EUCLIPSE First Period Report

Figure 18 : time series of
incoming short-wave flux at the
surface in 2006 at selected point
along the West African
climatological gradient, from a
wet Soudanian site (Djougou,
bottom) to the Southern
(Niamey, middle) and Northern
Sahel (Bamba, top). A 15-day
running mean has been applied.
Note that the cloud radiative
impact increases and extends in
time from the northern to
southern locations and is the
largest during the monsoon
period, from June to September.
Figure 18. shows a comparison of the annual cycle of the incoming solar radiation in models
and observations at selected locations. It illustrates the structure of the observed meridional
gradient of the cloud radiative impact (reaching more than 50 W/m2 within 7° of latitude
during the monsoon). This feature is at best qualitatively captured by NWPs, indeed, the
gradient is largely underestimated by the ECMWF models and much too strong in ARPEGE.
First examination of ARPEGE results show that these biases involve a too approximate
modelling of different cloud types, and notably mid-level clouds in the Sahel. These
clouds develop within an atmosphere characterized by a relatively weak stability up to
4 km (above the top of the boundary layer) – Fig.19 (left). The significance of such a
thermodynamic-cloud coupling was pointed out by Johnson (JAS 1999) albeit in the
very distinct moister climate of the Pacific warm pool and involved congestus clouds.
As illustrated in Fig. 19 (left), the model reproduces reasonably well the low-level
atmospheric structures, including the diurnal cycle of the boundary layer which is only
somewhat too cold. However, it fails to capture the weak stability layer above.
Similarly, it is able to capture the seasonal changes of the cloud cover vertical
structure, with an increase of the low cloud cover during the monsoon (Fig. 19 right).
Simulated mid-level clouds remains too few though.
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