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Applying the Rossby Centre Regional Climate Model (RCA3.5) over the
ENSEMBLES-AMMA region: Sensitivity studies and future scenarios
Patrick Samuelsson, Ulrika Willén, Erik Kjellström and Colin Jones
Rossby Centre, SMHI, SE-601 76 Norrköping, Sweden; patrick.samuelsson@smhi.se
1. Introduction
Western Africa is a region subject to decadal variability in
rainfall including devastating droughts in the Sahel region
during the last decades of the 20 th century. Changes in Sahel
precipitation may be related to the response of the African
monsoon to oceanic forcing amplified by land-atmosphere
interaction (Trenberth et al. 2007). Although AGCMs are
able to simulate basic patterns of rainfall trends during the
second half of the 20 th century there are uncertainties related
to their ability of simulating for instance interannual
variability in rainfall and the exact position of the African
Easterly Jet (Christensen et al. 2007). AOGCMs have even
larger problems including biases in sea surface temperatures
(SST), displacements in the intertropical convergence zone
and distortions in the monsoonal climate. Possibly RCMs
can be used to improve the understanding of the processes
regulating the climate in this region. So far only few RCM
simulations exist for this area although efforts are currently
undertaken in the AMMA (Redelsperger et al. 2006),
WAMME (Cook and Vizy 2006) and ENSEMBLES (Hewitt
and Griggs 2005) projects. In this paper we present results
from one RCM for this area.
2. RCA 3.5
We have used an updated version of the Rossby Centre
regional climate model RCA3 (Kjellström et al. 2005,
Samuelsson et al. 2006). RCA3 is developed at Rossby
Centre, SMHI, and has been extensively used in a number of
climate scenario studies. RCA3 is one of the RCMs in the
European projects PRUDENCE (Jacob et al. 2007) and
ENSEMBLES (Sanchez-Gomez et al. 2008). RCA3 includes
parameterizations for radiation (Savijärvi 1990; Sass et al.
1994), turbulence (Cuxart et al. 2000), large-scale clouds
and microphysics (Rasch and Kristjánsson 1998),
convection (Kain and Fritsch 1993; Jones and Sanchez
2002), and land surface (Samuelsson et al. 2006). In the
updated version RCA3.5 we have replaced the original lake
model PROBE (Ljungemyr et al. 1996) by FLake (Mironov
2008) and replaced the land-surface physiographic
information by ECOCLIMAP (Masson et al. 2003). The
convection parameterisation is based on Bechtold et al.
(2001) which is a development from the one by Kain and
Fritsch (1993) now separating shallow and deep convection.
3. RCA3.5 over ENSEMBLES-AMMA
The RCA3.5 ENSEMBLES-AMMA domain is shown in
Figure 1. The domain is set up on a regular grid resolved by
168x142 grid points covering -39 – 35°E and -23 – 39°N
which corresponds to 0.4° or 50km horizontal resolution. In
the vertical we have used 24 and 40 levels, respectively.
For hind-cast simulations we have used ERA INTERIM as
forcing for lateral boundary conditions and for sea-surface
temperature. The analyzed period cover 1990-1997 with one
year spin-up time before that.
For the scenario simulation we take lateral boundary
conditions and sea surface temperatures from the HadCM3
model (Collins et al. 2006) operating under the SRES A1B
emission scenario (Nakićenović et al. 2000). The scenario
simulation covers the period 1961-2100.
Figure 1. The RCA3.5 ENSEMBLES-AMMA
domain including the Sahel region as used for areaaveraging.
4. Sensitivity tests
In Figure 2 it is shown how the simulated annual cycle of
precipitation depends on the number of vertical levels
used. The results represent averaged values over the Sahel
region as shown in Figure 1.
The improvement of the model to capture the large
amount of precipitation during the rain period due to
increased number of vertical levels is obvious. However,
over this area, the too early on set of the rainy season gets
even more pronounced.
We will present results based on some more tests
including the sensitivity of the simulated African Easterly
Jet and precipitation patterns on albedo and SST.
Figure 2. Annual cycle of precipitation over the step
region. The lines represent: RCA3.5 40 levels (solid
red), RCA3.5 24 levels (dashed red), ERA
INTERIM (dashed blue), ERA40 (solid blue), CRU
(green), GPCP (black) and Willmott (cyan).