Week 3

Valerie P - February 26, 2025 12:00 pm

Good morning! Kind of actually reversing back on what I discussed in the last two posts, I want to discuss more how urban climate models are constructed and what factors can influence them.

So, when a lot of these researchers are constructing models to look at cities, many of them are based off the surface energy balance, which is the “available energy at any location, urban or rural, to evaporate water, to heat the air, or heat the ground, dependent on the balance of radiative fluxes.” So this can be shown with a simple conservation of energy equation:

Q^* + Q_F = Q_H+ Q_E+ ΔQ_S

Q^*: the net all wave radiation flux density; Q^* = (K_↓ – K _↑) + (L_↓ – L _↑) where K is the shortwave, L is the Longwave, ↓ represents incoming waves, and ↑ represents outgoing waves. Latitude can influence the incoming shortwave radiation because cloud formation and human pollution can influence scattering, resulting in different amounts of radiation reaching the surface. Surface morphology and materials can influence the shortwave outgoing radiation because it can alter the reflectivity of a surface.

Q_F: anthropengic heat flux; accounts for the energy released in cities, largely through buildings

Q_H: turbulent sensible heat flux; driven by the net available energy, the gradient in air temperature between the surface and the air above it, and the ability of the air to transport the energy away from the ‘warm’ location

Q_E: turbulent latent heat flux; dependent on the availability of moisture at the surface and the ability of the atmosphere to move the moisture away from the surface

ΔQ_S: net storage heat flux; the net uptake or release of energy by sensible heat changes in the urban canopy layer, buildings, vegetation, and the ground

So the goal of researchers is to use physics to create models that accurately reflect the values from the above equation. So for example, that’s why a researcher might want to collect data about the reflectivity or heat absorption of different building materials as well as what percent of a cities surface is covered by said materials to then create models measuring how much radiation is reflected back.

I hope that helped to shed a little more light on what these models are actually trying to calculate.

Until next week,

Valerie

Comments:

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Paul McClernon

How do these models account for varying emissivity?

February 26, 2025 at 2:02 pm - Reply

bhavitha_s

Hi Valerie! You did a great job explaining the equation behind the algorithms you're looking at. You mentioned latitude, surface morphology, and material can affect shortwave radiation—do you know what might affect longwave radiation, if anything?

February 26, 2025 at 4:43 pm - Reply

valerie_p

A lot of the same factors that impact shortwave radiation also impact longwave radiation. However, another factor that can increase longwave radiation is the trapping of longwave radiation in areas with low sky view factors (like buildings and vegetation with a larger height-to-width ratio).

March 3, 2025 at 6:26 pm - Reply