Cloud Amount Analysis at Yangbajing of Tibet in 2009-2010 Using All-Sky Images

At Yangbajing observatory, Tibet, an all-sky imager has been in operation for more than one year since April 2009, to measure and record cloud amounts and types. The daily cloud amount statistics over Yangbajing based on the high temporal and spatial resolution dataset is reported. The annual mean cloud amount is 5.2. The cloud-amount distributions show strong summer/winter contrasts. The mean cloud-amount is higher in summer than in winter. Lowest cloud frequencies are founded in the morning of winter and the highest cloud frequencies are founded in the afternoon of summer. Cloud amount distributions also show an obvious diurnal variation characteristic during the “winter” half-year (November, December, and January to April) that cloud amounts increase gradually from morning and reaches a peak in the late afternoon, afterward drops gradually. At later, another cloud amount estimation method (described as “temporal cloud amount”) that calculating the occurring frequency of cloud at a fixed direction during a period is compared with the true cloud amount. For clear and overcast conditions, both of them agree much better to each other, but they show great discrepancy for broken-cloudiness. Averaged over a longer time period, such as one day, the bias becomes lower. For heavy-cloudiness conditions, true cloud amount is higher than temporal cloud amount, but it is often lower in light-cloudiness conditions.