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.
Highlights
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 groundbased observations
Global climate models assumed that low-level clouds over the Antarctic Ice sheet essentially contained ice crystals, but Lawson and Gettelman (2014), and later Ricaud et al (2020) both showed from their observations that around 50% of clouds contained supercooled liquid water (SLW) during the austral summer
As part of the Australian Antarctic Division’s Precipitation over Land and The Southern Ocean (PLATO) field campaign, which operated during the Year of Polar Prediction (YOPP, Bromwich et al, 2020), a suite of ground-based remote sensing instruments was deployed at Davis (68.5762 oS, 77.9696 oE), one of the three permanent Australian Antarctic bases on the continent (Gehring et al, 2022), during the southern hemisphere summer 2018/2019
Summary
Mixed-phase clouds play a critical role in the earth radiation budget, through their complex interactions with incoming and outgoing shortwave and longwave radiation This effect is important at higher latitudes with variation in radiation affecting the snow or ice mass balance in the polar regions (Lawson and Gettelman, 2014). Despite their importance in the global climate system, the occurrence, amount, and nature of mixed-phase clouds remain poorly simulated in global climate models due to the paucity of reliable mixed-phase clouds observations, especially in remote regions of the globe such as Antarctica (Bodas-Salcedo et al, 2016; Hyder et al, 2018). Kay et al (2016) and Frey et al (2018) highlighted the importance of Southern Ocean mixed-phase clouds in global coupled climate models, under the predicted increase of greenhouse gases concentrations
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