Improving Hydrological Predictions

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1 Improving Hydrological Predictions Catherine Senior MOSAC, November 10th, 2011

2 How well do we simulate the water cycle? GPCP 10 years of Day 1 forecast Equatorial Variability on Synoptic scales (2-6 days) underestimated. S Milton

3 How will the water cycle change? Increased Precipitation Precipitation Intensity More Intense Rainfall More droughts Wet regions get wetter, dry regions get drier? Regional projections?? Dry Days Precipitation Change (%) IPCC AR4, 2007

4 Met Office Science Strategy: The Water Cycle Our goal must be to develop a more holistic approach to understanding, modelling and predicting the global and regional terrestrial water cycle and its role in the impacts of hazardous weather, climate variability and climate change. This must extend from the prediction of hydrological extremes (floods and droughts), to an integrated assessment of water, food and fibre Key Focuses Strength and variability of the global and regional hydrological cycles in a warmer world; Freshwater forcing and salinity budget of the global oceans; Terrestrial ecosystems and their dependence on water availability; The fate of polar ice caps and glaciers with consequent sea level rise.

5 Scientific Challenges Conservation Land-atmosphere interactions Uncertainty Large-scale phenomena (ENSO, monsoons, etc) s e l a c s e m i t l l A Surface energy balance Global and regional water budget Extremes Impacts Large-scale modes of variability Ocean-atmosphere interactions Cryosphere Systematic errors Rainfall variability

6 Improving Hydrological Predictions Aims: To improve the understanding and modelling of precipitation and hydrological processes, including variability and change. To pull together and synthesize knowledge and progress from the wider community through active collaboration and coordination A Working Group (IHPWG) includes scientists from within the Met Office and with external collaborators across the science questions. The Global Water Cycle group (4 staff) provides core effort and a key focal point.

7 Recent Science Highlights Model Improvement and evaluation across time and space scales Improved physics (light rain package, water loading, entrainment changes, diurnal cycle, ) Role of resolution on systematic biases (moisture convergence) Resolved convection and extreme rainfall Process understanding Key regional processes (Monsoons) Land-atmosphere coupling Transient Climate Change Hydrological Hysteresis Tropical circulation

8 Model Improvement

9 Improved physics: Light rain package From 1-day forecasts Control Dry days: observed to climate Dry days: control Dry days: LR package Light rain package Light rain package: Revised convection diagnosis Prognostic rain Abel/Shipway fall speeds PC2 bug fixes CAPTIVATE team

10 Improved physics: African Easterly Waves REANALYSIS/ OBS Curvature vorticity 700hPa (5-15N) GPCP rain rate (10-15N) SEASONAL FORECAST Curvature vorticity 700hPa (5-15N) Model rain rate (10-15N) Caroline Bain, CSRP

11 Improved Physics: Convective Water Loading JJA AMIP PPN: Water Loading Water Loading - Control Control - GPCP Water loading - GPCP Large reduction of Biases in Monsoon & ITCZ Martin Willet, GMED

12 Improved Physics: 1.5 x Entrainment: October 2008 MJO case Precipitation U850 U200 Control N. Klingaman and S. Woolnough

13 Improved Physics: 1.5 x Entrainment: October 2008 MJO case Precipitation U850 U x entrainment N. Klingaman and S. Woolnough

14 Resolution dependence: recycling ratio There is more evaporation (E) and precipitation (P) globally at higher resolution. The recycling ratio (E/P) over land decreases with resolution, bringing the ratio closer to the range of observed values estimated by Trenberth (1999). E P This is associated with an increase in the convergence of moisture at higher resolution. X 8 N4 6 N9 4 N1 4 1 N2 6 6 ro g 1 N 2 96 o N p ra hy In 1-5 day forecasts the EKE converges at higher resolution (up to N768) R. Schiemann, P-L. Vidale, M. Estelle Demory, JWCRP

15 Extreme Precipitation duration vs peak amount Radar RCM-radar year runs of UKV 1.5km Climatology of the temporal characteristics of hourly rainfall in UKV v 12km RCM mm/h mm/h UKV-RCM mm/h km RCM has too few short duration to mid-high peak intensity events; too many long duration low intensity events UKV-radar hr mm/h 1.5km model substantially better but still slight tendency for too little short duration rain hr Lizzie Kendon

