Defining and predicting the
West African Monsoon Onset
Rory Fitzpatrick1, Doug Parker1, John
Marsham1, Peter Knippertz2, Caroline
Bain3
1. University of Leeds, United Kingdom
2. Karlsruhe Institute of Technology, Germany
3. UK Met Office, United Kingdom
Motivation
Definitions of onset length scale
• Local onset – occurring at the grid‐cell to country wide scale • Regional onset –
occurring over a large longitudinal range (typically 10W‐10E). Data is zonally averaged across the region
• Semi‐local (hybrid) onset
The West African Monsoon
Figure 1 – Lafore et al. 2010
Figure 2 c and d – Sultan and Janicot 2003b
Definitions
Local
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Walter (1967)
Kowel and Knabe (1972)
Benoit (1977)
Olaniran (1983)
Sivakumar (1988)
Omotosho et al. (2000)
Le Barbé et al. (2002)
Zeng and Lu (2002)
Marteau et al. (2009)
Vellinga et al. (2012)
Yamada et al. (2012)
Regional
• Le Barbé et al. 2002
• Sultan and Janicot 2003
• Fontaine and Louvet 2006
• Fontaine et al. 2008
• Fontaine et al. 2008
• Gazeaux et al. 2011
• Vellinga et al. (2012)
• Nguyen et al. (2014)
This list does not include locally determined or traditional methods. There may be many more!
Datasets compared
– ERA‐Interim 1998‐2012 , 1.5°x1.5° resolution
– GPCP 1998‐2008, 1°x1° resolution
– TRMM 3B42 v6 1998‐2010, 0.25°x0.25° resolution
– TRMM 3B42 v7 1998‐2012, 0.25°x0.25° resolution
Sultan and Janicot (2003)
REGIONAL DEFINITION
TRMM v7 1998‐2012
Sultan and Janicot 2003 (SJ)
Onset occurs when zonally‐averaged, time‐smoothed precipitation (10W‐10E) decreases at 5N (red line) simultaneous with increases at 10N (blue line) and 15N (green line) Such a date is not always evident
Good Example TRMM7 2002
Bad Example TRMM7 2010
Selection of an onset date is not always unambiguous if definition is taken verbatim. It is therefore useful to create an objective version of regional onset date SJ. Objective onset: SJ‐10
• Objective onset of SJ:
– the first date for which precipitation at 5N is less than zonally averaged precipitation at 10N for 7 consecutive days (SJ‐10) (blue vertical line)
• Can differ greatly from the subjective version
(black line)
and the point where
precipitation at 5N falls
below precipitation values at
15N (green line)
TRMM v7 2010
SJ‐10 onset dates: Dataset comparison
Definition and Dataset
Mean onset date
Standard deviation
Sultan and Janicot 2003
24 June
8
ERA Interim
26 June
13.9
GPCP
29 June
9.3
TRMM v6
28 June
9.9
TRMM v7 5 July
9.6
GloSea5
26 June
7.0
Datasets can greatly disagree on onset dates
Marteau et al. 2009
LOCAL DEFINITIONS
TRMM v6 1998‐2010
Marteau et al. 2009 (Mart)
Local onset is triggered on the first rainy day (>1mm) of two consecutive rainy days (total rain > 20mm) with no 7 day dry spell of total rain < 5mm occurring in the subsequent 20 days
Inconsistency in onset across datasets
TRMM v7 1998‐2012
GPCP 1998‐2008
There are few locations across West Africa where TRMM v7 and GPCP local onset dates are correlated.
Spatially sporadic correlation between SJ‐10 and Mart
The regional onset date is only sporadically correlated with local onsets. Regional onset may only have limited use in predicting local onset.
Brief results
GLOSEA5 ONSET PREDICTABILITY
Mean onset of GloSea5 captures general local onset pattern
TRMM v7
GloSea5
But inter‐annual variability is not well captured
TRMM 2002
GloSea5 2002
TRMM 2003
GloSea5 2003
Although GloSea5 has a good representation of mean onset date, it does not capture inter‐annual variability.
Can GloSea5 correctly predict early or late local onsets?
Definition of prediction
Observed onset
Predicted onset
Complete hit
> 1 s.d. prior to mean
> 1 s.d. prior to mean
Partial hit
> 1 s.d. prior to mean
Between 0.5 and 1 s.d.
prior to mean
Partial miss
> 1 s.d. prior to mean
Between 0.5 s.d. prior and 0.5 s.d. after mean
Complete miss
> 1 s.d. prior to mean
> 0.5 s.d. after mean
False hit
< 1 s.d. prior to mean
> 1 s.d. prior to mean
s.d. – standard deviation
Not exceptionally. A similar pattern is found for late onsets.
Conclusions
• There is a big difference between local and regional onsets of the West African Monsoon. Are different dynamics involved?
• Datasets agree on mean regional onset date, but do not show inter‐annual correlation.
• Skilful prediction of regional onsets may have little practical use for local onset forecasts.
• GloSea5 can recreate the mean local onset date but does not capture inter‐annual variability.
Because everything can be described by The Simpsons
• “Lisa, your word to spell is weather.”
– “Can you use that in a sentence please?”
• “Certainly. I don’t know whether the weather will be good today.”