Momentum Transport by Organised Shallow Convection

Abstract

Atmospheric shallow convection (SC) is characterised by cumulus clouds that extend between a few hundred meters and a few kilometres. These clouds are more than passive tracers of the air flow in which they are embedded. They play an active role in atmospheric dynamics and the climate system by redistributing moisture, temperature, and momentum in the troposphere.
We focus on two recent field campaigns. 1. EUREC4A took place between January and February 2020 over the North Atlantic trade wind region, which provides perfect conditions for shallow cumuli. This campaign measured clouds and the wind flows on mesoscales. 2. CMTRACE took place in September 2021 around Cabauw, The Netherlands. During CMTRACE, a collocated wind lidar and cloud radar allowed, for the first time, to measure winds from the surface up to 2 km, with a frequency of about 1 minute (∼500 m).
Our results show that CMT crucially influences wind speed and direction throughout the lower troposphere, it also controls cloud organisation and mesoscale circulations. Observations reveal an excessively pronounced diurnal cycle of the winds in the IFS, with too strong winds at night and too weak winds during the day. The role of shallow convection parameterisation (SCP) in kilometre-scale models is explored using the HARMONIE-AROME model. Experiments with inactive SCP produce stronger mesoscale circulations, altering cloud cover and wind profiles.
Finally, the thesis examines novel momentum flux observations from the CMTRACE campaign. These data confirm errors in IFS momentum flux parameterisation, particularly under active convection.
The findings outlined in this thesis hold important implications for advancing our understanding of CMT and its role in atmospheric dynamics