Behavior of cloud base height from ceilometer measurements

Given the importance of clouds in the climate, and the difficulty in determining their behavior and their contribution to climate change, there is a need for improvement of methods for automatic and continuous description of cloud characteristics. Ceilometers constitute a priori a reliable instrumental method for sounding the atmosphere and describing cloudiness, specifically cloud base height (CBH), cloud cover, and even cloud vertical structure. In the present study, the behavior of CBH at different time scales has been investigated at Girona (Spain) including a statistical analysis of the frequency distributions of CBH. The study covers four years (2007–2010) of high resolution (both in time and in the vertical direction) ceilometer measurements. At this location, ceilometer measurements reveal a seasonal cycle, with important differences between “extreme” seasons (winter and summer) and the “transition” seasons (spring and autumn). Summer months in general and July in particular behave quite differently than other periods in the year, both regarding the presence of clouds (with a minimum cloud occurrence of about 20–30%) and the distribution of CBH (with more than 25% of clouds having CBH around 1400 m and 80% of clouds with CBH lower than 3000 m). The distributions of CBH are explained on the basis of some atmospheric situations that generate clouds, in particular conditions that produce the large number of low level clouds found.