Discerning Environmental Dependencies of Mixed-Phase Cloud Lifetime with a Focus on Ice Particle Habit Evolution
Open Access
- Author:
- Sulia, Kara Jo
- Graduate Program:
- Meteorology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- March 14, 2013
- Committee Members:
- Jerry Y Harrington, Dissertation Advisor/Co-Advisor
Johannes Verlinde, Committee Chair/Co-Chair
Eugene Edmund Clothiaux, Committee Member
Miriam Arak Freedman, Special Member
Dr Hugh Morrison, Special Member - Keywords:
- ice growth
glaciation
mixed-phase clouds
Arctic clouds
vapor diffusion
aspect ratio - Abstract:
- The influences of evolving ice habit on the maintenance and glaciation of layered mixed-phase clouds are examined theoretically. Unlike most current modeling methods where a single axis length is predicted, the primary habits, or two axis lengths, are computed explicitly. The method produces a positive non-linear feedback between mass growth and crystal aspect ratio evolution and is shown to have a distinct initial-size dependence. This feedback cannot be captured with simpler growth methods, leading to underestimates of ice growth and mixed-phase glaciation. Assuming spherical particles or mass-dimensional relationships derived from in-situ data cannot mechanistically evolve particle shape result in large variations in phase, leading to inaccuracies in estimates of mixed-phase lifetime. Aspect ratio prediction is most critical for mixed-phase maintenance at habit-prone temperatures (dendritic growth, T=-15 ºC, and needle growth, -6 ºC) and at ice concentrations >1 L^-1. At these temperatures and concentrations, rates of glaciation can be under-predicted by as much as an order of magnitude by equivalent density spheres. Lagrangian bin parcel studies show that habit prediction is less important for the maintenance of liquid at lower ice concentrations (< 1 L^-1) as the time-scale for liquid depletion is relatively long (days). Updraft strength also affects mixed-phase cloud maintenance primarily at ice concentrations between 1 L^-1 and 100 L^-1. It is theoretically possible for a multitude of vertical motions to maintain stratiform mixed-phase clouds indefinitely when temperatures are relatively high (>-10 ºC) and ice concentrations are relatively low (<1 L^-1). A bulk version of the habit evolution method called the adaptive habit model is implemented into a 2D kinematic model and the Large-Eddy Simulation version of WRF, allowing for sedimentation and separate advection of axis length mixing ratios for both dynamically-independent and dynamically-coupled simulations, respectively. Kinematic simulations with sedimentation increase cloud lifetime at higher ice concentrations, but can also lead to lower liquid amounts. Radiative cooling initially increases ice growth with a subsequent enhanced sedimentation flux, altering cloud-phase partitioning dependent on ice concentration. Surface latent and sensible heat fluxes of 50 W m^-2 result in an increase in overall water mass, while compensating fluxes that counteract radiative cooling and sedimentation establish sufficient energy and mass amounts for liquid and ice maintenance. Studies of habit evolution are completed using the LES version of WRF simulate Flight 31 of the ISDAC field campaign, in which a single mixed-phase layer was decoupled from the ice-covered ocean surface. Simulations with spherical ice, which is traditionally assumed in many models, confirm the dependency of liquid water depletion through increased ice concentrations. Similar to prior studies, simulations with spherical ice produce relatively stable mixed-phase clouds consisting of supercooled liquid layers that precipitate ice. In contrast to these studies, habit evolution produces stronger ice growth and slower sedimentation rates, leading to more rapid glaciation at higher concentrations. Additionally, aspect ratio evolution and subsequent ice sedimentation promote a net depositional warming that cannot be offset by cloud-top radiative cooling due to continually decreasing liquid mass for ice concentrations >1 L^-1. Further studies indicate that colloidally unstable clouds, regardless of habit, cannot be self-maintaining without an external source of moisture.