In situ and lidar observations of tropopause subvisible cirrus clouds during TC4

During the Tropical Composition, Clouds, and Climate Coupling (TC4) experiment in July-August 2007, the NASA WB-57F and ER-2 aircraft made coordinated flights through a tropopause subvisible cirrus (SVC) layer off the Pacific Coast of Central America. The ER-2 aircraft was equipped with a remote sensing payload that included the cloud physics lidar (CPL). The WB-57F payload included cloud microphysical and trace gas measurements, and the aircraft made four vertical profiles through the SVC layer shortly after the ER-2 flew over. The in situ and remotely sensed data are used to quantify the meteorological and microphysical properties of the SVC layer, and these data are compared to the limited set of SVC measurements that have previously been made. It is found that the layer encountered was particularly tenuous, with optical depths (τ) between about 10^-4 and 10^-3. From the in situ and other meteorological data, radiative heating rate perturbations of ∼0.05-0.1 K day^-1 are calculated. These heating rates are smaller than previous estimates for tropopause SVC, consistent with the smaller τ in the present study. Coverage statistics based on CPL data from other TC4 flights indicate that this cloud was not an outlier among the sampled population. SVC with properties similar to the one presented here are below the detection limit of space-based lidars such as CALIPSO, and a comparison with the TC4 statistics suggests that a majority (>50%) of tropopause SVC (with τ < 0.01) could be unaccounted for in studies using CALIPSO data.