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Planning for UTLS Science Using the new HIAPER Aircraft

William Randel (randel@ucar.edu) and Laura Pan (liwen@ucar.edu), NCAR, Boulder (CO), USA

A new high altitude research aircraft called the High-Performance Instrumented Airborne Platform for Environmental Research (HIAPER), acquired by the United States National Science Foundation (NSF), will soon become available to the atmospheric research community. The aircraft will be operated for the NSF by the National Center for Atmospheric Research (NCAR) in Boulder (CO), USA. HIAPER is a Gulfstream V aircraft, with high altitude and long-range capability (maximum altitude ~ 50,000 feet or ~15 km, maximum range ~11,000 km) making it a unique platform for sampling the upper troposphere and lower stratosphere (UTLS) using both in situ and remote sensing instruments. The aircraft is currently undergoing modification and is scheduled to become available for science applications in summer 2005. Details of the capabilities of the aircraft and its modifications for research can be found at http://www.hiaper.ucar.edu/.

With the acquisition of this new research platform, NCAR is developing a UTLS research initiative with the following goals: (1) to plan integrated UTLS research using HIAPER, and optimise integration with satellite programs and multi-scale models; (2) to formulate detailed science plans that can guide the instrument development for HIAPER and the protocol for field experiments; and (3) to enhance collaborations with national and international partners and within NCAR. Two recent developments of this programme are a draft White Paper describing some of the key issues and potential studies using HIAPER, and a 2-day workshop held at NCAR (October 27-28, 2003) to discuss these plans with the wider community. These topics are discussed briefly here, more detail can be found at http://www.acd.ucar.edu/UTLS/.

The NCAR Initiative on Integrated study of Dynamics, Chemistry, Clouds and Radiation of the UTLS

The NCAR UTLS initiative is motivated by a renewed appreciation for the importance of the UTLS region for understanding climate change and the evolving chemical composition of the atmosphere. Identifying and understanding the dynamical, chemical and physical processes that control water vapour, ozone, radical constituents, aerosols and clouds in the UTLS are critical for advancing the reliability of predictions of climate change or of trends in global air quality. The UTLS is a highly coupled region: dynamics, chemistry, microphysics and radiation are fundamentally interconnected. For example, the distribution of water vapour and ozone (two radiatively important gases) is controlled by the details of stratosphere-troposphere exchange (STE) and deep convection, chemical processing including multiphase chemistry, and cloud microphysics. These processes are in turn influenced by temperature and aerosol distributions. The coupling of UTLS processes is highlighted in Figure 1.

Figure 1. This schematic highlights important processes coupling dynamics, chemistry and cloud microphysics in the UTLS region (adapted from Figure 3 of Stohl et al. 2003). The green line denotes the time average tropopause. In the tropics, maximum outflow from deep convection occurs near ~12-14 km, while the cold point tropopause occurs near 17 km. The intervening region has characteristics intermediate between the troposphere and stratosphere, and is termed the tropical transition layer (TTL). Extratropical stratosphere-troposphere exchange occurs in tropopause folds and intrusions linked with synoptic weather systems; these events transport stratospheric ozone into the troposphere. In addition, synoptic scale uplift (‘warm conveyor belts’) and deep convection brings near-surface pollutants (from biomass burning or anthropogenic emissions) into the upper troposphere, strongly influencing global-scale chemistry.

The initiative is also motivated by the unprecedented observational opportunity provided by new or soon-to-be new satellite instruments. Currently, the UTLS is a relatively under-sampled region compared to the lower troposphere or stratosphere. The altitude range has typically been below the detection range of spaceborne instruments, and there are only a few high altitude airborne observing platforms available to the community. The strong gradients in stability and chemical structure near the tropopause are a challenge to current global and regional models. Future advancements will require coordinated use of high altitude aircraft for small-scale measurements and detailed process studies, combined with satellite data for the larger scale perspective, plus appropriately sophisticated large and small-scale models. The addition of the HIAPER aircraft to the available high altitude platforms, combined with the data from NASA A-train and European and Japanese satellite platforms, present an exciting new opportunity for UTLS studies. Of particular interest in planning for new aircraft measurements is the development of improved in situ techniques for measuring a suite of chemical and aerosol/cloud particles.

