EUCLIPSE First Period Report

perturbed future climate conditions are enforced by increasing the SST by 2K and by
weakening the subsidence.
Figure 6. Schematic of the stratocumulus to cumulus transitions as observed
during the First Lagrangian of ASTEX. Five aircraft flights were performed
between 12-14 June 1992.
a. Set up of the ASTEX transition case
For a model intercomparison case the time-dependent large-scale divergence rate, the
sea surface temperature (SST), and the geostrophic wind components need to be
prescribed. Because a Lagrangian trajectory is modelled the horizontal advection
terms can be neglected.
The SST is taken from Bretherton et al. (1999), who provide a best estimate of the
time-varying SST along the ASTEX First Lagrangian trajectory on the basis of ECWMF
SST fields that were obtained with a data assimilation procedure, direct
measurements from the ship R/V Oceanus and radiometric observations from aircraft.
In the subtropical parts over the oceans, the large-scale divergence of the horizontal
winds is characterized by positive values in the lower part of the atmosphere, which is
associated with a large-scale descending motion, which is often referred to as largescale
subsidence. For the ASTEX area, there is a considerable uncertainty not only in
the magnitude but also in the sign of the large-scale divergence. Bretherton et al.
analysed ERA-40 data and found that the large-scale divergence in the column
changed sign after approximately 15 hours (see Figure 7). By contrast, Ciesielski et
al. (1999) calculated the large-scale divergence directly from radiosonde observations
and found no change in the sign. Sigg and Svensson (2004) applied a spectral
23