Megacities are extensive, with large built surface, roughness, heat storage capacity and potentially immense anthropogenic heat emissions. These factors enhance turbulent surface fluxes and hence may have a profound impact on the structure of the urban boundary layer (UBL) and its mixing height (MH). This study combines ceilometer networks operated over several years within and around the megacity of London, UK, to evaluate the urban impact on the atmospheric structure, to analyse spatial variations within the city and to characterise the seasonal and diurnal variations based on a long-term climatology.

To obtain information about the depth of the mixed layer, profiles of attenuated backscatter data from several types of ceilometers were used: Vaisala CL31 and CT25K (Finland), and Jenoptik CHM15K (Germany). Calibrated attenuated backscatter backscatter profiles (vertical gradients and signal-to-noise ratio) obtained from the different systems are compared and analysed for strong changes in gradient to detect potentially significant internal boundary layers and MH. In addition, the strength of gradient and persistence in time are used to categorise different layers. Consideration is given to the implications of the strength and weaknesses of the observation techniques (e.g. performance under clear and cloudy conditions). The work confirms the importance of careful pre-processing of the attenuated backscatter profiles observed by the various instruments. Results further illustrate that, together with the day-time mixing height, ceilometer observations can also indicate the presence of potentially significant internal boundary layers, providing insight into the characteristics of the UBL under different synoptic conditions.

The focus of this paper is to highlight the findings from an observational analysis of wind, temperature, and relative humidity vertical profiles for the estimation of stability conditions and boundary layer heights in the very dense city of New York. A microwave radiometer and a RADAR wind profiler jointly measured the measured thermal and momentum conditions for the determination of the atmospheric static stability. The measured values cover a period from 2010 to 2014. The analysis focuses on seasonal variations of observed temperature, wind speed and direction, relative humidity, and other derived quantities. Planetary boundary layer (PBL) heights were derived from three methods that consider the virtual potential temperature, the potential temperature gradient, and the relative humidity gradient. The three approaches show greater agreement in their estimations when highly unstable conditions are present than when stable conditions are present. Stability conditions were determined from the gradient of the virtual potential temperature. Observed summer maximum and minimum values for the near-surface (less than or equal to 200 m) gradient were 0.4904 K m-1 (stable) and -0.0409 K m-1 (unstable), respectively, with an average summer seasonal value of 0.0024 K m-1. The corresponding winter season values are 0.1618 K m-1 and -0.0733 K m-1, with an average seasonal value of -0.0101 K m-1. Finally, these observations are compared to corresponding values found using model results from the urbanized-Weather Forecast and Research model, or u-WRF.

This study investigated vertical distributions of turbulent statistics at two different locations along a predominant wind direction. Two Doppler Lidars were used to observe the vertical profiles of mean wind velocity and vertical momentum flux at two locations along the predominant wind direction.

Doppler Lidars were installed at a top of I4-building in Tokyo Institute of Technology (Tokyo Tech), Tokyo, and a building top of Ota-ward office, Tokyo. A sonic anemometer was also installed at a top of M1-building in Tokyo Tech for compare with the Lidar observations. Ota-ward office is about 5.5 km away from the Tokyo Tech. Measurement was conducted for 2 months in Summer, 2013.

We estimated the internal boundary layer (IBL) height from the vertical momentum flux distribution as defined by a height where the momentum flux becomes zero. It resulted that the IBL height becomes about 300 m from the ground in the urban area irrespective of the atmospheric boundary layer height. Almost same value is estimated at both observation points. It is also shown that the IBL height increases with increasing mean wind velocity. This result has a potential to improve the surface parameterization of aerodynamic drag, and also modify the wall functions used in the mesoscale simulation model.

Typical countermeasures to urban heat islands (UHI) include the use of cool materials, urban greening or urban morphologies which favor regional winds. Such measures have been studied in the lab or on small-scale demonstrators for decades. Such tests have provided a basis for the simulation of larger scale implementations which provide assistance for decision-makers trying to reduce the impact of UHIs in cities. However, tools to validate predicted effects for larger scale field implementations are important to evaluate and follow-up on the effectiveness of anti-UHI policies put in place. Today, such tools mostly consist of direct comparisons between carefully-selected case and control sites.

Among UHI countermeasures, pavement-watering, which has been studied since the 1990’s in Japan, has sparked recent interest in Europe. In Paris, it is viewed as a potential climate change adaptation tool against increasing and intensifying heat waves and has been field tested since 2012. Since July 2013, the rue du Louvre, located in the 1st and 2nd Arrondissements, has been continuously monitored. Two paired sites were selected, one watered and one control, to evaluate the field effects of pavement-watering.

Long-term measurements from this campaign have revealed that direct case-control comparisons are not suited to identifying the effects of pavement-watering. Case-control sites are never perfectly paired in the complex environments found in dense cities such as Paris, and thus measurements made at paired case-control sites are only rarely identical. In the general case, differences between case-control measurements depend on preexisting factors. If these are overlooked, they may be mistakenly interpreted as the effects of the UHI countermeasure being tested.

This paper proposes a field method for quantifying UHI countermeasure effects based on the evaluation of preexisting differences between paired case-control sites and on the comparison of test results with the obtained reference profile.