Submillimeter-wave scattering indicatrices for oversized water drops and ice particles in a cloud

Abstract. The aeronautics industry has established that a threat to aircraft is posed by atmospheric conditions of substantial ice water content (IWC) where equivalent radar reflectivity (Ze) does not exceed 20–30 dBZ and supercooled water is not present; these conditions are encountered almost exclusively in the vicinity of deep convection. Part 1 (Fridlind et al., 2015) of this two-part study presents in situ measurements of such conditions sampled by Airbus in three tropical regions, commonly near 11 km and −43 °C, and concludes that the measured ice particle size distributions are broadly consistent with past literature with profiling radar measurements of Ze and mean Doppler velocity obtained within monsoonal deep convection in one of the regions sampled. In all three regions, the Airbus measurements generally indicate variable IWC that often exceeds 2 g m-3 with relatively uniform mass median area-equivalent diameter (MMDeq) of 200–300 μm. Here we use a parcel model with size-resolved microphysics to investigate microphysical pathways that could lead to such conditions. Our simulations indicate that homogeneous freezing of water drops produces a much smaller ice MMDeq than observed, and occurs only in the absence of hydrometeor gravitational collection for the conditions considered. Development of a mass mode of ice aloft that overlaps with the measurements requires a substantial source of small ice particles at temperatures of about −10 °C or warmer, which subsequently grow from water vapor. One conceivable source in our simulation framework is Hallett–Mossop ice production; another is abundant concentrations of heterogeneous ice freezing nuclei acting together with copious shattering of water drops upon freezing. Regardless of the production mechanism, the dominant mass modal diameter of vapor-grown ice is reduced as the ice-multiplication source strength increases and as competition for water vapor increases. Both mass and modal diameter are reduced by entrainment and by increasing aerosol concentrations. Weaker updrafts lead to greater mass and larger modal diameters of vapor-grown ice, the opposite of expectations regarding lofting of larger ice particles in stronger updrafts. While stronger updrafts do loft more dense ice particles produced primarily by raindrop freezing, we find that weaker updrafts allow the warm rain process to reduce competition for diffusional growth of the less dense ice expected to persist in convective outflow.

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Microwave and submillimeter wave scattering of oriented ice particles

10.5194/amt-2019-382 ◽

2019 ◽

Cited By ~ 3

Author(s):

Manfred Brath ◽

Robin Ekelund ◽

Patrick Eriksson ◽

Oliver Lemke ◽

Stefan A. Buehler

Keyword(s):

Tilt Angle ◽

Wave Scattering ◽

Conceptual Models ◽

Incidence Angle ◽

Scattering Data ◽

Random Orientation ◽

Standard Assumption ◽

Ice Cloud ◽

Ice Particles ◽

Angle Scattering

Abstract. Microwave dual-polarization measurements above 100 GHz are so far sparse, but they consistently show that larger ice hydrometeors tend to deviate from the standard assumption of total random orientation. This conclusion has been derived by conceptual models, while the first detailed simulations, recreating the observed polarization patterns, are presented in this study. The ice particles are assumed to be azimuthally randomly oriented with a fixed but arbitrary tilt angle. The scattering data for azimuthal random orientation is much more complex than for total random orientation. The scattering data of azimuthally randomly oriented particles depends in general on the incidence angle and two scattering angles compared to one angle scattering for total random orientation. The additional tilt angle adds an additional dimension. The simulations are based on the discrete dipol approximation in combination with a self developed orientation averaging approach. Data for two particle habits (51 hexagonal plates and 18 plate aggregates) and 35 frequencies between 1 GHz and 864 GHz were produced. The data is publicly available from Zenodo (https://doi.org/10.5281/zenodo.3463003). This effort is also an essential part of preparing for the upcoming Ice Cloud Imager (ICI), that will perform polarized observations at 243 GHz and 664 GHz, which will deliver new insights about clouds.

