- Overview
- The Data
- Characteristics
- Source
- Instrumentation
- Source
- The Files
- File characteristics
- Storage Requirements
- Data Access and Contacts
- FTP Site
- Points of Contact
- References
| Long EZ data | Oregon State analysis | Aircraft teams | Air boundary data |
Overview
Moisture statistics and fluxes were collected by the Long-EZ and Twin Otter aircraft along the flight path of the P-3 aircraft which carried the LASE and ESTAR instruments. In coordinated flights, the Long-EZ and Twin Otter measured the air-surface exchange in situ for relation to the soil moisture patterns observed by ESTAR. Measurements in coordination with LASE allow examination of the overall moisture budget of the atmospheric boundary layer and of individual turbulent entrainment events through the boundary-layer's top.
The Data
The data are in two formats. There are run averages of turbulent fluxes, where a 'run' is defined as a single pass over one of the pre-defined flux-aircraft flight lines (map available from Oregon State University.) These averages are divided into low-altitude passes, sampling surface fluxes, and passes higher in the boundary layer. The low passes are nominally at 35 m AGL, but 150 m AGL over Little Washita Watershed because of obstructions. Information provided is valid at the center of the pass. It includes the latitude/longitude, the time in day of year and fraction (Central Daylight Time), and fluxes of net radiation, sensible heat, latent heat, CO2, and momentum (drag) eastward and northward. Altitude is not provided on these files, but can be obtained from the more detailed set.
The more detailed information is 1-s averages of atmospheric state quantities over each run. There is one file per run, the columns of which are given in the table below. There are no column headings in the files themselves. The flux runs (files ending ".FLX" and ".BND") have the full set of data. Profiles (ending ".PRO") do not include the radiation measurements, since these runs were often spirals, where the airplane was significantly banked. Though it would be possible to correct the radiation for the nonzero roll angle, this has not so far been done. The units of photosynthetically active radiation are micro-Einsteins per square meter per second. An Einstein is an Avagadro's number of photons.
The DOY is Central Daylight Time, given with sufficient precision to resolve each second. Two variables have invalid offsets, unimportant for fluxes, but of interest for other purposes. The water mixing ratio can be related to absolute content using dew-point data. Absolute altitude, except for files on 19 and 24 June, can be determined from the pressure, and by noting that the low-altitude flux runs (except over Little Washita) were generally 35 m above ground. Files on 19 and 24 June have been recomputed with absolute altitudes, based on the US Geodetic Survey site at Lamont OK. Some confusion remains for these files, however. The Long-EZ's elevation on the runway is given in these files as 360 m (+/- 2 m), while the published elevation of the Will Rogers Airport is 395 m. This discrepancy has not yet been resolved.
The "flux fragments" are 1-s averages of products of the vertical component of the departure wind with the departure values of the scalars being transported. They can be composed into meaningful flux values by averaging over appropriate intervals. The base state from which these are departures is generated by a symmetric filter with half-power point at 60-s period. Features longer than about 3 km (50 m/s) are effectively part of the base state, while those shorter than about 3 km are effectively part of the departure. For the low-altitude runs this scale captures the bulk of the flux. At higher altitudes this is less certain, and other methods of computing flux are being investigated. It is recommended that the flux fragments be averaged as much as possible to obtain the largest possible sample of the turbulence. This can be accomplished by including fragments from runs neighboring in time as well as in space. The 1-s resolution of these fragments is meant to allow flexibility in choosing averaging interval's location.
One-second averages Parameters Units Description CDT DOY Day of Year and Fraction, Central Daylight Time Latitude deg GPS-derived coordinates Longitude deg Altitude m Flux runs only PAR (downwelling) 10-6En/m2/s Photosynthetically Active Radiation PAR (upwelling) Net Radiation W/m2 Surface temperature degC Dew Point degC Airplane's Heading deg U m/s Wind: eastward and northward V m/s Air temperature degK Potential temperature degK H2O mixing ratio g/kg Relative to dry air C02 mixing ratio ppm Ambient pressure mb Flux fragments, flux runs only H W/m2 Sensible heat flux LE W/m2 Latent heat flux C02 mg/m2/s CO2 flux dragEast N/m2 or Pa Momentum Flux: eastward and northward dragNorth
Run averages Parameters Units Description Filename MMDDhhmm Filename in the form of date & time + extension CDT DOY Day of Year and Fraction, Central Daylight Time Latitude deg GPS-derived coordinates Longitude deg NetRad W/m2 Net radiation H W/m2 sensible heat flux LE W/m2 latent heat flux C02 mg/m2/s Carbon dioxide flux dragEast N/m2 or Pa eastward and northward momentum flux dragNorth The Long-EZ flux aircraft, N3R, is an experimental airplane; modified from a Rutan design. Its aerodynamic characteristics have many advantages for high-fidelity turbulent flux measurement. The small, laminar-flow airframe minimizes flow distortion at the nose for high-fidelity measurements of winds, temperature and trace species. The pusher configuration frees the nose of propeller-induced disturbance, engine vibration, and exhaust. The canard design resists stalling and has excellent pitch stability in turbulent conditions. This, combined with its low wing loading, allows for safe low-speed (50 m s-1), low-altitude (10 m) flux measurement.
The Long-EZ has an empty mass of 430 kg and a maximum gross takeoff mass of 800 kg. Endurance significantly exceeds 10 hours, although pilot fatigue precludes routine 10-hour missions. Typical operations include two 4-hr or three 3-hr missions.
