Explicit forecasting of deep convection with the operational
convection-resolving model COSMO-DE

A. Seifert and M. Baldauf

Deutscher Wetterdienst, Kaiserleistr. 42, 63067 Offenbach, Germany

Since April 2007 the operational NWP system of the Deutscher Wetterdienst
(DWD) has been extended to the convection-resolving scale by the introduction
of the 2.8 km grid-spacing limited-area model COSMO-DE (formerly known as the
LMK project). The new high-resolution model COSMO-DE aims towards the explicit
forecasting of severe weather phenomena on the meso-gamma scale, for example
severe weather events related to deep moist convection or interactions of the
flow with topography like strong orographic precipitation or downslope
windstorms.

We will present several case studies of severe deep convection associated with
strong synoptic-scale forcing where COSMO-DE was able to successfully forecast
the development of supercells, e.g. on 13th May 2007 when two tornados were
observed in the area of Aachen. To detect supercells in the model forecasts
the supercell detection index (SDI) of Wicker et al. (2005) has been
implemented in the COSMO model as a new diagnostic output variable. The SDI
uses a correlation of vertical vorticity and updraft velocity within a control
volume as a measure of 'supercellness' of the simulated convective
storms. Using this model diagnostic it is shown that COSMO-DE is able to
simulate, and maybe even predict, long-lived severe supercell storms.

In the second part of the presentation an evaluation of the diurnal cycle of
deep convection in COSMO-DE forecasts will be discussed and it will be shown
that the currently operational version of COSMO-DE has some deficiencies in
forecasting small scale convection associated with the diurnal mode. These
problems are most likely due to problems in the COSMO boundary layer scheme
and some sensitivity studies are shown which support this hypotheses.