Understanding subtropical anvil cirrus: A coupled modeling study

This research investigates the sensitivity of anvil layer cirrus's characteristics to its mesoscale environment. A coupled modeling system composed of a mesoscale model and cloud model is used to represent the evolution of systems with different scales. Lagrangian trajectories in the mesoscale model are used to determine the mesoscale environment of the simulated anvil and calculate the mesoscale forcing. A new sedimentation parameterization for the cloud model is developed to better represent fall speeds for large particles. Convection and the resulting outflow cirrus occurring near Ft. Myers, Florida on July 16, 2002 are used for the case study. The mesoscale model produced convection and an ice cloud similar to what was observed that day. The cloud model was used to determine the set of cloud processes in response to the mesoscale forcing that produced more condensate and prolonged cloud lifetime. These simulations show that differential radiative heating and cooling is the key process. The cooling and moistening in response to the mesoscale forcing produces more ice, enhancing cloud-top radiative cooling and cloud-base infrared warming. This generates more buoyancy, strengthening the cloud's updrafts and producing many small crystals to further enhance the cloud-top cooling, until sedimentation removes enough mass to end the positive feedbacks. The presence and magnitude of the mesoscale forcing alters the amount of condensate formed, altering the cloud-top cooling rate and the cloud's response to the forcing. The anvil cloud simulation was relatively insensitive to the initial condensate once the cloud becomes optically thick enough to be considered a black-body. While longwave cloud-top cooling is necessary for the interactions between mesoscale forcing and cloud dynamics, the most turbulent anvil cirrus layers require shortwave in-cloud warming.;In response to recent measurements of the deposition coefficient, alpha d, I develop a parameterization that represents the effects of kinetically-limited growth in a bulk ice microphysics model. Using this, we explore a range of coefficients to determine how this would would affect our set of cloud processes. Values of alphad near those measurements significantly reduce vapor deposition, elevating supersaturation and the nucleation rates. The elevated nucleation rates sequester more condensate in non-precipitating ice and minimize sedimentation velocities. The large ice water contents so produced keep cloud-top infrared cooling rates high. These two effects directly and indirectly prolongs cloud lifetimes. These results are the first demonstration of the connection between alpha v and simulated cirrus dynamics.