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Progress report The Met. Office contribution to PROMISE Sep
2000
A pair of AGCM sensitivity
experiments have been designed and run, with
the
aim of assessing the impact
of SST anomalies in the Mediterranean on
seasonal
(July-September) rainfall
totals over the Sahel. They confirm that the
Mediterranean does indeed
have a significant impact on the Sahel, and are
currently being used to
understand the mechanisms involved. (Dave Rowell)
During the past year, a
new boundary layer mixing scheme has been
implemented in The Met.
Office's Unified Model (UM), and the vertical
resolution in the lower
troposphere has been increased nearly threefold.
Recently, a set of major
revisions to UM's convection scheme has been put
together to improve the
interaction between the convection and boundary
layer schemes and to reduce
the sensitivity of the convection scheme to
the increased vertical
resolution.γ The modifications
include improving
the diagnosis, triggering
and treatment of shallow and deep convection
(using information from
the boundary layer scheme), as well as correcting
long-standing errors in
the scheme. One of the most important aspects of
this revised version is
that convection is forced to trigger from the
lifting condensation level
when convection is diagnosed.
These revisions have now
been tested in the climate and global forecast
models. The distribution
of shallow and deep convection appears to be
realistic, and there are
some improvements in the monsoon circulation and
the precipitation distribution,
although there are problems with
excessive low level layer
cloud forming in the shallow convective layer
and an overall spin-upγ
of the model's Hadley circulation. A long (50+
years) run of the climate
model including these revisions will be started
shortly. This will be compared
with parallel runs of the previous model
version in order to assess
the impact of these changes on the mean and
interannual variability
of the summer monsoons. (Gill Martin)
Work has proceeded on finalising
the configuration of the Met.
Office coupled and uncoupled
GCMs to be run to seasonal range under the
EU DEMETER project. Installation
of the models on the ECMWF Fujitsu is
under way with testing
expected to start, as scheduled, in September.
In liaison with PROMISE
and DEMETER colleagues, a list of the required
model output diagnostics
has been drawn up.
A schedule of the coupled
and uncoupled experiments planned by
the Met. Office, and the
output that will be available for the PROMISE
archive, has been provided
for the PROMISE website. (Richard Graham)
Code was written to enable
seasonal variation of vegetation parameters
to be prescibed in the
new MOSES2 land surface scheme of the UM.
Analysis was carried out
on the dieback of the Amazon rainforest in
coupled climate-vegetation
simulations with the Hadley Centre GCM, and
preliminary results presented
at the EGS meeting in Nice, France
(25th-28th April 2000).γ
A literature review of Amazonian climate change
in other GCMs was made,
and it was found that several GCMs also produce
future drying of Amazonia.γ
In these models, Amazon drying appears to be
associated with El Nino-like
patterns of SST change in the Pacific under
a warming climate.γ (Richard Betts)γγγγγγγγγ