AFM-13 Aircraft Flux Analyses Analysis And Interpretation Of Airborne Flux Observations Over The BOREAS Site 1. INTRODUCTION For the BOReal Ecosystem-Atmosphere Study (BOREAS) in 1994 and 1996, the Airborne Fluxes and Meteorology (AFM) group was involved in measurements (from different platforms and heights within the atmospheric boundary layer) of key atmospheric variables and several surface-related parameters that can be used to describe the evolution of the atmospheric boundary layer and the boundary layer fluxes of sensible heat, latent heat, momentum, and CO2. Specifically, the AFM- 13 team was interested in analysis and interpretation of airborne flux observations over a 16-km by 16-km grid site in each of the BOREAS study areas. The primary data used in the investigation were collected using the Canadian Twin Otter aircraft, one among the many research aircraft flown in BOREAS. The main objectives of the AFM-13 investigations are to use the Twin Otter-based data with tower flux data to map spatial and temporal variations in the fluxes of heat, moisture, and CO2, and to define realistic footprint functions over the BOREAS sites, so that airborne observations are related to the correct ground surface with its biological and ecological characteristics. These maps are then compared to maps of remote sensing observations over the sites. It is hoped that these studies help to develop regional scale models of fluxes of sensible heat, latent heat, and CO2 for global monitoring of climate change. This document presents a brief summary of the Twin Otter grid sites, the measured data, the type of analysis carried out, and the preliminary results from the 1994 Intensive Field Campaigns (IFCs). 2. PRINCIPAL INVESTIGATOR Peter H. Schuepp McGill University Quebec, Canada (514) 398-7935 (514) 398-7990 (fax) pschuepp@NRS.mcgill.ca 3. CONTACT INFORMATION Segun O. Ogunjemiyo McGill University Quebec, Canada (514) 398-7950 (514) 398-7990 (fax) segun@NRS.mcgill.ca 4. DESCRIPTION OF THE GRID SITES The Twin Otter grid site of the Northern Study Area (NSA) is shown in the file NSA_CLASS.TIF. The surface cover classes of the site were extracted from the August 20, 1988 Landsat Thematic Mapper (TM) scene of the 129-km by 86-km NSA, using bands 1 through 5 and 7. The spatial resolution is 30 m. Nine of the 11 cover types defined by the National Aeronautics and Space Administration (NASA) classification scheme (Hall et al., 1996) can be identified. More than half of the area is covered by wet and dry conifers. The southern half of the site is dominated by burn areas, enclosing an unburned stand of primarily black spruce, where regeneration has been taking place in the past 7 to 15 years, and there is a significant density of bare rock outcroppings in the southwest corner. The terrain is generally flat to gently rolling, intersected by a few small escarpments and riverbeds. The Southern Study Area (SSA) site is shown in the file SSA_CLASS.TIF, which represents the land classification distribution based on bands 1 through 5 and band 7 of the 06-Aug-1990, Landsat TM scene. All 11 surface cover types were identified within the grid, with wet conifers constituting about 60 percent of the surface cover. Effects of logging, with new regeneration, can be seen in the vicinity of the young jack pine tower. The area beneath the northwest-southeast diagonal generally contains wetter surfaces, with mixed fen and forest. Forest cover is often controlled by small changes in relief and soil or drainage characteristics. 