Search

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−3to below the instrumental detection limit of 1 μg m−3. Geometric optical extinction (σ) values determined from cloud particle probe measurements ranged from 10−3m−1to <10−7m−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, Deffdecreased more strongly with decreasing T. From the ATTREX data, a new parameterization of the IWC‐σrelationship for TTL cirrus is derived that will improve the estimation of IWC from lidar and optical probe observations of these clouds. 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. The 2014 deployment of the ATTREX mission yielded an extensive data set of TTL cirrus IWC and microphysical propertiesThe effective diameter for TTL cirrus exhibited a weak temperature dependence above 192 K but decreased strongly between 192 and 186 KNew parameterization of the IWC‐extinction relationship will improve lidar‐derived IWC for climatically important TTL cirrus clouds

Dispersion of backward air parcel trajectories that are initially tightly grouped near the tropical tropopause is examined using three ensemble approaches: “RANWIND,” in which different ensemble members use identical resolved wind fluctuations but different realizations of stochastic, multifractal simulations of unresolved winds; “PERTLOC,” in which members use identical resolved wind fields but initial locations are perturbed 2° in latitude and longitude; and a multimodel ensemble (“MULTIMODEL”) that uses identical initial conditions but different resolved wind fields and/or trajectory formulations. Comparisons among the approaches distinguish, to some degree, physical dispersion from that due to data uncertainty and the impacts of unresolved wind fluctuations from those of resolved variability. Dispersion rates are robust properties of trajectories near the tropical tropopause. Horizontal dispersion rates are typically ~3°/d, which is large enough to spread parcels throughout the tropics within typical tropical tropopause layer transport times (30–60 days) and underscores the importance of averaging large collections of trajectories to obtain reliable parcel source and pathway distributions. Vertical dispersion rates away from convection are ~2–3 hPa/d. Dispersion is primarily carried out by the resolved flow, and the RANWIND approach provides a plausible representation of actual trajectory dispersion rates, while PERTLOC provides a reasonable and inexpensive alternative to RANWIND. In contrast, dispersion from the MULTIMODEL calculations is important because it reflects systematic differences in resolved wind fields from different reanalysis data sets. Dispersion rates are robust properties of trajectories near the tropical tropopauseDispersion is primarily carried out by the resolved flowPhysical dispersion can be distinguished from trajectory error

Although the tropopause is a well‐established concept, its definition and physical properties remain an active research topic. In the tropics, both the World Meteorological Organization established lapse rate tropopause definition and the minimum in the temperature profile (the cold point) are used to determine the tropopause height. We examine the differences produced by these two definitions using high‐resolution airborne in situ measurements of temperature, water vapor, and ozone in the tropical tropopause layer from a recent experiment over the western Pacific using the National Aeronautics and Space Administration (NASA) Global Hawk unmanned aircraft system. When the two definitions do not produce the same tropopause height, which is in about half of the cases, the combined temperature and trace gas analysis shows that the lapse rate definition better identifies the transition from the troposphere to the stratosphere. Discovered more than a century ago, the tropopause is known to mark the boundary of two dynamically and chemically distinct layers of atmosphere, the stratosphere, and the troposphere. In the tropics, the location, temperature, and physical/chemical gradients of the tropopause are important as part of the fundamental knowledge of the atmosphere and for regulating the amount of water vapor entering the stratosphere, which has a significant contribution to climate forcing. The tropopause over the tropical western Pacific, in particular, is known as the decisive regionfor determining the amount of stratospheric water vapor. High‐resolution measurements for this region are rare because the region is remote and tropopause altitudes are difficult to access. An airborne experiment targeting this decisive region was conducted in 2014, using the National Aeronautics and Space Administration (NASA) Global Hawk unmanned aircraft system. These high‐resolution temperature and trace gas data provided an unprecedented opportunity to examine the physical meaning of the two tropical tropopause definitions, known as the lapse‐rate tropopause and the cold‐point tropopause. In this work, we demonstrate how the relationship of two chemical tracers, ozone and water vapor, can unambiguously identify the transition from troposphere to stratosphere and therefore serve to diagnose the effectiveness of the different tropopause definitions. The tropical tropopause definitions are examined using airborne in situ measurements over the tropical western PacificO3and H2O relationship is used to identify the air mass change from the troposphere to the stratosphereThe lapse rate definition is shown to more consistently identify the tropopause based on the tracer diagnostic