Goals and Hypotheses

Goals and Hypotheses#

Goals#

The main focus of this COMBLE MIP is to evaluate the capability of LES and SCM simulations to reproduce the Lagrangian evolution (~1000 km distance) of Arctic convective mixed-phase cloud features under strong CAO conditions. We seek to understand the fundamental convective boundary layer and mixed-phase cloud properties, including the spread between state-of-the-art models, as well as explore which factors control mesoscale cloud organization and cloud physical properties in simulations under the observed conditions. Ground-based measurements taken during COMBLE, in addition to satellite observations, will be combined with various data analysis techniques to tackle the aforementioned goals.

Questions & Hypotheses#

  1. The initial transition from roll-like/stratiform to closed cellular structures is governed by dynamical processes (i.e., the relative importance of baroclinicity/vertical shear versus surface heat fluxes). What is the modeled scaling (i.e., zi/L where zi is the boundary layer depth and L is the Obukhov length) during this transition?

  2. Large, open cellular structures develop only once robust cold pools are generated from sufficiently negatively-buoyant air. Do precipitation rates increase sharply before or after open cells develop? Is rain production important for the development of open cells, as in previous studies of mixed-phase CAO clouds (e.g., ACTIVATE), or is snow/graupel production more important during COMBLE?

  3. Open cellular convection in CAOs undergoes a lifecycle which in part is affected by cloud (and aerosol) processes. Convective updrafts producing substantial liquid water develop as a result of the collapse of outflow boundaries driven by dissipating cells. The effectiveness by which cloud liquid water is converted to ice determines the rate of precipitation generation, cloud dissipation, and cold pool formation. This effectiveness in turn is affected by cloud top temperature (and aerosol). The higher rate of conversion results in proportionally more cloud free regions and an overall reduced albedo.

  4. Vapor growth of frozen hydrometeors is more important than accretional growth in roll-like/stratiform clouds near the ice edge when the liquid cloud layer is relatively shallow and vertical motions are relatively weak compared to cellular clouds near Andenes, when the liquid cloud is locally relatively deep and updrafts are intense. At which point along the trajectory does accretional growth become more important than vapor growth, if at all?

  5. CAOs have the potential to sustain enhanced microphysical processes, such as riming and precipitation formation, leading to strong cloud-aerosol-precipitation interaction. Can the use of a prognostic aerosol treatment and also a prognostic primary ice formation improve simulated cloud macrophysical properties and their changes in response to mixed-phase microphysical processes?

  6. For such cold CAOs, ice formation mechanisms will include primary ice formation, likely secondary ice formation, and possibly homogeneous freezing at cloud top. At which point along the trajectory do secondary ice processes or homogeneous freezing become more important than the others, if at all?

Coming soon: specific analyis approaches to address these questions & hypotheses.