Modelling Approaches at Different Scales

It should come as no surprise that not everyone would approach the UHI in the same manner. The interests of modelling a UHI to an architect for example is very likely to differ from that of a meteorologist, climatologist or business representative. As quoted by Parham A. Mirzaei, “The goal of a UHI study delineates the type of an adapted model”. 

The UHI is a complicated phenomenon and is not uniform over the entirety of its plume. Urban physics have a dynamic interaction which could range from minor influences such as human body thermal radiation up to city scale influences. The type of model that would be necessary to study the UHI is highly dependant on the scale of UHI formation and the goal of the study.

These can be broken down as follows:

Building-scale models:

Also known as building energy models (BEMs) operate under the notion of a building envelope existing as a closed system, isolated from the neighbouring buildings. External parameters such as temperature, moisture, solar and longwave radiation are incorporated into the model. BEM tools such as EnergyPlus are utilised to identify the influence of variations in the inputs, which then aids in identifying the potential effect of a warming climate on the building envelope in addition to the UHI influence. 

These types of models are very simplistic but are easily incorporated into larger scale models particularly when building energy performance is under investigation. 

~Further reading~

Micro-scale models:

A step up from BEMs, micro-scale models (MCMs) aim to identify the influence of the UHI at the urban canopy layer. Mainly of interest to architects, these include a range of models from computational fluid dynamics (CFD) models for wind flow patterns between buildings and streets to models dealing with the canopy energy budget (Urban canopy models, UCMs). Parameters of building orientation, pedestrian comfort, wind flow, vegetation, surface convection amongst other things could be investigated using MCMs, UCMs and CFDs. 

The biggest limitation would be the relatively small domain size (a few hundred meters) coupled with the steep computational costs.

~Further reading~

City-scale models:

The most well known scale of UHI modelling. Of biggest interest to meteorologists and climatologists, city-scale models operate over very large domains using meso-scale tools to identify the impact of pollution reduction approaches and surface ventilation strategies on the UHI in addition to the  natural meteorological influences on the UHI. They are based not he governing equations of fluid dynamics and incorporate other fundamentally important models such as those for: Cloud cover, soil moisture absorption and thermal radiation.

A large limitation of such city-scale models is they are modelled on very coarse cells giving a weak resolution within the surface layer (Hence the strong need for MSCs). Additionally, they cannot be easily extended into other regions due to being developed very specifically for their designated location and city structure.

~Further reading~

Figure 1: A schematic view of the UHI at multiple scales. (Source)

Models are developed to tackle particular issues related to the UHI. Most models serve a particular purpose and cannot extend to multiple scales of UHI formation. As a result, all scales carry their own share of importance with respect to who in particular is interested. Regardless of personal interest, the UHI in its entirety can only really be understood in a region where all scales have thoroughly been investigated.