At the current stage of planning, key UTLS issues are grouped into four interrelated themes. Each theme will potentially involve integrated use of field experiments, satellite measurements and state-of-the-art modelling tools. Models will also be used to help design the field experiments.

(1) Tropical UTLS water vapour, clouds, microphysics, and radiation.

The focus is to improve our ability to simulate the tropical UTLS region, which requires detailed understanding of the processes that maintain the observed distributions of water vapour and clouds, and their links with the large- and small -scale temperature structure. This includes observing and simulating the microphysics of cirrus formation and evaporation, and the role of deep convection and its effects on the radiation and chemical budgets. Water vapour is a major source of OH and is, thus, strongly coupled to chemical processing and composition in the tropical UTLS.

(2) Two-way stratosphere-troposphere exchange (STE) processes.

The overall objective is to better quantify the contribution of STE to the budgets of ozone and water vapour in the UTLS. There is a need to better characterize the role of multiple scale dynamical processes, from the large-scale planetary wave breaking, to synoptic scale baroclinic systems, and to small scales associated with convection and turbulence. Investigation of the effect of gravity wave breaking and turbulent mixing processes near the extratropical tropopause is an important component.

(3) Chemistry that controls the budgets of ozone and radical species in the UTLS.

One focus of this theme is to assess the impact of rapid convective upward transport of near-surface biogenic and anthropogenic emissions or oxidation products on radical budgets in the UTLS. Gaseous and multiphase processes in the UTLS control the sources and sinks of radical constituents (HO_x, NO_x, RO_x, ClO_x, BrO_x…), and hence the processes that control the budget of O₃ and removal of many chemical pollutants.

(4) Composition of aerosol and cloud particles in the UTLS.

The processes that control the formation of aerosols and cloud particles in the UTLS are poorly understood at present. Key topics include the chemical composition of aerosols and how the composition might influence the generation of cirrus particles. Identification and refined understanding of multiphase processing of chemical constituents on liquid and ice particles is of particular importance both for detailed microphysical/chemical models and for sub-grid scale arameterisations in global models.

NCAR Community Workshop

Inviting the wider community’s input and participation to the UTLS initiative was the impetus for holding a workshop at NCAR on October 27-28, 2003. Approximately 120 participants attended the two-day workshop, with ~ 55 NCAR scientists and ~ 65 others from ~ 20 universities, NOAA and NASA Laboratories and other research organizations. The objectives of the workshop were to: 1) identify and discuss key issues of UTLS research and to begin to define achievable goals; 2) form a science user community for the use of HIAPER in UTLS research, optimising the synergy with NASA satellites (AURA and A-train in particular); 3) work on a science plan and airborne experiment design and to form working groups to implement the plan; 4) form community consensus on instrumentation strategy. The workshop included a series of presentations and discussions on the four UTLS science themes discussed above. Additional topics included concepts for airborne experiments using HIAPER, the use of multi-scale models to help define the science objectives and strategy of the field campaigns, and required instrumentation that is critical for potential studies.

J. Holton and W. Randel led the opening session and gave brief overviews of UTLS science issues. The overviews emphasized the importance of convective transport into the UTLS, as illustrated by a simulation of midlatitude convection in Figure 2, plus the interconnection of transport, chemical, microphysical and radiative processes.

