This file contains a summary of the instrument documentation to help you use the CLAES data held at the BADC
This section contains a brief, simplified outline of the CLAES instrument and the measurement techniques which are employed. Users requiring a more detailed description of the design and operation of the instrument should consult the paper by Roche et al. (1993).
1.1 Instrument Description
CLAES is a cryogenically cooled infrared spectrometer which measures thermal emission from the Earth's limb. Characteristic vibration-rotation spectral line radiances are obtained between 3.5 and 13 microns and these are then fed into retrieval algorithms to obtain pressures, temperatures and species mixing ratios.
The instrument observes the Earth's limb in a direction normal to the orbital track on the anti-sun side of the spacecraft.The tangent point is a great circle distance of approximately 23° (2500km) away from the sub-satellite track, and is swept horizontally along the limb by the motion of the spacecraft. The field of view of the instrument is centred on a point at approximately 35km altitude at the limb tangent point. The optical design allows observations to be performed over an altitude range of 10 to 60 km with a spacing of 2.5km.
The instrument observes radiation through a 6-inch aperture Mersene telescope. The altitude range of the observations is adjusted by means of a motorised limb acquisition and adjustment mirror (LAAM). The CLAES spectrometer consists of one of four tilt-scanned Fabry-Perot etalons which are used in conjunction with one or more of nine interference filters which define the spectral channels. Radiation emerging from the spectrometer is focussed onto the focal plane assembly (FPA) consisting of two linear arrays of SiGa detectors. The main array has 20 elements and provides 2.5 km spacing at the limb. A separate three-element array is used with the 3.5 mm HCL channel, which sacrifices vertical resolution to increase signal-to-noise for the weak atmospheric thermal emissions at this short wavelength.
CLAES carries a full aperture black body calibration source on the inside of the telescope aperture door, which is passively heated by radiation from the Earth when the door is open during data acquisition. When the door is closed the black body slowly cools allowing an end-to-end absolute radiometric calibration over a large part of the instrument's dynamic range.
The Cryogens which slowly evaporated as they cooled the instrument, were designed to last about 18 months in orbit, allowing CLAES to make scientific measurements from 1st October 1991 through 5th May 1993, when the cryogens finally evaporated and the instrument warmed up.
1.2 Measurement Techniques
Spectroscopy of the incoming radiation is performed by tilt-scanning one of the four Fabry-Perot etalons between 0 and 23° in conjunction with blocking filters to define the spectral channel. The spectrometer can operate in one of two modes :
- Spectral Survey Mode in which each of the nine spectral intervals is spectrally scanned in 0.02 cm-1 steps over the etalon's free spectral range. Each scan takes 65.5 seconds making a total of 590 seconds for all channels.
- Primary Science Mode this is the usual CLAES operating mode, in which all parameters required to retrieve geophysical parameters must be acquired in a single 65.5 second period - the time taken for the spacecraft to move 500 km along track. This is achieved by targetting a limited number of spectral positions in each of the nine intervals.
1.3 Data Processing Techniques
The UARS data processing is carried out at the Central Data Handling Facility at the Goddard Space Flight Center using software supplied by the instrument's Principal Investigator group. The data processing for UARS instruments consists of a progression through a sequence of `levels' from the raw telemetry at level 0 to geophysical quantities interpolated onto standard grids at level 3. The processing steps for CLAES are outlined below :
- Level 1 processing: In the first processing step, the level 0 data (the raw telemetry) is processed to remove instrument-specific effects and a set of calibrated data is derived in physical units (eg. voltages and radiances) tagged with their locations which are written to a level 1 product.
- Level 2 processing : The level 1 data are then processed further to produce the level 2 product which contains vertical profiles of temperature, pressure and mixing ratios of chemical constituents at the measurement positions. This step involves a complex retrieval algorithm which consists of two principal stages : In the first step the pressure and temperature are retrieved as a function of the instrument's altitude grid, using features of the CO2 spectrum in the spectral channel at 789-793 cm-1. This region also contains a significant contribution from ozone, which must be retrieved simultaneously. The temperatures and pressures are then fed into the retrievals of mixing ratios from the radiances measured in other spectral channels. The algorithm used in the routine retrievals of geophysical parameters from CLAES data is an optimal mix of two different processes, one involving a relaxation technique to obtain a simultaneous retrieval over the whole altitude range and the second using an onion-peeling process. Both algorithms use some a priori information. For more details refer to Roche et al.(1993).
- Level 3A processing : The level 2 data are profiles located at the measurement positions which are determined by the scan pattern and by the track of the tangent point. The level 2-3A processing step takes these data and interpolates them onto a standard set of vertical levels, evenly spaced in log pressure, and onto standard times (level 3AT) and standard latitudes (level 3AL).
