Impact of elevated water vapor and aerosol layers on the stability of the sub-tropical atmosphere and the structure and evolution of shallow marine clouds

Clouds and convection play a key role in structuring atmospheric circulation and in determining the climate sensitivity. However, it is still not understood how clouds and convection will respond to warming of the atmosphere. This is due to an insufficient representation of clouds and moist convection in climate models. A better understanding of the coupling between water vapor, convection, cloud formation and circulation is needed. Shallow marine convection shows the largest frequency of occurrence amongst clouds. But besides being uniform clouds of similar structure, they can occur in different larger scale patterns of organization. The trade wind region is characterized by a complex structure of water vapor, aerosols and clouds. Depending on the season and larger scale circulation, it was found, that lofted layers of water vapor and aerosols can have a quite significant impact on the atmospheric stability, and with that on cloud structure and evolution.

Airborne lidar measurements with the combined water vapor differential absorption and high spectral resolution lidar system WALES provide simultaneous measurements of the water vapor mixing ratio and of aerosol properties. The WALES instrument was deployed in a series of airborne experiments aiming to better understand the coupling of clouds and convection over the sub-tropical and tropical Atlantic Ocean. The first campaign of this series, the NARVAL experiment, was conducted in wintertime out of Barbados. It was followed by the NARVAL-II experiment in August 2016, the EUREC4A experiment in January/February 2020 and the PERCUSION campaign in August and September 2024. The latter especially focused on the transition of shallow to deep convection and the ITCZ. Another add on of this campaign was the contrasting measurements over the east and west Atlantic Ocean. We used these measurements to investigate how the complex structure of water vapor and aerosol impact the stability of the atmosphere and with that the evolution and structure of clouds. We found that the impact is different, if the water vapor and aerosol are within distinct layers. During summertime, when they are well separated from the marine boundary layer, the radiative effect of the layers dominates. The evolution of shallow marine clouds below the SAL is suppressed. In wintertime, the e.g. dust is transported at lower altitudes and the dust layer is frequently mixed into the marine boundary layer. During this time of the year the effect of the layer on the evolution and lifetime of marine trade wind convection is much more complex, as the dust particles within the SAL might additionally act as cloud or ice nuclei.

In our presentation we will give an overview of the performed measurements and the radiative transfer calculations that were performed based on our findings. Those calculations together with the observations better help to understand the impact of lofted layers on cloud evolution and structure.