Climate Forecasts and Forecast Uncertainty

Transcription

1 Climate Forecasts and Forecast Uncertainty Holly Hartmann Department of Hydrology and Water Resources University of Arizona, Tucson CLIMAS-SAHRA press briefing August 26, 2002

2 How are climate forecasts made? NWS Climate Prediction Center (CPC) Official U.S. climate forecasting agency 4-5 primary forecasters, but they get input from ~20 people before deciding on a final forecast Long-range forecasters at work in 1946 Major Forecasting Tools Long-term trends Comparison with past situations Computer simulations Findings from recent research Human expertise CPC climate forecasts are created by combining results from several scientific techniques, using expert judgement to decide which should be given more or less importance.

3 NWS Seasonal Climate Outlooks Issued monthly Lead Times: months Probabilistic: Probabilistic: Predict the probability of conditions being among the warmest/coldest or wettest/driest terciles of years in

4 Press Interpretations are Often Wrong!

5 Press Interpretations are Often Wrong!

6 What do the Climate Outlooks Mean? Issued monthly Lead Times: months Probabilistic: Probabilistic: Predict the probability of conditions being among the warmest/coldest or wettest/driest terciles of years in

7 What do the Climate Outlooks Mean? The outlooks indicate probability of being in three specific categories. Legend Cold Norm Warm Climatology The categories are defined by having equal chances, historically.

8 What do the Climate Outlooks Mean? Legend Cold Norm Warm It does not just indicate above or below normal! 50% 50% Climatology

9 What do the Climate Outlooks Mean? Each colored contour indicates a shift in the normal chances. Legend Cold Norm Warm 3% 63% +30% Chance of Warm Climatology

10 What do the Climate Outlooks Mean? Near-Normal 27% 43% 27% Legend Cold Norm Warm 3% 63% +30% Chance of Warm Climatology

11 What do the Climate Outlooks Mean? Near-Normal 27% 43% 27% Unknown CL Legend Cold Norm Warm Sometimes forecasters don t know what the chances are 3% 63% +30% Chance of Warm Climatology

12 Wet, Dry, Normal Compared to What?.4,3,7. %49,! ,9 43.4,3,7. %49,!70. 5! , Wet Near- Dry Normal 0,7 Normal chances are based only on 30 years of data..00/,3.0!74-,- 9 66% The data are ranked and divided into three categories of equal probability, with roughly 10 years in each category.

13 Wet, Dry, Normal Compared to What?.4,3,7. %49,! ,9 43.4,3,7. %49,!70. 5! , Wet Near- Dry Normal 0,7 3.7 Wet Norm Dry /,3.0!74-, years had more than 3.7 inches 10 years had less than 1.9 inches 10 years were in the middle 1.9

14 Latest Precipitation Outlooks Sept-Oct-Nov CL CL Jan-Feb-Mar 2003 Issued Aug 15, % % 28% % 10+

15 How Good Are the Outlooks? Sept-Oct-Nov CL CL Jan-Feb-Mar 2003 The probability shift indicates the confidence of the forecasters. Issued Aug 15, % % 28% % 10+

16 How Good Are the Confidence Statements? Official Winter Climate Forecasts in West US( ) Precipitation Temperature Reliability Frequency of Obs Wet Dry Warm Cold Frequency Histogram (log scale) Strength of Forecast Message: increased chances of wet and warm conditions could have been even stronger. Forecasters were better than they thought!

17 Comparing individual outlooks can be misleading Climate Outlook: Precipitation June-August 2000 (issued April 2000) Observations: % of Normal Precipitation June-August 2000 Low spatial variability High spatial variability

18 Ranked Probability Skill Score RPSS: Best overall measure of forecast skill. Considers the chances assigned to all three categories. Outlooks issued July and August, andcovering ASO and SON. Temperature Precipitation SE AZ: +10% SW AZ: +21% NE AZ: -3% not good good SE AZ: +31% SW AZ: -15% NE AZ: -30% RPSS: % improvement over using equal chances (normal chances).

19 El Nino: A Wild Ride? Stay Tuned Outlooks are best during El Nino s major impact period thewinter. Winter outlooks for the Southwest are among the best.