In situ observations of supercooled liquid water clouds over Dome C, Antarctica by balloon-borne sondes

Abstract

Abstract. Clouds in Antarctica are key elements that affect radiative forcing and thus Antarctic climate evolution. Although the vast majority of clouds are composed of ice crystals, a non-negligible fraction is constituted of supercooled liquid water (SLW, water held in liquid form below 0 °C). Numerical weather prediction models have a great difficulty to forecast SLW clouds over Antarctica favouring ice at the expense of liquid water, and therefore incorrectly estimating the cloud radiative forcing. Remote sensing observations of SLW clouds have been carried out for several years at Concordia station (75° S, 123° E, 3233 m above mean sea level), combining active LIDAR measurements (SLW cloud detection) and passive HAMSTRAD microwave measurements (liquid water path, LWP). The present project aimed at in situ observations of SLW clouds using sondes developed by the company Anasphere, specifically designed for SLW content (SLWC) measurements. These SLWC sondes were coupled to standard meteorological pressure-temperature-humidity sondes from the Vaisala Company and released under meteorological balloons. During the 2021–2022 summer campaign, 15 launches were made, of which 7 were scientifically exploitable. Above a height of 400 m above ground level, we found that the SLWC sondes detected SLW clouds in a vertical range consistent with LIDAR observations. In nominal operation, the LWP values obtained either by HAMSTRAD or vertically-integrated from the SLWC sonde profiles were consistent in spite of their low values (< 10 g m-2). On some occasions far from nominal operation (surface fog, low vertical ascent of the balloon), the LWPs from the SLWC sonde were overestimated by a factor of 5–10 compared to the HAMSTRAD values. In general, the SLW clouds were observed in a layer close to saturation (U > 80 %) or saturated (U ~100–105 %) just below or at the lowermost part of the entrainment zone or capping inversion zone which exists at the top of the Planetary Boundary Layer and is characterized by an inflection point in the potential temperature vertical profiles. Our results are consistent with the theoretical view that SLW clouds form and pertain at the top of the Planetary Boundary Layer.