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William G. Finnegan
,
Steven K. Chai
, and
Andrew Detwiler

Abstract

Geometrically oriented riming was found in Formvar resin replicas of columnar ice crystals collected in cumulus clouds at −6°C during an aircraft field program in Texas. Rimed cloud droplets were found either on the ends of the crystals or in a girdle around the middle. Oriented riming is attributed to preferential collection on growing ice crystals with charge separations between the crystal body and growing ends. Droplet attraction to separated charge regions of growing ice crystals results in enhanced riming and increases the rate of precipitation development. Effects of this process on cloud electrification depend on whether the cloud droplets carry net charges or are polarized. The impact of this oriented riming process on several cloud electrification scenarios is discussed.

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Vaughan T. J. Phillips
,
Jun-Ichi Yano
,
Marco Formenton
,
Eyal Ilotoviz
,
Vijay Kanawade
,
Innocent Kudzotsa
,
Jiming Sun
,
Aaron Bansemer
,
Andrew G. Detwiler
,
Alexander Khain
, and
Sarah A. Tessendorf

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%.

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Vaughan T. J. Phillips
,
Marco Formenton
,
Vijay P. Kanawade
,
Linus R. Karlsson
,
Sachin Patade
,
Jiming Sun
,
Christelle Barthe
,
Jean-Pierre Pinty
,
Andrew G. Detwiler
,
Weitao Lyu
, and
Sarah A. Tessendorf

Abstract

In this two-part paper, influences from environmental factors on lightning in a convective storm are assessed with a model. In Part I, an electrical component is described and applied in the Aerosol–Cloud model (AC). AC treats many types of secondary (e.g., breakup in ice–ice collisions, raindrop-freezing fragmentation, rime splintering) and primary (heterogeneous, homogeneous freezing) ice initiation. AC represents lightning flashes with a statistical treatment of branching from a fractal law constrained by video imagery.

The storm simulated is from the Severe Thunderstorm Electrification and Precipitation Study (STEPS; 19/20 June 2000). The simulation was validated microphysically [e.g., ice/droplet concentrations and mean sizes, liquid water content (LWC), reflectivity, surface precipitation] and dynamically (e.g., ascent) in our 2017 paper. Predicted ice concentrations (~10 L−1) agreed—to within a factor of about 2—with aircraft data at flight levels (−10° to −15°C). Here, electrical statistics of the same simulation are compared with observations. Flash rates (to within a factor of 2), triggering altitudes and polarity of flashes, and electric fields, all agree with the coincident STEPS observations.

The “normal” tripole of charge structure observed during an electrical balloon sounding is reproduced by AC. It is related to reversal of polarity of noninductive charging in ice–ice collisions seen in laboratory experiments when temperature or LWC are varied. Positively charged graupel and negatively charged snow at most midlevels, charged away from the fastest updrafts, is predicted to cause the normal tripole. Total charge separated in the simulated storm is dominated by collisions involving secondary ice from fragmentation in graupel–snow collisions.

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