Limb-viewing Geometry Research Articles

Retrieval of stratospheric aerosol extinction coefficients from sun-normalized Ozone Mapper and Profiler Suite Limb Profiler (OMPS-LP) measurements

Abstract. A new retrieval approach for obtaining vertical profiles of the aerosol extinction coefficient from measurements of scattered solar light in the limb-viewing geometry made by the Ozone Mapper and Profiler Suite Limb Profiler (OMPS-LP) instrument is presented. In contrast to many other published limb-scatter retrievals, our new algorithm does not employ normalization by a limb measurement at an upper tangent height. Instead, the measured limb radiances are normalized to solar irradiance. The main advantage of this approach is an almost complete elimination of the dependence of the retrieval results on the prior aerosol extinction profile used in the retrieval. This makes the retrieval better suited for the analysis of observation scenes with highly elevated aerosol plumes, such as those that occurred after the Hunga Tonga–Hunga Ha′apai volcanic eruption in January 2022. The results from the new approach were compared to the vertical profiles of the aerosol extinction coefficients retrieved from the Stratospheric Aerosol and Gas Experiment III on the International Space Station (SAGE III/ISS) and the Optical Spectrograph and InfraRed Imaging System (OSIRIS). In general, agreement within 25 % between the different data products was observed in the 18–23 km altitude range, although larger differences were seen after very strong volcanic eruptions and wildfires. In comparison with OSIRIS, larger differences are seen at high southern latitudes (above 60° S). The new data product was used to investigate the evolution of the aerosol plume after the Hunga Tonga–Hunga Ha′apai volcanic eruption.

Synergy of Using Nadir and Limb Instruments for Tropospheric Ozone Monitoring (SUNLIT)

Abstract. Satellite measurements in nadir and limb viewing geometry provide a complementary view of the atmosphere. An effective combination of the limb and nadir measurements can give new information about atmospheric composition. In this work, we present tropospheric ozone column datasets that have been created using a combination of total ozone columns from OMI (Ozone Monitoring Instrument) and TROPOMI (TROPOspheric Monitoring Instrument) with stratospheric ozone column datasets from several available limb-viewing instruments: MLS (Microwave Limb Sounder), OSIRIS (Optical Spectrograph and InfraRed Imaging System), MIPAS (Michelson Interferometer for Passive Atmospheric Sounding), SCIAMACHY (SCanning Imaging Spectrometer for Atmospheric CHartographY), OMPS-LP (Ozone Mapping and Profiles Suite – Limb Profiler), and GOMOS (Global Ozone Monitoring by Occultation of Stars). We have developed further the methodological aspects of the assessment of tropospheric ozone using the residual method supported by simulations with the chemistry transport model SILAM (System for Integrated modeLling of Atmospheric coMposition). It has been shown that the accurate assessment of ozone in the upper troposphere and the lower stratosphere (UTLS) is of high importance for detecting the ground-level ozone patterns. The stratospheric ozone column is derived from a combination of ozone profiles from several satellite instruments in limb-viewing geometry. We developed a method for the data homogenization, which includes the removal of biases and a posteriori estimation of random uncertainties, thus making the data from different instruments compatible with each other. The high-horizontal- and vertical-resolution dataset of ozone profiles is created via interpolation of the limb profiles from each day to a 1∘×1∘ horizonal grid. A new kriging-type interpolation method, which takes into account data uncertainties and the information about natural ozone variations from the SILAM-adjusted ozone field, has been developed. To mitigate the limited accuracy and coverage of the limb profile data in the UTLS, a smooth transition to the model data is applied below the tropopause. This allows for the estimation of the stratospheric ozone column with full coverage of the UTLS. The derived ozone profiles are in very good agreement with collocated ozonesonde measurements. The residual method was successfully applied to OMI and TROPOMI clear-sky total ozone data in combination with the stratospheric ozone column from the developed high-resolution limb profile dataset. The resulting tropospheric ozone column is in very good agreement with other satellite data. The global distributions of tropospheric ozone exhibit enhancements associated with the regions of high tropospheric ozone production. The main datasets created are (i) a monthly 1∘×1∘ global tropospheric ozone column dataset (from ground to 3 km below the tropopause) using OMI and limb instruments, (ii) a monthly 1∘×1∘ global tropospheric ozone column dataset using TROPOMI and limb instruments, and (iii) a daily 1∘×1∘ interpolated stratospheric ozone column from limb instruments. Other datasets, which are created as an intermediate step of creating the tropospheric ozone column data, are (i) a daily 1∘×1∘ clear-sky and total ozone column from OMI and TROPOMI, (ii) a daily 1∘×1∘ homogenized and interpolated dataset of ozone profiles from limb instruments, and (iii) a daily 1∘×1∘ dataset of ozone profiles from SILAM simulations with adjustment to satellite data. These datasets can be used in various studies related to variability and trends in ozone distributions in both the troposphere and the stratosphere. The datasets are processed from the beginning of OMI and TROPOMI measurements until December 2020 and are planned to be regularly extended in the future.

