Influence of ambient environmental conditions and orography on the characteristics of deep convective cells as simulated with a sophisticated two-moment (bulk) microphysical scheme

The influence of orography and ambient environmental conditions (aerosol conditions, profile of temperature, humidity and wind) on the initiation, life cycle and precipitation efficiency of convective cells is a current topic in meteorology as well as in numerical weather prediction.

In order to elaborate parameters crucially affecting the development of convective cells high resolution cloud resolving 3D simulations (dx <=1 km) with a test version of the operational nonhydrostatic Lokalmodell (LM) of the German Weather Service are performed. In contrast to other studies, we use a sophisticated cloud microphysics parameterization, the two-moment bulk microphysical scheme by Seifert and Beheng (2006). In this way, the complex microphysical/(thermo)dynamical feedback processes in clouds are quite accurately described while keeping the numerical costs affordable for full 3D simulations. The scheme distinguishes six hydrometeor categories (cloud drops, cloud ice, rain, snow and two graupel classes) and represents each particle type by its respective number and mass density. It also allows the initial cloud droplet size distribution (determined by two moments) to represent either continental or maritime aerosol conditions. Note that the second graupel class, having comparatively high bulk density and fall velocity, was recently added to the scheme (see separate abstract by Noppel et al.).

In detail, idealized high resolution cloud resolving simulations are performed considering simplified orography (e.g., single bell-shaped mountain ridge). As influencing parameters, temperature- and humidity profiles, condensation- and 0-degree-C-level and maritime/continental conditions are varied as well as mountain width and height to investigate the combined effects of different (thermo)dynamic conditions and orographic flow modification on single convective systems. The ultimate goal is to find parameters allowing to discriminate different convective regimes, useful for convection parameterizations and for nowcasting purposes.

Some results are presented, which show, for example, a more intense development of convective systems if the 0-degree-C-level is low, but with less intense precipitation rate, compared to a higher 0-degree-C-level (at otherwise same conditions). This is interpreted in terms of cloud microphysical-dynamical feedback processes. Further, an idealized case study is shown where the modification of the temperature- and velocity fields by an idealized 2D-mountain ridge, oriented perpendicular to the flow, causes an existing convective system to dissolve after crossing the mountain, whereas - without the mountain ridge - the system develops into an intense squall line.

A second advantage of using the two-moment scheme is that it allows for a reliable calculation of radar reflectivities so that simulation results can be compared to radar measurements. The computed radar reflectivities rely on application of full Mie-scattering (single- or double-layered spheres) or Rayleigh approximation as well as on consideration of state-of-the-art effective medium theories for the particle's refractive index. An example of a comparison of model derived and measured reflectivity for a single convective system will be presented.