Laar, Thirza W. van ORCID: 0000-0002-6229-5014 (2019). Spatial patterns in shallow cumulus cloud populations over a heterogeneous surface. PhD thesis, Universität zu Köln.

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Abstract

Shallow cumulus clouds play an important role in the earth's climate. They are effective transporters of temperature, humidity and momentum and impact the radiation budget. It is therefore important to correctly represent them in numerical weather prediction and climate models. However, because of their small scale they are not directly resolved by the large scale models. Their presence and effect therefore is approximated by parametrization schemes. These parametrization schemes use many assumptions and have their uncertainties, therefore clouds are one of the major sources of uncertainty for climate prediction. To minimize the uncertainties associated with the different parametrization schemes, a more complete understanding of what influences cloud formation is needed. In this thesis two specific aspects of processes associated with shallow cumulus cloud populations are studied. The first one is their spatial organization. Through convective organization the mean state of the atmosphere is affected. Quantifying this behaviour could help in understanding the mechanisms behind organization. The second one is their dependence on surface conditions, more in particular the role of surface heterogeneity. Since shallow cumulus are strongly coupled to the surface, their formation and spatial distribution is influenced by heterogeneous surface conditions. In this thesis first some exploratory work is done on the description of a shallow cumulus cloud population in terms of size and spacing. The tools acquired are applied to asses the influence of a heterogeneous surface on the spatial patterns in shallow cumulus cloud populations. For a statistically reliable assessment of cloud size distributions, Large Eddy Simulations (LES) are used. 146 simulations are performed for days that feature shallow cumulus clouds. It is found that the cloud size distribution can be described by a power law-exponential function. The largest cloud in the field is found to correlate with the total cloud cover, meaning that larger clouds contribute most to a larger total cloud cover, and not an increase in number of smaller ones. To study cloud spacing, data is used from a large domain LES over the ocean in the subtropics. The data shows that the more numerous small clouds surround larger ones. The distances between clouds depend on the size of the cloud itself. The larger the cloud, the larger the distance to its nearest neighbor. The functional relation between cloud spacing and cloud size differs when either all clouds are taken into account, or only clouds of a similar size. For clouds of a similar size the spacing is found to increase exponentially with cloud size. To quantify the degree of organization of a complete cloud population, several parameters are evaluated. Taking into account advantages and disadvantages of all, it is concluded that I$_{org}$ is the best one to use, mainly because it describes the tendencies seen by eye and is useful over a range of scales. Finally the impact of surface heterogeneity on the cloud population is studied. For this, two different shallow cumulus days are simulated using a realistic set-up with cloud resolving resolutions. A sensitivity study is done, increasing and decreasing the topography, and changing the distribution of land use types. The cloud size distribution is not greatly affected by these changes in surface conditions. The slope stays the same, only the maximum cloud size differs slightly for the simulations, with the largest clouds for the simulations with increased topography. Judging by eye, the spatial distribution of clouds differs among the simulations, but this is not reflected in I$_{org}$. For the simulation with a different distribution of land use types a quasi secondary circulation can form if the wind direction allows for it. Even though quantification is not straightforward, using a realistic set-up shows that surface conditions do influence the spatial patterns in shallow cumulus cloud populations.

Item Type: Thesis (PhD thesis)
Creators:
CreatorsEmailORCID
Laar, Thirza W. vanvanlaar@meteo.uni-koeln.deorcid.org/0000-0002-6229-5014
URN: urn:nbn:de:hbz:38-102219
Subjects: Natural sciences and mathematics
Uncontrolled Keywords:
KeywordsLanguage
Shallow cumulus cloudsEnglish
Surface-atmosphere interactionEnglish
Cloud size distributionEnglish
Convective organizationEnglish
LES simulationEnglish
Faculty: Faculty of Mathematics and Natural Sciences
Divisions: Faculty of Mathematics and Natural Sciences > Department of Geosciences > Institute for Geophysics and Meteorology
Language: English
Date: 11 November 2019
Date of oral exam: 22 July 2019
Referee:
NameAcademic Title
Neggers, Roel A.J.Prof. Dr.
Refereed: Yes
URI: http://kups.ub.uni-koeln.de/id/eprint/10221

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