Observations of the morning development of the urban boundary layer over London, UK, taken during the ACTUAL project

Halios, C. H. ORCID: https://orcid.org/0000-0001-8301-8449 and Barlow, J. F. (2018) Observations of the morning development of the urban boundary layer over London, UK, taken during the ACTUAL project. Boundary-Layer Meteorology, 166 (3). pp. 395-422. ISSN 0006-8314 doi: 10.1007/s10546-017-0300-z

Abstract/Summary

The study of the boundary layer can be most difficult when it is in transition and forced by a complex surface, such as an urban area. Here, a novel combination of ground-based remote sensing and in situ instrumentation in central London, UK, is deployed, aiming to capture the full evolution of the urban boundary layer (UBL) from night-time until the fully-developed convective phase. In contrast with the night-time stable boundary layer observed over rural areas, the night-time UBL is weakly convective. Therefore, a new approach for the detection of the morning-transition and rapid-growth phases is introduced, based on the sharp, quasi-linear increase of the mixing height. The urban morning-transition phase varied in duration between 0.5 and 4 h and the growth rate of the mixing layer during the rapid-growth phase had a strong positive relationship with the convective velocity scale, and a weaker, negative relationship with wind speed. Wind shear was found to be higher during the night-time and morning-transition phases than the rapid-growth phase and the shear production of turbulent kinetic energy near the mixing-layer top was around six times larger than surface shear production in summer, and around 1.5 times larger in winter. In summer under low winds, low-level jets dominated the UBL, and shear production was greater than buoyant production during the night-time and the morning-transition phase near the mixing-layer top. Within the rapid-growth phase, buoyant production dominated at the surface, but shear production dominated in the upper half of the UBL. These results imply that regional flows such as low-level jets play an important role alongside surface forcing in determining UBL structure and growth.

Item Type	Article
URI	https://reading-clone.eprints-hosting.org/id/eprint/75392
Identification Number/DOI	10.1007/s10546-017-0300-z
Refereed	Yes
Divisions	Science > School of Mathematical, Physical and Computational Sciences > Department of Meteorology Science > School of the Built Environment > Energy and Environmental Engineering group
Publisher	Springer
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