The airborne flux instrumentation, and the data system with its associated software were specifically designed and built by ATDD (Crawford et al., 1990). Wind velocity and temperature fluctuations are measured with ATDD's turbulence probe (Crawford and Dobosy, 1992). The probe is mounted five chord lengths ahead of the wings, where flow distortion is small (Crawford et al., 1996). It carries pressure, temperature and acceleration sensors in a nine-hole pressure-sphere gust probe of ATDD design. This sensor suite is specifically designed for eddy-flux measurement at the higher frequencies required for low altitude flight. A thermistor in the central pressure port provides simultaneous temperature measurement, at a location symmetrical with respect to the flow, for accurate determination of true air speed and heat flux. Water-vapor and CO2 fluctuations are measured with an open-path , infrared gas absorption (IRGA) analyzer, developed at ATDD (Auble and Meyers, 1992).
Position, velocity and attitude of the Long-EZ are sensed by a mix of differentially-corrected GPS and integrated acceleration measurements, a departure from the inertial navigation systems (INS) typically used in this service. Differential correction of GPS provides a dramatic increase in accuracy by canceling errors common to both the base and mobile receivers. The receivers we use report ten times per second, a rate we extend to higher frequencies by mixing with integrated acceleration measurements. Thus we can cover, at significantly reduced cost, the same frequency range obtainable from a high-quality INS. Our upgrade to 10-Hz GPS receivers has largely overcome the few difficulties identified during our last intercomparison with similarly-rigged flux aircraft (Dobosy et al., 1997), yielding an effective and inexpensive tool for measurement of the spatial variation of air-surface exchange.
The data stream is dominated by high-frequency analog signals from the accelerometers, pressure sensors and the like. Analog signals are first electronically conditioned by 30-Hz lowpass Butterworth anti-aliasing filters. The conditioned signals are then sampled and digitized at 250 Hz. The 250-Hz data are digitally filtered and sub-sampled at 50 Hz. Low-frequency analog signals, such as from the net radiometer, are similarly handled, but the 250-Hz samples are filtered appropriately for subsample at 1 Hz. Meanwhile, GPS data are received at 10 Hz. All data streams are synchronized to a single clock frequency. Spectral and cospectral analysis show that the 50-Hz data rate (25 Hz Nyquist frequency) is adequate to measure fluxes at the Long-EZ's flight speed and altitude.
The Files
File characteristicsFilenames for the one-second averages indicate the Month, Day, Hour, and Minute of the start of the run in UTC, (CDT+5).
One-second averages Type File Extension Format # Parameters Avg file size
in bytes# Files Flux FLX ascii tables 22 ~82000 132 Boundary BND ascii tables 22 ~81500 101 Profile PRO ascii tables 11 ~5200 70
Run averages Type Name Format # Parameters File size
in bytes# Files Flux cumflx.asc ascii tables 10 15081 1 Boundary cumbnd.asc ascii tables 10 8503 1 The SGP97 Long EZ data set requires about 22MB of disk storage.
Data Access and Contacts
FTP SiteThe LONG EZ data is in the following GES DISC ftp site:
ERRATUM: Fluxes and flux fragments computed for 12 July and possibly 10 July 1997 have been found to be in error. Corrected values are to be supplied during October 1998. We appologize for any inconvenience.
Points of Contact
- For science and instrumentation questions contact:
R. J. Dobosy, Ph.D.,
Atmospheric Turbulence and Diffusion Division, NOAA/ARL
456 S. Illinois Ave, P. O. Box 2456
Oak Ridge, TN 37831-2456- E-mail: dobosy@atdd.noaa.gov Voice: 423-576-1233
- Fax: 423-576-1237
- For information about or assistance in using DAAC data, contact:
Hydrology Data Support Team
Goddard Earth Sciences- Data and Information Services Center (GES DISC)
E-mail: hydrology-disc@listserv.gsfc.nasa.gov Voice: 301-614-5165
Code 610.2
NASA Goddard Space Flight Center
Greenbelt, Maryland 20771
Fax: 301-614-5268
SGP97 Long EZ data online
References
Auble, D. A. and T. P. Meyers. An Open-Path Gas Analyser for Use with Eddy Flux Systems. Boundary-Layer Meteorology 59:243-255 (1992).
Crawford, T. L., R. T. McMillen, and R. J. Dobosy. Development of a "Generic" Mobile Flux Platform with Demonstration on a Small Airplane. NOAA Technical Memorandum ERL ARL-184, 81 pp. (1990).
Crawford, T. L. and R. J. Dobosy.A Sensitive Fast-Response Probe to Measure Turbulence and Heat Flux from any Airplane. Boundary-Layer Meteorology 59:257-278 (1992).
Crawford, T. L., R. J. Dobosy, and E. J. Dumas. Aircraft Wind Measurement Considering Lift-Induced Upwash. Boundary-Layer Meteorology 80: 79-94 (1996).
Dobosy, R. J., T. L. Crawford, J. I. MacPherson, R. L. Desjardins, R. D. Kelly, S. P. Oncley, and D. H. Lenschow. Intercomparison Among the Four Flux Aircraft at BOREAS in 1994. Journal of Geophysical Research 102(D): 29,101-29,111 (1997).
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Last update:Thu Jan 28 09:34:12 EST 1999 Page Info: Hydrology Data Support Team -- hydrology-disc@listserv.gsfc.nasa.gov Web Curator: Stephen W Berrick NASA official: Steve Kempler, DAAC Manager -- Steven.J.Kempler@nasa.gov |