Detection of supercooled liquid water clouds with ceilometers: Development and evaluation of deterministic and data-driven retrievals

Abstract

Abstract. Cloud and aerosol lidars measuring backscatter and depolarization ratio are most suitable instruments to detect cloud phase (liquid, ice, or mixed phase). However, such instruments are not widely deployed as part of operational networks. In this study, we propose a new algorithm to detect supercooled liquid water clouds based solely on ceilometers measuring only co-polarisation backscatter. We utilise observations collected at Davis, Antarctica, where low-level, mixed phase clouds, including supercooled liquid water (SLW) droplets and ice crystals remain poorly understood, due to the paucity of ground-based observations. A 3-month set of observations were collected during the austral summer of November 2018–February 2019, with a variety of instruments including a depolarization lidar and a W-Band cloud radar which were used to build a 2-dimensional cloud phase mask distinguishing SLW and mixed phase clouds. This cloud phase mask is used as the reference to develop a new algorithm based on the observations of a single polarisation ceilometer operating in the vicinity for the same period. Deterministic and data-driven retrieval approaches were evaluated: an extreme gradient boosting (XGBoost) framework ingesting backscatter average characteristics was the most effective method at reproducing the classification obtained with the combined radar-lidar approach with an accuracy as high as 0.91. This study provides a new SLW retrieval approach based solely on ceilometer data and highlights the considerable benefits of these instruments to provide intelligence on cloud phase in polar regions that usually suffer from a paucity of observations.

Detection of supercooled liquid water containing clouds with ceilometers: development and evaluation of deterministic and data-driven retrievals

“…A tool for converting Vaisala CL31 and CL51 data files to netCDF cl2nc is open-source and available at https://doi.org/10.5281/zenodo.4409716 (Kuma, 2020). The observational data (ALCF-processed netCDF ceilometer files and the radar-lidar mask netCDF files) are available on Zenodo at https://doi.org/10.5281/zenodo.5832199 (Guyot et al, 2022) and will also be available at the Australian Antarctic Division data centre by the end of 2022. The ERA5 data are available through the Copernicus data portal at https://cds.climate.copernicus.eu (last access: 2 May 2022; https://doi.org/10.24381/cds.adbb2d47, Hersbach et al, 2018).…”