Abstract:

The long-term changing characteristics of dry spells during 1960—2020 are analyzed based on the daily precipitation data from nine meteorological stations and National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research reanalysis data in the Inner Mongolia desert steppe of China in this paper. Additionally, the atmospheric circulation characteristics of extreme events (August 7^th-14^th, 1979 for extreme precipitation event and August 9^th-31^st, 2007 for an extreme dry spell event) are analyzed by the composite analysis method. The results show that: (1) dry spells mostly occur in summer, with the shortest mean and maximum length. Short dry spells and long precipitation events alternate in summer, whereas, long dry spells and short precipitation events appear alternatively in other seasons. (2) The annual mean non-precipitation day number is 304.1 d·a^-1, with no significant trend in the last 61 a. The long-term annual mean is 39.6 times·a^-1 for dry spell frequency, 8.0 d·a^-1 for the mean length of dry spells, and 43.7 d·a^-1 for the maximum length of dry spells. (3) All the factors show no decreasing or increasing trend during 1960—2020, whereas, during 2005—2020 the number of dry spells apparently increased and the mean length of dry spells decreased (all passed the 90% significant test). The scatter density analysis shows that the most commonly seen number of dry spells per year is 40, while the mean length of dry spells is mostly concentrated around 6-8 d. The maximum length of dry spells is mostly seen between 30-50 d. (4) The mean length of dry spells shows a strong negative correlation (-0.91) with the number of dry spells and a positive correlation (0.61) with the maximum length of dry spells, while the number of dry spells shows a negative correlation (-0.51) with the maximum length of dry spells. All the spells passed 99% significant level test. (5) The extreme long-lasting dry spell year saw significant anomalies in temperature, humidity, and geopotential height mid and high altitude. During abnormal long-lasting dry spell event, warm anomaly and anticyclonic circulation were seen over 500 hPa level. At the 850 hPa pressure level, the west Pacific high extended westwardly and northwardly. The relative humidity shows a negative anomaly. All the above conditions are to the disadvantage of precipitation development.

Key words: dry spell, average duration, maximum duration, desert steppe of Inner Mongolia