SGP99 DOE ARM SGP AERI Vertical Profiles Data 1.0 General Description The Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) Atmospheric Emitted Radiance Interferometer (AERI) Vertical Profiles is one of various data sets provided for the Southern Great Plains 1999 (SGP99) project. This data set contains vertical profiles of temperature, dew point and water vapor mixing ratio every eight minutes taken by the AERI instrument located at the Central Facility. This data set covers the period from 1 July through 31 July 1999. The SGP99 domain is approximately 97W to 99W longitude and 34.5N to 37N latitude. No additional quality control was performed by the University Corporation for Atmospheric Research/Joint Office for Science Support (JOSS). These data are in their original NetCDF format. 2.0 Note of this Documentation File The remainder of this document comes from the ARM program. An up-to-date version of this document (including figures mentioned within the text) is available on the WWW at: http://www.arm.gov/docs/instruments/static/aeri.html Additional information on the AERI instrument can be obtained directly from the University of Wisconsin as well on the WWW at: http://cimss.ssec.wisc.edu/aeriwww/aeri/index.html 3.0 General Purpose The Atmospheric Emitted Radiance Interferometer (AERI) measures the absolute infrared spectral radiance (watts per square meter per steradian per wavenumber) of the sky directly above the instrument. The spectral measurement range of the instrument is 500 to 3300 wavenumbers(cm-1) or 20 to 3 microns. Spectral resolution is 1.0 cm-1. Instrument field-of-view is 1.3 degrees. A calibrated sky radiance spectrum is produced every 10 minutes. The AERI data can be used for: 1) evaluation of line-by-line radiative transport codes; 2) detection/ quantification of cloud effects on ground-based measurements of infrared spectral radiance and; 3) calculation of vertical atmospheric profiles of temperature and water vapor. 4.0 Primary Quantities Measured with System Primary Measurement: 1. Absolute infrared spectral radiance of the sky in units of watts per square meter per steradian per wavenumber 2. Sky brightness temperature as a function of wavenumber in units of degrees Kelvin. Calculated Quantities: 1. Variance of sky infrared spectral radiance as a function of wavenumber 2. Vertical atmospheric profiles of temperature, potential temperature, mixing ratio, and relative humidity. 5.0 Detailed Description of Instrument 5.1 List of Components The AERI radiometer is comprised of six subsystems which include: 1) the interferometer; 2) the detector; 3) the scene scanning optics; 4) the calibration blackbodies with temperature controller; 5) a PC-based instrument control, data acquisition, data processing computer with custom software running under OS/2 and; 6) an automated viewport hatch which serves to protect the AERI optics in inclement weather. 1. Interferometer - The "heart" of the AERI radiometer is a Fourier- transform infrared spectrometer manufactured by Bomem Corp., Quebec, Canada. This spectrometer, dubbed the model MB-100, is a four-port Michelson interferometer employing a flex-pivot mirror translation mount and corner cube reflectors. This design is very robust against ambient vibrations and temperature fluctuations and is well-suited for a field-deployed instrument. In addition, the use of a flex-pivot mirror scanning system eliminates the need for bearing surface maintenance associated with conventional interferometer designs. 2. Detector - The AERI infrared detector is a two-layer "sandwich" design (InSb and HgCdTe) manufactured by Infrared Associates. 3. The scene scanning optics 4. Calibration black bodies 5. Instrument control Hardware 6. Hatch 5.2 Description of System Configuration and Measurement Methods The AERI is located in the optical trailer at the Southern Great Plains (SGP) site; the optical trailer elevation is 315.2 meters. The viewing mirror and the two calibration black bodies are located outside of the optical trailer, while the other half of the AERI: Bomem Interferometer, and the data acquisition computer are all inside of the optical trailer. The viewing mirror is rotated to view the sky and alternately the calibration sources. 