Abstract
Abstract. A comprehensive analysis of the water budget over the Dome C (Concordia, Antarctica) station has been performed during the austral summer 2018–2019 as part of the Year of Polar Prediction (YOPP) international campaign. Thin (∼100 m deep) supercooled liquid water (SLW) clouds have been detected and analysed using remotely sensed observations at the station (tropospheric depolarization lidar, the H2O Antarctica Microwave Stratospheric and Tropospheric Radiometer (HAMSTRAD), net surface radiation from the Baseline Surface Radiation Network (BSRN)), radiosondes, and satellite observations (CALIOP, Cloud-Aerosol LIdar with Orthogonal Polarization/CALIPSO, Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations) combined with a specific configuration of the numerical weather prediction model: ARPEGE-SH (Action de Recherche Petite Echelle Grande Echelle – Southern Hemisphere). The analysis shows that SLW clouds were present from November to March, with the greatest frequency occurring in December and January when ∼50 % of the days in summer time exhibited SLW clouds for at least 1 h. Two case studies are used to illustrate this phenomenon. On 24 December 2018, the atmospheric planetary boundary layer (PBL) evolved following a typical diurnal variation, which is to say with a warm and dry mixing layer at local noon thicker than the cold and dry stable layer at local midnight. Our study showed that the SLW clouds were observed at Dome C within the entrainment and the capping inversion zones at the top of the PBL. ARPEGE-SH was not able to correctly estimate the ratio between liquid and solid water inside the clouds with the liquid water path (LWP) strongly underestimated by a factor of 1000 compared to observations. The lack of simulated SLW in the model impacted the net surface radiation that was 20–30 W m−2 higher in the BSRN observations than in the ARPEGE-SH calculations, mainly attributable to the BSRN longwave downward surface radiation being 50 W m−2 greater than that of ARPEGE-SH. The second case study took place on 20 December 2018, when a warm and wet episode impacted the PBL with no clear diurnal cycle of the PBL top. SLW cloud appearance within the entrainment and capping inversion zones coincided with the warm and wet event. The amount of liquid water measured by HAMSTRAD was ∼20 times greater in this perturbed PBL than in the typical PBL. Since ARPEGE-SH was not able to accurately reproduce these SLW clouds, the discrepancy between the observed and calculated net surface radiation was even greater than in the typical PBL case, reaching +50 W m−2, mainly attributable to the downwelling longwave surface radiation from BSRN being 100 W m−2 greater than that of ARPEGE-SH. The model was then run with a new partition function favouring liquid water for temperatures below −20 down to −40 ∘C. In this test mode, ARPEGE-SH has been able to generate SLW clouds with modelled LWP and net surface radiation consistent with observations during the typical case, whereas, during the perturbed case, the modelled LWP was 10 times less than the observations and the modelled net surface radiation remained lower than the observations by ∼50 W m−2. Accurately modelling the presence of SLW clouds appears crucial to correctly simulate the surface energy budget over the Antarctic Plateau.
Highlights
Antarctic clouds play an important role in the climate system by influencing the Earth’s radiation balance, both directly at high southern latitudes and indirectly at the global level through complex teleconnections (Lubin et al, 1998)
We show the time evolution of the difference between surface radiation (W m−2) observed by Baseline Surface Radiation Network (BSRN) and calculated by ARPEGE-SH on 24 December 2018, in longwave downward (LW↓), longwave upward (LW↑), shortwave downward (SW↓), and shortwave upward (SW↑) components, superimposed on liquid water path (LWP) (Fig. 15, middle panel)
A comprehensive water budget study was performed during the Year of Polar Prediction (YOPP) SOP-SH at Dome C (Concordia, Antarctica) from mid-November 2018 to mid-February 2019
Summary
Antarctic clouds play an important role in the climate system by influencing the Earth’s radiation balance, both directly at high southern latitudes and indirectly at the global level through complex teleconnections (Lubin et al, 1998). Some in situ aircraft measurements exist over the Western Antarctic Peninsula (Grosvenor et al, 2012; LachlanCope et al, 2016) and nearby coastal areas (O’Shea et al, 2017) that provide ice mass fraction, concentration, and particle size relative to cloud temperature, cloud type, and formation mechanism, which have provided new insights into polar cloud modelling. These studies highlighted sea ice production of cloud-condensation nuclei and ice-nucleating particles, which is important in winter both coastally and at Dome C (see, e.g., Legrand et al, 2016). Over the Antarctic Plateau, where the atmosphere is colder and drier than along the coast, ice crystal clouds are mainly observed with crystal sizes ranging from 5 to 30 μm (effective radius) in the core of the cloud; mixed-phase clouds are preferably observed near the coast (Listowski et al, 2019) with larger ice crystals and water droplets (Lachlan-Cope, 2010; Lachlan-Cope et al, 2016; Grosvenor et al, 2012; O’Shea et al, 2017; Grazioli et al, 2017)
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