Boundary layer clouds make up a large part of the total cloud cover across the world. These clouds play an important role in the vertical transport of heat, moisture, and momentum from the surface through the boundary layer. Thus these clouds have a significant impact on the vertical structure of the boundary layer. They not only have an impact on the vertical structure, but also have a significant impact on the Earth's radiation budget. Normally boundary layer clouds generally have a higher albedo compared to the surface below them and as a result there is an increased reflectance of solar radiation. Due to these strong impacts on the atmospheric conditions it is important that these boundary layer clouds and their processes are taken into account when simulating (future) climates.
One of the largest uncertainties in climate projections is related to the uncertainty in how boundary layer clouds respond to climate change. This uncertainty in cloud feedback is primarily related to the use of general circulation models (GCMs) in climate projections. As GCMs have a very coarse resolution they require parameterizations to represent boundary layer processes and clouds. These parameterizations are imperfect and therefore the GCMs have difficulties in representing the radiative effects of clouds. Therefore high resolution models such as large-eddy simulations (LESs), which require less parameterizations are used to study boundary layer processes and clouds.
Several LES studies have been conducted on climate projections, where a perturbation of a future climate is applied to the model. These perturbations include increases in sea surface temperature and/or the concentration of CO2. In future climates it is anticipated that the atmosphere will become warmer and therefore it can contain a much larger concentration of moisture. This increased moisture can lead to the presence of very humid layers above the boundary layer, known as elevated moisture layers, which have already been observed in nature. This thesis investigates the response of boundary layer clouds to the presence of an elevated moisture layer, based on observed conditions during research flight 4 of the first Next Generation-aircraft Remote-sensing (NARVAL) campaign. This study is divided into three main sections. The first and second parts of the analysis focus on comparing the
LES to observations recorded during the campaign in order to test the representativeness of the model. Following this the response of boundary layer clouds to an elevated moisture layer perturbation is investigated.
To this purpose, LESs are initially generated at the locations of the 11 dropsondes launched during the fourth research flight of the NARVAL campaign, which took place on December 14th 2013. Initial comparisons indicate the LES shows good ability in representing the atmospheric conditions observed, showing a strong evolution of the boundary layer over time which has previously been observed at the Barbados Cloud Observatory. The results from the simulations also indicate that the LES has an ability to capture the height of the boundary layer inversion. There are some limitations in capturing the strength of the inversion, which is potentially related to the extremely dry conditions observed above the boundary layer.
The LES is then compared to retrievals from the High Altitude and Long Range Aircraft (HALO) Microwave Package (HAMP) instrument. In order to take the flight path into account the mean large-scale profiles, from the locations of 9 dropsondes, are used to derive a composite case. The aim of using the composite case was to investigate whether the LES has the ability to capture the large variability in the integrated water vapor and liquid water path retrieved throughout the flight path. Using a large domain LES, with horizontal extent reaching 51.2 km2 the variability in integrated water vapor and liquid water path does approach the retrieved values, while domains with a smaller domains have a larger underestimation of the variability. The simulations indicate a correlation between the degree of organization, Iorg and the precipitation flux, variability in integrated water vapor, and variability in liquid water path. A similar slope of dependency between the variability in integrated water vapor and Iorg is found, across all simulations. In comparison the slopes of dependency between the Iorg and both the variability in liquid water path and precipitation flux values differ between each of the simulations. This suggests that there are different structures in the clouds between simulations and that the Iorg is highly controlled by the water vapor distribution.
These studies give confidence that the LES has the ability to capture observed conditions, which is important for simulating future climates. For the investigation into the impact of an elevated moisture layer and the corresponding response of the boundary layer clouds, two sets of simulations were generated on a 25.6 km2 domain using the composite case setup from the HAMP comparison. These two sets of simulations include a control simulation and a set of 5 elevated moisture layer simulations with varying elevated moisture layer depth. While the elevated moisture layer has a significant impact on the atmospheric conditions in the free troposphere, while the largest impact in the boundary layer occurs in the cloud fraction. A decrease in the cloud layer depth is found with increasing elevated moisture layer depth. The impact is not however limited to the vertical structure of the clouds with a significant impact also found in the radiative fluxes throughout the lower troposphere. In order to determine the response of the boundary layer clouds to a change in climate, represented here by the elevated moisture layer, the cloud radiative effect is calculated at the top of the cloud layer. The results indicate there is a positive feedback from the boundary layer clouds produced in response to the elevated moisture layer, which indicates that these clouds have a warming effect on the boundary layer
