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                                                  TES Validation Results from the Lake Tahoe Special Observations

 Denis Tremblay                                                                                      Raytheon Information Solutions
 Susan Sund Kulawik, Simon Hook, Dave Rider                                                         Jet Propulsion Laboratory

 ABSTRACT: The Tropospheric Emission Spectrometer (TES) is a high-resolution infrared Fourier transform spectrometer on board the AURA spacecraft launched in 2004. As part of the science validation
 activities, four special "Stare" observations were acquired in August and September 2005 over Lake Tahoe located a the California-Nevada border in western US. Each "Stare" observation took 32 target
 observation scans each sharing almost the same footprint. The Tahoe site has several advantages for validation, namely it is at high altitude with negligible cloud cover, it has water as the surface with known
 spectral emissivity, and it has ground-based in-situ radiometric measurements. Statistics of the retrieved values of the target data set over water are compared with the observation error available in the TES product
 file. In most cases, the results show that the standard deviation of the volume mixing ratio (VMR 1 sigma) at a given pressure level is less that the observation error for H20, atmospheric temperature, and O3.
 VMR-sigma is higher than the observation error in the cases of H2O and atmospheric temperature for pressure levels very near the surface. The surface temperature difference of the in-situ radiometric
 measurements minus the TES retrieved values have a mean of -0.11K and a standard deviation of 0.30K. This exercise has provided important validation of the TES reported errors and the TES on-board
 radiometric calibration source.

      1. TES on EOS-Aura Satellite                                                                                                                                                                    Figure 2. TES Optical Schematic                        Table 1. TES Instrument Specifications
                                                                                             Figure 1. TES Instrument
     TES is a spaceborne Connes-type interferometer with 4 ports
    (2 inputs and 2 outputs). Radiance from the 2 outputs ports are
    splitted allowing the measurements of the same field-of-view
    over 4 different optical windows. Each of the 4 focal planes has
    16 HgCdTe PV detectors. A single scan will produces 64
    interferograms. A 4 seconds scan has an optical path difference
    (OPD) of 8.4 cm which gives a spectral resolution of 0.06
    cm-1. A 16 seconds scan has an OPD of 33.2 cm with a
    spectral resolution of 0.015 cm-1 (Limb view only).

     TES pointing control system (PCS) has along and crosstrack
    viewing capability with angles ranging from -45 to + 45
    degrees in each directions. TES has several viewing modes
    which include nadir (or downlooking), limb (backward
    looking), off-nadir, and “stare”. A typical nadir observation has
    a footprint of 5 Km wide by 8 Km long. Off-nadir view
    footprint will have larger footprint than that of nadir view and
    will depends on the viewing geometry.

     Figure 1 shows the TES instrument inside the clean room. The
    optical schematic shows the radiance path and the optical
    elements in Figure 2. Table 1 summarizes the TES instrument

                                                                                                                                                                                                                                  Figure 4. Lake Tahoe Buoy

   2. Lake Tahoe In-situ Measurements
       - Lake Tahoe: 1898 m. altitude, 20% less                                                                          Figure 3. Lake Tahoe buoys locations.

         atmosphere than sea level, never freezes.
       - 4 Buoys equipped with radiometers giving
         in-situ measurements of surface temperature (TSUR).
       - Buoy TB4 disregarded due to systematic bias
          and selected the 5-minutes averaged TSUR.
                                                                                                                                                                                                                        Figure 5. Temperature plot, all 4 buoys, August 20th 2005.
       - Other measurements includes wind direction
         and speed, relative humidity, and more.
            The far right plot shows the skin temperature on August 20th                                           Map Key
                                                                                                                   US Coast
                                                                                                                                              Longitude Measurements
          2005, date of the TES run 3049 which is one of the 4 special stare                                       Guard
                                                                                                                                 39 10.838
                                                                                                                                 39 09.180
                                                                                                                                              120 07.157 v, WD, Ps, SRu, SRd, AOD, ToV, TSI
                                                                                                                                              120 00.020 Tr, Ts, Ts, Ts, Ts, v, WD, Ps, AT, RH, NR
          observations. The greater variability observed in the afternoon, that                                    TB2
                                                                                                                                 39 09.290
                                                                                                                                 39 08.300
                                                                                                                                              120 00.020 Tr, Ts, Ts, Ts, Ts, v, WD, Ps, AT, RH, NR
                                                                                                                                              120 04.920 Tr, Ts, Ts, Ts, Ts, v, WD, Ps, AT, RH, NR
          is between 19H and 24H GMT, is due to increased wind velocity.                                           TB4           39 09.300    120 04.330 Tr, Ts, Ts, Ts, Ts, v, WD, Ps, AT, RH, NR,

          This plot was constructed using the on-line tools available on the
          JPL Lake Tahoe web site. Additional information on others                                        LR – Longwave Radiation (u-upward looking, d-downward looking), SR – Shortwave Radiation
                                                                                                           (u-upward looking, d-downward looking), v – Wind Speed, WD – Wind Direction, Ps – Pressure,
          measurements can be found at the following URL:                                                  AT - Air Temperature, RH - Relative Humidity, NR - Net Radiation, AOD - Aerosol Optical Depth,
                                                                                                           ToV - Total Column Water Vapor, TSI - Total Sky Imager, Tr - Temperature from radiometer, Ts -
                                                                                                           Skin temperature measured 1cm below skin by thermistor.

