Predicting Ice Shape Evolution in a Bulk Microphysics Model

Abstract

A novel bulk microphysics scheme that predicts the evolution of ice properties, including aspect ratio (shape), mass, number, size, and density is described, tested, and demonstrated. The scheme is named the Ice-Spheroids Habit Model with Aspect-Ratio Evolution (ISHMAEL). Ice is modeled as spheroids and is nucleated as one of two species depending on nucleation temperature. Microphysical process rates determine how shape and other ice properties evolve. A third aggregate species is also employed, diversifying ice properties in the model. Tests of ice shape evolution during vapor growth and riming are verified against wind tunnel data, revealing that the model captures habit-dependent riming and its effect on fall speed. Lagrangian parcel studies demonstrate that the bulk model captures ice property evolution during riming and melting compared with a bin model. Finally, the capabilities of ISHMAEL are shown in a 2D kinematic framework with a simple updraft. A direct result of predicting ice shape evolution is that various states of ice from unrimed to lightly rimed to densely rimed can be modeled without converting ice mass between predefined ice categories (e.g., snow and graupel). This leads to a different spatial precipitation distribution compared with the traditional method of separating snow and graupel and converting between the two categories, because ice in ISHMAEL sorts in physical space based on the amount of rime, which controls the thickness and therefore fall speed. Predicting these various states of rimed ice leads to a reduction in vapor growth rate and an increase in riming rate in a simple updraft compared with the traditional approach.

Global Statistics of Ice Microphysical and Optical Properties at Tops of Optically Thick Ice Clouds

“…However, such a conclusion is somewhat surprising as, within updrafts, r e at cloud top is expected to be an integral manifestation of processes at lower levels, as far down as the liquid phase. Aside we note that the slight increase of r e near 265 K over ocean, especially in midlatitude winter and at high latitudes, is consistent with laboratory measurements showing a local increase in growth rate at those temperatures (Fukuta & Takahashi, 1999; Jensen et al, 2017). Over land, r e quite abruptly increases with decreasing temperatures below about 225 K at middle and high latitudes, which is also observed using Atmospheric Infrared Sounder measurements and may be consistent with overshooting convection (Kahn et al, 2018).…”

“…At water saturation, growth rates derived from equations , , and increase with decreasing temperatures between 273 and about 255 K and decrease for lower temperatures (Fukuta & Takahashi, 1999; Jensen et al, 2017; Pruppacher & Klett, 1997). However, supersaturation levels in ice clouds are generally below that corresponding to liquid saturation but increase with decreasing temperatures (Korolev & Isaac, 2006).…”

How Does Riming Affect Dual‐Polarization Radar Observations and Snowflake Shape?

“…Since the ice particle growth rate depends on particle shape, the evolution of particle aspect ratio needs to be taken into account (Jensen & Harrington, 2015). Jensen et al (2017) have modeled the evolution of ice crystal shape during riming and compared it to wind tunnel measurements.…”

A Modeling Study on the Sensitivities of Atmospheric Charge Separation According to the Relative Diffusional Growth Rate Theory to Nonspherical Hydrometeors and Cloud Microphysics

“…As far as uncertainties in hydrometeor shape and density, that is, mass-size relationships, are concerned, new approaches to modeling mixed-phase cloud microphysics that replace the distinction between ice crystals, snowflakes, and graupel particles by an adaptive ice-phase category of process-dependent density (Morrison & Milbrandt, 2015) seem promising. In terms of crystal habit and graupel anisotropy, approaches like Jensen et al (2017), which take specific growth axis into account to describe anisotropic diffusional growth of ice-phase hydrometeors could replace the tuning parameter c a . Last but not least, the numerical simulations in this study are based on assuming water saturation in mixed-phase regions.…”