Title and abstract:

Member selection for a convective scale ensemble at the UK Met Office

The UK Met Office is developing an ensemble at the convective scale by
embedding the new 1.5km model over the UK within selected members of the
regional scale ensemble MOGREPS-R (~18km covering the North Atlantic and
Western Europe). The objective is to improve our forecasting capability of
high-impact local weather with a 12 to 36 hours lead time.

This seminar will describe the potential benefits of a convective-scale
ensemble and similar systems under development at other institutions.
However, the main focus will be the discussion of some of the difficulties
that need to be overcome. One issue we face is that computational constrains
will limit the number of convective scale members we can run in an
operational context to fewer than six, whilst MOGREPS-R provides 24 possible
initial and lateral boundary conditions.
This means there is a need to select a subset of members from the MOGREPS-R
ensemble that best represents the full 24 members at the scales of interest.

Results from case studies of convective events, some of which
flood-producing storms will be used to demonstrate the potential value of a
convective-scale ensemble. The MOGREPS-R regional-scale members provide
different  environments for the storms to form and develop and their
differences have been investigated at different scales using the Fractions
Skill Score of Roberts and Lean (2008) and in a more object-based way using
the SAL method of Wernli et al (2008). Both have been applied to a variety
of dynamical/thermodynamical variables such as potential vorticity and
wet-bulb potential temperature.
The differences can then be linked to the formation, development and
intensity of the flood producing  storms in the equivalent high resolution
(1.5km) precipitation forecasts at the lead time of choice and this
information used to inform the approach needed to choose appropriate
MOGREPS-R members in which to run the 1.5km forecasts.