The public CLAES data held at the BADC is at level 3A version 8. Some files from earlier processing version (v0007) are also held where there are gaps in the version 8 data. The version number refers to the processing algorithm which is used to create the level 3A files.
The data is stored in UARS binary format. Software is available to convert the files into ASCII data.
2.1 Spatial Coverage
The table below gives the vertical ranges over which geophysical parameters are retrieved. The retrieved quantities are not necessarily valid over this range, and users should consult the CLAES Data Set Change History Document before using any CLAES data.Parameter | Retrieved Range (mb) ------------------------------------------- Temperature | 150-0.1 O3 | 150-0.1 CH4 | 100-0.1 N2O | 100-0.15 NO2 | 100-0.1 HNO3 | 100-0.1 NO | 100-0.1 H2O | 100-0.1 ClONO2 | 100-1.0 CF2Cl2 | 100-2.0 AEROSOL | 68-20 N2O5 | 46-1.0 CFCl3 | 100-10
The vertical coverage of the CLAES instrument is shown schematically in UARS Figure 3 which compares the vertical coverage of the UARS instruments.
Views of the limb from the UARS orbit extend to 10° of the pole. To achieve maximum latitude coverage, the instrument views at right angles to the orbit track. The spacecraft performs a 180° yaw approximately every 36 days so that the spacecraft orientation relative to the velocity vector is reversed. (eg. the spacecraft flies "backwards" rather than "forwards").
Each spacecraft yaw manoeuvre changes the coverage from high latitudes in one hemisphere to high latitudes in the other. Thus, when the spacecraft files "forwards", the coverage is 80°S to 34°N, and when it flies "backwards" the coverage is 80°N to 34°S.
The latitudinal coverage of the CLAES instrument is compared with the other UARS instruments in UARS Figure 2.
2.2 Temporal Coverage
CLAES operated from shortly after the UARS launch in October 1991 until the 5th May 1993, when the instrument's cryogen was exhausted. The following list gives days for which data are missing or unavailable at version 7.Date UARS day Comments 13th-16th Jan 1992 124-127 Aperture door closed 31st Jan 1992 142 Aperture door closed 13th-17th Feb 1992 155-159 Aperture door closed 4th Mar 1992 175 Aperture door closed 8th Mar 1992 179 Processing failed 22nd-25th Mar 1992 193-196 Aperture door closed 13th Apr 1992 215 Aperture door closed 17th May 1992 249 Aperture door closed 30th May-10th Jun 1992 262-273 Solar array problem 19th Jun 1992 282 Processing failed 11th-16th Jul 1992 304-309 Aperture door closed 18th Jul 1992 311 Processing failed 29th Jul 1992 322 Aperture door closed 11th-15th Aug 1992 335-339 Aperture door closed 18th-23th Sep 1992 373-378 Aperture door closed 10th Oct 1992 395 Aperture door closed 27th-31st Oct 1992 412-416 Aperture door closed 13th Nov 1992 429 Aperture door closed 27th Nov-1st Dec 1992 443-447 Solar array problem 8th-11th Jan 1992 485-488 Aperture door closed 28th Jan 1992 505 Aperture door closed 7th-11th Feb 1992 515-519 Aperture door closed 17th-21st Mar 1992 553-557 Aperture door closed 27th Mar 1992 563 Processing failed 30th Mar 1992 566 Processing failed 4th Apr 1992 571 Aperture door closed 16th-20th Apr 1992 583-587 Aperture door closed 25th-29th Apr 1992 592-596 Aperture door closed 6th May 1992 603 Cryogen exhausted.
The UARS level 3A data are interpolated onto a standard set of logarithmically spaced pressure levels. There are 6 levels per decade in pressure given by the following relation :p = 1000 x 10(-i/6), i=0, 1, 2, ...
The level 3AT and level 3AL products differ in their horizontal gridding:
- Level 3AT: The 3AT products are interpolated along the tangent-point track at standard output times corresponding to intervals of one UARS Engineering MAjor Frame (1 EMAF = 65.536 s). This period represents an along- track displacement of approximately 400 km between measurements.The profiles are constructed by linear interpolation in observation time from the level 2 profiles. Note that the level 2 files have a higher resolution along track, each level 3 profile being constructed from two level 2 profiles.
- Level 3AL: The 3AL products are interpolated onto positions defined by the intersection of the tangent track with a latitude circle at standard 4° latitude intervals
2.4 Data Quality
The quality of the CLAES data is comprehensively reviewed in the CLAES Data Set Change History Document, which is available online in the BADC's CLAES documentation directory. Details of the data quality change from one data version to the next, so the contents of this document are not reproduced here.
The units for the parameters present in the CLAES data files are tabulated below :Parameter | Units ---------------------------------------------- Temperature | K Pressure | mb Gas Mixing Ratios | by volume Aerosol Extinction | km-1 ---------------------------------------------