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Coalescence and Secondary Ice Development in Cumulus Congestus Clouds

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-21-0188.1 ◽

2022 ◽

Keyword(s):

Prediction Models ◽

Process Development ◽

Supercooled Liquid ◽

Cloud Base ◽

Marine Environments ◽

Avalanche Effect ◽

Ice Particles ◽

Research Aircraft ◽

Water Drops ◽

Cumulus Congestus

Abstract Understanding ice development in Cumulus Congestus (CuCg) clouds, which are ubiquitous globally, is critical for improving our knowledge of cloud physics, cloud resolution and climate prediction models. Results presented here are representative of data collected in 1,008 penetrations of moderate to strong updrafts in CuCg clouds by five research aircraft in six geographic locations. The results show that CuCg with warm (> ∼20°C) cloud base temperatures, such as in tropical marine environments, experience a strong collision-coalescence process. Development of coalescence is also correlated with drop effective radius > ∼12 to 14 µm in diameter. Increasing the cloud-base drop concentration with diameters from 15 to 35 µm and decreasing the drop concentration < 15 µm appears to enhance coalescence. While the boundary-layer aerosol population is not a determinate factor in development of coalescence in tropical marine environments, its impact on coalescence is not yet fully determined. Some supercooled large drops generated via coalescence fracture when freezing, producing a secondary ice process (SIP) with production of copious small ice particles that naturally seed the cloud. The SIP produces an avalanche effect, freezing the majority of supercooled liquid water before fresh updrafts reach the −16°C level. Conversely, CuCg with cloud base temperatures ≤ ∼8°C develop significant concentrations of ice particles at colder temperatures, so that small supercooled water drops are lofted to higher elevations before freezing. Recirculation of ice in downdrafts at the edges of updrafts appears to be the primary mechanism for development of precipitation in CuCg with colder cloud base temperatures.

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Submillimeter-wave scattering parameter measurements with a sampled-line six-port reflectometer

60th ARFTG Conference Digest, Fall 2002. ◽

10.1109/arftgf.2002.1218688 ◽

2004 ◽

Cited By ~ 1

Author(s):

S. Ulker ◽

R.M. Weikle

Keyword(s):

Wave Scattering ◽

Submillimeter Wave ◽

Scattering Parameter

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High ice water content at low radar reflectivity near deep convection – Part 2: Evaluation of microphysical pathways in updraft parcel simulations

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-15-16551-2015 ◽

2015 ◽

Vol 15 (12) ◽

pp. 16551-16613

Author(s):

A. S. Ackerman ◽

A. M. Fridlind ◽

A. Grandin ◽

F. Dezitter ◽

M. Weber ◽

...

Keyword(s):

Water Vapor ◽

Water Content ◽

Deep Convection ◽

Radar Reflectivity ◽

Doppler Velocity ◽

Diffusional Growth ◽

Ice Particles ◽

Ice Water Content ◽

Water Drops ◽

Ice Water

Abstract. The aeronautics industry has established that a threat to aircraft is posed by atmospheric conditions of substantial ice water content (IWC) where equivalent radar reflectivity (Ze) does not exceed 20–30 dBZ and supercooled water is not present, encountered almost exclusively in the vicinity of deep convection. Part 1 of this two-part study presents in situ measurements of such conditions sampled by Airbus in three tropical regions, commonly near 11 km and −43 °C, and concludes that the measured ice particle size distributions are broadly consistent with past literature and with profiling radar measurements of Ze and mean Doppler velocity obtained within monsoonal deep convection in one of the regions sampled. In all three regions the Airbus measurements generally indicate variable IWC that often exceeds 2 g m−3 with relatively uniform mass median area-equivalent diameter (MMDeq) of 200–300 μm. Here we use a parcel model with size-resolved microphysics to investigate microphysical pathways that could lead to such conditions. Our simulations indicate that homogeneous freezing of water drops produces a much smaller ice MMDeq than observed, and occurs only in the absence of hydrometeor gravitational collection for the conditions considered. Development of a mass mode of ice aloft that overlaps with the measurements requires a substantial source of small ice particles at temperatures of about −10 °C or warmer, which subsequently grow from water vapor. One conceivable source in our simulation framework is Hallett–Mossop ice production; another is abundant concentrations of heterogeneous ice freezing nuclei acting together with copious shattering of water drops upon freezing. Regardless of production mechanism, the dominant mass modal diameter of vapor-grown ice is reduced as the ice multiplication source strength increases and as competition for water vapor increases. Both mass and modal diameter are reduced by entrainment and by increasing aerosol concentrations. Weaker updrafts lead to greater mass and larger modal diameters of vapor-grown ice, the opposite of expectations regarding lofting of larger ice particles in stronger updrafts. While stronger updrafts do loft more dense ice particles produced primarily by raindrop freezing, we find that weaker updrafts allow the warm rain process to reduce competition for diffusional growth of the less dense ice expected to persist in convective outflow.

