Elucidating the Multi-Timescale Variability of a Canopy Urban Heat Island by Using the Short-Time Fourier Transform

Taking the megacity of Beijing as an example, a short-time Fourier transform (STFT) method was employed to extract the multi-timescale evolution pattern of the canopy urban heat island intensity (CUHII) during 2000-2020. The STFT of CUHII showed a close relationship between the evolution of the CUHII in Beijing and the background meteorological forcing at intra-annual, weather and intra-daily scales. The intra-annual-scale spectrum of CUHII exhibited an increasing trend with obvious seasonal variation of the canopy urban heat island (CUHI). The intra-daily-scale spectrum of CUHII showed an increasing trend with the nighttime CUHI developing faster. Increasing Western Pacific Subtropical High intensity can enhance the seasonal and diurnal fluctuations of CUHII. The weather-scale spectrum of CUHII is controlled by weather system evolution, showing that the frequency of cold/heat waves (CWs/HWs) in Beijing was significantly negatively correlated with the weather-scale spectral intensity of the CUHII. CWs and HWs can increase the CUHII for a long duration. The canopy urban heat island (CUHI) phenomenon can affect human health and the ecological environment, and its multi-timescale variability brings great uncertainty to the study of urban climates worldwide. In this study, taking the megacity of Beijing as an example, a novel short-time Fourier transform (STFT) method was used to extract the multi-timescale pattern of the CUHI intensity (CUHII) during 2000-2020. The STFT of CUHII showed a close relationship between the CUHI and the background meteorological forcing at intra-annual, weather and intra-daily scales. The intra-annual spectrum of CUHII showed an increasing trend with a V-shaped mode. The local climatic backgrounds of different cities can lead to differences in the seasonal development of the CUHI. The intra-daily spectrum of CUHII showed an increasing trend due to the asymmetry in the day/night development of the CUHI. Increasing Western Pacific Subtropical High Intensity can enhance the seasonal and diurnal fluctuations of CUHII. The weather-scale spectrum of CUHII was mainly controlled by weather system evolution, showing that cold waves and heat waves can increase the CUHII over a long duration. Our findings indicate that the evolution of CUHII is a nonlinear and complex process that is directly related to multi-timescale background climate forcing. Using a short-time Fourier transform to study the multi-timescale evolution of the canopy urban heat island intensity (CUHII)Close relationships existed between CUHII and the background meteorological forcing at intra-annual, intra-daily, and weather scalesThe frequency of cold/heat waves in Beijing showed a significant negative correlation with the weather-scale spectral intensity of the CUHII