Analysis of the internal electric fields of pristine ice crystals and aggregate snowflakes, and their effect on scattering

McCusker, Karina; Westbrook, C. D.; Moiola, Andrea

doi:10.1016/j.jqsrt.2019.04.019

Cited by 3 publications

(6 citation statements)

References 56 publications

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“…Using DDA, McCusker et al . (2019) explored the internal electric fields of plane‐wave‐illuminated particles. It was shown that, for a given size parameter, aggregates have more weakly interacting dipoles and lower field magnitudes than single monomers.…”

Section: Experiments Designmentioning

confidence: 99%

“…As discussed previously, McCusker et al . (2019) used DDA to explore the internal fields of particles with size parameters up to 10. The authors of that paper showed that, as size parameter increases, constructive interference causes a region of enhanced electric field magnitude at the shadow‐side of the particle.…”

Section: Evaluation Of the Accuracy Of Imamentioning

confidence: 99%

“…Note that this requires input data identifying which monomer each dipole belongs to. More information on the DDA implementation used here may be found in McCusker et al (2019), where results were compared to calculations performed with an independent Boundary Element Method, BEM++. The difference between the internal field of a hexagonal ice plate obtained using our DDA code and the BEM++ set-up used by Groth et al (2015) was found to be within 1.2%, depending on the discretisation level.…”

Section: The Independent Monomer Approximation (Ima)mentioning

confidence: 99%

“…The purpose of this paper is to test an approximation that builds on the studies above, along with recent work by McCusker et al . (2019) who analysed DDA solutions for the internal electric fields within a large, low‐density aggregate composed of 10 dendrite crystals. Some of the individual monomer crystals were removed from the aggregate and the DDA calculations were repeated on each of the isolated monomers without the presence of the rest of the aggregate.…”

Section: Introductionmentioning

confidence: 99%

“…More information on the DDA implementation used here may be found in McCusker et al . (2019), where results were compared to calculations performed with an independent Boundary Element Method, BEM++. The difference between the internal field of a hexagonal ice plate obtained using our DDA code and the BEM++ set‐up used by Groth et al .…”

Section: Introductionmentioning

confidence: 99%

See 4 more Smart Citations

An accurate and computationally cheap microwave scattering method for ice aggregates: the Independent Monomer Approximation

McCusker

Westbrook

Tyynelä

2021

Quart J Royal Meteoro Soc

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The Discrete Dipole Approximation (DDA) is widely used to simulate scattering of microwaves by snowflakes, by discretising the snowflake into small ‘dipoles’ which oscillate in response to (a) the incident wave, and (b) scattered waves from all the other dipoles in the particle. It is this coupling between all dipole pairs which makes solving the DDA system computationally expensive, and that cost grows nonlinearly as the number of crystals n within an aggregate is increased. Motivated by this, many studies have ignored the dipole coupling (the Rayleigh–Gans Approximation, RGA). However, use of RGA leads to systematic underestimation of both scattering and absorption, and an inability to predict polarimetric properties. To address this, we present a new approach (the Independent Monomer Approximation, IMA) which solves the DDA system for each crystal ‘monomer’ separately, then combines them to construct the full solution. By including intra‐monomer coupling, but neglecting inter‐monomer coupling, we save a factor of n in computation time over DDA. Benchmarking IMA against DDA solutions indicates that its accuracy is greatly superior to RGA, and provides ensemble scattering cross‐sections which closely agree with their more expensive DDA counterparts, particularly at size parameters smaller than ∼ 5 . Addition of rime to the aggregates does not significantly degrade the results, despite the increased density. The use of IMA for radar remote sensing is evaluated, and we show that multi‐wavelength and multi‐polarisation parameters are successfully captured to within a few tenths of a dB for aggregates probed with frequencies between 3 and 200 GHz, in contrast to RGA where errors of up to 2.5 dB are observed. Finally, we explore the realism of the IMA solutions in greater detail by analysing internal electric fields, and discuss some broader insights that IMA provides into the physical features of aggregates that are important for microwave scattering.

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Section: Experiments Designmentioning

confidence: 99%

Section: Evaluation Of the Accuracy Of Imamentioning

confidence: 99%

Section: The Independent Monomer Approximation (Ima)mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

An accurate and computationally cheap microwave scattering method for ice aggregates: the Independent Monomer Approximation

McCusker

Westbrook

Tyynelä

2021

Quart J Royal Meteoro Soc

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The first microwave and submillimetre closure study using particle models of oriented ice hydrometeors to simulate polarimetric measurements of ice clouds

McCusker,

Baran,

Westbrook

et al. 2024

Atmos. Meas. Tech.

