Skip to content. | Skip to navigation

Frequently Asked Questions


What are the scientific objectives of the proposed project?

The scientific questions motivating HIPPO focus on (1) understanding the global sources and sinks for CO2, CH4, and other carbon cycle gases, and more broadly (2) determining large-scale rates of tracer transport in the atmosphere. The principal questions are:

  1. What are the rates for vertical exchange, mixing within a hemisphere, and inter-hemispheric transport, and how do these transport processes interact with source distributions to produce concentration gradients for key species (CO2, CH4, CO, O2) in the carbon cycle and for tracers used to diagnose the carbon cycle (SF6, HCFCs, COS)?  We will address this question by obtaining the first set of high-definition, seasonally resolved, global tracer data, nearly pole-to-pole and surface-to-tropopause, and interpreting the observed tracer distributions and fluxes with the GEOS-CHEM global 3-D chemical transport model and other diagnostic analysis tools.
  2. How do vertically resolved tracer data provide new constraints on global inverse models for sources and sinks of CO2 and related gases? We will use HIPPO data, plus remote sensing, with the GEOS-CHEM model to estimate global sources and sinks of CO2, CH4, and CO. We will specifically assess the performance of models in the TRANSCOM suite, all of which   simulate well the observed concentrations of trace gases at the surface but give very different results above the surface.
  3. What is the role of the Southern Ocean in global budgets of CO2 and O2? HIPPO data for CO2, the O2:N2 ratio, and the Ar:N2 ratio, will be assessed and inverted using GEOS-CHEM.
  4. How can we establish tracer “clocks”, and determine age spectra, for air in the Tropical Tropopause Layer and global remote troposphere? We will use our seasonally varying tracers in the tropics above 14 km ( = 46 kft) in a Green’s-function analysis of this question.
  5. What is the climatology of global pollutants in remote regions of the atmosphere? Data for tracers of biomass burning and industrial pollution will define the climatology of pollution in hitherto unstudied regions of the atmosphere, including the vertical gradients needed to interpret satellite data (e.g. MOPITT, SHCIAMACHY, AURA suite, OCO).

What are the hypotheses and ideas to be tested?

We will quantify the sources of major carbon cycle and greenhouse gases by region at the global scale. Hypotheses to be tested include, as examples:

  1. Nothern mid-latitude terrestrial ecosystems are a major sink for CO2.
  2. The Southern Ocean is a major sink for CO2 and the driver for global seasonality of the O2:N2 ratio
  3. Amazonia is a major source region for CH4, CO, and N2O.
  4. Upper tropospheric data can be used to falsify models used to derive inverse analysis of the global carbon cycle.

What previous experiments of similar type have been performed by you or other investigators?

Our team has 15 years of experience in airborne measurement of trace gases, including CO2, CO and many other gases. The most recent was the CO2 Boundary Layer Regional Airborne Study (COBRA-Maine) in 2004, “Continental, Landscape, & Ecosystem Scale Fluxes of CO2, CO and Greenhouse Gases (NSF ATM-0221850—Biocomplexity in the Environment [BE] 1/1/03—12/31/06,) PIs:  S. C. Wofsy,  P. R. Moorcroft, Harvard; CO-Is: D. Hollinger (USFS), J. C. Lin (Harvard; current Colo State), C. Gerbig (Harvard; current MPI Jena), A. Andrews (NOAA ESRL). This program of observations and modeling measured a suite of gases similar to the proposed suite for HIPPO during ~200 hours of flying the Wyoming King Air over northeastern North America.

In 2003 we flew "COBRA-North America" (NASA sponsored) using the UND Citation 2, twice covering the race track pattern from Colorado to Medford, OR, Campbell River BC, Edmonton, AB, Thompson, MB, Fraserdale, ON, Yarmouth, NS, Portsmouth, NH, Teterboro, NJ, Baltimore MD, Indiana, Park Falls, WI, and return to Jeffco. In addition a number of regional flights were flown, again with a similar set of tracers compared to HIPPO.

We have also flown almost 500 hours of measurements on the NASA ER-2, WB-57F, and stratospheric balloons, making very accurate CO2 measurements.

Almost all of the instrumentation on the HIPPO payload inherit the legacies of these flight programs.  Thus our sensors are automated, very robust, and require no in-flight operator. In fact, the sensors do not allow in-flight intervention.  They are built to exacting specifications for high-perfomance aircraft operations.

None of the previous experiments had global extent, and consequently none could address the scientific questions of HIPPO.  These measurement programs were focused on regional and continental scale observations and science questions.

How will the instruments/platforms requested be used to test the hypotheses and address each of the objectives?

The data will be analyzed using the GEOS-CHEM model, driven by assimilated wind fields: We will undertake extensive data analysis and conduct comprehensive GEOS-CHEM simulations using GEOS-5 meteorological fields for the flight years, focusing on diagnostic and inverse studies.

