On the meso versus large scale control of trade cumulus clouds

Abstract

Trade cumulus clouds are small clouds whose behaviour cannot be attributed to a single environmental factor. Previous studies have shown that some of their variability can be explained by large-scale environmental changes on daily to monthly timescales. At the same time, the clouds themselves exhibit substantial variability on hourly (meso- scales) timescales. Recent studies suggest that this mesoscale variability is associated with mesoscale circulations and rain-driven cold pools. However, it remains unclear how the two scales come together in their control the clouds. This gap is investigated in this thesis. Addressing this question requires examining the types of clouds observed by studies focusing on mesoscale processes versus those emphasising large-scale variability. Large-scale factors are typically studied using satellite observations of cloudiness, while mesoscale processes have been examined using airborne cloud observations. Using observations from the EUREC4 A campaign which happened in January-February 2020, east of Barbados, the study shows that mesoscale circulations under non-precipitating conditions are associated with projected cloud cover whose cloud tops lie between 900 m and 1400 m. Furthermore, projected cloud cover below 900 m, which accounts for more than 25% of trade wind cloudiness, is not explained by the cloud-base convective mass flux. Instead, it is strongly associated with wind shear within the cloud layer. Together, cloud-layer shear and convective mass flux explain nearly 80% of low-level trade wind cloudiness. For precipitating clouds at higher altitudes, I find a strong association with mesoscale vertical motion at approximately 1900 m. Using a case study of two days characterised by different types of mesoscale organisation, The study proposes the hypothesis that explaining the high cloud cover - which accounts for nearly 60% of trade wind cloudiness requires a cloud–circulation–rain framework that combines the effects of cold pools with mesoscale circulations. Such a framework provides a pathway toward understanding the mesoscale patterns of cloudiness observed from space.