High-frequency fluctuation of air temperature during a heatwave event in urban environment and the physical mechanism behind

Heatwaves threaten human health and power systems. Urban climate is non-stationary and wide-spectrum, with high-frequency temperature and wind-speed variations that could overload power grids and expose people to extreme heat. In this study, Hilbert-Huang transform (HHT) was unprecedentedly used to decompose the urban-scale temperature (IMFθ1 to IMFθ6) and wind-speed (IMFW1 to IMFW6) signals during a 5-day heatwave event into 6 intrinsic mode functions (IMFs). The spatio-temporal characteristics, physical mechanism, and effective ranges of high-frequency components (IMF1 to IMF4) were unveiled. Temperature (wind speed) IMFθ1 to IMFθ4 (IMFW1 to IMFW4) had a temporal scale of 2.63 h (2.53 h), 5.88 h (5.78 h), 13.16 h (9.84 h), and 22.72 h (19.05 h); as well as a spatial scale of 2.31 km (0.99 km), 4.29 km (1.65 km), 5.94 km (2.64 km), and 6.6 km (2.97 km), respectively. The physical mechanisms of IMF1 to IMF4 were composed of turbulence and heat storage/release; disturbance induced by mountainous terrain and slope flows; land/sea breeze, together with anthropogenic heat. Besides, the peaked amplitudes of IMFθ1 were most risky in compact/open high-rise urban (1.4 °C–1.6 °C) rather than rural (0.6 °C–1.0 °C) areas. The foothill areas within 8-km coverage were susceptible to IMFθ2 (1 °C–2.1 °C). IMFθ3 (0.6 °C–3.6 °C) was effective in urban areas within 10 km from coastline. IMFθ4 (2.5 °C–3.5 °C) exhibited the most intense fluctuation in urban/suburban areas. The outcome provides references for policy makers to mitigate heat-related risks.