Two paradigms for setting the urban heat island

Measurements of the screen-level urban heat island like warmer screen level temperatures in urban areas compared to their rural surrounds are among the oldest and most frequently measured signals of urban meteorology. Nevertheless, identifying the physical processes controlling its spatial pattern, magnitude and the diurnal cycle remain high research priorities in urban meteorology. Currently, the large urban heat storage capacity along with long wave radiative cooling differences between urban and rural surrounds are understood as key mechanism for setting the UHI. Here we suggest two paradigms for setting the screen level urban heat island. Firstly, we show that the magnitude of the urban heat island is determined by the coupling of the surface energy balance to the boundary layer during the evening transition, when the solar heating reduces in late afternoon. Differences in surface cooling by outgoing long-wave radiation between urban and rural areas after sunset seem to play a relatively minor role in setting the magnitude of the heat island. Secondly, we use scaling arguments and mesoscale high resolution model simulations to estimate the role of advection in setting the urban heat island magnitude and spatial pattern. Both suggest that urban fetches of order 50km are required before the surface energy balance and boundary layer are in equilibrium. This suggests that in practice the heat islands we see over most cities are likely to have a strong advective forcing.