The optical properties and longwave radiative forcing in the lateral boundary of cirrus cloud

Through observations from the Cloud-Aerosol Lidar with Orthogonal Polarization onboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation, we detected a common feature of narrow and subvisible lateral boundary layer in cirrus cloud. In this layer the lidar backscatter, the depolarization ratio, the ice water content, the effective radius of ice particles, and the cloud optical depth all decrease sharply toward the cloud edge. In general, the width of this layer (6.4 ± 3.1 km over land) decreases with increasing ambient temperature. The estimated longwave radiative forcing associated with the layer is about 10 W/m2. Due to its extremely small optical depth (less than 0.3), such lateral boundary layer may be missed by conventional satellite passive optical sensors. As a consequence, the mentioned radiative forcing has not been credited with its deserved share in the Earth's radiative energy budget.