system) need to include slowly varying changes/cycles Extremes particularly high return period events, useful for design/planning Spatial variability and coherence moving away from downscaling for individual locations/points Sub-daily variability crucial for many impact studies, i.e. urban flooding. Inter-variable relationships too many studies concentrate on just one variable (T or P); particularly important for hydrological impact studies

7 New opportunities Multi-model and perturbed-physics ensembles and probabilistic projections (pdfs) Weighting methods? How large an ensemble is needed to quantify uncertainty? Statistical emulators, i.e. used in UKCP09 High resolution, transient, GCM and RCM experiments added value in simulation of physical processes in relation to SD, e.g. storm track, extremes etc. Understanding evolution of changes over time Whole system models Embedding hydrological models within GCMs/RCMs

8 Impact studies and adaptation Leung et al. (2003) suggested that there is a need to involve stakeholders in determining the resolution of regional climate information required in different impact assessments How can results can be best used for robust management decisions leading to adaptation? Provision of tools based on the selection of robust, possibly impact specific, downscaling methods, or more generic, e.g. UKCP09 Weather Generator

9 Limitations in downscaling extreme rainfall

10 Widely held perception that flood risk has increased across Europe since the 1990s Why are we interested in downscaling extreme rainfall?

11 Can we provide useful information for adaptation decision-making? Following summer 2007 flooding, the UK Government announced increased annual budgets for flood risk management Higher spending on flood defence infrastructure prompts questions about when and where to prioritise the investment? The UK Government s Climate Change Bill places a statutory requirement upon competent authorities to undertake risk assessments as part of their duty to adapt to climate change Changes to extreme rainfall likely detectable in the near future (by 2050) and could be detectable by the 2020s (Fowler and Wilby, 2010, WRR) - many adaptation decisions will have to be made in the absence of formal detection

12 Observed changes to annual maximum rainfall in the UK Change in recurrence interval for a 50-year event in during East Scotland 1 in 8 year South Scotland 1 in 11 year North Scotland 1 in 25 year Northwest England 1 in 25 year Northeast England 1 in 25 year But, most trends insignificant, high inter-decadal variability, most do not continue into current decade Fowler and Kilsby 2003a, b Jones et al

13 Analysis of rainfall processes e.g. synoptic circulation, NAO, MSLP, humidity.. Using other historical data variables What are the predictors of extremes for different regions, seasons and spatial size of events?

14 Extreme rainfall scaling with temperature Daily extremes increase in observations approximately follows Clausius-Clapeyron (CC) 7% per C temperature rise. Hourly extremes increase in observations at a rate twice that of CC at temperatures higher than 12 C (by 10-15% per C) (Lenderink and van Meijgaard, 2008) (daily extremes increase at 5-10% per C) GCMs simulate a total precipitation increase of 1 to 3 % per C of surface warming (we expect global mean precipitation to increase at about half CC) (Allen and Ingram, 2002) Satellite observations show larger increases than those simulated by GCMs (approx CC), suggesting that projections of change to rainfall extremes may be underestimated (Allan and Soden, 2008)

15 Deficiencies in representation of rainfall in current RCMs RCMs provide regional detail, conditional on large-scale forcing from driving GCM. Resolve processes on spatial scales of >25km. Deficiencies in the representation of convection, including the diurnal cycle Represent rain on daily timescales or longer. Don t capture hourly extremes Tendency for too much persistent light rain and for underestimates of the intensity of heavy rain to be undere Some evidence that current models underestimate recent observed trends in heavy rain Crown copyright Met Office

16 Winter, 1 day 25 year return level Fowler and Ekström, 2009, IJC

17 Summer, 1 day 25 year return level Fowler and Ekström, 2009, IJC

18 Observed rainfall data (+ RCM change factors) The UKCP09 Weather Generator NSRP RAINFALL MODEL (1) Primary variable: Precipitation (mm) (2) Secondary variables: Mean temperature ( C) Daily temperature range ( C) Vapour pressure (hpa) Wind speed (ms -1 ) Sunshine duration (hours) Multiple Simulated Rainfall Series Observed daily weather data (+ RCM temperature change factors) CRU WEATHER GENERATOR Multiple series of simulated weather variables + PET + direct and diffuse radiation

19 Change Factor Perturbation Method Factors are multiplicative (except for mean temperature) Hourly stats derived using observed regression relations (fixed for future) IVRs also fixed for future So, no change information included at higher than daily resolution Observed statistics X Mean Proportion Dry Variance etc. RCM change factors X Mean Proportion Dry Variance

