New definition and new calculation method of effective average cloud amount

Different types of cloud amounts have different influences on the calculation of the radiation energy of the soil-air system. Based on the occurrence of the overlaying of high, medium and low types of clouds and the interaction and interrelationship of information provided by them, this paper puts forwards new concepts of "effective" frequency of occurrences of cloud and "effective" time-average cloud amount and their calculation formulas, while the commonly used calculation formula for time-average cloud amount is treated as a special case of the calculation formula for "effective" time-average cloud amount. Through a principal component analysis of the correlation coefficient matrix formed by conditional probabilities exhibited by any other type of cloud when a certain type of cloud occurs, the normalized weight of the conditional probability exhibited by any other type of cloud when this particular type of cloud occurs is ascertained. When this particular type of cloud is obscured by another type of cloud, the weight can be seen as the probability of this particular type of cloud being obscured by another type of cloud. With the formula for the "effective" frequency of occurrences of cloud and the formula for "effective" average cloud, this probability can be used to calculate the "effective" time-average cloud amount of this particular type of cloud. By applying this method to the analysis and calculation of the "effective" average cloud amount in the four different seasons as observed by 15 observation stations worldwide, this paper finds that the sum of the "effective" time-average cloud amounts of each type of clouds is far closer to the observed value of the total cloud amount than the sum of the time-average cloud amounts derived by using only the calculation formula for time-average cloud amount, with more than 87% of the relative errors of the total cloud amount corrections made by the former being smaller than 15% and all relative errors of the total average cloud corrections made by the latter exceeding 15%. (c) 2006 Elsevier Ltd. All rights reserved.