Water and nitric acid partitioning in cold cirrus clouds observed during CR-AVE 2006: observation-modeling closure studies

Model studies on the partitioning of H2O and HNO3 in a very cold cirrus cloud (T=180-190K) are performed by using a detailed micro-physical box model. The simulations are based on observations during the field campaign CR-AVE (Costa Rica 2006). Model sensitivity simulations are performed by varying different parameters along the trajectories of the air parcels: freezing mechanism, number of heterogeneous ice nuclei, temperature fluctuations, etc. Comparison between the model sensitivity studies and the field observations gives insight into which constellations of parameters are most likely to create the observed cloud. Further, the sensitivity runs accentuate the impact of the different parameters on the temporal evolution of RHice and HNO3 as well as the number and size of the ice crystals inside cold ice clouds.