Trends and sensitivities of low-cloud cover and top height from CALIPSO observations

In this contribution, we analyze 14 years of global spaceborne lidar observations for trends in the occurrence of low-level clouds and their cloud-top height, and derive their sensitivities to controlling factors.Low clouds play an important role in the Earth's energy budget because of their capability of reflecting large amounts of incoming sunlight. However, there is some ambiguity in the detection of low-level clouds in satellite observations which are mostly performed with passive sensors. Only active remote sensing with spaceborne lidar or radar can provide direct measurements of cloud-top height and, thus, lead to straightforward detection of low-level clouds. Here, we analyze 14 years of spaceborne lidar observations for trends in the occurrence of low-level clouds and their cloud-top height. We find that spatial trend patterns in low-level cloud cover and low-level cloud top height are negatively correlated, i.e., regions with a decrease in low-cloud cover tend to show an increase in cloud-top height. We find that spatial trend patterns of both parameters can be well explained by sea-surface temperature and estimated inversion strength trends. Low-level clouds in climatological stratocumulus regions are particularly sensitive to changes in sea-surface temperature (-3.4 to -3.7% K-1 in cloud cover and 48.8-52.3mK-1 in cloud-top height) and estimated inversion strength (3.3%K-1 in cloud cover and from -69.3 to -69.6mK-1 in cloud-top height).