5. METHOD OF DATA COLLECTION The Twin Otter aircraft of the National Research Council (NRC) of Canada was used to collect all the data presented in this documentation. A full description of instrumentation is given by MacPherson (1990, 1996), as detailed in AFM-04 documentation. The aircraft was flown in a grid pattern over the two study sites in either east-west or north-south trajectories. Flight patterns were chosen in response to the prevailing wind direction for closest approach to crosswind conditions. Each grid flight consisted of nine parallel straight lines, spaced 2 km apart, with each line sampled twice in opposite directions, in a sequence that ensured that all grid lines were sampled at the same mean time. Flight level was maintained at 30-35 m above ground level. The data were digitized at 16 Hz, resulting in a spatial resolution of about 4 m along the flight line at the aircraft speed of about 60 m/s. Where necessary, corrections were applied for delays between signals recorded at different sensor locations. Details of the grid flights flown in 1994, including the weather conditions under which they were flown, are given in Ogunjemiyo et al. (1997) and can be found in the AFM-04 data. A total of 23 grid fights were flown, 14 in the NSA and 9 in the SSA. The grid flights were spread over the three IFCs between May and September 1994; i.e., covering different physiological and phenological stages during the growing season of the boreal forest. Full descriptions of the 128 different parameters measured by the aircraft are given in the AFM-04 data. The parameters that were used (directly or indirectly) to obtain the results summarized here include air temperature, potential temperature, static pressure, incident short wave radiation, reflected short wave radiation, the three orthogonal components of the wind velocity, H2O mixing ratio, C02 mixing ratio, aircraft position, heading, altitude (radar and pressure), radiometric surface temperature, and greenness index (GI) (from the Simple Ratio of reflected red to infrared radiation). 6. FLUX CALCULATIONS AND MAPPING PROCEDURES Fluxes of sensible heat (H), latent heat (LE), and CO2 (C) were calculated along the flight lines using the eddy correlation method; i.e., estimating the covariance between the fluctuations of the vertical wind and the scalar of interest. The fluctuations were obtained from the original signals by detrending. The choice of the detrending method used was based on a scheme developed by Ogunjemiyo et al. (1997) that takes into consideration the physical properties of turbulent transports of scalar as observed over forests. It assumes that the most intensive turbulent transport occurs in the "gradient modes" (i.e., excess-up and deficit-down for H and LE or deficit-up and excess- down C) during sunlit, daytime convective conditions over photosynthesizing areas, and that optimum detrending yields the highest gradient/countergradient ratio for the dominant turbulent structures. To map fluxes, the flux events along the 16-km flight lines were block averaged over 2-km windows, with 1-km overlap between adjoining windows, giving 15 data points along each flight line. By combining the repeated passes over each grid line, a matrix of 135 points per grid is produced, representing a 4-km average per data point. Data smoothing is done by cubic spline interpolation, and interpolated data are used as input to Geographic Information System (GIS)-based IDRISI software to produce the flux maps of the grid sites. Due to the variability of individual maps, only composite maps representing all flights within the same IFC and over the given site are shown and discussed, as well as average maps over all three IFCs. The same procedure is used to make maps of surface characteristics, which are surface temperature excess Ts-Ta and GI. The reliability of the data to adequately represent the surface over which they were measured was evident from the good agreement between the observed surface classification at the site and those within the estimated flux 'footprint' developed for the sites from tracer gas release studies (Kaharabata et al., 1997). 