5.3 Assessment of System Uncertainties for Primary Quantities Measured Errors in retrieved temperature are estimated by comparing radio sonde values. This figure compares values taken during the water vapor intensive operation period (IOP). As seen in the figure the dew point and temperature both have maximum residual (retrieved minus sonde) at 1.6 kilometers above the ground level (AGL). The maximum temperature residual is 1.6 degrees centigrade, while the maximum dew point residual is 2.5 degrees centigrade. At this time it is not known where the source of the error is or how much error is due to the reference values (radiosonde). Variance of the AERI data with respect to the line-by-line radiative transfer model (LBLRTM), Brown et. al, is reported in reference 3. Also in this same reference a comparison of brightness temperature from the AERI spectra in the window from 1142 wn to 1147 wn. is made with the brightness temperature in the window from 2506 to 2511 wn. In the shorter wavelength window (2506 to 2511 wn.), more solar scattering from clouds and atmospheric aerosol is thought to be the cause of a substantial (up to 30 degrees, higher observed brightness temperature in the shorter wavelength window) offset. Cumulus clouds are observed to have a single scattering albedo near 90% at 2500 wn. while the single scattering albedo of a cumulus cloud at 1100 wn is close to 40% (reference 4). If the solar insulation curve is also considered, far more solar scattering from clouds will be observed in the 2506 to 2511 wn window. This image provides a 1-year comparison of sonde data and AERI retrievals for temperature and mixing ratio. 5.4 Description of Observational Specifications Resolution of instrument is one wavenumber (1/cm). Range of wavelengths is 500 to 3300 wavenumbers. Maximum range is top of atmosphere on clear sky day. Measurements are taken every 10 minutes. The instrument views straight up into the atmosphere with a 1.3 degree field of view. 6.0 Theory of Operations To arrive at the temperature profile of the atmosphere, the radiative transfer equations are inverted and then an iterative scheme is used to compute the best estimate of the atmospheric temperature profile, reference 2. The infrared spectra is gathered by the AERI instrument and the operation of the interferometer follows. Fourier transform infrared (FTIR) spectrometers measure light absorbed or emitted from a sample as a function of wavelength. They consist of an optical system for collecting light and concentrating it, an interferometer for algebraically combining the light from the two light paths, a detector to change the light intensity into an electrical signal, signal conditioning electronics, and a computer for extracting spectral data from the signal using FTIR methods. In general, interferometers combine light from two light paths algebraically resulting in variations in light intensity across the aperture of the interferometer called interference fringes (for non-coincident or non-identical wavefronts). One light path is scanned to vary the optical path length. The other path is a reference path. Consider a Michelson interferometer looking at monochromatic light from a collimated expanded laser beam, in which the incident beam is split into two equal length paths by a beam-splitter. Also assume that each path ends in a plane front surface mirror, which is aligned such that the surfaces are normal to the beam, as shown in Figure 4. If the mirrors are aligned exactly so that the distance traveled by light is point-for-point identical over the beam for the two paths, the observer will see a uniformly bright entrance aperture through the interferometer. If the paths differ by a half-wavelength, the observer will see a uniformly black aperture. For intermediate positions, the intensity will be proportional to the cosine of the phase angle (relative fraction of a half-wavelength path difference). This observation is true only for monochromatic light. If a second monochromatic wavelength is added, the cross-section will have different intensity for each of the two wavelengths because the difference in path lengths between the two paths will be a different multiple (or fraction) of wavelengths for each wavelength. For additional wavelengths, intensity contributions are algebraically summed. If light entering the interferometer is an unknown combination of wavelengths, like light from a source having a broadband spectrum, the result will be a complex combination of intensities due to the multiple wavelengths. As the optical path length of one path is slowly, but uniformly changed, the difference in path length for each wavelength will change. Since the wavelengths are different, the path difference expressed as a factor of the wavelength will be different for each wavelength, and will change at a different rate. Path differences, resulting in a variation in output intensity, will change more quickly for short wavelengths than for long wavelengths. If a detector converts the intensity variations into electrical variations, temporal signal will be a superposition of cosines with periods representing the time variations in intensity. Analysis of this series into its component frequency components (with coefficients characteristic of the relative intensities of the individual wavelength components present in the incident light) is accomplished using a FTIR algorithm. The algorithm is ideally suited to breaking down signals comprising a series of sines or cosines, resulting in the electromagnetic spectrum of the incident light. The function of the Helium Neon laser in a modern FTIR is often misunderstood. Its sole purpose is to measure the position(x) of the moving mirror, the so-called retardation distance. The helium neon laser is used in a separate interferometer, called the reference interferometer, that shares the moving mirror with the infrared interferometer. In this way, fringes are counted in the reference interferometer, which allows a precise measurement of the retardation position, x. With the interferrogram, I(x), from the infrared interferometer, and the retardation position, x, the spectra can be obtained by a fast fourier transform. 