Understanding Diurnality and Inter-Seasonality of a Sub-tropical Urban Heat Island

We quantify the spatial and temporal aspects of the urban heat-island (UHI) effect for Kanpur, a major city in the humid sub-tropical monsoon climate of the Gangetic basin. Fixed station measurements are used to investigate the diurnality and inter-seasonality in the urban-rural differences in surface temperature () and air temperature () separately. The extent of the spatial variations of the nighttime and is investigated through mobile campaigns and satellite remote sensing respectively. Nighttime values dominate during both the pre-monsoon (maximum of 3.6 ) and the monsoon (maximum of 2.0 ). However, the diurnality in is different, with higher daytime values during the pre-monsoon, but very little diurnality during the monsoon. The nighttime value is mainly associated with differences in the urban-rural incoming longwave radiative flux ( during the pre-monsoon; 0.65 during the monsoon), which, in turn, causes a difference in the outgoing longwave radiative flux. This difference may modulate the nighttime value as suggested by significant correlations ( for the pre-monsoon; 0.50 for the monsoon). The magnitude of may also be modulated by advection, as it is inversely related with the urban wind speed. A combination of in situ, remotely sensed, and model simulation data were used to show that the inter-seasonality in , and, to a lesser extent, in , may be related to the change in the land use of the rural site between the pre-monsoon and the monsoon periods. Results suggest that the degree of coupling of and may be a strong function of land use and land cover.