Modeling of inelastically scattered radiation: Rotational Raman scattering in the spherical Earth’s atmosphere

This study presents an approximative approach to solve the radiative transfer equation accounting for the rotational Raman scattering in a spherical atmosphere. The solution is obtained employing the method of characteristics in combination with the discrete-ordinates technique and the forward-adjoint perturbation theory. For a non-limb observational geometry, impact of the atmospheric sphericity on the filling-in of solar Fraunhofer lines is discussed. For a limb viewing geometry, the dependence of the filling-in on the surface elevation, surface albedo, and cloudiness below the instrument line of sight as well as the effect of the multiple scattering in the spatial domain is analyzed. A comparison of modeled and measured Ring spectra for a limb-viewing geometry is shown. The considered solution technique is implemented in the radiative transfer model SCIATRAN and is ready to use for atmospheric applications.

The Radiative Transfer Characteristics of the O2 Infrared Atmospheric Band in Limb-Viewing Geometry

The O2(a1Δg) emission near 1.27 μm provides an important means to remotely sense the thermal characteristics, dynamical features, and compositional structures of the upper atmosphere because of its photochemistry and spectroscopic properties. In this work, an emission–absorption transfer model for limb measurements was developed to calculate the radiation and scattering spectral brightness by means of a line-by-line approach. The nonlocal thermal equilibrium (non-LTE) model was taken into account for accurate calculation of the O2(a1Δg) emission by incorporating the latest rate constants and spectral parameters. The spherical adding and doubling methods were used in the multiple scattering model. Representative emission and absorption line shapes of the O 2 ( a 1 Δ g , υ ′ = 0 ) → O 2 ( X Σ g 3 , υ ″ = 0 ) band and their spectral behavior varying with altitude were examined. The effects of solar zenith angle, surface albedo, and aerosol loading on the line shapes were also studied. This paper emphasizes the advantage of using infrared atmospheric band for remote sensing of the atmosphere from 20 up to 120 km, a significant region where the strongest coupling between the lower and upper atmosphere occurs.

Stratospheric aerosol characteristics from space-borne observations: extinction coefficient and Ångström exponent

