Transcript Document

Regional climate model simulation
of the West African Monsoon precipitation
during 1980-1990
Wilfran Moufouma-Okia, David Hassell and David Hein
Hadley Centre for Climate Prediction and Research
1. Motivation
4 . South westerly monsoon flow
West Africa is a vulnerable region where the scientific
community is facing challenges in estimating the timing
and the magnitude of climate changes, and their
environmental and socioeconomic impacts. The interactions
between continental surfaces and atmospheric forcing are
crucial drivers for it climate, and the variability of rainfall
is a key issue. However, the global climate models (GCMs)
which traditionally provide useful climate projections at
continental scale of several thousand kilometres, lack the
regional scale details needed for adequate predictions of
precipitation. Atmosphere regional climate models (RCMs)
have shown promising performances in reproducing
observed regional surface climate characteristics for many
tropical regions, but to date have not been widely used over
West Africa.
Figure 2 illustrates the seasonal excursion of the horizontal wind at
925-hPa averaged over March-May (MAM), June-August (JJA) and
September-October (SON) in both the reanalysis and RCM. The black
solid line represents the zero isoline of the zonal wind component, so as
to delineate the south-westerly monsoon (SWM) flow and detect the
location of the inter-tropical front (ITF). The RCM captures reasonably
well the location of the confluence along the ITF of the moist SWM
wind and the north-easterly dry wind (Harmattan). The RCM simulated
SWM flow is also consistent with the results of HadAM3P GCM (not
shown).
Sahel (most prominent in MAM) and a dry bias over the highlands of
Cameroon and Guinea. Table 1 summarises the seasonal means, biases,
and standard correlations of simulated precipitations over the land points.
Although the RCM overestimates rainfall, the bias in JJA and SON is less
than 10%. The main deficiencies of the RCM are present and exacerbated
in HadAM3P (not shown) implying they are in part locally forced due to
deficiencies in the model formulation.
2. Abstract
This study presents the ability of the Hadley Centre regional
climate model HadRM3P to simulate the West African
monsoon precipitation. The model is evaluated with 1978–
1990 continuous baseline integration, at 50 km spatial
resolution, driven by the European Centre for MediumRange Weather Forecasts reanalysis ERA-15. The causes for
major model biases (differences from observations) are
examined through a supplementary experiment where
HadRM3P is nested into the global climate model HadAM3P.
The results are compared against both the surface
observations of the climate research unit (CRU) and the
driving atmospheric conditions. HadRM3P demonstrates
pronounced rainfall downscaling skills in complex costal and
orographic locations. The seasonal cycle of precipitation is
simulated realistically, in space and time. The rain
installation phase in the south, the high rain phase in the
north, and the retreat of rain southward are well reproduced.
On the other hand, HadRM3P shows some biases common to
the driving model in this region. This suggests some
deficiencies in the model’s representation of rainfall
processes over West Africa.
Figure 4: Observed and simulated mean MAM and JJA precipitation
5.2 Interannual variability
Figure 2: Climatology of seasonally averaged 925-hPa wind fields in
HadRM3P(bottom) and ERA40 (top)
over West Africa
Figure 3 depicts the vertical cross-section of the mean JJA air
temperature and specific humidity, associated with the SWM, for the
reanalysis and the RCM. The thermodynamic vertical structure of the
simulated atmosphere is similar to the observations. The main
discrepancies are the northward displacement of the surface maximum
of moisture and the overestimation of its magnitude.
The skills of the RCM at simulating the interannual variability of rainfall
is illustrated through the computation of seasonal (JJA) rainfall indices
over the Sudan and Guinea coast regions (Fig. 5). The RCM captures the
main features of the interannual variability in the Sudano-Sahelian and
Guinean coast regions including reasonable simulations of the extreme
years 1982, 1984 and 1988. The first two of these years are markedly dry,
whereas 1988 is the wettest observed year over the Sudan. As expected,
the RCM performs better than the GCM, as the latter is only constrained
by observed SSTs.
