Jenny Stocker

The comfort, energy and health implications of London's urban heat island

The urban heat island (UHI) is a well-known effect of urbanisation and is particularly important in world megacities. Overheating in such cities is expected to be exacerbated in the future as a result of further urban growth and climate change. Demonstrating and quantifying the impact of individual design interventions on the UHI is currently difficult using available software tools. The tools developed in the LUCID (‘The Development of a Local Urban Climate Model and its Application to the Intelligent Design of Cities’) research project will enable the related impacts to be better understood, quantified and addressed. This article summarises the relevant literature and reports on the ongoing work of the project. Practical applications: There is a complex relationship between built form, urban processes, local temperature, comfort, energy use and health. The UHI effect is significant and there is a growing recognition of this issue. Developers and planners are seeking advice on design decisions at a variety of scales based on scientifically robust, quantitative methods. The LUCID project has thus developed a series of tools that (1) quantify the effect of urbanisation processes on local environmental conditions, and (2) quantify the impact of such conditions on comfort, energy use and health. The use of such tools is vital, both to inform policy but also to be able to demonstrate compliance with it.

The effectiveness of retrofitted green and cool roofs at reducing overheating in a naturally ventilated office in London: Direct and indirect effects in current and future climates

Mitigating summertime overheating is increasingly viewed as a key issue in urban planning – a warming climate and increasing urbanisation will exacerbate the problem. The effectiveness of green and cool roofs at reducing summertime overheating was assessed for a naturally ventilated, poorly insulated office roof in London. This was contrasted to the application of retrofitting traditional insulation. The new Chartered Institute of Building Service Engineers overheating criteria was used to assess the level of overheating as predicted by a whole building thermal simulation model. The impacts of the roofing strategies were split into the direct and indirect effects. The indirect effects of the roofs were modelled using microclimatic modelling software. The results indicate the direct effects of green and cool roofs at reducing overheating are much greater than the indirect cooling effect. A non-insulated cool roof was found to be the most effective strategy. By insulating the roof, the level of overheating was slightly reduced. Non-insulated green and cool roofs were more effective than insulated roofs at reducing levels of overheating. When using a 2050 weather file, the building frequently overheated without the use of green or cool roof.

ADMS-Urban: developments in modelling dispersion from the city scale to the local scale

Many countries perform national air quality assessments using grid-based numerical air dispersion models, generally referred to as ‘regional’ models. Advantages of these models include the ability to use temporally and spatially varying meteorology and model chemical reactions over large temporal and spatial scales. These models usually perform reasonably well against rural and urban background monitors, but predictions at roadside monitors are underestimated. City-scale air dispersion models have been developed to give high spatial resolution, but are usually restricted to use spatially homogeneous meteorological data and to model simplified chemical reactions over short time scales. Thus, regional and city-scale air dispersion models have complementary strengths and a system where a city-scale model is nested within a regional model allows accurate air dispersion modelling over a range of spatial scales. This paper presents preliminary modelling results from a system where the local model ADMS-Urban is nested within the regional model, CMAQ.

The impact of the London Olympic Parkland on the urban heat island

The London Olympic Parkland represents a substantial area of redevelopment with the potential to significantly modify urban temperatures. This paper illustrates a neighbourhood-scale model of the type that can be used to analyse the impact of large developments on the urban heat island, using the London Olympic Parkland as an example. Using the Atmospheric Dispersion Modelling System Temperature and Humidity model, the impact of the urban surfaces for the Parkland area (∼16 km2) is modelled at a 400 m2 grid resolution for the pre-Olympic, Olympic and Legacy periods. Temperature perturbations from upwind values are simulated for the periods to estimate the contribution the Parkland has on local air temperatures. The results illustrate the impact that large impermeable features such as the concourse might have on increased air temperatures during Olympic period design conditions. In comparison, a Legacy scenario shows temperature reductions from the pre-Olympic period due to an increase in vegetation coverage.

Evaluation of an explicit NOx chemistry method in AERMOD

ABSTRACT An explicit NOx chemistry method has been implemented in AERMOD version 15181, ADMSM. The scheme has been evaluated by comparison with the methodologies currently recommended by the U.S. EPA for Tier 3 NO2 calculations, that is, OLM and PVMRM2. Four data sets have been used for NO2 chemistry method evaluation. Overall, ADMSM-modeled NO2 concentrations show the most consistency with the AERMOD calculations of NOx and the highest Index of Agreement; they are also on average lower than those of both OLM and PVMRM2. OLM shows little consistency with modeled NOx concentrations and markedly overpredicts NO2. PVMRM2 shows performance closer to that of ADMSM than OLM; however, its behavior is inconsistent with modeled NOx in some cases and it has less good statistics for NO2. The trend in model performance can be explained by examining the features particular to each chemistry method: OLM can be considered as a screening model as it calculates the upper bound of conversion from NO to NO2 possible with the background O3 concentration; PVMRM2 includes a much-improved estimate of in-plume O3 but is otherwise similar to OLM, assuming instantaneous reaction of NO with O3; and ADMSM allows for the rate of this reaction and also the photolysis of NO2. Evaluation with additional data sets is needed to further clarify the relative performance of ADMSM and PVMRM2. Implications: Extensive evaluation of the current AERMOD Tier 3 chemistry methods OLM and PVMRM2, alongside a new scheme that explicitly calculates the oxidation of NO by O3 and the reverse photolytic reaction, shows that OLM consistently overpredicts NO2 concentrations. PVMRM2 performs well in general, but there are some cases where this method overpredicts NO2. The new explicit NOx chemistry scheme, ADMSM, predicts NO2 concentrations that are more consistent with both the modeled NOx concentrations and the observations.

Model inter-comparison and validation of ADMS plume chemistry schemes

Two schemes for the determination of NO2 concentrations in the atmospheric dispersion model ADMS are evaluated using data from two sites in Alaska. Both schemes take account of the rate of oxidation of NO and photolysis of NO2 in the plume using identical chemical formulations. The differences lie in the approaches used for the entrainment and mixing of ambient ozone into the plume. In the standard scheme it is assumed that ozone is mixed instantaneously into the plume at source; in the entrainment limited scheme ozone is entrained into the plume at a rate determined by the rate of dilution of the instantaneous plume. A methodology comprising a scatter plot of the ratio of modelled to observed NO2 vs. modelled to observed NOx is used to distinguish errors in the chemistry schemes from errors in the prediction of NOx. Both schemes show good performance statistics with the standard scheme predicting higher NO2 concentrations.