The NASA Airborne Tropical Tropopause Experiment (ATTREX) deployment in January–March 2014 yielded more than 34 h of cirrus cloud sampling in the tropical tropopause layer (TTL) over the western Pacific. Cirrus were encountered throughout the TTL, at temperatures between 185 and 207 K, with ice water content (IWC) ranging from >10 mg m
3 to below the instrumental detection limit of 1 μg m
3. Geometric optical extinction (σ) values determined from cloud particle probe measurements ranged from 10
3 m
1 to <10
7 m
1. The median effective diameter (Deff) for cirrus sampled at T > 192 K was 40–50 μm and exhibited a weak temperature dependence, while colder than 192 K, Deff decreased more strongly with decreasing T. From the ATTREX data, a new parameterization of the IWC-σ relationship for TTL cirrus is derived that will improve the estimation of IWC from lidar and optical probe observations of these clouds. Plain Language Summary Cold, high-altitude tropical cirrus clouds are an important component of the climate system but are significantly underconstrained in climate models. Lidar measurements, especially from satellites, have the spatial and temporal coverage to produce statistically meaningful observations for model comparison and validation but do not directly measure quantities such as cloud ice water content that are important predicted quantities in the models. We have used an extensive data set of cloud ice water content and microphysical properties collected during a 2014 aircraft campaign in the western Pacific to derive a new parameterization that will improve the estimation of ice water content from lidar remote sensing measurements of tropical cirrus. With this parameterization, lidar observations can be used to derive a more accurate ice water content for tropical cirrus, in order to improve its treatment in models.