Inferring Ice Crystal Shapes from ARM Polarimetric Radar Measurements

The Science

Precipitation that falls to the ground as either rain or snow often starts off as ice within the cloud. Ice particles within in clouds can have different shapes (often referred to as habits) and the specific details about those shapes can determine how large a particle has to be before it falls and how quickly it falls, as well as how fast the particle grows and how it absorbs, emits, and scatters atmospheric radiation. Quantifying particle shapes is critical for refining ice microphysical representations in numerical models and improving estimation of snowfall amount from weather radars. Direct in situ measurements, such as from cameras and other types of particle imagers, are, so far, the main source of the data on particle habits, although radar-based approaches of identifying general particle types (e.g., aggregates versus pristine crystals) have been developed.

The Impact

This study tests and validates an advanced method to retrieve a parameter describing ice crystal shape from measurements made by the Scanning ARM Cloud Radar (SACR). The advanced polarimetric capabilities of these radars provide additional information for deriving ice crystal shape. This new shape parameter includes more information about complex crystal shapes than previously used parameters. The new radar retrievals thus have the potential to address known uncertainties in regional and global earth system models and to improve predictions of snowfall.

Summary

In this study, we propose, test, and validate a remote-sensing method to retrieve a quantitative parameter describing ice hydrometeor shape from polarimetric measurements conducted by the Scanning ARM Cloud Radar (SACR). This parameter is the particle mean aspect ratio that characterizes general non-sphericity of ice hydrometeors and is defined as the ratio of smallest particle dimension to its largest dimension. By retrieving aspect ratios quantitatively, the suggested method goes beyond the existing approaches that use polarimetric radar data to distinguish between several ice hydrometeor types. The new method accounts for the effects of changing particle bulk density, which influences shape retrievals, and minimizes the effects of particle orientations, which enhances the accuracy of the aspect ratio estimates. Since ice particle aspect ratio for spheroidal shapes is an important prognostic parameter in advanced cloud microphysical models, the retrievals obtained with this method can be used in future model validation efforts.

The new method to retrieve ice hydrometeor aspect ratios is used with scanning polarimetric measurements from the Ka-band (~35 GHz) channel of the cloud radar deployed at the third ARM Mobile Facility at Oliktok Point, Alaska. For a case study of a weakly precipitating mixed-phase cloud observed on 21 October, 2016, the results of the radar-based retrievals are compared with closely co-located in situ microphysical measurements from the tethered-balloon-system-based video ice particle sampler (VIPS) and the ground-based multiangle snowflake camera (MASC). The observations reveal that ice particles had mostly irregular shapes, which is common for arctic clouds. Assuming a spheroidal ice particle shape, the radar retrievals indicate ice hydrometeor aspect ratios varying between 0.3 and 0.8 with retrieval uncertainties of around 0.1 to 0.15. The radar-based retrievals agree well with in situ microphysical measurements of particle aspect ratios given the estimated uncertainties.

Principal Investigator(s)

Sergey Y. Matrosov
University of Colorado Boulder, and NOAA/Earth System Research Laboratory
[email protected]

Funding

This study was sponsored by the U.S. Department of Energy Atmospheric System Research Program under Award DE-SC0013306. Funding for VIPS measurements and analysis was provided by the DOE ARM/ASR through Subcontract 298377 with Pacific Northwest National Laboratory of the Battelle Memorial Institute. The SACR, MASC, radiosonde, and VIPS data are available from the ARM archive.

References

Matrosov S., C. Schmitt, M. Maahn, and G. de Boer. “Atmospheric Ice Particle Shape Estimates from Polarimetric Radar Measurements and In Situ Observations.” Journal of Atmospheric and Oceanic Technology 34(12) (2017). [DOI: 10.1175/JTECH-D-17-0111.1]