Figure 2. Vertical cross-section through an idealised supercell anvil at 2 hours (left panel) and 6 hours (right panel). The coloured contours show the concentration of a boundary layer tracer, initialised with a mixing ratio of 1 kg/kg in the lowest 1 km at model time zero. The thick black line shows the location of the tropopause, defined as a surface of constant gradient in potential temperature. [Simulation and figure are courtesy of G. Mullendore]

A. Ravishankara (co-chair of SPARC) presented an overview of SPARC perspectives, emphasizing the link of UTLS processes to climate change issues. D. Fahey presented NASA plans and activities for UTLS research (on behalf of D. Anderson and M. Kurylo of NASA Headquarter). This presentation focused on the common scientific objectives of the UTLS initiative and AURA satellite measurements, and potential links to HIAPER deployment with the planned airborne AURA validation missions. This was followed by a series of brief discussions on new generations of satellite data in the UTLS region, given by team members from AURA/HIRDLS (A. Lambert), AURA/MLS (G. Manney), AURA/TES (H. Worden), and AQUA/AIRS (A.-M. Eldering).

An overview of HIAPER status, funding opportunities for HIAPER instrumentation, and the plan for initial science missions were given by D. Carlson (HIAPER PI). He told the community that HIAPER will be ready for initial science payload by the summer of 2005. An initial testing period of six months (July to December 2005) has been designated as the “Progressive Science” period. Solicitation for Letters of Intent will soon be released, and additionally NSF will soon announce the funding opportunity for HIAPER Aircraft Instrumentation.
Following these overview discussions, sessions discussed the following specific topics:

(1) Mechanisms controlling tropical UT humidity (by I. Folkins, A. Dessler, E. Jensen, Q. Fu and A. Gettelman)

The common thread of the discussions was that the “deep convection detrainment layer”, spanning altitudes of 10-14 km in the tropics, is a critical region for understanding the processes that control the UT humidity, cirrus formation and their radiative impact. There is good progress in simulating ice crystal formation associated with tropical deep convection, as shown in comparisons with recent tropical measurements (Figure 3). A better understanding of how dynamical, microphysical and radiative processes couple in this region is required to reduce the uncertainty in climate models. HIAPER, with its altitude capability, is well suited to contribute to investigations of this region.

Figure 3. Results of a 3-D cloud model simulation of a deep cumulonimbus cloud sampled on July 29, 2002. The simulation was initialised with aircraft aerosol measurements and with mesoscale meteorological fields provided by D. Wang. The left panel shows a cross section of condensate mixing ratio (ice + water, colour shading) and 100% relative humidity with respect to ice (white contours) in a mature anvil. The right panel shows the ice crystal size distributions measured (coloured lines) by a combination of instruments on the WB-57 [unpublished data courtesy of D. Baumgardner] and simulated (black line) at the location indicated by the black diamond in the left panel. [Unpublished model results courtesy of A. Fridlind and A. Ackerman]. The discrepancy in the 1-10 micrometer region is unresolved at this time. Problems with the particle size distribution retrieval and possible model shortcomings are being explored.

(2) Multi-scale dynamics and STE (by A. Stohl, L. Pan, T. Lane, J. Sun, for D. Lenschow, O. Cooper, J. Moody, P. Wang, T. Marcy and J. Gille)

The discussions recognized recent progresses in STE climatology using Lagrangian models, but pointed out the need to verify these model results by observations. Stratospheric intrusions and mixing between the stratosphere and troposphere is frequently observed but poorly modelled. Characterization of mixing between the stratosphere and troposphere is facilitated by use of chemical tracers (Figure 4), and global characterization will require multiscale observations from aircraft and spaceborne instruments, coupled with multi-scale models. Better characterization of gravity wave breaking and turbulent mixing was emphasized; an example of mixing from a high resolution simulation is shown in Figure 5. It was also recognized that there is increasing observational evidence on the importance of vertical transport by mid- to high-latitude deep convection. The relative contribution of these processes to the lowermost stratospheric composition, compared to that produced by isentropic mixing, needs to be quantified.

