Numerical simulation of the microphysics of an orographic cloud: Comparison with measurements and sensitivity studies

The formation and evolution of orographic clouds are modeled using a parcel model with sectional microphysics based on the Linear Discrete Method and a size-dependent representation of the soluble particle fraction. The model results are compared to observations from three periods of the field experimental campaigns FEBUKO 2001 and 2002 covering about 150 single cases. Processing of aerosol is sensitive to cloud droplet number and size. Therefore, droplet nucleation is emphasized. Sensitivity studies concerning the soluble particle fraction ε, the water accommodation coefficient αC, and model dynamics were carried out. The size-dependent representation of ε turned out to be very important for a correct nucleation description whereas a shift of the soluble fraction by ±0.1 induces much smaller effects. Decreasing αC and increasing vertical velocity both lead to enhanced droplet formation due to higher supersaturations reached. This effect often occurred for the same parameter configuration. Entrainment was shown to be important to reach better agreement between the calculated and the observed data, reducing the liquid water contents below the respective adiabatic values and leading to a broadening of drop size distributions including an increase of small droplets.