Abstract: The annual and decadal variations, anomalous and abrupt change of the frost days are analyzed in this paper, using the daily minimum temperature of 38 stations over Tibet from 1981 to 2010 and modern statistical diagnosis methods, such as linear trend analysis, accumulative anomaly, signal noise ratio and rescaled range analysis (R/S analysis). The results showed that, (1) in recent 30 years, the frost days decreased with a rate of (3.3-14.6) d/ 10a (P<0.01, at 37 stations), and the damping of frost days increased with increasing altitude. (2) In terms of decadal variations, the frost days presented a negative anomaly in the 1980s and a positive anomaly in the 2000s, the positive anomaly range of frost days was larger than the negative anomaly in the 1990s. (3) It was found with abrupt change test that frost days at eight stations had abrupt change, which occurred in the 1990s with the year 1997 having the largest number. (4) The results of R/S analysis showed that changes of frost days had the persistence with a Hurst index larger than 0.5 at most of the stations, and indicated that it will assume a continuous decrease in future and the decreasing rate will became larger. (5) In addition, it was found that the frequency of anomalous (more than normal) frost days was 0 to 3 times and occurred mainly in the 1980s, while the anomalous (less than normal) frost days occurred in the 2000s with a frequency of 0 to 4 times. There is little correlation between altitude (or latitude, longitude) and anomalous more frost days frequency, while altitude has negative correlation with anomalous less frost days frequency.

Key words: annual and decadal variations, climate abrupt, Tibet, frost days, anomalous