Figure 4. Diagnosis of mixing of stratospheric and tropospheric air observed in the vicinity of a tropopause fold. The tracers and temperature measurements are made onboard the DC-8 on October 29, 1997 during SONEX. Lower panel shows an ozone curtain measured by lidar, together with analysed potential vorticity (white lines) and the aircraft flight track (black dotes). Top panel shows a scatter plot of O3 vs. CO along the flight track (from in situ measurements). In both panels, letters A, B, C, and D help identify the segment of flight where the tracer mixing ratios form “mixing lines”. Purple crosses represent the location of the thermal tropopause calculated from MTP measurements. [from Pan et al., 2003]

Figure 5. High resolution simulation of transport and mixing associated with deep midlatitude convection [from the work of Lane et al., 2003]. Contours show potential temperature at 2K intervals, and blue indicates cloudy air. This close-up view of turbulence near the cloud top highlights a small-scale intrusion of stratospheric air downwards, which is subsequently irreversibly mixed. [Figure courtesy T. Lane].

(3) UTLS radical budgets, ozone and convective influence (by B. Brune, J. Logan, M. Barth, D. Toohey and R. Cohen)

Discussions focused on ozone and radical budget issues in the tropical and extra-tropical UTLS regions, and the possible impacts of deep convective redistribution, production, or multiphase uptake of constituents. Specific topics included the behaviour of HO_x at high NO_x mixing ratios, multiphase interactions of HO_x, identifying the role of OVOCs as HO_x precursors, the sources and sinks of halogen radicals, and the impact of deep convection, cloud processing and lightning NO_x on the ozone budget. Simulations of lightning generated NO_x in high resolution models show reasonable agreement with aircraft observations (Figure 6), while global model comparisons with satellite data show some interesting differences (Figure 7).

Figure 6. Comparison of NOx from the University of Maryland Cloud-Scale Chemical Transport Model (UMD CS-CTM) with measurements for the July 29, 2002, CRYSTAL-FACE storm in South Florida. Model is driven by cloud-resolved MM5 fields in this case and contains a parameterization for lightning NOx. Production of NO by lightning is assumed to be 490 moles/flash for both intracloud and cloud-to-ground flashes. Anvil NO measurements were performed aboard the NASA WB-57 aircraft by B. Ridley of NCAR. NO2 was estimated from photostationary state (PSS) calculations using observed O3 data and assumptions of NO2 photolysis rates (j(NO2) for clear sky and for enhancement due to cloud reflections (j(NO2) x 2). (Ott et al., 2003). [Figure courtesy K. Pickering].

Figure 7 (top) Climatology of NOx (ppbv) at 100 hPa in July, derived from HALOE satellite measurements. (bottom) Simulation of July NOx at 158 hPa, derived from a MOZART chemical transport model simulation. Both figures show localized maxima in NOx near the tropopause, associated with the Northern Hemisphere summer monsoons over South Asia and North America. Note the somewhat larger values inferred from the HALOE measurements. (Figure from Park et al., 2003).

(4) Interaction of chemistry and particle/cirrus formation (by S. Massie, J. Wilson, D. Murphy, A. Heymsfield, R. Gao, M. Fromm, and T. Clarke).

The composition of UTLS aerosol has an organic content (~ 50%) that is higher than previously realised. It is known that organic aerosol is produced at the Earth’s surface by urban pollution and biomass fires. Boreal forest fires are a (recently recognized) source of particles in the UTLS. Aerosol is potentially a controlling factor for humidity and the cirrus formation criteria near the tropopause. There is observational evidence (INCA) that Northern Hemisphere (NH) cirrus formation nucleates at relative humidities lower than those in the Southern Hemisphere (SH), a result attributed to the presence of more ice nuclei in the NH, leading to heterogeneous nucleation of cirrus. Furthermore, recent observations from the NASA CRYSTAL-FACE program were interpreted (R-S Gao, D. Fahey) to show that a nitric acid “coating” on ice near the tropopause interferes with H₂O uptake.