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Abstract. The first closure study involving passive microwave and submillimetre measurements of ice clouds with the consideration of oriented particles is presented, using a unique combination of polarised observations from the ISMAR spectral-like radiometer, two radars with frequencies of 35 and 95 GHz, and a variety of in situ instruments. Of particular interest to this study are the large V–H polarised brightness temperature differences measured from ISMAR above a thick frontal ice cloud. Previous studies combining radar and passive submillimetre measurements have not considered polarisation differences. Moreover, they have assumed particle habits a priori. We aim to test whether the large V–H measurements can be simulated successfully by using an atmospheric model consistent with in situ microphysics. An atmospheric model is constructed using information from the in situ measurements, such as the ice water content, the particle size distribution, and the mass and shape of particles, as well as background information obtained from dropsonde profiles. Columnar and dendritic aggregate particle models are generated specifically for this case, and their scattering properties are calculated using the independent monomer approximation under the assumption of horizontal orientation. The scattering properties are used to perform polarised radiative transfer simulations using ARTS to test whether we can successfully simulate the measured large V–H differences. Radar measurements are used to extrapolate the 1-D microphysical profile to derive a time series of particle size distributions which are used to simulate ISMAR brightness temperatures. These simulations are compared to the observations. It is found that particle models that are consistent with in situ microphysics observations are capable of reproducing the brightness temperature depression and polarisation signature measured from ISMAR at the dual-polarised channel of 243 GHz. However, it was required that a proportion of the particles were changed in order to increase the V–H polarised brightness temperature differences. Thus, we incorporated millimetre-sized dendritic crystals, as these particles were observed in the probe imagery. At the second dual-polarised channel of 664 GHz, the brightness temperature depressions were generally simulated at the correct locations; however, the simulated V–H was too large. This work shows that multi-frequency polarisation information could be used to infer realistic particle shapes, orientations, and representations of the split between single crystals and aggregates within the cloud.

show abstract

The first microwave and submillimetre closure study using particle models of oriented ice hydrometeors to simulate polarimetric measurements of ice clouds

McCusker

Baran

Westbrook

et al. 2023

Preprint

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Abstract. The first closure study involving passive microwave and submillimetre measurements of ice clouds with the consideration of oriented particles is presented, using a unique combination of polarised observations from the ISMAR spectral-like radiometer, two radars with frequencies of 35 and 95 GHz, and a variety of in-situ instruments. Of particular interest to this study are the large V-H polarised brightness temperature differences measured from ISMAR above a thick frontal ice cloud. Previous studies combining radar and passive submillimetre measurements have not considered polarisation differences. Moreover, they have assumed particle habits a-priori. We aim to test whether the large V-H measurements can be simulated successfully by using an atmospheric model consistent with in-situ microphysics. An atmospheric model is constructed using information from the in-situ measurements, such as the ice water content, the particle size distribution, and the mass and shape of particles, as well as background information obtained from dropsonde profiles. Columnar and dendritic aggregate particle models are generated specifically for this case, and their scattering properties are calculated using the Independent Monomer Approximation under the assumption of horizontal orientation. The scattering properties are used to perform polarised radiative transfer simulations using ARTS to test whether we can successfully simulate the measured large V-H differences. Radar measurements are used to extrapolate the 1D microphysical profile to derive a time-series of particle size distributions which are used to simulate ISMAR brightness temperatures. These simulations are compared to the observations. It is found that particle models that are consistent with in-situ microphysics observations are capable of reproducing the brightness temperature depression and polarisation signature measured from ISMAR at the dual-polarised channel of 243 GHz. However, it was required that a proportion of the particles were changed in order to increase the V-H polarised brightness temperature differences. Thus we incorporated mm-sized dendritic crystals, as these particles were observed in the probe imagery. At the second dual-polarised channel of 664 GHz, the brightness temperature depressions were generally simulated at the correct locations, however the simulated V-H was too large. This work shows that multi-frequency polarisation information could be used to infer realistic particle shapes, orientations, and representations of the split between single crystals and aggregates within the cloud.

show abstract

Analysis of the internal electric fields of pristine ice crystals and aggregate snowflakes, and their effect on scattering

Cited by 3 publications

References 56 publications

An accurate and computationally cheap microwave scattering method for ice aggregates: the Independent Monomer Approximation

An accurate and computationally cheap microwave scattering method for ice aggregates: the Independent Monomer Approximation

The first microwave and submillimetre closure study using particle models of oriented ice hydrometeors to simulate polarimetric measurements of ice clouds

The first microwave and submillimetre closure study using particle models of oriented ice hydrometeors to simulate polarimetric measurements of ice clouds

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