Climate Diagnostics and Prediction Workshop Lincoln, NW October 22, 2008

1. Drought Monitoring and Prediction Systems at the University of Washington and Princeton University Dennis P. Lettenmaier Department of Civil and Environmental Engineering University of Washington Climate Diagnostics and Prediction Workshop Lincoln, NW October 22, 2008

2. Outline of this presentation • Motivation for experimental hydrological prediction systems • Evolution of the UW and Princeton systems • Current components • UW west-wide prediction system • UW surface water monitor • Princeton eastern U.S. and CONUS systems • Integration • Outstanding issues

3. Motivation for experimental hydrological prediction systems: Traditional “bottom up” hydrologic modeling approach (subbasin by subbasin)

6. Snoqualmie River at Carnation, WA Flood of record • Principal calibration locations were the Skykomish at Gold Bar and the Snoqualmie at Carnation

8. How important is calibration for seasonal hydrologic prediction?

9. How important is calibration: ensemble mean (from ESP) vs obs for April-July forecasts on six forecast dates, Gunnison River, CO uncalibrated uncalibrated bias corrected calibrated

10. Why do we need an experimental hydrological prediction system? • From Wood and Lettenmaier (BAMS, 2006): • Despite the potential benefits of improved hydrologic forecasts, most operational hydrologic prediction at seasonal lead times … are based on methods and data sources that have been in place for almost half a century. • The skill of western U.S. seasonal streamflow forecasts has generally not improved since the 1960s. • While forecast accuracy improvements would likely result from observing system densification, the need for long data records in regression-based methods would take decades to realize, and would be complicated by a changing climate. • We believe that a more promising pathway lies in the development of methods … for assimilating new sources of observational data into land surface energy and water balance models, which can then be forced with modern climate and weather forecasts.

11. One reason for the slow progress in hydrologic prediction has been the lack of real-time testing of new prediction models and methods …

12. Will help to address emerging water resources operation and planning issues (e.g., nonstationarity) Better exploit predictability in weather and climate (which is inherently at progressively larger scales with lead time) Make better use of methods, like data assimilation, that can use large scale data sources to improve hydrologic initial conditions The need for a national perspective on hydrologic prediction

13. Evolution of the UW and Princeton (near) real-time hydrologic forecast systems From Wood et al (2002) – development of a hydrologically based statistical downscaling method

14. GSM Regional Bias:a spatial example Bias is removed at the monthly GSM-scale from the meteorological forecasts (so 3rd column ~= 1st column)

15. Downscaling Test • Start with GSM-scale monthly observed met data for 21 years • Downscale into a daily VIC-scale time series • Force hydrology model to produce streamflow • Is observed streamflow reproduced?

16. start of month 0 end of month 6 1-2 years back VIC forecast ensemble VIC model spin-up VIC climatology ensemble NCDC met. station obs. up to 2-4 months from current LDAS/other met. forcings for remaining spin-up climate forecast information (from GSM) data sources Forecast Products streamflow soil moisture runoff snowpack A B C Simulations

17. Model forecasting domain

18. East Coast hindcast

19. Pilot scale implementation Pacific Northwest Updates Dec 28, 2002 ESP Jan 15, 2003 ESP Feb 1 ESP, GSM, NSIPP Feb 15 ESP Mar 1 ESP, GSM, NSIPP Mar 16 ESP Apr 1 ESP, GSM, NSIPP <disk crash> Approach: 1/8 - 1/4 degree implementation

20. Pilot Forecasts: Initial Conditions Dec 28, 2002 Jan 15, 2003 This past winter, alarmingly low PNW December snowpacks mostly recovered by April, although some locations are still well off their long term averages Feb 1, 2003 Mar 1, 2003 Apr 1, 2003

21. Winter 2002/03 forecasts: UW/NRCS comparison UW pilot results were comparable to the official streamflow forecasts of the National Resources Conservation Service (NRCS) streamflow forecast group (one location shown).

22. UW West-wide forecast system – current domain and streamflow forecast points • ~250 forecast points, including ~15 in Mexico • Forecast models/methods include CPC “official” forecasts, ESP, and stratified ESP • Forecasts for 6-12 month lead issued twice monthly (winter), monthly otherwise

23. UW West-wide forecast system soil moisture nowcast (8/6/08) • Daily updates, 24 hour lag effective ~2 pm Pacific • Based on ~2000 index stations, adjusted to long-term (1915 – present) climatology

24. Princeton University drought monitoring and prediction system ~weekly nowcast update, eastern U.S. domain Uses NLDAS forcings Focus on (soil moisture) drought nowcast and forecast Forecasts based on Bayesian MME merging of GFS and ESP

25. UW National Surface Water Monitor • ½ degree spatial resolution • Updates daily (same lag as west-wide system) • Same index station approach as west-wide system • Climatology 1915-present

26. UW Multi-model monitor • Same approach as VIC-based SWM • Models include VIC, Noah, CLM, Sac

27. Model i Cumulative Probability, 1916-2004 100 % Multi-Model Cumulative Probability, 1916-2004 0 100 50 Soil Moisture (mm) 800 % 0 50 Soil Moisture (mm) 800 Multi-model Ensemble Model i soil moisture For each model, re-express current soil moisture as percentile of climatology for this day of year Average all models’ percentiles = 1/N Σ (i=1 to N) percentile i Model i percentile Multi-Model percentile Multi-model ensemble result is the percentile of the average of model percentiles This procedure occurs separately for each grid cell

28. Soil Moisture Percentiles w.r.t. 1920-2003 2008-07-01 VIC CLM SAC NOAH ENSEMBLE US Drought Monitor

29. US Drought Monitor UW Surface Water Monitor Multimodel Ensemble Jul 1 Agreement: Dry west coast Aug 5 Disagreement: Dry conditions in N.,S. Carolina? Sep 2 Agreement: WI drying trend Agreement: Gulf wetting trend

30. Soil Moisture Percentiles w.r.t. 1916-2004 2008-07-01 VIC CLM Multimodel results with drought monitor color scheme (truncated at 30th percentile) SAC NOAH ENSEMBLE US Drought Monitor

31. US Drought Monitor UW multimodel SWM Summer 2008 Jul 1 Aug 5 Sep 2

32. Ongoing unification of UW and Princeton systems a) unified nowcast (completed, in testing) b) expansion of multimodel SWM domain into Mexico (in progress) c) merger of forecast methods (esp. multimodel Bayesian MME) – planned d) improved data assimilation – planned e) multiple (land) model forecasts – planned f) reservoir storage forecasts -- planned

33. Conclusions and challenges • Need for national scale hydrological prediction (including streamflow) • Need for better ways of including a historical perspective (what historical period?) post-data assimilation • Need for site-specific calibration (MOS-type approaches?) and verification • Mechanisms for inclusion of local information?