Abstract. Stratospheric aerosols are of a great importance to the scientific community, predominantly because of their role in climate, but also because accurate knowledge of aerosol characteristics is relevant for trace gas retrievals from remote-sensing instruments. There are several data sets published which provide aerosol extinction coefficients in the stratosphere. However, for the instruments measuring in the limb-viewing geometry, the use of this parameter is associated with uncertainties resulting from the need to assume an aerosol particle size distribution (PSD) within the retrieval process. These uncertainties can be mitigated if PSD information is retrieved. While occultation instruments provide more accurate information on the aerosol extinction coefficient, in this study, it was shown that limb instruments are more sensitive to the smaller particles in the visible–near-infrared spectral range. However, the sensitivity of occultation instruments improves if the UV part of the wavelength spectrum is considered. A data set containing PSD information was recently retrieved from SCIAMACHY limb measurements and provides two parameters of the unimodal lognormal PSD for the SCIAMACHY operational period (2002–2012). In this study, the data set is expanded by aerosol extinction coefficients and Ångström exponents calculated from the retrieved PSD parameters. Parameter errors for the recalculated Ångström exponents and aerosol extinction coefficients are assessed using synthetic retrievals. For the extinction coefficient the resulting parameter error is within ±25 %, and for the Ångström exponent, it is better than 10 %. The SCIAMACHY aerosol extinction coefficients recalculated from PSD parameters are compared to those from SAGE II. The differences between the instruments vary from 0 % to 25 % depending on the wavelength. Ångström exponent comparison with SAGE II shows differences between 10 % at 31 km and 40 % at 18 km. Comparisons with SAGE II, however, suffer from the low number of collocated profiles. Furthermore, the Ångström exponents obtained from the limb-viewing instrument OSIRIS are used for the comparison. This comparison shows an average difference within 7 %. The time series of these differences do not show signatures of any remarkable events (e.g., volcanic eruptions or biomass burning events). In addition, the temporal behaviour of the Ångström exponent in the tropics is analyzed using the SCIAMACHY data set. It is shown that there is no trivial relation between the Ångström exponent value at a single wavelength pair and the PSD because the same value of Ångström exponent can be obtained from an infinite number of combinations of the PSD parameters.

Aerosol particle size distribution in the stratosphere retrieved from SCIAMACHY limb measurements

Abstract. Information about aerosols in the Earth's atmosphere is of a great importance in the scientific community. While tropospheric aerosol influences the radiative balance of the troposphere and affects human health, stratospheric aerosol plays an important role in atmospheric chemistry and climate change. In particular, information about the amount and distribution of stratospheric aerosols is required to initialize climate models, as well as validate aerosol microphysics models and investigate geoengineering. In addition, good knowledge of stratospheric aerosol loading is needed to increase the retrieval accuracy of key trace gases (e.g. ozone or water vapour) when interpreting remote sensing measurements of the scattered solar light. The most commonly used characteristics to describe stratospheric aerosols are the aerosol extinction coefficient and Ångström coefficient. However, the use of particle size distribution parameters along with the aerosol number density is a more optimal approach. In this paper we present a new retrieval algorithm to obtain the particle size distribution of stratospheric aerosol from space-borne observations of the scattered solar light in the limb-viewing geometry. While the mode radius and width of the aerosol particle size distribution are retrieved, the aerosol particle number density profile remains unchanged. The latter is justified by a lower sensitivity of the limb-scattering measurements to changes in this parameter. To our knowledge this is the first data set providing two parameters of the particle size distribution of stratospheric aerosol from space-borne measurements of scattered solar light. Typically, the mode radius and w can be retrieved with an uncertainty of less than 20 %. The algorithm was successfully applied to the tropical region (20° N–20° S) for 10 years (2002–2012) of SCIAMACHY observations in limb-viewing geometry, establishing a unique data set. Analysis of this new climatology for the particle size distribution parameters showed clear increases in the mode radius after the tropical volcanic eruptions, whereas no distinct behaviour of the absolute distribution width could be identified. A tape recorder, which describes the time lag as the perturbation propagates to higher altitudes, was identified for both parameters after the volcanic eruptions. A quasi-biannual oscillation (QBO) pattern at upper altitudes (28–32 km) is prominent in the anomalies of the analysed parameters. A comparison of the aerosol effective radii derived from SCIAMACHY and SAGE II data was performed. The average difference is found to be around 30 % at the lower altitudes, decreasing with increasing height to almost zero around 30 km. The data sample available for the comparison is, however, relatively small.

