Vertical profile of cloud amount over Poland: Variability and uncertainty based on CloudSat–CALIPSO observations

Abstract

Historically, climatologists have found it difficult to access reliable information on cloud vertical structure. This is because both surface‐based observers and imaging/sounding satellite sensors have limited capability to report superposed cloud. However, lidar–radar cloud profiles, collected between 2006 and 2011 by the experimental CloudSat and CALIPSO missions, offer an alternative source of data. This data set is explored to provide the very first analysis of cloud vertical structure over Poland. The study shows that the sky over Poland is dominated by low‐level cloud (33–35% at an altitude of 1–3 km), while cloud amounts at 5–10 km altitude are relatively constant (20%). Low‐level cloud is found to have the greatest seasonal (20% in July, 50% in January) and diurnal (30% daytime, 40% night‐time) variability. Uncertainty in the estimate of mean cloud amounts is shown to vary as a function of the number of profiles analysed, notably the time‐window used for data averaging. Uncertainty (measured as the width of the 95% confidence interval) ranged from a minimum of ±2% for the annual mean, to a maximum of ±10% for monthly means. The analysis demonstrated that uncertainty of below ±1% for the annual mean was only achievable if data were averaged over an area of 1,300 km2. A further finding was that surface‐based observers in Poland are unable to determine whether there are high‐level clouds up to 64% of the time, as these clouds are obscured by clouds at lower levels. Similarly, middle‐level clouds cannot be observed 55% of the time. These findings lead to the conclusion that CloudSat–CALIPSO observations remain the most reliable source of data on cloud vertical structure for Poland.