7. DISCUSSIONS The results presented in the form of composite maps of GI, Ts-Ta, LE, C, and H represent sampling within the same IFC, during intervals ranging from 3 to 11 days, with the exception of IFC-3, where a last sample of the NSA grid was added 13 days after the initial 7-day period. The composite maps and the maps for each variable averaged over the three IFCs are listed in Section 8. It must be mentioned that GI and Ts-Ta were measured from downward looking airborne sensors with a narrow view angle, restricting measurements to a strip approximately 3 m wide beneath the aircraft. The surface temperature patterns at the sites are tied to the surface cover distributions. At the NSA grid site, the largely forested area in the northern half of the site is cooler than the southern half of the site, which is dominated by regenerating burn areas. The same trend is observed in the SSA grid site, where the NW-SE diagonal structure on the surface classification map is strongly reflected in the surface temperature signal. The dominant surface temperature patterns were persistent throughout the three IFCs, though the range of temperature differs significantly from one IFC to another. The GI maps show inverse relationship with surface temperature. This relationship is more pronounced in the NSA than in the SSA. Overall, the most clearly observed relationship between fluxes and surface features is that between GI and CO2 flux, with a good correlation between the CO2 maps and GI maps. By contrast, the relationship between H and Ts-Ta is much more complex, with a significant degree of decoupling between H and Ts-Ta, particularly at the NSA. In the NSA, the main features in sensible heat flux maps are a local maximum in the NW corner, around the dry old jack pine site, and a broader maximum over the much cooler mature black spruce stands. Over the SSA grid, sensible heat fluxes are highest in the northern part, where the old jack pine sites and various clearings and regenerating stands affect the energy balance. The high value of H over the forest and the decoupling of H from Ts-Ta have also been observed by others (e.g., Hall et al., 1995; Mahrt and Sun, 1996). It has been partly associated with viewing angle problems of the radiometers, which see varying fractions of shaded, cool surface between the often sparse trees from which heat is primarily exchanged. While the latent heat flux shows the highest maximum during IFC-2 at both sites, the higher values at the SSA could be associated with intense precipitation at the site, prior to the start of IFC-2. In the NSA, no characteristic patterns appeared to emerge from the LE maps, which are subject to spatially and temporally varying precipitation and drying rates. The local maximum in LE at the SSA, in the southeastern region of the burn area, may be associated with a relative abundance of shallow surface water during most of the IFCs in that area. The dependence of flux patterns on the relative distribution of surface covers is exemplified by the differences between the SSA and the NSA grids. Differences in flux patterns from one IFC to another are more likely caused by changes in soil moisture distribution and plant physiological properties than by changes in radiant energy. By providing the maps listed below, it is assumed that the general patterns of averaged flux density and their persistence may provide the link-up point with geographically referenced models, at the high-resolution end of their spatial scales. 