Understanding the spring cloud onset over the Arctic sea-ice

Abstract. Using 13 years of space-based lidar cloud observations over the Arctic sea-ice, we show that the low cloud cover increases from 34 % to 71 % between 7 April and 7 May, mainly due to the increase of liquid-containing clouds below 1 km altitude. This study assesses the relative roles of moisture transport from the mid-latitudes and the spring warming driven by the seasonal increase of solar radiation in setting this transition called “spring cloud onset”. We highlight that daily moisture mass advected over the sea-ice in March represents 14 % of the moisture mass already present, exceeding the increase of atmospheric moisture carrying capacity due to the rising temperatures. Consistently, MOSAiC campaign data suggest that moisture is not limiting the spring cloud onset as most of the radiosoundings in spring contained layers saturated with respect to ice (96 %). Instead, we identify a temperature dependency in the ratio of ice to liquid-containing layer occurrence, confirmed by ground-based MOSAiC lidar. While the proportion of ice layers over all atmospheric layers is poorly dependent on temperature below 0 °C, the occurrence of liquid-containing layers increase steeply between −20 and −10 °C. As a result, March lower troposphere temperatures (−20 °C) favor more ice clouds, while May (−13 °C) favors more liquid-containing clouds. Overall, our results suggest that moisture availability is not the limiting factor and that spring warming induced by solar radiation sets the transition toward more liquid-containing low clouds.