7.0 ARM Data Quality Reports Nor reports issued. 8.0 Assessment of Instrument Calibration and Maintenance Procedures from work for site scientist Preventative Maintenance Procedure Summaries for the AERI at the Southern Great Plains (SGP) Site. JOSS NOTE: SEE http://www.db.arm.gov/emp-bin/MDS/ListPM.pl?instrument=AERI01 for this information. 9.0 Calibration Theory Two black body sources, one at 290K, the other at 330K are used to calibrate the instrument. The two sources are used to determine the slope and offset which define the linear instrument response at each wavenumber. (see reference 1) The AERI views these two black bodies every two minutes. Magnitude of the difference between these black body spectra is then formed to compute the responsivity and offset for the instrument. After application of the responsivity and offset, the brightness temperature vs. wavenumber agrees with the known temperatures (290K and 330K) within 1 Kelvin. The residual error is thought to originate from two sources:angular dependence in the beam splitter coatings and emission from the beam splitter coatings. Of these two sources, the largest error appears to originate from emission in the beam-splitter. (see reference 1). 10.0 Calibration History As this instrument takes a calibration run every 2 minutes, the best history is to obtain views of the black body brightness temperature curves (available only from MADS software, instrument mentor). Inspections of these curves by the mentor in the past has revealed no anomalies in calibration. 11.0 Current Status and Locations The first AERI for the ARM project is located in the optical trailer at the Southern Great Plains (SGP) site in Oklahoma. The instrument is located in the optical trailer at an altitude of 362.5 meters. Due to the successful deployment of the first ARM AERI at the central facility near Lamont, four more AERI's were purchased for each of the boundary facilities at the SGP. These instruments are due to be deployed in 1998. They will be linked by the Multiple Aeri Data System (MADS), which will allow the status of each instrument (some of which are 200 miles from the central facility) to be monitored remotely by the mentor at PNNL. AERI instruments were procured for the Tropical Western Pacific (TWP), North Slope of Alaska (NSA), and for the SHEBA ice station. The AERI-ER at the SHEBA ice station is currently deployed at 75 degrees North latitude on an ice flow and is collecting data as of October 28, 1997. The AERI for the NSA site is an extended range (AERI-ER: 2.5 to 25 microns, 4000 to 400 wavenumbers) suited for the reduced water vapor concentration in the Arctic atmosphere and viewing into the so-called "dirty window" from 18 to 25 microns (556 to 400 wavenumbers). 12.0 Calculated Data 12.1 Value Added Procedures AERI PROF, compares retrieved profiles from AERI with the profiles from radiosondes. JOSS NOTE: See http://www.arm.gov/docs/research/vap_homepage/details/aeriprof.html for more information. AERI/LBLRTM, compares radiances retrieved by AERI with those calculated by the line-by-line radiative transfer model (LBLRTM) using sonde data for pressure and temperature broadening calculations. JOSS NOTE: See http://www.arm.gov/docs/research/vap_homepage/details/lblrtm.html for more information. RLAER, profiles of aerosol backscatter and extinction, together with "best estimate" profiles of water vapor mixing ratio, relative humidity, and aerosol scattering ratio. JOSS NOTE: See http://www.arm.gov/docs/research/vap_homepage/details/rlaer.html for more information. LSSONDE, radiosonde profiles, where the moisure profile is scaled to match the MWR's total precipitable water vapor. JOSS NOTE: See http://www.arm.gov/docs/research/vap_homepage/details/lssonde.html for more information. Water vapor and temperature profiles are available at http://cimss.ssec.wisc.edu/aeriwww/aeri/sgpcart/ 12.2 Quality Measurements Experiments The value-added products listed above are also quality measurement experiments (QME). 1) QME AERI PROF, compares retrieved profiles from AERI with the profiles from radiosondes. JOSS NOTE: See http://www.arm.gov/docs/research/vap_homepage/details/qmeaeriprof.html for more information. 