Ice Multiplication by Breakup in Ice–Ice Collisions. Part II: Numerical Simulations

Abstract

In Part I of this two-part paper, a formulation was developed to treat fragmentation in ice–ice collisions. In the present Part II, the formulation is implemented in two microphysically advanced cloud models simulating a convective line observed over the U.S. high plains. One model is 2D with a spectral bin microphysics scheme. The other has a hybrid bin–two-moment bulk microphysics scheme in 3D. The case consists of cumulonimbus cells with cold cloud bases (near 0°C) in a dry troposphere. Only with breakup included in the simulation are aircraft observations of particles with maximum dimensions >0.2 mm in the storm adequately predicted by both models. In fact, breakup in ice–ice collisions is by far the most prolific process of ice initiation in the simulated clouds (95%–98% of all nonhomogeneous ice), apart from homogeneous freezing of droplets. Inclusion of breakup in the cloud-resolving model (CRM) simulations increased, by between about one and two orders of magnitude, the average concentration of ice between about 0° and −30°C. Most of the breakup is due to collisions of snow with graupel/hail. It is broadly consistent with the theoretical result in Part I about an explosive tendency for ice multiplication. Breakup in collisions of snow (crystals >~1 mm and aggregates) with denser graupel/hail was the main pathway for collisional breakup and initiated about 60%–90% of all ice particles not from homogeneous freezing, in the simulations by both models. Breakup is predicted to reduce accumulated surface precipitation in the simulated storm by about 20%–40%.

The Role of Ice Splintering on Microphysics of Deep Convective Clouds Forming Under Different Aerosol Conditions: Simulations Using the Model With Spectral Bin Microphysics

“…The goal of the present study is to determine the role of different mechanisms of ice multiplication, as well as primary nucleation in ice particles production in deep mixed‐phase clouds with warm cloud base developing under the thermodynamic conditions close to those in ICE‐T experiment. There are several substantial differences between the studies of Phillips, Yano, Formenton, et al, (, ) and the present study: In contrast to parcel model used by Phillips, Yano, Formenton, et al, (, ), we use the 2‐D mixed‐phase HUCM with spectral bin microphysics, which allows us to take into account many microphysical processes, including entrainment and mixing of clouds with environment, variability of vertical velocities, and other microphysical parameters, as well as precipitation formation. HUCM simulates the entire cloud evolution of a mixed‐phase cloud, which allows us to simulate and analyze all the three ice multiplication mechanisms mentioned above and determine the comparative role of each mechanism at different stages of cloud evolution. The role of aerosol number concentration and shapes of aerosol size distributions on the process of ice multiplication is analyzed. …”

Simulating a Mesoscale Convective System Using WRF With a New Spectral Bin Microphysics: 1: Hail vs Graupel

“…The lack of ice fragmentation by ice‐ice collisions in the current simulations may be another reason for the small ice particles concentration below 6 km in the simulations. The role of this mechanism will be investigated in subsequent studies, where ice multiplication by ice‐ice collisions will be described following Phillips et al (,b). Rain size distributions simulated by FSBM‐2 are in good agreement with observations, especially in H43. Aerosol regeneration by complete droplet evaporation is a significant additional source of atmospheric aerosols in the area surrounding the squall line.…”

Effects of solid aerosols on partially glaciated clouds

“…The aerosol–cloud microphysics scheme ((Phillips et al. ); ; (Kudzotsa, ; Kudzotsa et al. )) is a bin‐emulating bulk scheme with a two‐moment prognosis of sulphate aerosols, cloud ice, and cloud droplets, while a single‐moment approach for rain, snow, and graupel is implemented, in order to reduce computational power and time.…”

“…The microphysics scheme predicts the supersaturation and diffusional growth of all five categories of hydrometeors being treated in the model, using the linearized supersaturation scheme of Phillips et al. (). All the aerosol types included in this model can initiate cloud droplets, while only solid aerosols can nucleate ice crystals.…”

“…The ascent‐dependent fraction of droplets that evaporate without freezing near −36 to −37 °C is resolved using a lookup table created by Phillips et al. (). This preferential evaporation of smaller droplets during homogeneous freezing is triggered by larger droplets that freeze first and cause subsaturation and was first modeled by Phillips et al.…”