EFFECTS OF ICE NUCLEI CONCENTRATIONS, ICE NUCLEATION MECHANISMS AND ICE CRYSTAL HABITS ON THE DYNAMICS AND MICROPHYSICS OF ARCTIC MIXED-PHASE CLOUDS

Open Access
Author:
Komurcu, Muge
Graduate Program:
Meteorology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 16, 2011
Committee Members:
  • Jerry Y Harrington, Dissertation Advisor
  • Jerry Y Harrington, Committee Chair
  • Johannes Verlinde, Committee Member
  • Marcelo Chamecki, Committee Member
  • Andrew Mark Carleton, Committee Member
Keywords:
  • ice nuclei concentrations
  • ice crystal habits
  • ice nucleation
  • cloud dynamics
  • cloud microphysics
  • Arctic mixed-phase clouds
Abstract:
There is a significant warming in the Arctic that is evident in both observations and in the future climate predictions. The Arctic warming is greater than any other region on Earth, however, the degree of warming is inconsistent among the climate models even for the same emission scenarios. Clouds, especially low-level clouds, are a prevailing feature of the Arctic atmosphere. They strongly affect the surface radiative and energy budgets, which make them a key component of the Arctic climate. Recent inter-comparison studies using regional climate models show that models are incapable of reproducing the supercooled liquid water observed in clouds during the cold season. Large discrepancies exist in the partitioning of phase between ice and liquid water among different models. It is currently thought that these discrepancies are due to the uncertainties in ice nuclei concentrations, ice nucleation, and ice crystal habits used in models. Predicting these physical processes controls the partitioning between liquid and ice, and hence the impact of mixed-phase clouds on the surface energy budget. There is a need to improve model cloud predictions in the Arctic, however, the microphysical uncertainties mentioned above are tied directly to the cloud dynamics that help maintain persistent mixed-phase clouds. Therefore, this dissertation analyzes and inter-compares the impacts of different ice nuclei concentrations, ice nucleation mechanisms and ice crystal habits on mixed-phase cloud dynamics. Separate simulations using different ice nuclei concentrations, ice nucleation mechanisms, and crystal habits are performed. It is found that the choice of habits in models alters the water paths and cloud dynamics strongly. Next, the relative importance of and interactions among the processes that influence the dynamics of the cloud, such as the radiative cooling at cloud top, and the ice precipitation induced cloud-base stabilization are investigated. To examine these processes in detail, sensitivity studies are performed by fixing the radiative cooling, and the diabatic influences of ice precipitation. In addition, simulations with increasing ice nuclei concentrations, different nucleation mechanisms, and crystal habits are repeated with surface fluxes and large-scale forcing included. The influence of surface fluxes is important as it can compensate for the water mass that is lost through ice precipitation if the ice precipitation is weak. Surface fluxes can also lead to the coupling of the liquid cloud layer with the sub-cloud layer. The cloud-base stability is diminished with the inclusion of the surface fluxes, and the effect of entrainment is enhanced. Sensitivity tests are also repeated with the added surface fluxes. Using the results of the sensitivity analysis, a ratio identifying the decoupling of the cloud and subcloud layers is generated, and also with the sensitivity analysis cloud dynamic and microphysical interactions within Arctic mixed-phase clouds are explained.