Retrieval of O<sub>2</sub>(<sup>1</sup>Σ) and O<sub>2</sub>(<sup>1</sup>Δ) volume emission rates in the mesosphere and lower thermosphere using SCIAMACHY MLT limb scans

Abstract. We present the retrieved volume emission rates (VERs) from the airglow of both the daytime and twilight O2(1Σ) band and O2(1Δ) band emissions in the mesosphere and lower thermosphere (MLT). The SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY (SCIAMACHY) onboard the European Space Agency Envisat satellite observes upwelling radiances in limb-viewing geometry during its special MLT mode over the range 50–150 km. In this study we use the limb observations in the visible (595–811 nm) and near-infrared (1200–1360 nm) bands. We have investigated the daily mean latitudinal distributions and the time series of the retrieved VER in the altitude range from 53 to 149 km. The maximal observed VERs of O2(1Δ) during daytime are typically 1 to 2 orders of magnitude larger than those of O2(1Σ). The latter peaks at around 90 km, whereas the O2(1Δ) emissivity decreases with altitude, with the largest values at the lower edge of the observations (about 53 km). The VER values in the upper mesosphere (above 80 km) are found to depend on the position of the sun, with pronounced high values occurring during summer for O2(1Δ). O2(1Σ) emissions show additional high values at polar latitudes during winter and spring. These additional high values are presumably related to the downwelling of atomic oxygen after large sudden stratospheric warmings (SSWs). Accurate measurements of the O2(1Σ) and O2(1Δ) airglow, provided that the mechanism of their production is understood, yield valuable information about both the chemistry and dynamics in the MLT. For example, they can be used to infer the amounts and distribution of ozone, solar heating rates, and temperature in the MLT.

Two-dimensional radiative transfer for the retrieval of limb emission measurements in the martian atmosphere

The remote sounding of infrared emission from planetary atmospheres using limb-viewing geometry is a powerful technique for deriving vertical profiles of structure and composition on a global scale. Compared with nadir viewing, limb geometry provides enhanced vertical resolution and greater sensitivity to atmospheric constituents. However, standard limb profile retrieval techniques assume spherical symmetry and are vulnerable to biases produced by horizontal gradients in atmospheric parameters. We present a scheme for the correction of horizontal gradients in profile retrievals from limb observations of the martian atmosphere. It characterizes horizontal gradients in temperature, pressure, and aerosol extinction along the line-of-sight of a limb view through neighboring measurements, and represents these gradients by means of two-dimensional radiative transfer in the forward model of the retrieval. The scheme is applied to limb emission measurements from the Mars Climate Sounder instrument on Mars Reconnaissance Orbiter. Retrieval simulations using data from numerical models indicate that biases of up to 10K in the winter polar region, obtained with standard retrievals using spherical symmetry, are reduced to about 2K in most locations by the retrieval with two-dimensional radiative transfer. Retrievals from Mars atmospheric measurements suggest that the two-dimensional radiative transfer greatly reduces biases in temperature and aerosol opacity caused by observational geometry, predominantly in the polar winter regions.

First observations of H2O and CO2vapor in comet 67P/Churyumov-Gerasimenko made by VIRTIS onboard Rosetta

