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. Explanations for this transition, called the spring cloud onset, involve (1) increasing advection of warm moist air masses from mid-latitudes in spring and (2) reduced ice production efficiency as spring temperatures rise. 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 °C 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, this study suggests that while moisture transport from mid-latitudes is already sufficient in March to support a spring cloud onset, the temperature increase above the Arctic sea-ice, induced by the increase of solar radiation, enables the increased formation of liquid-containing clouds in April.