2) QME AERI/LBLRTM, for analysis of the QME AERI/LBLRTM residuals. Compares radiances retrieved by AERI with those calculated by the LBLRTM (line-by-line radiative transfer model), using sonde data for pressure and temperature broadening calculations. JOSS NOTE: See http://www.arm.gov/docs/research/vap_homepage/details/qmeaerilbl.html for more information. 13.0 Examples of Data Quick Looks: Daily Plots, from ARM Science Applications Group (See: http://wetfly.llnl.gov/plots/aeri/) AERI Water Vapor and Temperature Profiles, from the University of Wisconsin AERI HOME Page (See: http://cimss.ssec.wisc.edu/aeriwww/aeri/sgpcart/) This plot shows two spectra taken with the AERI. Recalling that the spectra is emission spectra from the atmosphere, the absence of emission at 950 and the weak emission at 550 wavenumbers observed in the lower panel, is evidence for a very dry air mass. In fact, the lower panel is a continental polar air mass (very dry). Compare these same spectral regions with the upper panel taken at Point Magu, CA on the southern California coast line. This link provides data from the deployment of the AERI-ER at the SHEBA ice station. Note the extended wavelength coverage in the low wavenumber end (long wavelengths). As you click on the "bear paws" next to each available date, note occurrences of dry air masses (weak emission at 950 and 550 wavenumbers: November 28, 1997). However, on some days there is a smooth spectral feature indicative of low clouds (eg., November 11, 1997; actually snowed on this day). Ten-minute spectra are available from Penn State University's Web site at http://aero.essc.psu.edu/pub/data2/sgp_gifs/C1/sgpaeri01ch1/. Enter this http address then select the directories with "AERI" in the name and view a variety of gif images: sky brightness temperature as a function of wavelength, standard deviations, and averaged spectra. More data examples appear in the value-added products section below. 14.0 Instrument Mentor notes on data quality control procedures QC frequency: daily, every 2 days QC delay: real-time QC type: multi-parameter display with MADS software from U. Wisconsin Inputs: monitoring information extracted directly from the instrument Outputs: MADS log files Reference: None. PCs dedicated to the AERI and AERI-X instrument systems apply calibration factors prior to data ingest by the SGP site data system. The calibrations are not adjusted subsequently. The collection and ingest modules producing netcdf files have built-in alarms to alert instrument mentor Connor Flynn about missing or corrupted data. Data quality monitoring of both these instruments is done primarily with the Multiple AERI Display System (MADS) supplied by University of Wisconsin. This software package runs under the OS/2 operating system. It permits virtually real-time monitoring of the AERI and AERI-X. The AERI-X instrument, which was produced by the University of Denver, has a data format that is compatible with University of Wisconsin's MADS system. The MADS package consists of two logically separate components. One component is the AERI-sitter which is a panel of between 33 and 42 status buttons that are illuminated green, yellow, red, or blue depending on whether the indicated quantity is okay, marginal, alarming, or missing, respectively. The second component consists of automated displays of the calibrated spectra from detector channels 1 and 2, as well as time series graphs of temperatures, calculated sky-brightness temperatures, and house-keeping quantities. In addition to the automated plots provided by MADS, it can be used to make additional plots, either on specific occasions or for automatic generation by script. Within the MADS environment, limited analysis of the data can be carried out with scalar and vector addition, subtraction, and element-to-element multiplication and division. The MADS system has been configured to automatically collect the calibrated data from each accessible AERI every ten minutes. The displays can be configured for real-time update or for review of historical data. MADS is used by Connor at Pacific Northwest National Laboratory (PNNL) to display real-time data from the SGP AERI-01 and AERI-X. MADS has been used for real- time data from North Slope of Alaska extended range AERI (ER-AERI) during testing at the University of Wisconsin and is set up to handle data collected with the ER-AERI as deployed at the Surface Heat Budget of the Arctic Ocean experiment. Each day, the MADS system collects and stores the data in date-stamped directories with a first-in-first-out (FIFO) "revolving door" system. If there is not enough disk-space to store new data, old data are removed, one day at a time, until there is enough room for the entire new day. Connor reserves an entire 1-GB disk for AERI data, so he has over a week of storage capacity. When he is at PNNL, he usually inspects AERI data every day or every other day. When he is traveling, he has student assistant Brian Ermold inspect the data. The MADS system does not use