(5) Airborne experiment strategy (by R. Cohen, P. Wennberg, B. Ridley, A. Fried, J. Kuettner, J. Whiteway, A. Heymsfield, and M. Coffey)

Initial concepts of several categories of HIAPER-led airborne experiments were discussed, including: (1) Tropical experiments: it is recognized that HIAPER has capability for sampling the 10-14 km altitude range, which is the region of main convective outflow in the tropical UT. A plan for a tropical mission using HIAPER in early 2007 as part of the NASA-led TC4 experiment was presented by P. Wennberg. (2) Mid- to high latitude experiments: a number of mid- to high latitude experimental themes were discussed that can potentially be combined into joint campaigns. These are considered to be multi-aircraft campaigns, including HIAPER and other platforms. The prospective experiments covered: (a) STE in the extratropics, characterization of the tropopause transition in the region of the subtropical jet and the coupling of the tropics and the extratropics across the jet; (b) Vertical redistribution of chemical species via deep convection, the production and distribution of lightning-produced NOx, and investigation of the downwind impact on ozone production over several days; (c) detailed process studies of the role of convection on the UTLS radical budgets, particularly the role of peroxides, aldehydes and OVOCs; (d) Characterization of UT aerosol composition, distribution and cirrus formation processes near the tropopause; (e) airborne studies of polar stratospheric clouds (PSCs) in the Arctic.

(6) Roles of multi-scale models (by D. McKenna, K. Pickering, J. Powers, M. Olson, P. Hess, and A. Gettelman).

Discussions focused on the range of relevant models, from detailed microphysical/aerosol models, 3-D cloud models, to local and global-scale chemistry transport models. Detailed process studies in the UTLS will be required to quantify dynamical, chemical and aerosol behaviour in models. Models can in-turn be used to focus potential field studies and provide input for the design of aircraft missions.

(7) Instrumentation issues (by P. Wennberg, J. Stith, T. Campos, L. Avalone, C. Gerbig, for S. Wofsy, J. Hair, for E. Browel), (C. Senf, D. Rogers, R. Shetter, and E. Apel).

It is recognized that the success of the HIAPER related science missions critically depends on a strategy for the progressive development of basic instruments to a more complex suite for aerosol and radical studies. Following a review of HIAPER Advisory Committee (HAC)’s recommendation on instrument development, discussions of critical needs and the status of instrument development were given, including 1) fast sampling in situ O₃ and CO instruments; 2) water vapour and total water instruments; 3) long-lived tracer instruments; 4) cloud microphysics instruments; 5) small lightweight LIDARs for aerosol, ozone, water vapour and wind measurements; 6) radiation instruments; and 7) instrumentation for a variety of VOC measurements.

Over the next few months, working groups are to be identified and organised to begin the more difficult task of formulating detailed plans for field studies and their links with satellite and modelling partners. The NCAR UTLS project welcomes comments or suggestions on possible experiments or research strategies.

Reference

Lane, T.P., R.D. Sharman, T.L. Clark, and H.-M. Hsu, An investigation of turbulence generation mechanisms above deep convection. J . Atmos. Sci., 60, 1297-1321, 2003.

Pan, L. L., W. J. Randel, E. Browell, B. J. Gary, M. J. Mahoney, and E. J. Hintsa, Definitions and the Sharpness of the Extratropical Tropopause: A Trace Gas Perspective, submitted to J. Geophys. Res, 2003.

Park, M., W. J. Randel, D. E. Kinnison, R. R. Garcia, and W. Choi, Seasonal variation of Methan, water vapour, and nitrogen oxides near the tropopause: satellite observations and model simulations, submitted to J. Geophys. Res, 2003.

Ott, L. E., K. E. Pickering, G. L. Stenchikov, R.-F. Lin, B. Ridley, M. Loewenstein, E. Richard, Trace gas transport and lightning NOx production during a CRYSTAL-FACE thunderstorm simulated using a 3-D cloud-scale chemical transport model, AGU Fall Meeting, 2003.