UHI layers were shown to be capped by one or more weak and shallow elevated temperature inversions and by a “reverse heat island” layer of about 100 m, in which a negative UHI existed. Various explanations have been put forth as to the origin of urban elevated inversions, e.g., they are remnants of the surface based rural inversion as it is advected over a city or they form from radiative cooling from the top of the urban pollution layer. Various explanations have also been put forth as to the origin of the “reverse heat island” layer aloft, e.g., radiative flux divergence from the top of the urban pollution layer or UHI-induced convergence, which produces rural downward motion and subsidence warming over surrounding rural areas.
The presentation thus presents a summary of these classic observational studies, along with a history of the modeling attempts (from 1-D analytical to 3-D numerical) that tried to explain these observed features of the urban boundary layer. It finishes with suggestions for required future observational and modeling studies that could answer some still-existing questions concerning the vertical structure of PBL UHIs.