Observations of Ice Multiplication in a Long Island Snowstorm

Secondary ice production processes can play a significant role in precipitation formation. One of the most commonly observed and studied processes of ice multiplication is rime splintering, also known as the Hallett-Mossop process. Processes like this can be hard to identify when observations are limited to radars and lidars. The NASA IMPACTS field campaign provides a rare opportunity to have numerous airborne instruments and in situ observations available to better understand the conditions and processes occurring.

On 18 January 2020, in the early stages of the NASA IMPACTS field campaign, Long Island, NY, experienced a snowstorm. The storm was well observed by IMPACTS assets, including the Ka-band Scanning Polarimetric Radar (KASPR) located at Stony Brook University, as well as the P-3 aircraft, which carried instruments observing the ice and liquid water content as well as recording images of particles aloft, and the ER2, which flies at higher altitudes and recorded electric field measurements, among other observations.

Fully polarimetric variables, while less commonly studied, contain useful information on the shapes, orientations, and make-up of the observed scatterers. For example, LDR is useful for identifying columnar and canted ice crystals, and the cross-polar phase shift Φ𝑋𝐻 may also be associated with canted ice crystals. KASPR data reveal a band of enhanced Φ𝑋𝐻 values shortly after the ER2 recorded an electric field of 14.8 V/m near Long Island. The Φ𝑋𝐻 signature is thought to be related to the canting of ice crystals, arising from a strong in-cloud electric field. This signature was collocated with a secondary mode in the Doppler spectra, comprising ice crystals. Liquid droplets and needles were both observed by the SPEC Hawkeye Cloud Particle Imager (CPI) aboard the P3 aircraft in proximity of the SBU site at the same elevation, suggesting the likely mechanism for the secondary spectral mode may have been secondary ice multiplication via the Hallett-Mossop rime splintering process. In this presentation, we show how the combined use of fully polarimetric radar, spectra, and aircraft in situ data can be used to provide evidence of ongoing secondary ice processes.