C. D. Rodgers

Remote sensing of atmospheric structure and composition by pressure modulator radiometry from space : the ISAMS experiment on UARS

The scientific objectives of the improved stratospheric and mesospheric sounder (ISAMS) experiment involve the measurement of global temperature and composition profiles from an instrument on the Upper Atmosphere Research Satellite (UARS). This paper describes the instrument concept, its design, and its performance as calculated and as measured in the laboratory. The data retrieval technique, operating modes, observing strategy, and the error budget are briefly discussed.

A retrieval method for atmospheric composition from limb emission measurements

Efficient and flexible algorithms for the retrieval of atmospheric constituent and aerosol profiles from a limb-sounding satellite radiometer are presented. The radiative transfer scheme ( “forward model”) has been generalized to cope with departures from local thermodynamic equilibrium (LTE), modelling emission from fundamental bands as well as hot bands. Its formulation makes possible a fast method of accurately calculating the radiance derivatives (“weighting functions”) required for the nonlinear optimal estimation algorithms. The retrieval schemes can be used with a wide variety of signal to noise ratios and with measurements from more than one wavelength. Multiple retrieval products are possible. A complete error analysis method emerges naturally from the theory, permitting an extensive prelaunch characterization of the retrieval products. The algorithms have been implemented for the improved stratospheric and mesospheric sounder (ISAMS), an infrared limb sounder on the Upper Atmosphere Research Satellite. The retrieval algorithms and error analysis are illustrated with some of the prelaunch tests performed for ISAMS. The O3 retrieval exemplifies those using measurements with a very high signal to noise ratio for which the quality of the retrieved product will be limited by spectroscopic and other forward model deficiencies. Preliminary calculations indicate that it should be possible to perform retrievals of daytime CO in the mesosphere and lower thermosphere because of rather than in spite of the strong non-LTE emission in that region due to solar pumping at 4.6 μm. The “optimal onion-peeling” algorithm used in the joint retrieval of CH4 and N2O is a very accurate means of solving strong cross contamination problems and proves to be much quicker than any iterative sequence of single-product retrievals.

The separated polar winter stratopause: A gravity wave driven climatological feature

Abstract An examination of satellite-derived temperatures reveals that the winter polar stratopause is usually elevated and warmer than the adjacent midlatitude stratopause. This “separated stratopause” occurs in both hemispheres, but is more pronounced and persistent in the southern winter. It descends with time towards spring and exhibits week to week variability. Observational diagnostics and results from a two dimensional (2-D) model suggest that gravity wave driving can account for this separated polar stratopause by driving a meridional circulation with downwelling over the winter pole. In the model, the solar heating pattern induces stronger winter westerlies than summer easterlies, which leads to a stronger gravity wave driven circulation in the winter hemisphere. Spherical geometry and the high latitude location of the winter westerly jet combine to yield a concentrated region of downwelling. Model results suggest that descent of the temperature maximum with time is probably caused by wave–mean f...

Microwindow selection for high-spectral-resolution sounders

The recent development of satellite instruments that obtain spectrally resolved measurements of the atmosphere has highlighted the problem of how to determine the best subsets, or microwindows, of such spectra for retrievals of temperature and composition. A technique is described that maximizes the information content (or some other figure of merit) based on the modeling of the propagation of systematic as well as random error terms through the retrieval process. Apart from selecting microwindows, this technique can also prioritize existing microwindows for different circumstances and provides a full error analysis of the retrieval. A practical application is demonstrated for the Michelson Interferometer for Passive Atmospheric Sounding limb-viewing interferometer, but the technique is equally applicable to nadir-viewing instruments.

Retrieval of tropospheric carbon monoxide for the MOPITT experiment

A retrieval method for deriving the tropospheric carbon monoxide (CO) profile and column amount under clear sky conditions has been developed for the Measurements of Pollution In The Troposphere (MOPITT) instrument, scheduled for launch in 1998 onboard the EOS-AM1 satellite. This paper presents a description of the method along with analyses of retrieval information content. These analyses characterize the forward measurement sensitivity, the contribution of a priori information, and the retrieval vertical resolution. Ensembles of tropospheric CO profiles were compiled both from aircraft in situ measurements and from chemical model results and were used in retrieval experiments to characterize the method and to study the sensitivity to different parameters. Linear error analyses were carried out in parallel with the ensemble experiments. Results of these experiments and analyses indicate that MOPITT CO column measurements will have better than 10% precision, and CO profile measurement will have approximately three pieces of independent information that will resolve 3-5 tropospheric layers to approximately 10% precision. These analyses are important for understanding MOPITT data, both for application of data in tropospheric chemistry studies and for comparison with in situ measurements.

Measurements of the evolution of the Mt. Pinatubo aerosol cloud by ISAMS

Measurements by the Improved Stratospheric and Mesospheric Sounder (ISAMS) on the Upper Atmosphere Research Satellite (UARS) are being used to study the spatial and temporal evolution of the volcanic stratospheric aerosol from Mt. Pinatubo. The maximum opacity of the aerosol cloud moved from a position south of the Equator at an altitude of about 26 km in early October 1991, became located over the Equator by mid-January 1992, and descended in altitude to about 21 km by July 1992. Dispersal of the cloud was more rapid in the Southern Hemisphere and penetration to the southern polar region occurred earlier than transport to the corresponding northern polar area. The area weighted global mean stratospheric optical thickness between 15 km and 35 km at 12.1 μm remained at about 5.5×10−3 from November 1991 through to April 1992. The estimated aerosol mass loading is 19–26 megatonnes for this period and by the end of July 1992 it had declined to 15–21 megatonnes.

