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

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

Michael Vellinga, Richard Graham and Alberto Arribas, 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


Motivation


Sahelian rainfall


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)


Observed onset W Africa (JAS) GPCP mean 1996-2009

Calendar date (month/day)


Observed onset W Africa (JAS) GPCP standard deviation

St Dev (days)


Observed onset (SOND) GPCP standard deviation – Substantial!

St Dev (days)


Onset forecastsDefinitions, skill and examples


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’)


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


Forecast skill for onset

• ROC score for earlier-than-average onset• 25 April hindcasts, i.e. ~2 months’ lead

indicator 1)


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


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


Other regions…

GHA (SON)

SE Asia (MJJ)

S Africa (OND)


Seasonal forecasts for onsetExamples from 2011/12


2012 Onset W Africa (JAS)

Glosea4 6 May forecast: probability for early (lower tercile) onset

Observed onset anomaly 2012 (CPC/FEWS)


2011 Short Rains onset GHA (SON)

Observed (CPC/FEWS) anomaly

Glosea4 18 July forecast: probability for early (lower tercile) onset


2012 Short Rains onset GHA (SON)

Observed (CPC/FEWS) onset anomaly (days)

Glosea4 22 July forecast: probability for early onset


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?

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

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