Thaopaul V. Bui

Aircraft observations of thin cirrus clouds near the tropical tropopause

This work describes aircraft-based lidar observations of thin cirrus clouds at the tropical tropopause in the central Pacific obtained during the Tropical Ozone Transport Experiment/Vortex Ozone Transport Experiment (TOTE/VOTE) in December 1995 and February 1996. Thin cirrus clouds were found at the tropopause on each of the four flights which penetrated within 15° of the equator at 200–210 east longitude. South of 15°N, thin cirrus were detected above the aircraft about 65% of the time that data were available. The altitudes of these clouds exceeded 18 km at times. The cirrus observations could be divided into two basic types: thin quasi-laminar wisps and thicker, more textured structures. On the basis of trajectory analyses and temperature histories, these two types were usually formed respectively by (1) in situ cooling on both a synoptic scale and mesoscale and (2) recent (a few days) outflow from convection. There is evidence from one case that the thicker clouds can also be formed by in situ cooling. The actual presence or absence of thin cirrus clouds was also consistent with the temperature and convective histories derived from back trajectory calculations. Notably, at any given time, only a relatively small portion (at most 25%) of the west central tropical Pacific has been influenced by convection within the previous 10 days. The structures of some of the thin cirrus clouds formed in situ strongly resembled long-wavelength (500–1000 km) gravity waves observed nearly simultaneously by the ER-2 on one of the flights. Comparison with in situ water vapor profiles made by the NASA ER-2 aircraft provide some observational support for the hypothesis that thin cirrus clouds play an important role in dehydrating tropospheric air as it enters the stratosphere.

The NASA Airborne Tropical Tropopause Experiment: High-altitude aircraft measurements in the Tropical Western Pacific

AbstractThe February–March 2014 deployment of the National Aeronautics and Space Administration (NASA) Airborne Tropical Tropopause Experiment (ATTREX) provided unique in situ measurements in the western Pacific tropical tropopause layer (TTL). Six flights were conducted from Guam with the long-range, high-altitude, unmanned Global Hawk aircraft. The ATTREX Global Hawk payload provided measurements of water vapor, meteorological conditions, cloud properties, tracer and chemical radical concentrations, and radiative fluxes. The campaign was partially coincident with the Convective Transport of Active Species in the Tropics (CONTRAST) and the Coordinated Airborne Studies in the Tropics (CAST) airborne campaigns based in Guam using lower-altitude aircraft (see companion articles in this issue). The ATTREX dataset is being used for investigations of TTL cloud, transport, dynamical, and chemical processes, as well as for evaluation and improvement of global-model representations of TTL processes. The ATTREX da...

On the Susceptibility of Cold Tropical Cirrus to Ice Nuclei Abundance

AbstractNumerical simulations of cirrus formation in the tropical tropopause layer (TTL) during boreal wintertime are used to evaluate the impact of heterogeneous ice nuclei (IN) abundance on cold cloud microphysical properties and occurrence frequencies. The cirrus model includes homogeneous and heterogeneous ice nucleation, deposition growth/sublimation, and sedimentation. Reanalysis temperature and wind fields with high-frequency waves superimposed are used to force the simulations. The model results are constrained by comparison with in situ and satellite observations of TTL cirrus and relative humidity. Temperature variability driven by high-frequency waves has a dominant influence on TTL cirrus microphysical properties and occurrence frequencies, and inclusion of these waves is required to produce agreement between the simulated and observed abundance of TTL cirrus. With homogeneous freezing only and small-scale gravity waves included in the temperature curtains, the model produces excessive ice con...

Air parcel trajectory dispersion near the tropical tropopause

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.

Ice water content‐extinction relationships and effective diameter for TTL cirrus derived from in situ measurements during ATTREX 2014

The NASA Airborne Tropical Tropopause Experiment (ATTREX) deployment in January–March 2014 yielded more than 34 hours 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   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.