Stratosphere-Troposphere Analyses of Regional Transport 2008 (START08) Experiment: an Overview
Stratosphere-Troposphere Analyses of Regional Transport 2008 experiment (START08) was a NSF sponsored field campaign for investigating climate relevant chemical and dynamical processes in the upper troposphere and lower stratosphere (UTLS). Using the NSF research aircraft Gulfstream V (GV), also known as HIAPER (High-Performance Instrumented Airborne Platform for Environmental Research), a total of 18 research flights was conducted from April to June 2008. The flights covered a wide range of North America from the Gulf of Mexico to the Canadian Arctic, with a vertical range from the surface to lower stratosphere. Total of 19 chemical tracer and microphysics instruments were onboard, representing the most complex chemistry payload of this new research aircraft. A large suite of chemical species, dynamical and microphysical variables was measured under targeted meteorological conditions. START08 was designed to provide new and more detailed chemical and microphysical measurements in the extratropical tropopause region that will test a new generation of chemistry-climate models. An overview including observational highlights and initial results of data analyses will be presented.
Context for START08 Using Satellite Data and Meteorological Analyses
In support of the Stratosphere-Troposphere Analyses of Regional Transport (START08) campaigns in April, May and June 2008, ozone, water vapor, nitric acid, and carbon monoxide fields in the upper troposphere/lower stratosphere (UTLS) from the Aura Microwave Limb Sounder (MLS) were mapped and analyzed to provide global context for the START08 flights. The Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS) also measured trace gases in the UTLS at northern middle and high latitudes during the period of the START08 campaign. To place the START08 measurements in the context of larger-scale meteorology and transport processes, we use information derived from the Goddard Earth Observing System- Version 5.2.0 (GEOS-5) assimilated meteorological analyses to analyze MLS and ACE-FTS data in relation to the tropopause and the upper tropospheric jets during the period of the STARTO08 campaigns. The contributions of these analyses of the MLS and ACE-FTS data to understanding transport and mixing processes in the extra-tropical tropopause layer are discussed. Some START08/MLS comparisons will be shown and discussed in light of the sampling and resolution of the satellite and aircraft measurements. (Work at the Jet Propulsion Laboratory, California Institute of Technology is done under contract with the National Aeronautics and Space Administration.)
Origin of Tropospheric Air in the Extratropical Lower Stratosphere during START08
The Stratosphere-Troposphere Analyses of Regional Transport 2008 (START08) experiment took place during
April, May, and June of 2008. The project used the NCAR Gulfstream V research aircraft to provide in situ
measurements of trace consitituents in the extratropical upper troposphere and lower stratosphere. One of the
primary goals of START08 is to investigate the mechanisms responsible for the existence of low-stability air
layers and double tropopauses in the extratropical lower stratosphere. A number of START08 flights sampled
these low-stability layers. This study describes the chemical characteristics of the low-stability layers and the
very stable layers sandwiched between them and the tropospause. Trajectory analysis for Flight 1 of START08
reveals that the air in the low-stability layer was transported from the tropical upper troposphere by a large
amplitude Rossby wave during the period 7 to 10 days before the flight. The role of large-scale stirring by
Rossby waves in the extratopical lower stratosphere is discussed.
Trace gas distributions and correlations observed during START08
The Stratosphere -Troposphere Analyses of Regional Transport (START08) experiment was conducted during the spring and early summer of 2008 over continental North America. A major goal of the mission was to better understand the processes that control the chemical composition of the UT/LS (upper troposphere/lower stratosphere) region. The NSF Gulfstream V (GV) aircraft was instrumented for in-situ measurements of an array of trace gases and aerosol properties, including the Advanced Whole Air Sampler (AWAS), which is the focus of this presentation. During the mission, the AWAS collected over 1000 air samples which were analyzed at the University of Miami for 56 trace gas components. These compounds include a range of hydrocarbons, CFCs, HCFCs, solvents, methyl halides, organic nitrates, and sulfur species. The chemical species measured from the AWAS span several orders of magnitude in chemical lifetimes and cover major types of surface emission (anthropogenic/industrial, biomass burning, ocean emission, etc.). Thus, the trace gas composition in the UT/LS provides information on the sources, transport, and mixing of air masses near the tropopause. Trace gas correlations observed during START08 suggest several major pathways and mixing regimes for the spring-time UTLS. The presentation will discuss the observed distributions, composition, and relationships in these different regimes.
Dynamic Variability of the UTLS Chemical Composition from Satellite and in situ Measurements during START08
Understanding the role that large-scale dynamics plays in the chemical composition of the upper troposphere - lower stratosphere (UTLS) is critical for assessing current and future changes in the climate system. The START08 campaign successfully sampled the extratropical UTLS region over North America during April - June 2008. The campaign targeted a range of dynamical processes that contribute to the redistribution of radiatively important chemical species such as ozone and water in the UTLS. In this study, we use a combination of space-borne and in situ measurements of chemical tracers to investigate the dynamic variability of the UTLS. Tracers and tracer correlations from the START08 campaign are used to characterize the transport pathways. Space-borne measurements of ozone from Aqua/AIRS and MetOpA/IASI are used to broaden the perspective of the aircraft sampling and to investigate the spatial and temporal variability in relation to dynamic parameters. In particular, we explore the use of potential vorticity in integrating the measurements from these two different platforms and in quantifying the impact of stratosphere-troposphere exchange in UTLS ozone.
In situ Measurements of Ice supersaturation in the Upper Troposphere in START08 Campaign
Accurate measurements of water vapor in the upper troposphere are very important for predicting future climate. Supersaturations of water vapor with respect to ice (relative humidity over ice more than 100 percent) are critical for understanding ice cloud formation and radiative forcing. Aircraft data is an important tool for understanding of the extent and frequency of ice supersaturations at cloud scales which provide more detailed datasets that would not be observed in satellite data on the sub-km vertical scale. Here we investigated RHice by the Vertical Cavity Surface Emitting Laser (VCSEL) hygrometer on the NSF G-V aircraft during START08 campaign in 2008. The campaign covers the extratropical area in Northern Hemisphere from the surface to extratropical lower stratosphere. During the 90 h measurements of the VCSEL during START08, the thickest vertical layers with RH more than 100 percent are observed around 3 km. The average thicknesses of the RH more than 100 percent and RH more than 150 percent vertical layers are around 500 m and 200 m, respectively. In UT, supersaturations of RH more than 100 percent and RH more than 120 percent have average frequencies of 15 percent and 5 percent, and RH more than 150 percent have a maximum frequency of 2 percent. This means that even if there is uncertainty of 20 percent RH from the combined VCSEL and temperature measurements, ice supersaturations are still present in UT. Comparisons between the VCSEL and the UCATS hygrometers show that the VCSEL hygrometer's H2O mixing ratio data are lower than UCATS at low level of water vapor concentration (less than 15 ppmv) and higher at high level of water vapor concentration (more than 250 ppmv). For the 25 to 250 ppmv range, both instruments show excellent agreement (5 percent) where ice supersaturated regions are frequently observed. An in-flight intercomparison between the NASA DC-8 laser hygrometer will also be presented. The ice supersaturated regions will be discussed in the context of ice particle characteristics, tropopause height, and vertical wind speed.