Ceilometers are operated by KNMI at several stations throughout the Netherlands. The individual cloud base measurements of the Vaisala Impulsphysik LD40 ceilometers are transmitted to the central office in De Bilt and processed to obtain cloud base and total cloud cover as well as amounts and heights of up to 3 cloud layers. This automated process is performed in parallel to manual observations at some stations in order to get information on the quality of automated cloud reports. For that purpose the automated cloud parameters are compared with routine hourly SYNOP reports made by observers. This presentation will discuss: (i) details of the cloud algorithm that transforms 1-minute cloud base data into cloud reports; (ii) a comparison of automated and visual cloud reports for several stations; (iii) the effect of using the measurements of multiple ceilometers at one site in the cloud algorithm.

The table shows an example of the comparison for total cloud cover at De Bilt in 2000. The table shows that the reports of the observer and the automated system agree within 0, 1 and 2 octa for respectively 41, 75 and 86% of the time (the bands are denotes by the coloured areas in the table). The table also shows that for 6% of the time differences in total cloud cover are more than 4 octa. The differences are caused by a lack of spatial representativeness of the ceilometer measurements of the last 30-minutes and also due to the limited vertical range of the ceilometer under some conditions. Making a selection for situations with all clouds below 15000ft largely reduces the numbers in the lower left part of the table. The fraction of cloud reports on the diagonal or within 1 and 2 octa is 59, 90 and 94%, respectively. A recently available new software release of the ceilometer gives a better detection of high clouds.