Comparison of carbon monoxide measurements by TES and MOPITT: Influence of a priori data and instrument characteristics on nadir atmospheric species retrievals

[1] Comparisons of tropospheric carbon monoxide (CO) volume mixing ratio profiles and total columns are presented from nadir-viewing measurements made by the Tropospheric Emission Spectrometer (TES) on the NASA Aura satellite and by the Measurements of Pollution in the Troposphere (MOPITT) instrument on the NASA Terra satellite. In this paper, we first explore the factors that relate the retrieved and the true species profiles. We demonstrate that at a given location and time the retrieved species profiles reported by different satellite instrument teams can be very different from each other. We demonstrate the influence of the a priori data and instrument characteristics on the CO products from TES and MOPITT and on their comparisons. Direct comparison of TES and MOPITT retrieved CO profiles and columns show significant differences in the lower and upper troposphere. To perform a more proper and rigorous comparison between the two instrument observations we allow for different a priori profiles and averaging kernels. We compare (1) TES retrieved CO profiles adjusted to the MOPITT a priori with the MOPITT retrievals and (2) the above adjusted TES CO profiles with the MOPITT profiles vertically smoothed by the TES averaging kernels. These two steps greatly improve the agreement between the CO profiles and the columns from the two instruments. No systematic differences are found as a function of latitude in the final comparisons. These results show that knowledge of the a priori profiles, the averaging kernels, and the error covariance matrices in the standard data products provided by the instrument teams and understanding their roles in the retrieval products are essential in quantitatively interpreting both retrieved profiles and the derived total or partial columns for scientific applications.

Stratospheric aerosol effective radius, surface area and volume estimated from infrared measurements

A technique is presented for estimating the effective radius, surface area density, and volume density of stratospheric aerosols from infrared emission measurements. These parameters are required to assess the perturbation of the climate and chemical balance of the stratosphere following the largest volcanic eruption so far this century: that of Mount Pinatubo in the Philippines. The method uses a relationship between the surface area density and the volume density derived from balloon-borne measurements of the Mount Pinatubo aerosol cloud made at Laramie, Wyoming. It is shown that the aerosol emission value is well approximated by a linear function of effective radius and aerosol volume density. The technique relies on knowing the refractive index of the aerosol cloud, which is assumed to be composed of liquid spheres of sulphuric acid and water. It is shown that the uncertainties in the current knowledge of the refractive index of sulphuric acid solutions limit the accuracy of the inversion technique. As a case study, the aerosol effective radius, surface area density, and volume density are determined from emission measurements at 12.1 μm of the Mount Pinatubo aerosol cloud made by the improved stratospheric and mesospheric sounder (ISAMS) carried on the Upper Atmospheric Research Satellite (UARS). From these measurements it is shown that five months after the eruption the core of the Mount Pinatubo cloud had a size distribution with an effective radius of 0.5 μm, a surface area density of 35 μm2 cm−3, and a volume density of 6 μm3 cm−3.

Forward model and Jacobians for Tropospheric Emission Spectrometer retrievals

The Tropospheric Emission Spectrometer (TES) is a high-resolution spaceborne sensor that is capable of observing tropospheric species. In order to exploit fully TESs potential for tropospheric constituent retrievals, an accurate and fast operational forward model was developed for TES. The forward model is an important component of the TES retrieval model, the Earth Limb and Nadir Operational Retrieval (ELANOR), as it governs the accuracy and speed of the calculations for the retrievals. In order to achieve the necessary accuracy and computational efficiency, TES adopted the strategy of utilizing precalculated absorption coefficients generated by the line-by-line calculations provided by line-by-line radiation transfer modeling. The decision to perform the radiative transfer with the highest monochromatic accuracy attainable, rather than with an accelerated scheme that has the potential to add algorithmic forward model error, has proven to be very successful for TES retrievals. A detailed description of the TES forward model and Jacobians is described. A preliminary TES observation is provided as an example to demonstrate that the TES forward model calculations represent TES observations. Also presented is a validation example, which is part of the extensive forward model validation effort.

Infrared absorption by volcanic stratospheric aerosols observed by ISAMS

The Improved Stratospheric and Mesospheric Sounder (ISAMS) aboard the Upper Atmosphere Research Satellite (UARS) senses in 14 wideband channels in the infrared. The absorption by the Mt. Pinatubo aerosol cloud for nine of the channels was averaged over heights from 20 km to 30 km for a 60° latitude band centred on the Equator. The absorption spectrum for sulphuric acid-water aerosols was calculated for wavelengths from 4 μm to 17 μm and investigated as a function of the particle size distribution and the particle composition. The infrared spectrum is shown to be more sensitive to changes in particle composition than to drop size; the ISAMS results are consistent with drops composed of a 59% to 77% solution of sulphuric acid in water.