Probability of Exceedance Simon Mason simoniri columbia edu
Probability of Exceedance Simon Mason simon@iri. columbia. edu Seasonal Forecasting Using the Climate Predictability Tool
Forecasting Is the following a good forecast for the maximum temperature in Kingstown on Wednesday (the day of the Cari. COF)? • 27°C • 33°C • 30°C If we forecast a temperature that is very high (33°C) it is more likely that it will be colder than forecast compared to warmer than forecast. The forecast should be most accurate when we think the chance that it will be hotter than the forecast = the chance that it will be colder. But 33°C is not impossible (record is 39°C); it may be useful to know what are the chances of exceeding 33°C And 27°C is not impossible (record is 17°C); it may be useful to know what are the chances of exceeding 27°C 2 Seasonal Forecasting Using the Climate Predictability Tool
Climatological probabilities Barbados CIMH rainfall JFM Year JFM 1971 190 1981 110 1991 133 2001 46 1972 234 1982 124 1992 106 2002 75 1973 98 1983 88 1993 138 2003 115 1974 167 1984 1994 137 2004 110 1975 131 1985 131 1995 197 2005 182 1976 223 1986 105 1996 109 2006 251 1977 47 1987 97 1997 108 2007 146 1978 55 1988 169 1998 96 2008 202 1979 141 1989 279 1999 112 2009 150 1980 144 1990 194 2000 218 2010 54 What is the driest year? 3 Seasonal Forecasting Using the Climate Predictability Tool
Climatological probabilities Barbados CIMH rainfall JFM Year JFM %> 2001 46 97. 5 1984 184 22. 5 1977 47 95. 0 1971 190 2010 54 92. 5 1990 194 17. 5 1978 55 90. 0 1995 197 15. 0 2002 75 87. 5 2008 202 12. 5 1983 88 85. 0 2000 218 10. 0 1998 96 82. 5 1976 223 7. 5 1987 97 80. 0 1974 234 5. 0 1973 98 77. 5 2006 251 2. 5 1986 105 75. 0 1989 279 0. 0 4 … Seasonal Forecasting Using the Climate Predictability Tool
Probability of exceedance 5 Seasonal Forecasting Using the Climate Predictability Tool
Climatological probabilities Barbados CIMH rainfall JFM Year JFM %> 2001 46 97. 5 1977 47 95. 0 2010 54 92. 5 1978 55 2002 This procedure is exactly how the terciles are determined. Below: Year JFM %> 90. 0 1996 109 67. 5 ? 75 87. 5 lower 109 66. 7 ? 1983 88 85. 0 1981 110 65. 0 ? 1998 96 82. 5 1987 97 80. 0 1973 98 77. 5 1986 105 75. 0 6 Above: The percentile is the probability Year JFM %> of non-exceedence 2007 146 35. 0 upper 148 33. 3 2009 150 32. 5 Seasonal Forecasting Using the Climate Predictability Tool
Probability of exceedance 7 Seasonal Forecasting Using the Climate Predictability Tool
Gaussian approximation 8 Seasonal Forecasting Using the Climate Predictability Tool
Climatological rainfall Barbados CIMH rainfall JFM Average: 140 mm Std Dev: 56 mm 9 Seasonal Forecasting Using the Climate Predictability Tool
Climatological rainfall Barbados CIMH rainfall JFM Normal distribution 10 Seasonal Forecasting Using the Climate Predictability Tool
Climatological rainfall Barbados CIMH rainfall JFM Gamma distribution 11 Seasonal Forecasting Using the Climate Predictability Tool
Gamma approximation 12 Seasonal Forecasting Using the Climate Predictability Tool
Gaussian distribution 13 Seasonal Forecasting Using the Climate Predictability Tool
Gamma distribution 14 Seasonal Forecasting Using the Climate Predictability Tool
Exercises • Construct probability of exceedence forecasts for your stations. • Confirm the tercile-based probabilities from the probability of exceedence diagrams. • Identify the probability of exceedance for some possible thresholds of interest. 15 Seasonal Forecasting Using the Climate Predictability Tool
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