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Andrew G. Detwiler
and
Bernard Vonnegut

Abstract

The ice saturation ratio at which 1% of aged silver iodide and lead iodide aerosol particles nucleate ice from moist air is observed to depend on temperature. Between roughly −30 and −67°C the threshold for both aerosol types rises slowly with decreasing temperature in agreement with a simple classical nucleation theory. Between −6 and −30°C the threshold for the silver iodide aerosol rises more rapidly than predicted by simple theory while the threshold for lead iodide decreases.

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Patrick C. Kennedy
and
Andrew G. Detwiler

Abstract

An armored T-28 research aircraft made direct observations of the hydrometeors present at approximately the −3°C temperature level in the inflow region of a multicell thunderstorm. During the penetration, both the Colorado State University (CSU)–University of Chicago and Illinois State Water Survey (CHILL) 11-cm-wavelength dual-polarization research radar and the Denver, Colorado, Front Range Airport (KFTG) Weather Surveillance Radar-1988 Doppler (WSR-88D) were scanning this storm. Polarimetric radar indications of hail (high reflectivity and low differential reflectivity) appeared near the surface in the echo core adjacent to the aircraft track approximately 6 min after the T-28's inflow transit. Radial velocity data from the KFTG radar were combined with those recorded at CSU–CHILL to synthesize the airflow fields in the storm around the time of the T-28 penetration. Hail trajectories were initiated from a location at which the T-28 encountered a burst of approximately 1-cm-diameter, low-density graupel particles within the general storm inflow region. Forward-time trajectory calculations indicated that these graupel particles subsequently grew slightly into small hailstones and ended up within a few kilometers of the near-surface polarimetric radar hail-signature location. Trajectories computed backward in time imply that these hail embryos originated aloft in the forward portion of the echo complex. These are the first quantitative, direct in situ observations of recirculating precipitation becoming embryos for hail development.

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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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Rand E. Feind
,
Andrew G. Detwiler
, and
Paul L. Smith

Abstract

Comparisons are made between liquid water concentration (LWC) readings obtained from a Johnson–Williams (J–W) cloud water meter and a King (Commonwealth Scientific and Industrial Research Organisation) liquid water probe, both mounted on the armored T-28 research aircraft during penetrations of springtime convective storms in Oklahoma and Colorado. The King probe readings are almost always higher, being up to twice those of the J–W instrument in clouds with narrower cloud droplet spectra. In clouds with broader droplet spectra, the ratio often climbs to three or greater. The King probe responds partially to drops larger than cloud droplet size, and also to some ice particles, so its reading can be higher than the cloud LWC present. However, this and earlier comparisons by others indicate that the primary reason for this discrepancy is that the J–W probe often underestimates the cloud LWC due to incomplete response to larger cloud droplets. Thus, published studies involving cloud LWC in convective storms based on readings of the T-28 J–W probe have often overestimated the effects of entrainment and precipitation scavenging on depletion of updraft liquid water, particularly in those areas characterized by clouds with broad droplet size spectra.

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Paul R. Field
,
Andrew J. Heymsfield
,
Andrew G. Detwiler
, and
Jonathan M. Wilkinson

Abstract

Hail and graupel are linked to lightning production and are important components of cloud evolution. Hail can also cause significant damage when it precipitates to the surface. The accurate prediction of the amount and location of hail and graupel and the effects on the other hydrometeor species depends upon the size distribution assumed. Here, we use ~310 km of in situ observations from flights of the South Dakota School of Mines and Technology T-28 storm-penetrating aircraft to constrain the representation of the particle size distribution (PSD) of hail. The maximum ~1-km hail water content encountered was 9 g m−3. Optical probe PSD measurements are normalized using two-moment normalization relations to obtain an underlying exponential shape. By linking the two normalizing moments through a power law, a parameterization of the hail PSD is provided based on the hail water content only. Preliminary numerical weather simulations indicate that the new parameterization produces increased radar reflectivity relative to commonly used PSD representations.

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Paul L. Smith
,
Dennis J. Musil
,
Andrew G. Detwiler
, and
Rahul Ramachandran

Abstract

Various procedures for inferring hydrometeor characteristics from polarimetric radar data have indicated that regions with echoes exhibiting relatively high linear depolarization ratios along with relatively low differential reflectivity contain wet graupel or hail. Such particles could be found either in a melting zone below the 0°C level in a cloud or in a region of wet growth where the rate of supercooled cloud water accretion overwhelms the rate at which the latent heat associated with complete freezing can be dissipated. In subtropical clouds such as those studied in the Convection and Precipitation/Electrification (CaPE) project in Florida, at the −5°C level or higher, neither condition is obtained. Yet similar polarimetric radar signatures were nevertheless observed at such levels during CaPE.

Examination of in situ observations by the T-28 aircraft in the Florida clouds, along with results from previous laboratory and theoretical studies, suggests that the signature regions were characterized by raindrops formed at lower levels in the clouds in the process of freezing while being carried up in the updraft. This interpretation is supported by some model calculations of the process of raindrops freezing in a supercooled cloud where the particles continue to accrete cloud water. Complete freezing of millimeter-sized drops can take a few minutes after nucleation, and the particles can ascend 1 or 2 km in the updraft during that period.

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Michael W. Huston
,
Andrew G. Detwiler
,
Fred J. Kopp
, and
Jeffrey L. Stith

Abstract

Observations made by three instrumented aircraft, a Doppler radar, and other data sources were used to follow the initiation and development of precipitation in a small cumulus congestus cloud. The cloud was seeded at its base using an airborne silver iodide solution burner. Sulfur hexafluoride tracer gas was released along with the seeding material. Analyzers on two instrumented aircraft detected the tracer gas during subsequent cloud penetrations as it was carried up into the cloud along with the seeding agent. Ice developed initially in the upper regions of the cloud near the −10°C level ∼15 min after the commencement of seeding. This is consistent with primary nucleation by the seeding agent. The cloud developed millimeter-size graupel within the following few minutes. A radar echo approaching 40 dBZ subsequently developed. The echo was observed to descend through the cloud as the cloud dissipated.

One-dimensional, steady-state and two-dimensional, time-dependent bulk water models were used to simulate this cloud. The one-dimensional model produced realistic values for updraft speeds allowing credible estimates of time required for transport from cloud base to upper regions of the cloud. The development of precipitation in the two-dimensional simulation resembled that in the observed cloud. Precipitation developed through riming of snow to graupel. In both the observed and simulated clouds, precipitation development was limited by cloud lifetime. Both clouds collapsed at a time when they were still generating ample supercooled water in their updrafts. Total precipitation on the ground from the seeded cloud simulations was ∼5 times the radar estimated rainfall total of 0.5 mm from the observed seeded cloud. This occurred despite the fact that the simulated cloud went through an accelerated life cycle compared to the observed cloud. A comparison between simulations with a natural ice process and with cloud base release of silver iodide shows that seeding accelerated precipitation formation in the model cloud leading to a fourfold increase in total precipitation for the seeded cases compared to the natural one.

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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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