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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 - PowerPoint PPT Presentation
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© Crown copyright Met Office
Extended-range forecasts for onset of the African rainy seasonsexamples and ideas for future work
Michael Vellinga, Richard Graham and Alberto Arribas, Met Office
© Crown copyright Met Office
Outline
1. Motivation
2. Onset forecasts
• Criteria and definitions
• Longrange forecast skill: level and sources
• Example forecasts
3. What next: thoughts and discussion
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Motivation
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Sahelian rainfall
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Motivation
• Rain-fed agriculture in Africa vulnerable to fluctuations in timing of arrival time of rains, as well as rainfall amounts:
a) Long-range decision window:• choice for slow or fast cropping varieties/crops• agricultural logistics (field preparation, mobilisation of
work force,..)
b) 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)
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Observed onset W Africa (JAS) GPCP mean 1996-2009
Calendar date (month/day)
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Observed onset W Africa (JAS) GPCP standard deviation
St Dev (days)
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Observed onset (SOND) GPCP standard deviation – Substantial!
St Dev (days)
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Onset forecastsDefinitions, skill and examples
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Onset definitions:two classes
1. Local criteria:• time of local rainfall threshold exceedance (e.g.
Sivakumar 1988, AGRHYMET)
2. 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’)
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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
Time
Percentage of season total rainfall
onset=20%
Average time of onset
•Heavy line: accumulated precip. from climatology
•Thin line: accumulated precip. for individual year
example: early onset in individual year
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Forecast skill for onset
• ROC score for earlier-than-average onset• 25 April hindcasts, i.e. ~2 months’ lead
indicator 1)
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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
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HadiSST and GPCP-derived onset
Observed teleconnection
Source of predictability: June SST
GloSea4 teleconnection
Fore
cast
ski
ll (a
nom
. cor
r.)
For more details see: Vellinga et al 2012 Clim Dyn
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Other regions…
GHA (SON)
SE Asia (MJJ)
S Africa (OND)
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Seasonal forecasts for onsetExamples from 2011/12
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2012 Onset W Africa (JAS)
Glosea4 6 May forecast: probability for early (lower tercile) onset
Observed onset anomaly 2012 (CPC/FEWS)
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2011 Short Rains onset GHA (SON)
Observed (CPC/FEWS) anomaly
Glosea4 18 July forecast: probability for early (lower tercile) onset
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2012 Short Rains onset GHA (SON)
Observed (CPC/FEWS) onset anomaly (days)
Glosea4 22 July forecast: probability for early onset
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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?