Effect of urbanization on weather events

Question

At what stage is the studies on effect of urbanization on weather events (eg: Rainfall, Temperature, UHI) are now? There has been a number of observational and modelling studies around the world and it has been found (In my understanding) that urbanization cause increase in frequency and intensity of heavy rainfall, higher temperature, heat island, changes in boundary layer etc. in urbanized region, mainly due to land-surface change. Now, What challenges and possibilities are there in this particular research field?

Answer 1

I did my undergraduate capstone on observational moisture changes around cities. It's an interesting field, though it certainly seems fairly challenging to draw out and explain changes given the incredible spatial and temporal variability. So that's definitely one of the challenges to it. Another is the current observational density, as there are very few micronets setup and satellite resolution probably isn't helpful to diagnose variations on small time/distance scales.

But, while it definitely doesn't have near the attention (or money) that climate modeling itself does, there are a few sharp folks working in the field at least. From my experience, having dabbled on the topic but having incomplete research familiarity particularly over the past decade, it would seem that theoretical microscale modelling might be the most fruitful research direction. Computing power and models continue to step up, and there's much better room for experimentation. But then, it'll still be very hard to demonstrate that those idealized simulations match reality, and impacts to connect changes definitively to theory/causes.

Indeed, if somehow sharp links can be made that are powerfully convincing, then work could be directed into reshaping urban planning to address such changes.

Answer 2

This is a very interesting topic, since climate and land surface model is becoming more powerful and with higher resolution data available, it is possible to include urban land tiles in the model.

The biggest difference of urban region and natural region is the surface albedo and heat capacity. Buildings and roads with dark surfaces have a lower albedo and absorb more sunlight and heat. Building material especially concrete has a lower specific heat capacity than the natural vegetation, it can heat up easily and warm the surrounding at night, which create the heat island effect.

High rise buildings cluster in the city centre also create an urban boundary layer on the near surface atmosphere, this has altered the wind speed and hence affect evapotranspiration and the heat diffusion. Consequently it also traps air pollutants causing photochemical smog.

Traffic and factories in urban area create high concentration of aerosol, NOx, CH4 and it produces ozone through secondary pollution. Ozone gases at the troposphere is extremely dangerous to living organism, it can trigger asthma and reduce plants' photosynthesis. The dust and PM2.5 particles could also induce higher precipitation because it acts as hygroscopic nuclei for rain droplets to form.

Urban area is usually built with efficient drainage system to avoid flooding. This however reduce the evaporation and transpiration from rain event and cause lower relative humidity.

These properties of urban area are incorporate into the land surface model. One of the model that did this is the Joint UK Land Environment Simulator (JULES). It has a land tile that is specifically dedicated to urban area [Young, 2014, presentation]. It can be used to test the urban canyon effect, heat island effect and the impact of building green roofs and sky garden etc.

More detail about the model can be found here: http://jules.jchmr.org/content/about#overview

Answer 3

Now, What challenges and possibilities are there in this particular research field?

Now that urban effects on regional weather can be observed, modeled, and to a large degree verified, all that remains on the science side is improving forecast resolution.

Serious work, however, is being done on the policy side given these new cause and effect scenarios, although how seriously this effort is being taken is largely nation-dependent: China's response to Beijing's consequences of creating its UHI is a lot different than how the US responds to Atlanta's.

To give you another example, consider how basic mortality data for a heat wave is tabulated for Paris versus New York City. For similar events, Paris records heat-related deaths that are orders of magnitude higher than in NYC, for the simple reason that in NYC, according to city records, the elderly die of "natural causes" regardless of weather. By contrast, in Paris, if a spike in deaths is concurrent with a weather event, the weather event is held accountable.