Figure 5: Time-series of JAS rainfall anomalies for CRU dataset (blue),
HadRM3P (white) and HadAM3P (red)
5.3 Intraseasonal variability
The performances of HadRM3P for simulating the intraseasonal
variability of West African rainfall are highlighted by the 1988 timelatitude diagram of daily precipitation averaged over 10° W–10° E,
where a meridional land-sea contrast exists (Fig. 6). Although the RCM
simulates the monsoon preonset and onset stages, there are some
important discrepancies in the timing and location of rainfall maxima.
Figure. 1: Model domain including the relaxation zone
3. Experimental setup
Figure 3: Mean JJA vertical cross section of temperature (top) and
humidity (bottom), averaged from 20° W–15° E
IRD data
HadRM3P
3.1 Regional climate model
 Horizontal resolution: 0.44° x 0.44° (~50 km) grid spacing
 Vertical resolution: 19 hybrid levels, from ~50 m to 0.5 hPa
 Time step: 5 minutes
 Duration: 12 years, 1 December 1978 to 31 December 1990
 Spinup time: two years
 Experiment 1: HadRM3P driven by ERA-15 reanalysis and
observed sea-surface temperatures (SSTs)
 Experiment 2: HadRM3P driven by HadAM3P, both driven
by observed SSTs
5. Precipitation
5.1 Seasonal cycle
Figure 4 shows the mean climatological spatial distribution of seasonal
rainfall simulated and observed. The RCM reproduces the northward
and the southward movement the band of high rainfall (i.e. > 5 mm/day)
associated with the ITCZ, from March to October. The main
deficiencies of the model include a wet bias over the eastern part of the
3.2 Observational data
Season
Average
Bias (RCM – CRU)
Correlation
The data used for the validation of the RCM are mainly
extracted from the Climatic Research Unit (CRU) analyses
and the IRD/ASECNA/CIEH where available. Additionally,
ECMWF ERA40 reanalysis are also used for the validation of
the atmospheric circulation in the interior domain.
MAM
JJA
SON
2.6
3.9
2.4
1.2
0.2
0.3
0.81
0.80
0.86
Table 1: 1981–1990 basic statistics of simulated rainfall over land points
Met Office Hadley Centre FitzRoy Road Exeter Devon EX1 3PB United Kingdom
Tel: 01392 886079 Fax: 01392 885681
Email: [email protected]
Figure 6: 1988 Time-latitude diagrams of daily rainfall, filtered to remove
variability lower than 10 days
6. Conclusions
The HadRM3P RCM has demonstrated good skill in simulating the
climatology of the south-westerly monsoon flow and its associated
thermodynamic vertical structure of the atmosphere over West Africa.
When driven by ECMWF reanalysis, HadRM3P simulates reasonably
well the seasonal cycle and the interannual variability of rainfall. In
comparison to its parent GCM, HadAM3P, HadRM3P simulates mean
precipitation better (and the evolution of seasonal anomalies due, in
part, to its use of reanalysis boundary conditions). However, these two
models share some common biases over the Sahel region.
References:
Le Barbé, L., T. Lebel, and D. Tapsoba, 2002: variability in West Africa during the years 1950-90. J.
Climate, 15, 187-202.
Gallé, H., W. Moufouma-Okia, P. Bechtold, O. Brasseur, I. Dupays, P. Marbaix, C. Messager, R. Ramel,
and T. Lebel, 2004: A high-resolution simulation of a West African rainy season using a regional climate
model. J. Geophys. Res., 109, D05108, doi:10.1029/2003JD004020.
Jones, R.G., Noguer, M., Hassell, D.C., Hudson, D., Wilson, S.S., Jenkins, G.J., Mitchell, J.F.B.: 2004,
‘Generating high resolution climate change scenarios using PRECIS’, Met Office Hadley Centre, Exeter,
UK/UNDP, New York, USA: 35pp.
Sultan, B. and S. Janicot, 2003: The West African Monsoon dynamics. Part II: The preonset and the onset
of the summer monsoon. J. Climate,16, 3407-3427.
Rowell, D., C.K. Folland, K. Maskell and M.N. Ward, 1995:Variability of summer rainfall over tropical
north Africa (1906-92): Observation and modelling. Q. J. R. Meteorol. Soc., 121, 669-704.
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