Title: Ice Parcel Versus Layer Glaciation in Shallow Mixed-Phase Clouds
Speaker: Dr. Ann Fridlind, NASA Goddard Institute for Space Studies (GISS)
Date: Wednesday, November 1, 2017
Time: 1:30-2:30 p.m.
Location: NASA/LaRC, Bldg 1250, Room 116
Abstract: Shallow mixed-phase boundary layer clouds can be considered as cold versions of their warm counterparts, wherein boundary layer and mesoscale structure is determined primarily by the role of liquid-phase processes. Based on conditions observed over the Arctic and Southern Ocean, we propose that it is useful to consider a continuum from well-mixed to weakly coupled to cumuliform conditions. In the well-mixed limit, which generally corresponds to shallower and non-precipitating conditions, large-eddy simulation studies show that layer glaciation occurs without significant horizontal variability. Under such well-mixed conditions, supercooled cloud-top conditions are highly supersaturated with respect to ice, horizontal mean ice number concentrations are found to be nearly vertical uniform, and the liquid phase in individual cloud parcels is determined by small deviations from the relative humidity with respect to liquid rather than variability in ice loading. Cloud desiccation by ice is therefore a layer glaciation process rather than a parcel glaciation process. When a cloud-topped boundary layer is too deep to remain well-mixed, for instance, or with onset of drizzle, cloud layer coupling to the sub-cloud layer and surface may weaken. Under such conditions, large-eddy simulations indicate that warm cloud mesoscale structures tend to increase in horizontal dimension, consistent with increasing total water variance within the weakly coupled cloud layer. Under cold air outbreak conditions, cumuliform conditions may correspond to an absence of continuous turbulence in the upper cloud-containing elevations. Along the weakly coupled to cumuliform continuum, observations indicate that both liquid and ice in the cloud layer become increasingly patchy, with ice commonly formed where liquid is also present because freezing occurs through a liquid-phase heterogeneous nucleation process. Under such conditions cloud parcel glaciation could become a pertinent concept if ice production is extremely efficient. Recent field measurements suggest that updraft parcel glaciation is surprisingly efficient in tropical deep convection, likely owing to unestablished ice multiplication mechanisms. There is evidence that multiplication mechanisms do operate in well-mixed stratiform clouds when liquid-phase precipitation is present (under layer glaciation conditions), but their typical strength in mixed-phase cumuliform clouds remains to be established. Within the context of well-mixed to cumuliform conditions, which are commonly represented by separate stratiform and convective cloud schemes in climate models, we discuss the relevance of parcel versus layer glaciation to parameterization components.
Note: This invited seminar is partly sponsored by the NIA Internal Research and Development (IRAD) program. Those who would like to meet with Dr. Fridlind individually during her visit (Nov. 1 – noon Nov. 2) may contact Hongyu Liu (1250/209, email@example.com) or Patrick Taylor (1250/179, firstname.lastname@example.org).