20 Summary Extreme rainfall appears to be increasing Observational increases scale with T through CC (but GCMs underestimate) Greater confidence in ability of models to represent large-scale precipitation extremes on daily timescales or longer (i.e. winter extreme rainfall) There are deficiencies in representation of convective rainfall particularly issue in summer for UK. For these events, we have low confidence in current model projections no information on sub-daily extremes

21 Summary SD methods cannot be used to resolve problems as reliant on relationships inherited from GCMs/RCMs so errors propagated Skill for the present day climate is a necessary but not sufficient condition for reliable future projections In addition we need to have confidence that models are representing the key processes responsible for future change

22 High resolution regional climate modelling the CONVEX project Initial Results

23 The CONVEX team CONVEX: Using Observational Evidence and Process Understanding to Improve Predictions of Extreme Rainfall Change Newcastle University Hayley Fowler Stephen Blenkinsop Steven Chan (based at Met Office) Met Office Hadley Centre Elisabeth Kendon Richard Jones Nigel Roberts (MO-JCMM) University of Exeter Chris Ferro David Stephenson Pat Sessford

24 CONVEX: Objectives 1. Explore observed rainfall data 3. Assess the deficiencies of climate and NWP models 2. Better understand the causes of extreme rainfall 4. Assess the influence of model detail resolution and structure 100km

25 CONVEX: Objectives 5. Provide new estimates of change to extreme rainfall inform future flood risk adaptation strategies improve climate and NWP models

26 Recent advances in global model resolution N640/N768 20/17km NWP in development Current Numerical Weather Prediction global resolution N512 25km UPSCALE project resolution ORCA025 1/4 Climate in development N320 40km Climate resolutions N216 60km ORCA025 1/4 Higher resolution climate model/monthly forecast/2012 GloSea4 resolution Current GloSea4 seasonal forecast resolution N96 130km ORCA1 1 Crown copyright Met Office Atmosphere Ocean/sea-ice

27 Benefits of very high resolution regional modelling Hourly rainfall rates from radar Model forecasts (a) 12km (b) 4km (c) 1km 1km model forecast Crown copyright Met Office Case study: Boscastle, 16 th Aug 2004; Courtesy: Nigel Roberts

28 Improved representation of orographic enhancement of rain Rain gauge observations and model forecasts Model orography 12 km 12km 1 km 4 km 1 km Case study: Carlisle flood, Jan 2005 Crown copyright Met Office

29 First regional climate simulations at 1.5km resolution over UK First climate simulations at convection permitting scales, being run as part of CONVEX project. Span southern England and Wales at 1.5km resolution. Driven by 12km European model, which is in turn driven by ERAinterim or 60km global model. Explicitly represents convection without need for parameterisation scheme. Completed , and just started climate change experiments. Crown copyright Met Office Kendon EJ, Roberts NM, Senior CA, Roberts MJ (2012) Realism of rainfall in a very high resolution regional climate model, J Climate, in press.

30 Daily precipitation ( ) Obs Bias Bias Rain gauge 1.5km-gauge 12km-gauge Mean precip Rain gauge 1.5km-gauge 12km-gauge Dry day occurrence Rain gauge 1.5km-gauge 12km-gauge Heavy precip Crown copyright Met Office

31 Diurnal cycle of convection Radar = Models = (solid) and (dotted) Crown copyright Met Office

32

33

34 New understanding from CONVEX The 1.5km climate model gives the most realistic representation yet of rainfall over the UK, giving confidence in its ability to project future changes. Long-standing problems in climate models, such as too much persistent light rain and errors in the diurnal cycle, are considerably reduced. First climate-change experiments at convection-permitting scales (expected completion summer 2013) Assessment of importance of representing local storm dynamics for future changes in precipitation extremes First estimate of future changes in convective summer storms Updated guidance on strengths and limitations of current climate projections Crown copyright Met Office

35 Finding out more The CONVEX project website: Follow us on twitter: #CONVEX_PROJECT Newsletters twice annually Publications: Kendon EJ, Roberts NM, Senior CA, Roberts MJ (2012) Realism of rainfall in a very high resolution regional climate model, J Climate, in press. Chan, SC, Kendon, EJ, Stephenson, D, Ferro, C, Blenkinsop, S, Fowler, HJ. Does increasing resolution improve the simulation of UK daily precipitation in a Regional Climate Model? Submitted to Climate Dynamics.