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Extended-range forecasts for onset of the African rainy seasons examples and ideas for future work

Extended-range forecasts for onset of the African rainy seasons examples and ideas for future work. Michael Vellinga, Richard Graham and Alberto Arribas, Met Office. Outline. Motivation Onset forecasts Criteria and definitions Longrange forecast skill: level and sources Example forecasts

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Extended-range forecasts for onset of the African rainy seasons examples and ideas for future work

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  1. Extended-range forecasts for onset of the African rainy seasonsexamples and ideas for future work Michael Vellinga, Richard Graham and Alberto Arribas, Met Office

  2. Outline • Motivation • Onset forecasts • Criteria and definitions • Longrange forecast skill: level and sources • Example forecasts • What next: thoughts and discussion

  3. Motivation

  4. Sahelian rainfall

  5. Motivation • Rain-fed agriculture in Africa vulnerable to fluctuations in timing of arrival time of rains, as well as rainfall amounts: • Long-range decision window: • choice for slow or fast cropping varieties/crops • agricultural logistics (field preparation, mobilisation of work force,..) • Medium-range to-monthly range decision window: • optimal sowing time • A strong information need among regional stake-holders, not well addressed at present (also non-agricultural applications)

  6. Observed onset W Africa (JAS) GPCP mean 1996-2009 Calendar date (month/day)

  7. Observed onset W Africa (JAS)GPCP standard deviation St Dev (days)

  8. Observed onset (SOND) GPCP standard deviation – Substantial! St Dev (days)

  9. Onset forecasts Definitions, skill and examples

  10. Onset definitions:two classes • Local criteria: • time of local rainfall threshold exceedance (e.g. Sivakumar 1988, AGRHYMET) • Regional criteria: • Large-scale key changes to regional meteorology (wind field, convection, OLR, MSE, etc) • We have developed trial longrange forecasts using such definitions, presented at African RCOFs in 2011/12 • Formulated as tercile probabilities (‘earlier than average’, ‘average’ or ‘later than average’)

  11. Local indicator used in our trial forecasts: • Describe temporal evolution with rainfall accumulations between 21 May-27 October • Scale by long-term average season total accumulation. Example: 1 member Percentage of season total rainfall example:early onset in individual year • Heavy line:accumulated precip. from climatology • Thin line: accumulated precip. for individual year Average time of onset onset=20% Time

  12. indicator 1) Forecast skill for onset • ROC score for earlier-than-average onset • 25 April hindcasts, i.e. ~2 months’ lead

  13. Skill scores for onset over W Africa in ENSEMBLES • Using standard definition for ‘regional’ onset (Fontaine Louvet 2006) • ENSEMBLES hindcasts (1979-2005) in red circle From Vellinga et al 2012 Clim Dyn

  14. Source of predictability: June SST Forecast skill (anom. corr.) For more details see: Vellinga et al 2012 Clim Dyn GloSea4 teleconnection Observed teleconnection HadiSST and GPCP-derived onset

  15. SE Asia (MJJ) GHA (SON) S Africa (OND) Other regions…

  16. Seasonal forecasts for onset Examples from 2011/12

  17. 2012 Onset W Africa (JAS) Glosea4 6 May forecast: probability for early (lower tercile) onset Observed onset anomaly 2012 (CPC/FEWS)

  18. 2011 Short Rains onset GHA (SON) Glosea4 18 July forecast:probability for early (lower tercile) onset Observed (CPC/FEWS) anomaly

  19. 2012 Short Rains onset GHA (SON) Glosea4 22 July forecast: probability for early onset Observed (CPC/FEWS) onset anomaly (days)

  20. Discussion points:Possible interaction with S2S Strong demand for this type of onset forecast information Not only at seasonal lead times, but also monthly Could set up multi-model prediction page for onset diagnostics: East/West/southern Africa What detail is possible at monthly lead time (range of dates)? What is the role of MJO/other tropical systems in delaying/advancing onset? How predictable are these? Is there multi-model agreement about years with high vs. low predictability for onset etc? Other aspects of intraseasonal variability - how accurately can a dry spell or wet spell be predicted in week 3?

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