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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)

West African Monsoon onset

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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)

Examples of onset indicators

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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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New ways of displaying information for decision

making

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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

Thank you