Modeling of daytime convective development over land with new prototype model COSMO-EULAG

Anelastic model EULAG [Prusa et al., Computers & Fluids, Vol. 37, 2008] has been recently implemented as a new dynamical core of the COSMO weather prediction framework. EULAG features a high numerical robustness confirmed in number of benchmark tests and is widely documented in scientific literature. Its dynamical core is based on fully 3D, i.e. non-direction splitted, conservative advection scheme MPDATA and advanced elliptic solver GCR. EULAG has considerable advantages concerning conservation properties and does not impose severe constraints on the maximal allowable steepness of the surface orography.

The new prototype model COSMO-EULAG has been successfully tested in number of idealized dry experiments including stationary flow over mountains, expansion of linear gravity waves, strongly nonlinear and non-stationary falling bubbles. The current efforts are focused on accurate modeling of convective processes such as cloud and precipitation formation and further optimization of the prototype model. In particular, our goal is to calibrate coupling between the dynamical core and COSMO physical parameterizations to make the code suitable for resolving explicitly convective processes. The optimizations involve tuning of turbulence scheme, shallow convection parameterization, moist processes and other cloud processes. The efforts made allow more deeply explore the numerical features of the COSMO-EULAG.

Setup of the numerical experiments is similar to study by Grabowski at al. [Q.J.R. Meteorol. Soc., Vol. 132, 2006] and is based on observations of the diurnal cycle and convection during rainy season in Amazonia. The focus is on the 6 h period between sunrise and early afternoon. This period is critical for the diurnal cycle over summertime continents in numerical weather prediction models.

A number of simulations have been performed for different parameterization of moist processes for different resolutions of computational grids (up to 100m horizontally) and different subgrid scale parameterizations. The simulations evaluate model's representation of daytime convective development over land that features convection initiation in the morning hours followed by the transition from shallow to deep convection. The study also involves the comparison of COSMO-EULAG results with results of standard compressible COSMO-Runge-Kutta model to test the suitability of an anelastic dynamical core for operational mesoscale high-resolution NWP.