The Goddard Cumulus Ensemble (GCE) model with bin spectral microphysics is used to simulate convection systems in different environments. The model uses explicit bins to represent size spectra of cloud nuclei, water drops, ice crystals, snow, and graupel. Each hydrometeorite category is described by 33 mass bins. The simulations provide a unique data set of simulated raindrop size distributions in a realistic dynamical frame. Calculations of radar parameters using the simulated drop size distribution can serve as an evaluation of numerical model performance when model results are compared with observations. In addition, the GCE spectral bin model is a very useful tool in studies of uncertainties related to radar observations because all the environmental parameters are precisely known in model simulations. In this presentation, we concentrate on the discussion of Z-R (ZDR-R) relation in simulated systems.

Due to computational limitations, the spectra bin model has been run in two dimensions, with 33 stretched vertical layers and 1026 horizontal grid points (1km resolution). Two different cases, one in the midlatitude continent, the other in the tropical ocean, have been simulated in this study. The continental case has strong convection which lasted for about two hours. The oceanic case is a persistent system with more than ten hours' life span. It is shown that the simulated Z-R (ZDR-R) relations generally agree with observations using radar and rain gauge data. The spatial and temporal variations of the Z-R relation in different locations are also analyzed.

The impact of aerosols on cloud formation and raindrop size distribution has been studied by simulating both clean (low CCN) and dirty (high CCN) cases. The Z-R relation is shown to vary considerably with the initial CCN concentrations. Further details will be presented at the conference.