Toward designing strategies for urban heat island mitigation based on multiscale flow considerations

Much of the on-going discussion on urban heat island mitigation and proposed measures for cooling is based on case-studies taken at a specific scale and settings; the evaluation of the effectiveness of proposed cooling measures is therefore made using performance criteria derived for that specific scenario.  The transferability of this knowledge to other sites and climatologies is not ensured. This is because the phenomena dictating the urban climate are inherently multi-scale and the contribution of heating sources or cooling mechanisms as well as their interaction with other ongoing-possibly physical phenomena can be different.  Therefore, strategies for urban cooling in a city should consider the mutli-scale nature of urban climate, based on which mitigation actions and costs must be considered.  In this work we report results from a multi-scale field experiment conducted in Nicosia-Cyprus in July 2010 to investigate the Urban Heat Island (UHI) in Nicosia capital city and its interaction with multi-scale meteorological phenomena that take place in a broader region over Cyprus. Specifically, the results are analysed and interpreted in terms of a non-dimensional/scaling parameter dictating the urban heat island circulation reported from laboratory experiments (Fernando, 2010). We find that the field measurements obey the same scaling law during the day, in the absence of any other flow phenomena apart from the urban heating. During the night  we find that the deduced non-dimensional value reduces to half  (compared to that during the day); this is due to the presence of katabatic winds from Troodos mountains into the urban center of Nicosia and their cooling effect superimposed on diurnal urban heating. Based on this deduction, we evaluate the impact of various proposed heat island mitigation measures in urban planning.