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Forecasting the onset of the African rainy seasons
Michael Vellinga, Alberto Arribas and Richard Graham
S2S Conference, Washington, Feb 2014
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Background
• In numerous regions timing of rainfall events is at least as important to users as rainfall amounts:
• Onset and cessation of rainy season
• Occurrence of dry/wet spells
• Strong user demand for subseasonal rainfall forecasts for a range of lead times: months-weeks-days
What climate information do African decision makers need?
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• Highest priority: predictions of rainy season onset, cessation and dry spell risk (temporal distribution of rainfall)
• Predictions needed well ahead of the season
Consultation with 9 climate service providers in Africa (across 8 countries)
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Approach
1. Develop rainy season onset fc, 1-3 months’ lead
2. Use multi-variate approach to define onset, to give more reliable forecasts
• Combine rainfall, OLR and dynamical onset indicators
3. Formulate onset indicators in a way that minimises sensitivity to model rainfall bias
• Developed for sub-Saharan Africa (Vellinga, Clim. Dyn. 2012), presented at RCOFs across Africa and Asia
• Forecast skill analysed in multiple systems (Met Office and ENSEMBLES database)
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Example: Precipitation-based method (isochrones)
Rainfall accumulation
Time
Average onset
Late onset
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Observed mean evolution: 20th isochrone for OND (from 1st Aug.)
•Connect points of ‘equal arrival time’
•Colours indicate time of local arrival of 20% of average season total rainfall
• GPCP average for 18th Sep - 1st Jan (1996-2009/10)
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GloSea4 mean evolution: 20th isochrone for OND (from 1st Aug.)
•Colours indicate time of local arrival of 20% of average season total rainfall
• GloSea4 average for 18th Sep - 1st Jan (1996-2009/10)
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GloSea4 Forecast for 2011Arrival of 20th isochrone
Probability of early arrival: Probability of late arrival:
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GloSea4 forecast skill ROC scores 20th isochrone for 1 August hindcasts
Early arrival: Late arrival:
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Conclusions and future work
• Skilfull forecast of subseasonal rainfall characteristics [onset] are possible in spite of mean rainfall biases
• Needs to be judicious in defining fc variable: combine fc skill with user relevance. Iterative process between ‘modeller’ and users
• Need to translate ‘tercile maps’ into PDF for onset dates
• Provision of seamless forecasts from days to months ahead: Mechanisms and sources of fc skill are different (MJO, midlatitude intrusions, etc).
• Skill largely unexplored