Cloud macro-physical properties in Saharan-dust-laden and dust-free North Atlantic trade wind regimes: a lidar case study

The Next-generation Aircraft Remote-Sensing for Validation Studies (NARVAL) aimed at providing a better understanding of shallow marine trade wind clouds and their interplay with long-range-transported elevated Saharan dust layers over the subtropical North Atlantic Ocean. Two airborne campaigns were conducted – the first one in December 2013 (winter) and the second one in August 2016, the latter one during the peak season of transatlantic Saharan dust transport (summer). In this study airborne lidar measurements in the vicinity of Barbados performed during both campaigns are used to investigate possible differences between shallow marine cloud macro-physical properties in dust-free regions and regions comprising elevated Saharan dust layers as well as between different seasons. The cloud top height distribution derived in dust-laden regions differs from the one derived in dust-free regions and indicates that there are less and shallower clouds in the dust-laden than in dust-free trades. Additionally, a clear shift of the distribution to higher altitudes is observed in the dust-free winter season, compared to the summer season. While during the summer season most cloud tops are observed in heights ranging from 0.5 to 1.0 km, most cloud tops in winter season are detected between 2.0 and 2.5 km. Moreover, it is found that regions comprising elevated Saharan dust layers show a larger fraction of small clouds and larger cloud-free regions, compared to dust-free regions. The cloud fraction in the dust-laden summer trades is only 14 % compared to a fraction of 31 % and 37 % in dust-free trades and the winter season. Dropsonde measurements show that long-range-transported Saharan dust layers come along with two additional inversions which counteract convective development, stabilize the stratification and may lead to a decrease in convection in those areas. Moreover, a decreasing trend of cloud fractions and cloud top heights with increasing dust layer vertical extent as well as aerosol optical depth is found.