Study On Accumulative Distribution Of Sand-dust Storm Duration Times

The number of sand-dust storms (N) with the duration time greater than t, is obeyed by 1gN=a-bt. This phenomenon with name of the Log-linear Accumulative distribution of Sand-dust storm Duration times (LASD) was called LH phenomenon in some previous researches. LASD can be deduced in theory with the conceptual model of the threshold wind speed for sand-dust storm and the b value increases with the threshold wind speed for sand-dust storm (Vt). The conceptual model can be described as that the sand-dust storms occur only when the wind speed (V)≥the threshold wind speed for sand-dust storm (Vt).This paper introduces the author’s new research on LASD, both in theory and in practice. The sand-dust storms data of the 56 stations in the Loess Plateau for the period of 1958 to 2000 from the China Meteorological Administration (CMA) are used in this study. The duration times of sand-dust storms are analyzed, and the theory of LASD is improved with the analyzing results. Moreover, the effect of the desertification on the sand-dust storms is analyzed by the theory of LASD.The analysis shows that the b values agree with that of the previous researches, with maximum of 0.3197 hr-1 for summer in Yanchi and minimum of 0.1417 hr-1 for spring in Yanchi. The differences of b values of four seasons are reasonable with b values in Yanchi of 0.1417 hr-1,0.3197 hr-1,0.1656 hr-1 and 0.1982 hr-1 for spring, summer, autumn and winter, respectively. The differences agree with the result that the b value increases with Vt.It is also found that the regression equation of N (the number of sand-dust storms with the duration time greater than t) and t is more preferred N=N0exp(-(t/C)), which can be explained by analyzing above conceptual model with Vt varying with time. As the same time the condition for LASD is obtained as that the variation of Vt with time can be ignored. A practical substitute for the condition is obtained as that K-1 and the correlation index R2 (with lgN=a-bt to fit N and/) is big enough (with value> 0.95). While the conditions for LASD were meet, the analysis also shows that the b value for a single station can be calculated by 1/T times a factor, where T is the mean sand-dust storms duration time.From the theory of LASD, it can be deduced that the b value of LASD decreases with desertification. The analysis of the spring sand-dust storms data in the Loess Plateau is a vindication to this deduction. The analysis also shows that the decreasing sand-dust storm frequencies and increasing desertification are inferred from the same sand-dust storms data, while they seem to be in confliction. This is an unexpected discovery, and it may provide a reference for convincing the people who are still suspicious of the sand-dust storms data from CMA.