ASR scientists use ARM-tethered balloons and snowflake cameras in Alaska to test a new way to describe the shape of falling ice crystals.
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.
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.
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.
Contacts (BER PM)
ASR Program Manager
ARM Program Manager
Sergey Y. Matrosov
Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, and NOAA/Earth System Research Laboratory, Boulder, Colorado
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.
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]
ASR Highlight: Inferring Ice Hydrometeor Shapes from Polarimetric Radar Measurements
SC-23.1 Climate and Environmental Sciences Division, BER
BER supports basic research and scientific user facilities to advance DOE missions in energy and environment. More about BER
Aug 24, 2019
New Approach for Studying How Microbes Influence Their Environment
A diverse group of scientists suggests a common framework and targeting of known microbial processes [more...]
Aug 08, 2019
Nutrient-Hungry Peatland Microbes Reduce Carbon Loss Under Warmer Conditions
Enzyme production in peatlands reduces carbon lost to respiration under future high temperatures. [more...]
Aug 05, 2019
Amazon Forest Response to CO2 Fertilization Dependent on Plant Phosphorus Acquisition
AmazonFACE Model Intercomparison. The Science Plant growth is dependent on the availabi [more...]
Jul 29, 2019
A Slippery Slope: Soil Carbon Destabilization
Carbon gain or loss depends on the balance between competing biological, chemical, and physical reac [more...]
Jul 15, 2019
Field Evaluation of Gas Analyzers for Measuring Ecosystem Fluxes
How gas analyzer type and correction method impact measured fluxes. The Science A side- [more...]
List all highlights (possible long download time)