Transcript Document

The effects of regional orography
on the West African monsoon system
Wilfran Moufouma-Okia and David Hassell
Hadley Centre for Climate Prediction and Research
Results
Background
The West African monsoon system (WAM) is characterised by the monsoon onset, which is
an abrupt latitudinal shift of the inter-tropical convergence zone (ITCZ) and its associated
rainfall maxima, from a quasi-stationary location at 5° N in May–June to another quasistationary in July–August (Le Barbé et al, 2002). Numerical studies with global climate
models (GCMs) and linear models have suggested a possible interaction between the North
African large-scale orography and the WAM at the time of the onset. Due to the lack of
regional scale details within the above mentioned models, the mechanisms responsible for
these interactions remain not as clearly understood as the role of other regional mountain
ranges. However, the recent and wide development of high spatial resolution regional
climate models (RCMs) has now provided an opportunity to further study the influence of
orographic forcing on the climate in West Africa.
The main objective of this study is to further investigate the influence of orographic forcing
on the West African climate with a high resolution RCM, focusing on the abrupt northward
shift of the ITCZ in summer.
Methodology
Method
The method used here is the one-way nesting of a limited area climate model into a coarse
resolution GCM. The principle behind this technique is that given a detailed representation of
physical processes, and high spatial resolution that resolves complex orography, land-sea
contrast, and land-use, a limited area model can generate realistic regional climate
information consistent with the driving large-scale circulation, supplied either by global
reanalysis data or a GCM.
1. Atmospheric circulations
The orographic forcing influences the features of the
mean summer atmospheric circulations (Fig. 2). The
Hoggar-Air-Tibesti
complex
controls
the
northernmost excursion of the south west monsoon
(SWM) flow over the land and its exclusion moves
the ITF northward by 2°. The shift of the eastern
lateral boundary eastwards, so that the orographic
features of East Africa are included, strengthens the
AEJ, TEJ and the SWM flow significantly.
Figure 2: Mean JJA cross-vertical section of zonal wind component
2. Monsoon pre-onset
The latitude of the ITF — represented by the northern boundary of the 925-hPa zonal wind zero isoline — is generally a good
indicator of the meteorological signal associated with the monsoon preonset. The date of the pre-onset can be defined as the date
when the 925-hPa zonal wind component averaged over 10° W–10° E equals zero, going from negative (Harmattan wind) to
positive values (SWM flow). Figure 3 depicts the time series of daily outgoing longwave radiation (OLR) and 925-hPa zonal
wind at 15° N, filtered to remove variability below 10 days. The exclusion of orographic forcing clearly does not modify the
date of the preonset (early April) but does modify the magnitude and the variability of deep convection and zonal wind.
Regional climate model
The RCM used here is HadRM3P developed by the Hadley Centre. This model is used in the
regional climate modeling system “Providing Regional Climates for Impact Studies
(PRECIS)” (Jones et al., 2004). HadRM3P is similar to its immediate predecessor,
HadRM3H, for which an extensive description has been given by Hudson and Jones (2002),
and Frei et al. (2003). The main differences reside in the details of the representation of
dynamical and convective cloud, and thresholds associated with the formation of
precipitation. HadRM3P has a horizontal resolution of 0.44° latitude x 0.44° longitude (~50
km), 19 hybrid vertical levels and a time-step of 5 min.
Experiments
Orographic features
Horizontal
resolution
Domain
CTRL
All include
50 km
standard
NOFOU
No Fouta-Djalon
50 km
standard
NOCAM
No Cameroon
50 km
standard
NOBAU
No Bauchi
50 km
standard
NOHOG
No Hoggar-Air-Tibesti
50 km
standard
HIRES
All include
25 km
standard
EAFR
All include
50 km
extended
Table 1: Conducted RCM experiments
3. Monsoon onset
Figure 4 shows the time-latitude diagrams of daily rainfall values, averaged over the longitudes 10° W–10° E. The rapid shift
of the ITCZ is identified in all the experiments around the end of June. The orographic forcing influences the rainfall
distribution over the Sahel in June-September. This influence is particularly strong when the Hoggar-Air-Tibesti and the
Cameroon highlands are excluded, and the extended domain is used.
Experimental design
Seven, one-year long integrations of the RCM are
performed for the year 1988. Each simulation is
driven by the ECMWF reanalysis ERA-15 and two
domains are used for the integrations, one standard
and one extended (Fig. 1). Table 1 summarises the
integrations we have conducted, which consist of one
control and six sensitivities experiments. In the first
four experiments, the highlands of Fouta-Djalon,
Cameroon, Bauchi, and Hoggar-Air-Tibesti have
been removed from the simulation, one at the time,
by replacing the height of the topography in grid
boxes with the mean height of the areas adjacent to
the highland. The fifth experiment corresponds to an
enhanced representation of the topography by
doubling the RCM horizontal resolution from 50 km
to 25 km. In the last experiment, the eastern boundary
of the model domain is shifted eastwards, allowing
the inclusion of orographic features of North East
Africa such as the Darfur and the Ethiopian
highlands.
Figure 3: Time series of daily zonal wind and OLR, averaged over 10° W–10° E along 15° N
Figure 1: Model domains and distribution of
orographic height (m)
Figure 4: Time-latitude diagrams of daily rainfall, averaged over 10° W–10° E
It is worth noting that the control
experiment has been first
compared against the 1988 RCM
outputs extracted from a previous
12-year (1978-1990) continuous
integration of HadRM3P. The
results reveal no significant
differences between the two
RCM integrations. Therefore, the
results presented here are
considered to be unaffected by
the model spinup.
Figure 5: Time-longitude diagrams of daily rainfall along 15° N
4. Sensitivity to model resolution
The sensitivity of the simulated West African monsoon to model resolution is detected in the representation of mesoscale
convective systems (MCSs). Figure 5 illustrates the time-longitude diagram of daily rainfall from 15° W–15° E and at 15° N,
and from June to September. The monsoon is developing rapidly from the second half of June and there is a band of high
convective activity. The daily rainfall increases with the increase of the horizontal resolution. However, the influence of
model resolution is less important than that of the domain size shifted in the eastwards direction.
Conclusions and future plans
The West African orography influences the south west monsoon flow, the intensity of deep convection and rainfall. Over the Sahel, the influence of the Hoggar-AirTibesti complex on the West African monsoon is stronger than that of Fouta-Djalon, Bauchi Plateau, and Cameroon highlands. The increase of the model horizontal
resolution from 50 km to 25 km leads to an increase in the simulated convective system in June-September. The simulated West African monsoon is particularly sensitive
to the extension of the eastern boundary eastwards, so as to include orographic features of North East Africa. In the future, further work is needed to understand the
influence of North East Africa orography and domain size on simulated convective systems. A series of model integrations over the extended domain, where the Darfur
and Ethiopian highlands are removed, are planned.
Met Office Hadley Centre FitzRoy Road Exeter Devon EX1 3PB United Kingdom
Tel: 01392 886079 Fax: 01392 885681
Email: [email protected]
References:
Le Barbé, L., T. Lebel, and D. Tapsoba, 2002: variability in West Africa during the years 1950-90. J. Climate,
15, 187-202.
Hudson, D. and R. Jones, 2002: Regional climate model simulation of present-day and future climate of
southern Africa. Hadley Centre Technical Note, 39, 41pp.
Frei, C., J. Christensen, M. Dequé, R. Jones, and P. Vidale, 2003: Daily precipitation statistics in regional
climate model. Evaluation and intercomparison for the European Alps. J. Geophys. Res., 108, 4124-4142.
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.
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