Temporal and spatial variation of 10-day mean air temperature in Northwestern China

Both inter- and intra-annual changes in air temperature are impacted by global climate change. Sine curves, such as T = A sin(ω t + φ), adjust the 10-day mean air temperature T contributed by the corresponding number of 10-day t within a year. Based on the curve, relative indices were defined as follows: A , indicating the annual maximum 10-day air temperature; t LIP (0) and t RIP (0) , representing the starting and ending time of the warm season; t LIP (10) and t RIP (10) , indicating the starting and ending time of the growing season; l ws and l gs , denoting the length of the warm and growing seasons. The objective of this study was to evaluate the inter- and intra-annual distribution characteristics of the 10-day mean air temperature and the associated spatial patterns, changing trends, and abrupt points in 128 meteorological stations across Northwestern China during 1961 to 2010 by using the selected indices. The results showed that the 2010s had the fastest increase in air temperature during the study period across the study area. Altitude was an important factor affecting the selected indices. Annual mean air temperature ( T mean ) and A were high in low-altitude areas, where the warm and growing season came earlier and ended later, and vice versa. The values of T mean and A had increased by 1.65 and 0.65 °C during the past 50 years, respectively. The warm and growing season had come earlier by 9 and 7.5 days and ended later by 7.5 and 6 days during the past 50 years, respectively. Therefore, the warm season has gotten longer by 16 days and the growing season by 14 days in the study area during the past 50 years. The abrupt increase of T mean , A , t LIP (0) , t RIP (0) , t LIP (10) , t RIP (10) , l ws , and l gs detected at the 0.05 significance level took place in the late 1960s and that of A occurred in the late 1980s. The inter- and intra-annual distribution of air temperature in Northwestern China has changed during the last five decades, and it is reasonable to expect those changes to continue for a period under global climate change.