   3. TES “Stare” Mode Special Observations
     - 4 “Stare” mode runs taken, each comprising of 32 target scans.     Scan 31: Backward Looking

     - Along and cross track viewing angle range from +45 to -45 degrees.                                                                                                                                                         Figure 6. Run 3049 (August 20th 2005) TES footprint
                                                                                                                                                                                                                                           of all 32 scans.
     - Scans of the same run look at the same ‘nominal’ atmosphere
       and share approximately the same footprint (5x8 Km at Nadir).
     - Last 5 scans of run 3049 disregarded due to possible land overlay.                                                                                                                                                                          Scan 31

     - Data processed with TES Software Version 9.3.
     - The retrieval algorithm (ELANOR) uses a non-linear optimal                     Scan 15: Nadir                                                                                                                                                  Scan 0

       estimation method where the forward model is based on the Line-By-Line
       Radiative Transfer Model (LBLRTM) from AER corporation.
                                                                        Figure 4. TES Ground Data System data processing flow

                                                             L1B                      Radiometrically                      L2 Processing
                             L0 Data                                                                                                                                profiles, surface
                                                          Processing                    calibrated                         (Atmospheric
                          (raw packets)                                                                                                                              temperature,
                                                         (Calibration)                   spectra                             Retrieval)
                                                                                                                                                                      and others.

                                                                                                                                                                                                              Scan 0: Forward Looking
                                                    Table 2. TES “Stare” special observations time with duration of 2 min 20 seconds.
                                                                         Run                Date               GMT Starting Time
                                                                         3049          August 20th 2005               21:17:36
                                                                         3060          August 22nd 2005               21:05:14
                                                                                      September 5th 2005
                                                                                      September 7th 2005
4. Results from Atmospheric Retrievals and TSUR comparison.
    - Are the atmospheric profiles consistent within the same run?
    - Is the retrieval error analysis modeling adequate?
    - How do the retrieved and in-situ surface temperatures compare?

                                                                           Air Temperature (TATM)         H2O                         O3                     CO

    A. Each plot shows the mean atmospheric profiles of the air
       temperature (TATM), H2O, O3, and CO of scans flagged as
       “good quality”. In general, the values are consistent from
       one run to another. The values of O3 at about 200 HPa show
       a decline in time.

                                                                              TATM Deviation        H2O Deviation (%)             O3 Deviation (%)     CO Deviation (%)
                                                                                 Run 3049               Run 3049                     Run 3049             Run 3049

   B. These plots show the deviation of the individual profiles
      with respect to the mean profiles shown in section A above. Two
      scans in H2O plot have large deviation and were flagged as
      valid retrievals. This appears to be anomalous. All other
      plots show reasonable values.

                                                                                TATM Error             H2O Error                   O3 Error               CO Error
                                                                                 Run 3049              Run 3049                    Run 3049               Run 3049
    C. Total retrieval error (ST) comprises 4 terms: 1) Smoothing,
       2) Cross-state, 3) Systematic, 4) measurement (Sm). The
       standard deviation (RUN) of the valid profiles of each runs
       is shown along ST and Sm. In general, we have

                          Sm < RUN < ST .

       Values of RUN for TATM and H2O appears to be large for the
       2 pressure levels nearest to the surface. ST and Sm are the
       averaged of the individual TES scans and the horizon bar is the
       standard deviation.
                                                                              Surface Temperature   Surface Temperature       Surface Temperature    Surface Temperature
                                                                                   Run 3049              Run 3060                  Run 3104               Run 3110
   D. The retrieved surface temperature (TSUR) of the individual TES
      observation scans are compared with the in-situ TSUR measured
      by the radiometers mounted on the buoys. The in-situ TSUR mean
      was computed by averaging 6 values, that is 3 buoys with the 2
      averaged values taken at the closest time of the TES observations.
      The difference between the in-situ and the retrieved has a bias
      of -0.11K and  = 0.30. All retrieved TSUR values were
      accounted for to the exception of the 5 scans in run 3049 with
      land overlay.                                                                                   Histogram Surface Temperature Difference
                                                                                                              (Retrieved minus In-situ)


                        - TES special observations over Lake Tahoe provide a unique opportunity to measure the same nominal
                         atmosphere and surface multiple times within three minutes. From these measurements, we learn that
                         the variability of the retrieved profiles is consistent with the reported errors for the air temperature,
                         H2O, O3, and CO.

                       - The TES retrieved surface temperature is consistent with the in-situ measurements, the TES minus
                         in- situ difference has a bias of -0.11K and a standard deviation of 0.30K.


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