netcdf files but this functionality might be added. The University of Wisconsin scientists are considering porting the MADS system to a Java platform. If they do move towards Java, they will also add in the netcdf functionality. Connor occasionally uses the AERI netcdf files to examine data that have revolved off of his MADS system. In this event, he obtains the desired netcdf files from the archive and open them with Matlab software. With regard to automation, an expansion might be considered of the capabilities of the ingest module, which produces warnings if data are missing or if the netcdf encapsulation fails due to data corruption. Towards this end, the values of the status lights displayed by the AERI-sitter would be fairly easy to capture with an automated procedure. Connor can provide the limits currently used by the AERI-Sitter display for each relevant quantity. 15.0 Explanation of Flags Applied During Data Ingest The following flag will occur during ingest if the data is suspect: There was some AERI 01 samples that were flagged as having a high standard deviation in the opaque regions of 675-680 and 2295-2300 cm-1 wavenumbers. The thresholds that were exceeded were 1.000 and 0.025 mW/(m2 ster cm-1), respectively. The times of the samples are listed below. This flag will be sent directly to the instrument mentors. 16.0 Frequently Asked Questions FAQs 1. Are data taken continuously? Spectra are taken every 10 minutes. 2. Are data taken at night or on cloudy days? Data are taken as long as the precipitation sensors on the instrument hatch are not triggered. This guards the steering mirror from water and snow that would obscure the optical throughput. 3. Are independently acquired data on temperature and water vapor profiles available for comparison? If so, from what source? Radiosonde data (from weather balloon launches) are available in 3-hour time intervals. A quality measurement experiment (qme), called qmeaeriprof compares the AERI-derived profiles against radiosonde data. See the quality measurement experiment section above. 4. Are there any other references you can recommend? Reference four has a good discussion of the calibration methods. 5. Much more frequent weather balloon launches were made during the Water Vapor Intensive Operation Period (IOP). Are there comparisons available for this period of time? Yes, I refer you to ftp://fire.arm.gov/pub/VAPs/WV_IOP/ 17.0 Contacts 17.1 Instrument Mentor Connor J. Flynn, Postdoctoral appointee Pacific Northwest National Laboratory PO Box 999 Mail Stop K7-22 Richland, WA 99352 Phone (509) 375-2041 FAX (509) 375-4545 Email: cj_flynn@pnl.gov 17.2 Connor's assistant, web page author: Peter A. Eschbach, Senior Research Scientist Pacific Northwest National Laboratory P.O. Box 999, MS. K5-25 Richland, WA 99352 Phone (509) 375-2678 FAX (509) 372-4725 Email: pa_eschbach@pnl.gov 17.3 Vendor/Instrument Developer Wayne Feltz AERI Research Specialist (608) 265-6283 wayne.feltz@ssec.wisc.edu Bob Knuteson University of Wisconsin 608 263-7974 Hank Revercomb University of Wisconsin 608 263-6758 18.0 Glossary Adam: data acquisition system FTIR: Fourier Transform Infrared Wavenumber: the inverse of the wavelength in centimeters. For example, one micron wavelength (.0001 cm, 1e-4) becomes 10,000 wavenumbers when inverted. Wavenumber is useful because the photon energy is equal to the wavenumber times planck's constant times two pi. That is wavenumber is proportional to the photon energy. HgCdTe: Mercury Cadmium Telluride Detector for long wavelength infrared detection (5 to 15 microns). InSB: or 'insbee' detector optimized for near to mid infrared 1 to 5 microns. 19.0 Acronyms AERI: Atmospheric Emitted Radiance Interferometer AERI-ER: Atmospheric Emitted Radiance Interferometer Extended Range FTIR: Fourier Transform Infrared IOP: Intensive Operation Period VAP: Value Added Product CART: Cloud and Radiation Testbed SGP: Southern Great Plains MADS: Multiple Aeri Data System SHEBA: Solar Heat Energy Budget of the Arctic Ocean 20.0 Citable References 1) H.E. Revercomb, H Buijs, H.B. Howell, D.D. LaPorte, William L. Smith, and L.A. Sromovsky. "Radiometric Calibration of IR Fourier transform spectrometers: solution to a problem with the High- Resolution Interferometer Sounder" Applied Optics, Vol. 27 no. 15 pp. 3210-3218. 2) W.L. Smith, "Iterative Solution of the Radiative Transfer Equation for the Temperature and Absorbing Gas Profile of an Atmosphere." Applied Optics, Vol. 9, No. 9, September 1970, pp. 1993-1999 3) P.D. Brown, S.A. Clough, N.E. Miller, T.R. Shippert, D.R. Turner, R.O. Knuteson, H.E. Revercomb, and W.L. Smith "Initial Analyses of Surface Spectral Radiance Between Observations and Line-by-Line Calculations." Proceedings of the Fifth Atmospheric Radiation Measurement (ARM) Science Team Meeting", March 19-23, 1995. pp. 29-32. 4) K.N. Liou, "Radiation and Cloud Processes in the Atmosphere", Oxford, 1992, p. 267