Meso-scale patterns of shallow convection in the trades

• Main Author: Schulz, H.
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Universität Hamburg 2021
• Subjects: North Atlantic
• Online Access: http://hdl.handle.net/21.11116/0000-0009-A02C-1; http://hdl.handle.net/21.11116/0000-0009-A02E-F

How will marine low-level cloudiness change in a warming climate? To answer this ques- tion a better process understanding of low-level cloudiness is needed. This dissertation uses a multitude of observations and large-eddy simulations to explore how meso-scale patterns of shallow convection relate to this challenging question. This study focuses on the downwind trades and its meso-scale patterns that only recently raised interest based on the work of Stevens et al. (2020) who supplemented the traditional classes of meso-scale patterns of the upstream trades. These new classes are named based on their visual impression Sugar, Gravel, Flowers and Fish. Here they are further investigated in terms of their climatic relevance, physical characteristics, atmospheric environment and emergence. The core of these investigations consists of deep neural networks that have been trained to identify these patterns in satellite images. At the same time, the deep neural networks proved to be a valuable tool to derive a common perception of subjectively defined classes that do not have a ground truth. Although the crowd-sourced labels were therefore very noisy, the neural networks ranked among the highest in inter-annotator agreements. The classification of the neural network reveals that the patterns are common to the trades beyond the winter season in the western North Atlantic and can represent more than 40 % of the observed variability depending on season and region. This variability expresses itself not only in changes of the visual appearance but also physically in the cloud cover. A linear relationship between the cloud cover and the cloud radiative effect makes the processes leading to the patterns relevant for the climate. The underlying physical processes of each meso-scale pattern are related to the air- mass origin with an influence of diurnal variations that are potentially modulating the large-scale factors. One large-scale factor that is most distinct among the patterns is wind speed. Other factors are only related ...