Diurnal and interannual variations of canopy urban heat island (CUHI) effects over a mountain–valley city with a semi-arid climate

Abstract

Taking Lanzhou (a typical mountain–valley city in Northwest China) as an example, the diurnal and interannual variation characteristics of the canopy urban heat island (CUHI) and its relationship with heat waves (HWs), local climate zones (LCZs), mountain–valley wind circulation and aerosol pollution were explored. Results showed that the peak CUHI intensity (CUHII) in Lanzhou occurred at nighttime, while the minimum appeared in the daytime. The interannual variation of the CUHI fluctuated greatly, showing an upward trend. The average CUHII in HW periods was 103.6% higher than that in non-heat wave (NHW) periods. Longer-lasting HWs amplified the CUHII, in turn stronger CUHII led to more frequent HWs. The spatial variation of CUHIIs mainly varied with LCZ types. In addition, relative to NHW periods, the mountain–valley wind circulation was strengthened during HW periods, favoring an increase in CUHII. On the one hand, the increased urban wind speed contributed to the enhancement of vertical turbulent heat transfer during HW periods. On the other hand, the ventilation conditions were improved, resulting in the reduced aerosol concentration, the urban canopy received more shortwave radiation, and the heat storage increased during HW periods. Both of these situations were conducive to enhanced CUHII.