Wavelet characterizing the non-stationary features of mountain valley winds both in frequency and time domain

This study aimed to quantify and evaluate the non-stationary characteristics of wind speed in mountain canyon terrain. A non-stationarity index was proposed based on continuous wavelet transform to quantitatively evaluate the non-stationarity of measured wind speed samples in both time and frequency domains. Subsequently, the effect of sample length and observation time interval of wind speed samples on the non-stationarity index was investigated. In addition, the non-stationarity of mountain canyon winds was compared with that of normal climate winds under the same sample length and observation time interval. The results indicate that the nonstationarity index based on wavelet transform can quantitatively evaluate the non-stationary characteristics of wind speed. With an increase in the sample length of wind speed series, the non-stationarity of wind speed increase gradually, and its probability distribution becomes more positively skewed. When the sample length of wind speed does not exceed 10 min, the probability distribution of non-stationarity index for mountain valley terrain and normal climate winds is not significantly different, which implies that it is reasonable for most countries to use a 10-min statistical length to calculate average wind speed. However, the difference in the probability distribution of non-stationarity index between mountain and normal climate winds significantly increases when the sample length exceeds 10 min. Besides, the non-stationarity index can reflect the nonstationary characteristics of wind speed series at different frequency components. Higher frequency components in wind speed exhibit stronger non-stationary characteristics. Moreover, mountain winds exhibit stronger non-stationary characteristics than normal climate winds in amplitude and frequency, especially the highfrequency components in wind speed series with long sample lengths.