Abstract:
Cloud Resolving Models (CRMs) which are used increasingly
to make operational forecasts, employ Bulk Microphysics
Schemes (BMSs) to describe cloud microphysical processes. In this
study two BMSs are employed in a new Nonhydrostatic σ-coordinate
Model to perform two hour simulations of convection initiated by a
warm bubble, using a horizontal grid resolution of 500 m. Different
configurations of the two BMSs are applied, to test the effects of the
presence of graupel with one scheme (2-configurations) and of
changing the cloud droplet sizes in the second scheme (4-configurations),
on the simulation of idealised thunderstorms. Maximum updrafts
in all the simulations are similar over the first 40 minutes, but
start to differ beyond this point. The first scheme simulates the
development of a second convective cell that is triggered by the cold
pool that develops from the outflow of the first storm. The cold pool
is more intense in the simulation with graupel because of melting of
graupel particles, which results in relatively large raindrops, decreases
the temperature through latent heat absorption, causing stronger
downdrafts, which all contribute to the formation of a more intense
cold pool. The second scheme simulates the development of a second
cell in two of its configurations, while two other configurations do
not simulate the redevelopment. Two configurations that simulate the
secondary redevelopment produce a slightly stronger cold pool just
before redevelopment. Our results show that small differences in the
microphysics formulations result in simulations of storm dynamics
that diverge, possibly due nonlinearities in the model.
