PNNL

Abstract

Ground based active and passive remote sensing instrumentation are combined to derive radiative and macrophysical properties of tropical cirrus clouds. Eight months of cirrus observations at the Department of Energy Atmospheric Radiation Measurement site located on Nauru Island provide independent retrieval of cloud height and visible optical depth using lidar and radar techniques. Comparisons reveal the millimeter cloud radar does not detect 13% of cirrus clouds with a cloud base higher than 15 km that are detected by the lidar. Lidar and radar cloud heights demonstrate good agreement when the cloud lies below 15 km. Radar and lidar retrievals of visible optical depth also compare well for all but the optically thinnest clouds. Cloud occurrence at Nauru as measured by lidar, reveal clear sky conditions occur on average 40%, low clouds 16%, and high clouds 44% of the time. Analysis of observed cirrus macrophysical and radiative properties suggests that two different types of cirrus exist in the tropical western Pacific: high, thin, laminar cirrus with cloud base higher than 15 km, and lower, physically thicker, more structured cirrus clouds. Differences in cirrus types are likely linked to their formation mechanisms. Radiosonde profiles of temperature and equivalent potential temperature near the tropical tropopause show a clear transition between neutrally stable and stable air at ~15 km, which may also explain the presence of two distinct cirrus types. Radiative heating rate and cloud forcing calculations for specific cirrus cases reveal the impact of tropical cirrus clouds on the earth?s radiation budget.