The Turbulent Structure of the Urban Boundary Layer Judith McConnell, Prof. Janet Barlow, Dr Omduth Coceal, The ClearfLo Consortium

Background The Planetary Boundary layer (PBL) is the lowest layer of earth’s atmosphere whose behaviour is directly influenced by interaction with the surface. Highly turbulent variations of meteorological parameters and its nature of being highly responsive to surface changes can make it challenging to model and observe. A PBL being influenced by an urbanised surface can be called an Urban Boundary layer (UBL)

Motivation Human health can be negatively impacted by repeated exposure to pollutants found in high concentrations in cities, such as Small particulates (PM10 and PM2.5 size ranges) Nitrogen Oxides Near-surface ozone And also by the additional heat stress associated with the Urban Heat Island (UHI) effect With the percentage of the global population living in cities on the increase, improved forecasting of urban weather and pollution will be desirable.

The ClearfLo Project “ClearfLo” = “Clean Air for London” The combined work of several academic institutions, which aims to Synthesise both long-term data observations of pollution loading and the urban meteorology of London. Assessing both the local and regional/non-local processes that influence London’s air quality. Evaluating how well current models can represent the evolution of a UBL. To this end there were 2 Intensive Observation Periods (IOP’s) in 2012. One carried out from 6th Jan – 11th Feb 2012 and one from 23rd July - 17th August 2012.

Current Aims Analysing the two primary case studies of pollution events that emerges from the first IOP and identify what met factors contributed to them. Investigating the use of a simple boundary layer model coupled to an urban surface and how it could be modified to include pollution dispersion. My primary long term goal is to describe the climatology of how UBL structure and turbulent statistics evolve, diurnally and seasonally.

The BT Tower in London (Photograph by Adrian Welch) IOP Instruments The weather station at the top of the BT tower, along with an eddy correlation system, an infra-red hygrometer and a net radiometer at 190 m from the surface. Rooftop data from a Halo Photonics scanning Doppler lidar which records the backscatter and vertical turbulence through the UBL, located in Central London. -> “Stare”, “Doppler Beam Swinging” and “Range-Height Indicator” scans. The BT Tower in London (Photograph by Adrian Welch) HALO Photonics Doppler lidar (Photograph courtesy of King’s College London)

Case Study 15th-16th Jan – Lidar Observations With the exception of a few clouds clear conditions and a deeply convecting UBL. The aerosol layer depth drops on the evening of the 15th Aerosol layer depth remains very shallow on the 16th – possible strong capping inversion?

Case Study 4th-5th Feb – Lidar Observations Conditions much more unsettled with cloud cover in February. Snowfall on evening of 4th and morning of 5th appears as backscatter streaks on lidar. Possible mist/fog during 5th – snow lying on the ground?

15th-16th Jan – Airmass Source Primarily an oceanic source?

4th-5th Feb – Airmass Source Continental source, increase in particulate pollution?

Future Work Looking at seasonal tendencies in UBL development by comparing the summer and winter IOP data from this year. Calculating the total horizontal wind speeds from the DBS-type lidar scans could prove useful in identifying the structure and formation of nocturnal jets in the UBL. Try simulating UBL growth by including an urban canopy parameterization in a simple boundary layer numerical model.

Any Questions? Thanks for listening! For more information on the ClearfLo project please visit www.clearflo.ac.uk