16 Process Understanding

17 Understanding Monsoon processes: role of Arabian sea SST bias Strong monsoons depend on Arabian Sea moisture in observations large systematic cold SST errors in Arabian Sea, reduce monsoon rainfall (up to 30%) and delay onset by weakening local evaporation cold errors develop in winter due (in part) to excessive winter monsoon northerlies Seasonal model initialisation doesn t allow these biases to develop and monsoon spins up due to equatorial SST bias which develop during the season Levine and Turner (2011, Climate Dynamics, in press)

18 Understanding Monsoon processes: Arabian sea SSTs in CMIP5 Arabian sea SST All-India rainfall Similar pattern and timing of cold Arabian Sea SST bias seen in CMIP5 models Weak monsoon rainfall, particularly at beginning of monsoon, problem in almost all CMIP5 models Future work: impact of mean state bias on future monsoon predictions, which currently predict small increase in monsoon rainfall Richard Levine

19 Met Office Seasonal forecast GloSea4 West African monsoon: Jump in the onset rains mean Mean observed rainfall (TRMM ) Latitude Sahel Guinea coastal region July October TIME Good monsoon climatology enables accuracy in seasonal forecasts of timing of onset Focus on understanding sensitivity to model improvements Requires correct relative importance of latent and non-latent heat sources and Atlantic/Pacific teleconnnections Caroline Bain, Michael Vellinga, CSRP

20 West African Monsoon: Land atmosphere interactions Recent model configurations show stronger land-atmosphere coupling than previous versions. This appears to be mainly due to improvements in the atmospheric parameterizations and possibly also the model s vertical resolution. When soil moisture is prescribed, there is a clear reduction in the spread of West African monsoon onset dates. Free Prescribed Ruth Comer, WATCH, CSRP

21 Transient climate change

22 Hydrological cycle hysteresis HadCM3 Changes in water vapour content in response to idealised ramp-up/ramp-down CO2 scenarios are reversible, while global precipitation shows hysteresis (HadCM3 and HadGEM2-ES). dp/dt depends on the CO2 concentration. Non-linear responses in outgoing long-wave radiation, sensible heating, short-wave and cloud responses all contribute to the precipitation behaviour. Including a non-linear term to statistical fit allows very good agreement with various scenario experiments Peter Good, Peili Wu

23 Hydrological cycle hysteresis: Tropical precipitation asymmetries 4xCO2: Ramp down Ramp up for same T1.5m El Nino-like shift in rainfall into the central equatorial Pacific related to shift in Walker Circulation in response to East Pacific warm anomaly Sub-thermocline water still warming in ramp-down experiment reduces efficiency of upwelling-related surface cooling in East Pacific Rob Chadwick, Peili Wu

24 Collaborations NERC-CWC projects: Constraining the response of the hydrological cycle, land surface and regional weather to global change (HYDRA) Hydrological Cycle Understanding via Process Attribution and prediction (Horyuji PAGODA) Soil water Climate Feedbacks in Europe in the 21st century (SWELTER-21) Using observational evidence and process understanding to improve predictions of extreme rainfall change (CONVEX) South Asian precipitation: a seamless assessment (SAPRISE) JWCRP: Influence of horizontal resolution on simulations of the global water cycle Monsoon variability and extremes, representation in models and changes in future climate Monsoon working group EU FP7 projects: Earth system model bias reduction and assessing abrupt climate change (EMBRACE) [under review] Water and global change (WATCH) CMIP5 CSRP: DfID-funded project to improve knowledge and predictions of weather and climate over Africa.

25 Future focus Model development and evaluation across timescales Parametrisation development (GA4, diurnal cycle, ) Resolution studies (CASCADE, PrACE, UoTexas, ) Evaluation using EO across timescales (JPL, NCEO,..) PEGs (MJO and convectively coupled equatorial waves, Tropical cyclones, Monsoon systems - Asia and Africa, Teleconnections...) Process understanding: Global and regional processes (Monsoon, MJO, TCs, ) Observations and idealised modelling studies (YOTC/GCSS, CINDY/DYNAMO..) On-going evaluation -> Feedback to developers Climate change CMIP5 projections Sensitivity to parameterizations Idealised and/or policy relevant scenarios

26 Questions