Thomas Eden

Global estimates of gravity wave momentum flux from High Resolution Dynamics Limb Sounder observations

analyzed to derive global properties of gravity waves. We describe a wavelet analysis technique that determines covarying wave temperature amplitude in adjacent temperature profile pairs, the wave vertical wavelength as a function of height, and the horizontal wave number along the line joining each profile pair. The analysis allows a local estimate of the magnitude of gravity wave momentum flux as a function of geographic location and height on a daily basis. We examine global distributions of these gravity wave properties in the monthly mean and on an individual day, and we also show sample instantaneous wave events observed by HIRDLS. The results are discussed in terms of previous satellite and radiosonde observational analyses and middle atmosphere general circulation model studies that parameterize gravity wave effects on the mean flow. The high vertical and horizontal resolution afforded by the HIRDLS measurements allows the analysis of a wider range of wave vertical and horizontal wavelengths than previous studies and begins to show individual wave events associated with mountains and convection in high detail. Mountain wave observations show clear propagation to altitudes in the mesosphere.

Initial validation of ozone measurements from the High Resolution Dynamics Limb Sounder

Comparisons of the latest High Resolution Dynamics Limb Sounder (HIRDLS) ozone retrievals (v2.04.09) are made with ozonesondes, ground-based lidars, airborne lidar measurements made during the Intercontinental Chemical Transport Experiment–B, and satellite observations. A large visual obstruction blocking over 80% of the HIRDLS field of view presents significant challenges to the data analysis methods and implementation, to the extent that the radiative properties of the obstruction must be accurately characterized in order to adequately correct measured radiances. The radiance correction algorithms updated as of August 2007 are used in the HIRDLS v2.04.09 data presented here. Comparisons indicate that HIRDLS ozone is recoverable between 1 and 100 hPa at middle and high latitudes and between 1 and 50 hPa at low latitudes. Accuracy of better than 10% is indicated between 1 and 30 hPa (HIRDLS generally low) by the majority of the comparisons with coincident measurements, and 5% is indicated between 2 and 10 hPa when compared with some lidars. Between 50 and 100 hPa, at middle and high latitudes, accuracy is 10–20%. The ozone precision is estimated to be generally 5–10% between 1 and 50 hPa. Comparisons with ozonesondes and lidars give strong indication that HIRDLS is capable of resolving fine vertical ozone features (1–2 km) in the region between 1 and 50 hPa. Development is continuing on the radiance correction and the cloud detection and filtering algorithms, and it is hoped that it will be possible to achieve a further reduction in the systematic bias and an increase in the measurement range downward to lower heights (at pressures greater than 50–100 hPa).

Overview and characterization of retrievals of temperature, pressure, and atmospheric constituents from the High Resolution Dynamics Limb Sounder (HIRDLS) measurements

Received 18 February 2009; revised 22 April 2009; accepted 26 June 2009; published 23 October 2009. [1] The retrieval algorithm for the High Resolution Dynamics Limb Sounder (HIRDLS) instrument onboard NASA’s Earth Observing System (EOS) Aura satellite is presented. The algorithm is based on optimal estimation theory, using a modified Levenberg-Marquardt approach for the iterative solution. Overview of the retrieval scheme, convergence criteria, and the forward models is given. Treatments of clouds and aerosols as well as line-of-sight gradients in temperature are described. The retrievals are characterized by high vertical resolution of 1 km and negligible a priori contribution for all products in regions of high signal-to-noise ratio (SNR) (most of the retrieval ranges). It is shown that these characteristics hold for all latitudes along a HIRDLS orbit. The weighting functions are narrow and show good sensitivity to temperature or gas perturbations in regions of high SNR. The retrieval error predicted by the algorithm consists of radiometric noise, pointing jitter error, smoothing error, and forward model error. For temperature, these components are shown for a midlatitude profile as well as for a full orbit. The predicted temperature error varies from 0.5 K to 0.8 K from the upper troposphere to the stratopause region, consistent with the empirical estimates given by Gille et al. (2008). For O3 and HNO3, the predicted errors and their useful pressure ranges are, respectively, 10–5% from 50 to 1 hPa and 5–10% from 100 to 10 hPa. These results are based on version V004 of the retrieved data, released in August 2008 to the Goddard Earth Sciences Data and Information Services Center (http://daac.gsfc.nasa.gov).

Development of special corrective processing of HIRDLS data, and early validation

The High Resolution Dynamics Limb Sounder (HIRDLS) instrument was launched on NASAs Aura spacecraft on 15 July 2004. When activation was completed 25 days later, it was discovered that the measured radiances were very different from those that were expected. After a long series of analyses and diagnostic tests, the cause was confirmed to be a blockage that covers much of the front aperture, preventing even one completely clear view of the atmosphere. In this paper the steps required to correct the radiances for the effects of the blockage are noted. These are calibrating the radiances, removing the effects of the blockage oscillating, and the radiance coming from the blockage, correcting for the effects of the partial aperture, and filtering the noise. The paper describes the algorithms needed, and presents the results of their application. The success of the procedures will be demonstrated by the quality of the resulting radiances and retrieved profiles of temperature and trace species. The difficulties that have been eliminated, and that still remain are noted, along with plans for further improvement. Finally, the scientific implications are briefly discussed.

