Characteristics and main influence factors of heat waves in Beijing-Tianjin-Shijiazhuang cities of northern China in recent 50 years

Heat wave is serious natural disaster that can harm human health and affect social economy, transportation and ecological environment. This paper investigates the long term trends of high temperature events in three major cities (Beijing, Tianjin and Shijiazhuang) of northern China during 1970-2019, and quantifies the contributions of main influencing factors to the variability of high temperature days. High temperature events in Beijing-Tianjin-Shijiazhuang cities mainly occur from June to July and account for 60-65% of the annual total, showing a significant upward trend in the interannual change. There is a trend of high temperature events starting early and ending late in recent 50 years, and this trend is intensified by the increasing urbanization. Due to geographical location and city scale, the frequency and intensity of high temperature events show difference among Beijing, Tianjin and Shijiazhuang cities. Variance analysis shows that climate warming contributes 20.2-25.5% to the variability of high temperature days, while urban heat island accounts for 14.7-24.2%, equal to the sum of the contributions from atmospheric circulation and solar activity.