8. LIST OF MAPS NORTHERN STUDY AREA GRID SITES: Classification image of the NSA (nsa_class.tif) CO2 flux map, IFC-1 (nsa_ifc1_carb.tif) CO2 flux map, IFC-2 (nsa_ifc2_carb.tif) CO2 flux map, IFC-3 (nsa_ifc3_carb.tif) Average CO2 flux map for the three IFCs (nsa_avg_carb.tif) Greenness index map, IFC-1 (nsa_ifc1_green.tif) Greenness index map, IFC-2 (nsa_ifc2_green.tif) Greenness index map, IFC-3 (nsa_ifc3_green.tif) Average greenness index map for the three IFCs (nsa_avg_green.tif) Sensible heat flux map, IFC-1 (nsa_ifc1_sens.tif) Sensible heat flux map, IFC-2 (nsa_ifc2_sens.tif) Sensible heat flux map, IFC-3 (nsa_ifc3_sens.tif) Average sensible heat flux map for the three IFCs (nsa_avg_sens.tif) Latent heat flux map, IFC-1 (nsa_ifc1_latent.tif) Latent heat flux map, IFC-2 (nsa_ifc2_latent.tif) Latent heat flux map, IFC-3 (nsa_ifc3_latent.tif) Average latent heat flux map for the three IFCs (nsa_avg_latent.tif) Surface temperature excess map, IFC-1 (nsa_ifc1_tempx.tif) Surface temperature excess map, IFC-2 (nsa_ifc2_tempx.tif) Surface temperature excess map, IFC-3 (nsa_ifc3_tempx.tif) Average surface temperature excess map for the three IFCs (nsa_avg_tempx.tif) SOUTHERN STUDY AREA GRID SITE: Classification image of the SSA (ssa_class.tif) CO2 flux map, IFC-1 (ssa_ifc1_carb.tif) CO2 flux map, IFC-2 (ssa_ifc2_carb.tif) CO2 flux map, IFC-3 (ssa_ifc3_carb.tif) Average CO2 flux map for the three IFCs (ssa_avg_carb.tif) Greenness index map, IFC-1 (ssa_ifc1_green.tif) Greenness index map, IFC-2 (ssa_ifc2_green.tif) Greenness index map, IFC-3 (ssa_ifc3_green.tif) Average greenness index map for the three IFCs (ssa_avg_green.tif) Sensible heat flux map, IFC-1 (ssa_ifc1_sens.tif) Sensible heat flux map, IFC-2 (ssa_ifc2_sens.tif) Sensible heat flux map, IFC-3 (ssa_ifc3_sens.tif) Average sensible heat flux map for the three IFCs (ssa_avg_sens.tif) Latent heat flux map, IFC-1 (ssa_ifc1_latent.tif) Latent heat flux map, IFC-2 (ssa_ifc2_latent.tif) Latent heat flux map, IFC-3 (ssa_ifc3_latent.tif) Average latent heat flux map for the three IFCs (ssa_avg_latent.tif) Surface temperature excess map, IFC-1 (ssa_ifc1_tempx.tif) Surface temperature excess map, IFC-2 (ssa_ifc2_tempx.tif) Surface temperature excess map, IFC-3 (ssa_ifc3_tempx.tif) Average surface temperature excess map for the three IFCs (ssa_avg_tempx.tif) The following files contain the values (in tabular form) for the various grid points in the maps listed above: NSA IFC-1 NSA_94_06-07.CSV NSA_94-06-08A.CSV NSA_94-06-08B.CSV NSA_94-06-10.CSV NSA-94-06-13.CSV NSA IFC-2 NSA_94-07-28.CSV NSA_94-08-01.CSV NSA_94-08-04.CSV NSA_94-08-08.CSV NSA IFC-3 NSA_94-08-31.CSV NSA_94-09-02.CSV NSA_94-09-03.CSV NSA_94-09-06.CSV NSA_94-09-19.CSV SSA IFC-1 SSA_94-05-26.CSV SSA_94_05-31.CSV SSA_94-06-04.CSV SSA IFC-2 SSA_94-07-20.CSV SSA_94-07-21.CSV SSA_94_07-24.CSV SSA_94-07-26.CSV SSA IFC-3 SSA_94-09-13.CSV SSA_94-09-16.CSV 9. Data Description 9.1 Data Characteristics Data characteristics are defined in the companion data definition file (afm13afr.def). 9.2 Sample Data Record Sample data format shown in the companion data definition file (afm13afr.def). 10. Data Organization 10.1 Data Granularity All of the airborne flux data are contained in one dataset. 10.2 Data Format(s) The data files contain numerical and character fields of varying length separated by commas. The character fields are enclosed with a single apostrophe marks. There are no spaces between the fields. Sample data records are shown in the companion data definition file (afm13afr.def). 11. Data Access 11.1 Contact Information Ms. Beth Nelson NASA GSFC Greenbelt, MD (301) 286-4005 (301) 286-0239 (fax) Elizabeth.Nelson@gsfc.nasa.gov 11.2 Data Center Identification See Section 9.1. 11.3 Procedures for Obtaining Data Users may place requests for the data on line or by telephone, electronic mail, or fax. 11.4 Data Center Status/Plans The Analysis And Interpretation Of Airborne Flux Observations Over The BOREAS Site are available from the Earth Observing System Data and Information System (EOS-DIS) Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC). The BOREAS contact at ORNL is: ORNL DAAC User Services Oak Ridge National Laboratory (865) 241-3952 ornldaac@ornl.gov ornl@eos.nasa.gov 12. REFERENCES Desjardins, R.L., J.I. MacPherson, P.H. Schuepp, and F. Karanja. 