Proceedings - C-SRNWP Project

The Convective-Scale UM: Current Status, Verification
and Physics Developments
Peter Clark, Richard Forbes, Humphrey Lean, Nigel Roberts,
Sue Ballard, Mark Dixon, Zhihong Li,
Rachel Capon, Carol Halliwell, Andrew Macallan, Yongming Tang
Met Office, Joint Centre for Mesoscale Meteorology, UK
1. Current Status
High resolution NWP is an approach which has potential to improve our ability to forecast
deep convection, though precisely how high the resolution needs to be is an open question.
Experience suggests that an important step is allowing explicit convection to form in flows
which adequately resolve the mesoscale features which lead to triggering and organisation of
storms, rather than fully resolving the detail of convective cells. It is anticipated that high
resolution will also address other high-impact weather forecasts, such as orographically
forced strong winds, fog and low cloud, urban and other surface induced variations in
temperature and wind.
At the Met Office, we have developed the Unified Model (UM) for use at convective-scale
resolutions, focussing on configurations with grid resolutions of around 1 km. As an
intermediate step, work on a 4km grid resolution configuration has shown the benefit of
resolving convective storms (at least partially) compared to purely parametrizing the effects
of convection at lower resolutions, and a 4km UM has been operational over the UK since
2005. A 1.5km grid resolution configuration of the UM will be operational for a choice of
limited area domains over part of the UK (450km x450km) “on-demand” from January 2007
(i.e. the forecaster decides whether or not to run the model for a particular domain dependent
on the severity of the weather conditions). A 1.5km model for the whole of the UK is planned
to be operational in 2009, after the next major supercomputer upgrade at the Met Office. In
the meantime, research is ongoing to increase our understanding of processes at the
convective-scale and improve the formulation of the model and data assimilation system.
Figure 1 shows the standard domains that have been used for much of the testing and
assessment of the high resolution models. The focus on the southern UK is primarily due to
the availability of observational data from the Convective Storms Initiation Project (CSIP), an
observational campaign over the southern UK in the summer of 2005, providing a wealth of
data on convective storms and their initiation. The initiation of convection is particularly
important in our models as different initiation mechanisms, such as surface forced sea-breeze
convergence or mid-level initiation by gravity waves, may have very different inherent
predictability, which is important to understand when designing and assessing the model. If
the model does not correctly initiate convection, then subsequent evolution will be in error
and, at best, data assimilation systems will have to address correction of the model error.
Recent evaluation of the high resolution modelling system (see section 2) has concentrated on
CSIP cases due to the fact they are well observed and represent most of the 2005 summer
convection. A brief (subjective) assessment of performance for the CSIP cases is described in
Clark and Lean (2006). As an example, Figure 2 shows a 1.5 km resolution UM forecast of a
case study of deep convective storms breaking out in a westerly over most of the UK on
25/08/2006 (CSIP IOP 18). The comparison with the satellite image and radar-derived rainfall
show a high degree of consistency with the different meteorological regimes being captured
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