Radiance Validation for the MOPITT Instrument

The validation phase of the MOPITT mission is well underway.  Two classes of products from MOPITT will become available. The Level-1 Product consists of the sixteen geolocated radiances measured by the instrument at each pixel.  Of these, twelve are sensitive to CO, and four to CH4.  The Level-2 Product consists of the retrieved trace gas quantities corresponding to each MOPITT pixel (in addition to several retrieval byproducts, including surface temperature).  Since the Level-1 Product is a required input to the Level-2 Product, it must be validated first.  The MOPITT retrieval algorithm is based on the maximum likelihood technique, and makes heavy use of a fast operational radiative transfer model known as MOPFAS.  The degree to which this model accurately calculates the MOPITT radiances is a main determinant of the accuracy of the MOPITT Level-2 Product.

MOPITT  Radiances

The twelve CO-sensitive MOPITT radiances include four solar-channel signals (in the 2.3 um band) and eight thermal-channel signals (in the 4.6 um band).  The thermal-channel signals, which are the subject of the current study, are evenly divided between 'Average' and 'Difference' signals.  'A' signals are sensitive mainly to the spectral regions between the CO absorption lines, and are therefore very insensitive to CO concentration.  Conversely, 'D' signals are sensitive only to spectral regions very close to the absorption lines and are therefore very sensitive to varying CO.  Both the 'A' and 'D' thermal-channel radiances are generally very sensitive to surface temperature.
 

Radiance Validation Processing

The goal of this research project is to compare satellite-measured radiances with model-calculated radiances in situations where the most relevant inputs to the operational forward radiative transfer model can be accurately specified.  Quantitative agreement between the measured and modeled radiances over widely varying atmospheric conditions is essential to producing reliable trace-gas retrievals.  Initially, we have chosen to validate the MOPITT thermal-channel radiances using the following sources as inputs to the forward radiative transfer model.

- Meteorological data is taken from NCEP, and is temporally and spatially interpolated to the individual MOPITT pixels.

- Surface  temperature , to which all of the MOPITT thermal-channel signals are very sensitive, and which exhibits large variability in space and time, is retrieved from the MOPITT 'A' signals.  Generally, this forces agreement between the calculated model 'A' signals and the observed satellite 'A' signals.

- The input CO profiles are generated by combining the CMDL in-situ profiles (measured from aircraft) for the lower- to mid-troposphere (p > 350 mb) with mixing-ratio models to represent the range of expected CO uncertainty in the upper troposphere and lower stratosphere.

For one in-situ validation profile, all the available MOPITT clear-sky data within a given temporal and spatial 'window' centered on the time and location of the in-situ profile are collected.  For each MOPITT pixel found, the in-situ profile is fed to the forward radiative transfer model (MOPFAS) along with the relevant ancillary data for that pixel (described above).  This produces two datasets, representing the observed (satellite) radiances and a corresponding set of model-calculated radiances.  Each set of radiances corresponding to one in-situ validation profile is averaged to produce a mean observed radiance and a corresponding mean modeled radiance.

Validation Profile Data

In support of MOPITT, NOAA's Climate and Monitoring Diagnostics Laboratory (CMDL) is currently engaged in a program to make regular in-situ CO profile measurements at five sites:  Carr, Colorado; Harvard Forest, Massachusetts; Poker Flats, Alaska; Molokai, Hawaii; and Raratonga in the Cook Islands.  In addition, CMDL is often involved in mission-specific field campaigns, such as SAFARI-2000 (which took place in August-September, 2000, in Southern Africa).

During October, 2000, NCAR personnel obtained access to a Citation aircraft, which is able to fly (and make in-situ measurements) considerably higher than the aircraft generally used at the five CMDL anchor sites.  The aircraft acquired four useful in-situ profiles over Colorado and California during this period. Validation radiances for these profiles are particularly interesting because they include data at upper levels where the mixing ratio can only be estimated (by extrapolation) for the standard CMDL anchor sites.  These data are most important for validating the 'D' signals for channels 3 and 7, which are most sensitive to CO in the upper troposphere and lower stratosphere.
 

Validation Quality Issues

Altitude Limits of CMDL Aircraft

The MOPITT weighting functions exhibit significant sensitivity to CO throughout the troposphere and into the lower stratosphere (up to ~ 100-200 mb).  In contrast, the CMDL aircraft from which the in-situ profiles are recorded typically have an upper altitude limit of about 350 mb. To fill this 'data void', the in-situ profiles are extended by interpolating between the monthly mean climatological CO concentration (from model output) at 100 mb, and the highest actual in-situ level (typically between 350 and 400 mb).  Low and high scaling factors are then applied to this extrapolated part of the profile to represent the expected range of CO variability, which could cause error in the validation radiances.

The upper altitude limit for the Citation aircraft is above the 200 mb level.  For the in-situ profiles measured from this platform, the validation radiance errors associated with the high-altitude 'data void' should be expected to be significantly less than for profiles with lower altitude limits.

Temporal and Spatial Matchup Quality

Generally, MOPITT observations used for validation will be displaced relative to the in-situ observations both spatially and temporally.  For the validation radiances reported here, only MOPITT pixels recorded within 3 hours and within 300 km of the in-situ data are retained.  Nevertheless, significant validation radiance errors might still occur if the measured in-situ profile is influenced by localized phenomena (e.g. plumes from agricultural or industrial emissions).   The validation radiances for the SAFARI-2000 experiment (during which there was significant biomass burning activity) are probably most susceptible to errors of this type.
 

Results

Validation Results for CMDL Anchor Sites

Thermal-channel validation radiances for the five CMDL anchor sites and SAFARI-2000 profiles are shown in Figure 1.  As expected, satellite and modeled radiances for all four thermal-channel 'A' signals agree very well.  More importantly, however, the satellite and modeled radiances for all four thermal-channel 'D' signals also agree very well.  Generally, the agreement for the 'D' signals is highest for channels 1 and 5, which are most sensitive to CO in the lower and middle troposphere, and somewhat less for channels 3 and 7, which are relatively more sensitive to CO in the upper troposphere and lower stratosphere.  This is most likely due to the fact that the upper part of the assumed in-situ profile (obtained by extrapolation) is much more prone to error than is the lower part of the profile (see section above on 'Altitude Limits of CMDL Aircraft').  Most importantly, no significant bias is apparent in any of the thermal-channel A or D signals.

Validation Results for Citation Profiles

Validation radiances for the four profiles measured from the Citation aircraft are shown in Figure 2. In comparison with Figure 1, it is apparent that the the observed and modeled radiances agree somewhat better for these profiles than for the CMDL anchor site profiles.  It is believed that the better agreement for these profiles is directly attributable to the inclusion of in-situ measurements of CO at upper levels unattainable with the aircraft used at the CMDL anchor sites.
 

Summary

- Radiance validation is an essential prerequisite to validation of the MOPITT CO and CH4 trace-gas retrievals.

- Current focus of MOPITT radiance validation is on CO thermal-channel 'A' and 'D' signals.

- Radiances are validated by feeding forward radiative transfer model 'known' inputs, and comparing model-calculated radiances with actual satellite radiances.

- Radiance validation errors (unrelated to the performance of the forward radiative transfer model) may arise from temporal and spatial displacements between the MOPITT pixels and CO in-situ data, and from the lack of in-situ data for the upper troposphere / lower stratosphere region.
 

For more information, contact Merritt Deeter.
7 Mar 2001