Thin and subvisual tropopause tropical cirrus: Observations and radiative impacts

In situ microphysical, remote sensing, and satellite observations of thin and subvisible cirrus have been used to establish their frequency of occurrence, determine their mean optical depths and radiative forcings, and to analyze their association with deep convection. A spatially thin layer of cirrus, with both base and top above 15 km, was observed in the central Pacific Tropics 29% of the time, with a mean thickness of 0.47 km, using a nadir-pointing Nd: YAG lidar operating at 1.064 μm during the Central Equatorial Pacific Experiment (CEPEX). In situ microphysical data collected in the mid-1970s and mid-1980s by a WB-57 and Learjet near Kwajalein, Marshall Islands, are revisited to determine typical ice crystal sizes and shapes that occur in this cloud type. Three observed vertical profiles, obtained from ascents/descents through cloud, are used with a δ-four-stream radiative transfer model to calculate observed heating rates of up to 1.0 K day^-1, principally in the infrared, and cloud radiative forcings of up to 1.2 W m^-2. These calculations are extended using remotely sensed optical depths acquired with the airborne lidar on four days during CEPEX; the average τ estimated was 0.01, and the corresponding heating rates and cloud radiative forcings were 1.66 K day^-1 and 1.6 W m^-2, respectively. Using a visibility threshold for τ of 0.03, this suggests that the majority of the thin cirrus observed are subvisible. The calculated radiative effects depend principally on the observed optical thickness of the cloud layers. Altitude-dependent average extinction coefficients between 0.001 and 0.004 km^-1 were calculated from the limb-viewing Stratospheric Aerosol and Gas Experiment (SAGE) II satellite, confirming that the lidar estimated τ is representative for the Tropics. A comparison of the SAGE II observed occurrence times with cloud properties estimated from collocated International Satellite Cloud and Climatology Project retrievals for the closest time shows that 28% of the subvisible cirrus occurred within the same 2.5°by 2.5°grid box as deep convection. Although the effect of subvisible cirrus on the radiative budget of the Tropics is not as large as for other ice clouds, their effects are not negligible and their other impacts, such as enhancing upper-tropospheric vertical motions and the lower-stratospheric water vapor, should not be ignored.