Supercooled liquid water and secondary ice production in Kelvin–Helmholtz instability as revealed by radar Doppler spectra observations

Haoran Li,Dmitri Moisseev,Alexei Korolev

doi:10.5194/acp-21-13593-2021

Abstract

Abstract. Mixed-phase clouds are globally omnipresent and play a major role in the Earth's radiation budget and precipitation formation. The existence of liquid droplets in the presence of ice particles is microphysically unstable and depends on a delicate balance of several competing processes. Understanding mechanisms that govern ice initiation and moisture supply are important to understand the life cycle of such clouds. This study presents observations that reveal the onset of drizzle inside a ∼ 600 m deep mixed-phase layer embedded in a stratiform precipitation system. Using Doppler spectral analysis, we show how large supercooled liquid droplets are generated in Kelvin–Helmholtz (K–H) instability despite ice particles falling from upper cloud layers. The spectral width of the supercooled liquid water mode in the radar Doppler spectrum is used to identify a region of increased turbulence. The observations show that large liquid droplets, characterized by reflectivity values larger than −20 dBZ, are generated in this region. In addition to cloud droplets, Doppler spectral analysis reveals the production of columnar ice crystals in the K–H billows. The modeling study estimates that the concentration of these ice crystals is 3–8 L−1, which is at least 1 order of magnitude higher than that of primary ice-nucleating particles. Given the detail of the observations, we show that multiple populations of secondary ice particles are generated in regions where larger cloud droplets are produced and not at some constant level within the cloud. It is, therefore, hypothesized that K–H instability provides conditions favorable for enhanced droplet growth and formation of secondary ice particles.

Highlights

Clouds strongly influence the Earth’s radiation budget (Baker, 1997; Baker and Peter, 2008; Morrison et al, 2012; Tan et al, 2016) and hydrological cycle (Mülmenstädt et al, 2015)
On 18 April 2014 between 20:50 and 21:40 UTC, a K–H cloud embedded in stratiform precipitation was observed over the University of Helsinki measurement site in Hyytiälä, Finland
The observations were collected using three radars, two vertically pointing C- and W-band radars located at the site and a scanning weather radar operated by Finnish Meteorological Institute (FMI)

Summary

Introduction

Clouds strongly influence the Earth’s radiation budget (Baker, 1997; Baker and Peter, 2008; Morrison et al, 2012; Tan et al, 2016) and hydrological cycle (Mülmenstädt et al, 2015). A large fraction of clouds are mixed phase (Hogan et al, 2004), i.e., contain both liquid water droplets and ice particles. Climate and numerical weather prediction models, struggle to accurately represent mixed-phase clouds (Klein et al, 2009; McCoy et al, 2016; Barrett et al, 2017) They tend to underestimate cloud liquid water content (Klein et al, 2009; Barrett et al, 2017), which seems to be linked to the modeled ice production (Klein et al, 2009; Barrett et al, 2017).

Methods

Discussion

Conclusion