  • Identifying transport model error : The range of species measured by HIPPO provides a unique opportunity to characterize errors in vertical and inter-hemispheric transport. We will identify sources of transport error using a range of methods including the residual relative error (RRE) (difference between model and observations after subtracting mean model bias).
  • Observed and Modeled Species Correlations: Concentration correlations between tracer species provide information on common sources/sinks and on atmospheric transport. Our previous analysis of CO2:CO correlations showed the utility of these enhancement ratio constraints in partitioning combustion and biospheric CO2 fluxes in a coupled CO2:CO inverse analysis. Using measured and modeled correlations among the suite of HIPPO tracers, we will identify constraints on common sources and sinks to address the questions of the Northern mid-latitude and Southern Ocean sources and sinks for CO2 and O2.
  • We will use correlations among species to help define the error covariance matrices for multiple-species inversions.
  • Inverse Analyses for Carbon Cycle Sources and Sinks. We will use GEOS-CHEM in Bayesian synthesis inversions to identify the value added of vertically resolved HIPPO data in constraining CO2 and other tracer fluxes. Our initial analysis will follow TransCom3 methodology [e.g. in selection of source regions, see Gurney et al. 2002; Gurney et al. 2003], enabling comparison of our inversions with the many TransCom inversions, which were restricted to use GLOBALVIEW-CO2 surface data. We plan:
    • A ‘Base-Case’ inverse analysis using the GLOBALVIEW-CO2 surface data for the HIPPO period, obtaining estimates of annual mean and monthly fluxes, then
    • Inversions that incrementally add constraints from HIPPO data; i.e., (1) vertically resolved CO2 measurements, and (2) other tracer species (e .g., O2/N2, CO).

We will also conduct inversions using different versions of GEOS assimilated meteorology, as well as the GISS GCM transport fields, to assess sensitivity to transport parameterizations. The diverse species measured simultaneously in HIPPO offer new inversion strategies to address carbon cycle issues. For example, partitioning of thermally and biologically driven oceanic CO2 exchange can be addressed using measurements of CO2, O2/N2 and Ar/N2 in a parameter estimation analysis focused on underlying processes. Such questions will be ripe for future efforts.

  • We will carry out tagged tracer releases to assess tropospheric age spectra at HIAPER altitudes.

What results do you expect and what are the limitations?

The deliverables for HIPPO will be:

  1. Publicly available global data sets for CO2, O2:N2 ratio, 13CO2:12CO2, CH4, CO, N2O, CFCs, HFCs, HCFCs, O3, PAN, and other tracers. There will be five or six missions over 2 years spanning the seasonal cycle, covering almost pole-to-pole and the full depth of the troposphere, through the Central Pacific and the East Pacific.
  2. Tests of global models used to assess the distribution of sources and sinks of CO2 over the globe, using these data.
  3. Analysis of the seasonal exchange of O2 with the oceans.
  4. Assessment of CO2 and O2 exchange in the Southern Ocean.
  5. Analysis of transport and mixing rates in the TTL.
  6. Green’s functions for the vertical propagation of the seasonal cycle through the middle and upper troposphere.
  7. New constraints on global sources and sinks for CH4 and other gases.
  8. Climatology of pollution layers in the global atmosphere.

There will be limitations imposed by our inability to measure more frequently, over a longer period, at more longitudes.

How far will the Gulfstream V travel?

The G-V will travel almost 30,000 miles (48,000 kilometers) during the 27 day mission.

Where will the GV land to refuel and resupply?

The G-V will land in a few pre-arranged locations where it can be refueled, have any needed maintenance performed, get resupplied, as well as give the flight crew and scientists a rest.

The locations are:

What is the flying altitude range of the GV?

The G-V can fly up to altitudes of just over 45,000' and as low as 1000'. This ability to fly at such high and low altitudes is one of the reasons this particular type of aircraft was chosen for atmospheric studies. The range of altitude gives scientists a very broad "view" into the composition of the Earth's atmosphere.

How is HIPPO suported?

HIPPO is supported by the National Science Foundation (NSF) and its operations are managed by the Earth Observing Laboratory (EOL) of the National Center for Atmospheric Research (NCAR). Its base of operations is EOL's Research Aviation Facility (RAF) at the Rocky Mountain Municipal Airport (RMMA) in Jefferson County, Colorado.

How can I get the data or collaborate with the PIs?

Please contact the HIPPO Principal Investigators if you would like to access the data or inquire about collaborating with them.

You can also view the data catalog from current and previous HIPPO field campaigns.


HIPPO is a landmark study for many reasons, not the least of which is it is the first time scientists have systematically mapped global distribution of carbon dioxide and other greenhouse gases in the atmosphere, covering the full troposphere in all seasons and multiple years.

  • HIPPO I :: 8 January-30 January 2009
  • HIPPO II :: 31 October-22 November 2009
  • HIPPO III :: 24 March-16 April 2010
  • HIPPO IV :: 14 June-11 July 2011
  • HIPPO V :: 9 August-9 September 2011