1989. An Evaluation of Airborne Eddy Flux Measurements of CO2, Water Vapour, and Sensible Heat. Boundary-Layer Meteorology 47:55-69. Desjardins, R.L., P.H. Schuepp, J.I. MacPherson, and D.J. Buckley. 1992. Spatial and Temporal Variations of the Fluxes of Carbon Dioxide and Sensible Heat Over the FIFE Site. Journal of Geophysical Research 97(D17): 18,467-18,475. Hall, F.G., K.F. Huemmrich, S.J. Goetz, P.J. Sellers, and J.E. Nickeson. 1992. Satellite Remote Sensing Of Surface Energy Balance: Success, Failures, and Unresolved Issues in FIFE. Journal of Geophysical Research 97(D17): 19,061- 19,089. Hall, F.G., Y.E. Shimabukuro, and K.F. Huemmrich. 1995. Remote Sensing of Forest Biophysical Structure Using Mixture Decomposition and Geometric Reflectance Models. Ecological Applications 5(4): 993-1013. Hall, F.G. and D.E. Knapp. 1998. BOREAS TE-18 Landsat TM Maximum Likelihood Classification of the NSA. Oak Ridge National Laboratory Distributed Active Archive Center. Hall, F.G. and D.E. Knapp. 1998. BOREAS TE-18 Landsat TM Maximum Likelihood Classification of the SSA. Oak Ridge National Laboratory Distributed Active Archive Center. Kaharabata, S.K., P.H. Schuepp, S. Ogunjemiyo, S. Shen, M.Y. Leclerc, R.L. Desjardins, and J.I. MacPherson. 1997. Footprint considerations in BOREAS. Journal of Geophysical Research 102(D24): 29,113-29,124. MacPherson, J.I. 1990. Wind And Flux Calculations On The NAE Twin Otter. Rep. Ltr-Fr-109, National Research Council, Ottawa, Ontario, Canada. MacPherson, J.I. 1996. NRC Twin Otter Operations in BOREAS, Rep. Ltr-Fr-129, National Research Council, Ottawa, Ontario, Canada. Mahrt, L. and M. Ek. 1993. Spatial Variability of Turbulent Fluxes and Roughness Lengths in HAPEX-MOBILHY. Boundary-Layer Meteorology 65: 381-400. Mahrt, L. and Sun, J. 1996. Formulation of Heat Flux over the Boreal Forest. Proc. 22nd Conference on Agric. and Forest Meteorology, American Meteorology Society, Atlanta, GA, Jan. 28 - Feb. 2, 1996, p. 114-117. Mahrt, L., J.I. MacPherson, and R. Desjardins. 1994. Observations of Fluxes over Heterogeneous Surfaces. Boundary-Layer Meteorology 67:345-367. Ogunjemiyo, O.S., P.H. Schuepp, J.I. MacPherson, and R.L. Desjardins. 1997. Analysis of Flux Maps vs. Surface Characteristics from Twin Otter Grid Flights in BOREAS 1994. Journal of Geophysical Research 102(D24): 29,135-29,145. Sellers, P., F. Hall, H. Margolis, B. Kelly, D. Baldocchi, G. den Hartog, J. Cihlar, M.G. Ryan, B. Goodison, P. Crill, K.J. Ranson, D. Lettenmaier, and D.E. Wickland. 1995. The Boreal Ecosystem-Atmosphere Study (BOREAS): An Overview and Early Results from the 1994 Field Year. Bulletin of the American Meteorological Society 76(9): 1549-1577. Sellers, P.J., F.G. Hall, G. Asrar, D.E. Strebel, and R.E. Murphy. 1992. An overview of the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE). Journal of Geophysical Research 97 (D17): 18,345-18,371. Sellers, P.J., F.G. Hall, R.D. Kelly, A. Black, D. Baldocchi, J. Berry, M. Ryan, K.J. Ranson, P.M. Crill, D.P. Lettenmaier, H. Margolis, J. Cihlar, J. Newcomer, D. Fitzjarrald, P.G. Jarvis, S.T. Gower, D. Halliwell, D. Williams, B. Goodison, D.E. Wickland, and F.E. Guertin. 1997. BOREAS in 1997: Experiment Overview, Scientific Results and Future Directions. Journal of Geophysical Research, 102 (D24): 28,731-28,770. 13. ACRONYMS AFM - Airborne Flux and Meteorology BOREAS - BOReal Ecosystem and Atmosphere Study GI - Greenness Index GIS - Geographic Information System IFC - Intensive Field Campaign NASA - National Aeronautics and Space Administration NRC - National Research Council NSA - Northern Study Area SSA - Southern Study Area TM - Thematic Mapper AFM13.doc 03/03/99