Observations and Numerical Simulations of Precipitation Development in Seeded Clouds over the Sierra Nevada

Abstract

The evolution of precipitation in seeded wintertime orographically induced convective and stratiform clouds with embedded convection were studied using in situ observations and particle growth and trajectory models. The particle growth model of Heymsfield embedded in a kinematic flow field representative of the Sierra barrier was used to study the ice particle growth by diffusion, accretion and subsequent fall trajectories. The particles observed by the aircraft were classified into habits. The growth of observed particles were compared with the model predicted evolution. Using the aggregation model of Heymsfield, the observation of formation of aggregates in <10 minutes was verified. The key findings of this study were.. (i) Aggregates (>1 mm) form in 4?8 minutes after seeding a convective cloud. (ii) Riming is important close to the barrier in a stratiform cloud when large cloud droplets and liquid water up to 0.3 g m?3 are present. (iii) Diffusional growth is extremely important for temperatures near ?15°C in these low liquid water content clouds. The particles grow to ?2 mm when released from just colder than ?15°C, and to ?1 mm when falling from warmer than ?15°C.