One of the key questions when considering the interaction between the atmosphere and the surface is “what surface?” – most climate models are not equipped to handle urban areas, and in the earliest days of climate modelling, entire cities, from New York to London and Tokyo and beyond, were treated as a simple slab of concrete.
This wasn’t a bad first attempt actually.
Let’s imagine you have two shallow boxes (as illustrated below) one you packed with some grass (and the soil its growing out of) and the other you filled with concrete and left it to harden. Now if we took two containers full of water (say each holds 100ml) and two empty containers, we tilt the boxes containing the grass/soil and concrete forward and slowly pour the water onto each surface. If we measured the contents of the empty glasses below each surface we would note there are significant differences in the amount of water that travelled over the surface and into the empty container. Why does the container beneath the concrete surface have more water?
The answer isn’t quite as simple as you may think. Firstly, yes, the grass/soil surface is what we term “pervious”, so it acts more like a sponge, absorbing and storing more water in the soil beneath, whereas the concrete is what we term “impermeable” or “hydrophobic”, it doesn’t store water so more of it “runs-off” into the waiting glass below.
However, in addition to whether or not a surface is impermeable or pervious, some water will be evaporated off both surfaces due to sunlight striking the surface, because vegetation uses sunlight, water and carbon dioxide to create food for itself, it will evaporate more water from that surface meaning less reaches the glass below.
This has been proven as the case for most cities – they absorb less water (hence can be prone to flooding) and use less heat energy for evaporation than their surrounding non-urban hinterland. Capturing these key differences in climate models therefore maybe regarded as a promising first step, however, how many cities actually resemble a concrete slab?
Think about where you live and work – cities are three dimensional and they can also be really complex – what we would term as “extremely heterogeneous” meaning lots of different materials, building shapes, and building uses exist in every city.
Whereas if you think about grass (and even to a lesser extent, forests) they all look rather alike, what we would term as “extremely homogeneous”.
So in simplifying the processes between the atmosphere and the surface (which is essentially what modelling and climate models do) we have to determine what kind of surface a) is appropriate and b) is usable by the climate models.
The problem encountered by the Urb-ADAPT project was we have a land cover data-set known as CORINE which has lots of different non-urban surfaces, and only a few urban surfaces.
Since non-urban areas are homogeneous whereas the urban area is heterogeneous, we decided to modify the CORINE dataset to remove some level of detail from the non-urban types while keeping the same level of detail for the urban types.
So in our project, the models we use to look at the urban climate of the Greater Dublin Region recognise the following kinds of urban surfaces:
Of course this neglects variations (e.g. very tall offices, or apartment blocks) but these 3 types of urban surfaces account for about 75% of the urban land cover in the Greater Dublin Region