HIRDLS proto-flight model radiometric calibration from pre-launch calibration data

The High Resolution Dynamics Limb Sounder (HIRDLS) flight instrument, which is currently in orbit on the NASA Aura Satellite, went through a pre-launch calibration at Oxford University during Autumn 2002. One of the calibration exercises was to characterize the radiometric signals of the HIRDLS proto-flight model (PFM). It was discovered during the data-analysis phase, that the radiometric data required special treatment. Because of the stringent radiometric requirements imposed on HIRDLS, these additional analyses were necessary. This manuscript will detail these specific analysis techniques that were used on the data and present results based on a full analysis of the data, including a complete accounting of the statistical error analysis.

Spectral Characterization of the HIRDLS Flight Instrument From Prelaunch Calibration Data

Results from an instrument-level spectral characterization of the 21-channel High Resolution Dynamics Limb Sounder (HIRDLS) flight instrument will be presented. These data were obtained during the prelaunch calibration of HIRDLS at the University of Oxford (fall 2002). A monochromator, equipped with a controllable diffraction grating, was used to produce monochromatic light for these tests. The monochromator was housed, along with HIRDLS, in a large vacuum chamber. The monochromator was also equipped with a polarizer, which allowed data to be acquired at known polarizations for each channel. A calibration detector, with a flat spectral response, was used to measure the output from the monochromator. This paper will document the analysis procedures used to obtain a measured instrument spectral response for each channel, along with the associated error analyses for these measurements.

Prelaunch calibration of the NASA AURA HIRDLS instrument

The High Resolution Dynamics Limb Sounder (HIRDLS) instrument is scheduled for launch on the NASA AURA satellite in January 2004; it is a joint project between the UK and USA. HIRDLS is a mid-infrared limb emission sounder which will measure the concentration of trace species and aerosol, and temperature and pressure variations in the Earths atmosphere between about 8 and 100 km altitude on a finer spatial scale than has been achieved before. This will depend upon both a high quality of instrument build, and very precise pre-launch calibration. Proto Flight Model calibration was performed in a purpose-built laboratory at Oxford University during an 13-week period in 2002. The tests were made in vacuum under cryogenic conditions close to the space environment. The measurements were divided into spectral, spatial and radiometric, with the HIRDLS pointing capability being used to control which item of test equipment was viewed. A large degree of automation was achieved, and this combined with 24-hour/7-day working enabled a large quantity of information to be obtained.

Spectral characterization of the HIRDLS flight instrument from prelaunch calibration data

Results from a pre-launch in-band spectral characterization of the 21-channel HIRDLS flight instrument will be presented. These data were obtained during a pre-launch calibration of HIRDLS at Oxford University (Fall 2002). A monochromator, equipped with a controllable diffraction grating, was used to produce monochromatic light for these tests. The monochromator was enclosed, with HIRDLS, in a large vacuum chamber. The monochromator was also equipped with a polarizer, which allowed for data to be procured at known orthogonal polarizations for each channel. A calibration detector, with a flat spectral response, was used to monitor the output from the monochromaotr. This report will consist of a description of the analyiss methodlogy, leading to an unpolarized instrument spectral response function for each channel.

Prelaunch Radiometric Calibration of the HIRDLS Flight Instrument: Results and Use in On-Orbit Data Processing

Results from the prelaunch radiometric calibration of the 21-channel High Resolution Dynamics Limb Sounder (HIRDLS) flight instrument are presented. The calibration was carried out in the Department of Physics of Oxford University. Two large aperture external blackbody cavities were used to generate stable radiances at target temperatures between ~90 and ~320 K. These blackbodies were located, along with the HIRDLS instrument, inside a large vacuum chamber. Data were taken at three different focal-plane temperatures (~61, ~66, and ~71 K). To complicate matters beyond the initial scope of the prelaunch calibration, a failure of some contamination close-out material (Kapton) that lined the inner fore-optics cavity occurred during launch, which made the original in-flight radiometric calibration procedure impossible. Accordingly, the radiometric conversion algorithm had to be changed, requiring more information from prelaunch calibration to be used than first envisioned. This paper discusses a variety of details, such as data-taking procedures, analysis methodology, associated error analyses, and necessary changes to the radiometric conversion algorithm needed for inflight data processing.

HIRDLS functional performance in orbit: a summary

The functional performance of the NASA Aura HIRDLS instrument since launch on the 15th July 2004 is presented and discussed. The HIRDLS (High Resolution Infra-red Limb Sounder) is a 21-channel infra-red radiometer, using actively cooled MCT detectors on a common focal plane. It has many features that provide considerable flexibility of the commanding, control and the format and content of the telemetry. HIRDLS also features a precision 2-axis scan mirror and gyroscopes that are attached to the optical bench and together they provide additional data on the line of sight on small time scales. The stability of the temperature control of the focal plane and critical optical components is also presented and discussed. To-date the instrument has performed functionally without fault and in many aspects well within specifications. The only problem (and a serious one) so far encountered has been the optical blockage of the main aperture, which is discussed in other papers. Some aspects of the instrument that have been utilised to help characterise the blockage are outlined.