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Secondary ice production during the break-up of freezing water drops on impact with ice particles

10.5194/acp-2021-557 ◽

2021 ◽

Author(s):

Rachel L. James ◽

Vaughan T. J. Phillips ◽

Paul J. Connolly

Keyword(s):

Laboratory Study ◽

Water Drop ◽

Supercooled Water ◽

Glass Slide ◽

Ice Formation ◽

Production Mechanism ◽

Temperature Range ◽

Ice Particles ◽

Water Drops ◽

Ice Production

Abstract. We experimentally investigated collisions of supercooled water drops (∼ 5 mm in diameter) with ice particles of a similar size placed on a glass slide at temperatures T ≥ −12 °C. Our results showed that secondary drops were generated during both the spreading and retraction phase of the supercooled water drop impact. The secondary drops generated during the spreading phase were emitted too fast to quantify. However, quantification of the secondary drops generated during the retraction phase with diameters > 0.1 mm showed that 5–10 secondary drops formed per collision, with approximately 30 % of the secondary drops freezing over a temperature range of −4 °C ≤ T ≤ −12 °C. Our investigation provides the first dedicated laboratory study of collisions of supercooled water drops with ice particles as a secondary ice production mechanism. Our results suggest that this secondary ice production mechanism may be significant for ice formation in atmospheric clouds containing large supercooled drops and ice particles.

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The freeezing behaviour of supercooled water drops

Journal of Glaciology ◽

10.1017/s0022143000030756 ◽

1976 ◽

Vol 17 (75) ◽

pp. 99-109 ◽

Cited By ~ 4

Author(s):

M. J. Gay ◽

J. Latham

Keyword(s):

Relative Humidity ◽

Mass Loss ◽

Supercooled Water ◽

Additional Mechanism ◽

Ice Particles ◽

Containment System ◽

Water Drops ◽

Almost All ◽

Visual Observations

Abstract An electrodynamic containment system has been used to study the freeezing behaviour of supercooled water drops, of radius range 25 to 90 μm. The drops were freozen at temperatures between 0 and — 29°C in an environment whose relative humidity was approximately 90% with respect to ice. Freezing events were observed visually and photographically, and measurements were mager of the accompanying freactional mass loss Δm/m. The most common moger of freeezing (70% of the drops studied) resulted in an apparently spherical ice particle. However, 18% exhibited spikes or other protuberances and the freeezing of 3% was accompanied by the ejection of numerous ice particles. In each of these situations values of Δm/m ranged freom about 5 to 15%. A further 9% of the drops exhibited one or more secondary mass-loss events, occurring several seconds after the freeezing process was complete; these were thus indicative of the ejection of ice particles. Almost all of the values of Δm/m were significantly in excess of those predicted on the basis of evaporation during freeezing, suggesting that an additional mechanism of mass loss was also present. The measured freeezing times were consigerrably shorter than the classical values—at least, for the larger drops freeezing at warmer temperatures. Some visual observations were consistent with the “supersaturation wave” around a freeezing drop, which has been predicted by Nix and Fukuta (1974).

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