Context. Outgassing from cometary nuclei involves complex surface and subsurface processes that need to be understood to investigate the composition of cometary ices from coma observations. Aims. We investigate the production of water, carbon dioxide, and carbon monoxide from the nucleus of comet 67P/Churyumov-Gerasimenko (67P). These species have different volatility and are key species of cometary ices. Methods. Using the high spectral-resolution channel of the Visible InfraRed Thermal Imaging Spectrometer (VIRTIS-H), we observed the ν3 vibrational bands of H2O and CO2 at 2.67 and 4.27 μm, respectively, from 24 November 2014 to 24 January 2015, when comet 67P was between 2.91 and 2.47 AU from the Sun. Observations were undertaken in limb-viewing geometry at distances from the surface of 0 to 1.5 km and with various line-of-sight (LOS) orientations in the body-fixed frame. A geometry tool was used to characterize the position of the LOS with respect to geomorphologic regions and the illumination properties of these regions. Results. The water production of 67P did not increase much from 2.9 to 2.5 AU. High water column densities are observed for LOS above the neck regions, suggesting they are the most productive in water vapor. While water production is weak in regions with low solar illumination, CO2 is outgassing from both illuminated and non-illuminated regions, which indicates that CO2 sublimates at a depth that is below the diurnal skin depth. The CO2/H2O column density ratio varies from 2 to 60%. For regions that are in sunlight, mean values between 2 and 7% are measured. The lower bound value is likely representative of the CO2/H2O production rate ratio from the neck regions. For carbon monoxide, we derive column density ratios CO/H2O < 1.9% and CO/CO2< 80%. An illumination-driven model, with a uniformly active surface releasing water at a mean rate of 8 × 1025 s-1, provides an overall agreement to VIRTIS-H data, although some mismatches show local surface inhomogeneities in water production. Rotational temperatures of 90–100 K are derived from H2O and CO2 averaged spectra.

Differentiability and retrievability of CO2 and H2O clouds on Mars from MRO/MCS measurements: A radiative-transfer study

Since the 1970s, it has been predicted that both CO2 and H2O clouds can form in the Martian atmosphere, and many remote-sounding instruments have directly observed layers of extinction asserted to be clouds composed of either CO2 or H2O ice on Mars. The Mars Climate Sounder, onboard the Mars Reconnaissance Orbiter (MRO/MCS), entered orbit around Mars in 2006, and has been providing near-continuous coverage of the full planet since, at wavelengths from visible through to the mid-infrared, primarily in limb-viewing geometry, making it a suitable candidate to study the parameters of these clouds.In this work, the multiple scattering radiative-transfer tool NemesisMCS has been used to create a large dataset of simulations of CO2 and H2O clouds on Mars as would be measured by MRO/MCS, using a range of atmospheric conditions as well as cloud parameters derived from literature suitable for upper atmospheric clouds, and building specifically on the work of Sefton-Nash et al. (2013). This ensemble of simulations has been used to characterise the spectral signature of plausible CO2 and H2O clouds, as well as to assess the suitability of MRO/MCS to observe, to differentiate between, and to derive properties of such clouds.It has been found, given the noise levels expected for MRO/MCS and the range of atmospheric and cloud parameters sampled in this study, that radiance signals introduced by upper atmospheric clouds having nadir optical depths greater than about 10−5 should be distinguishable, with S/N≥1. This corresponds to specific concentrations greater than about 105particles/g, particle radii greater than around 0.5μm, and cloud depths greater than about 2km.MRO/MCS measurements should be able to be used with confidence to differentiate between upper atmospheric cloud and dust in the lower atmosphere, and clear conditions, with high success (≈100%). Lower reliability classification is accomplished for CO2 clouds, with only 60% being correctly identified as CO2, and the remainder classified instead as H2O cloud, in the case of optical depths in the expected range for upper atmospheric cloudswhich are detectable by MRO/MCS, although this result is highly dependent upon the sampled selection of optically thin and thick clouds and the atmospheric model employed. Although almost all the H2O clouds are correctly identified, the fact that such a large proportion of CO2 clouds are misclassified as H2O clouds shows that the spectral information alone from MRO/MCS is insufficient to differentiate between CO2 and H2O clouds when optically thin—but detectable—clouds are included in the analysis.Using a simple look-up table (LUT) scheme and simulated data, retrieval of properties of upper atmospheric clouds of sufficient opacity is possible, with preliminary estimates indicating that H2O cloud and dust parameters can be correctly reproduced between 48% and 100% of the time, and between 18% and 92% of the time for CO2 cloud test cases, although it must be noted that these values must be taken as a qualitative measure which does not capture the full range of atmospheric and cloud conditions on Mars which would be present in real MRO/MCS data. Furthermore, because of the optical properties of H2O and CO2, on a like-with-like selection, the H2O clouds always produce greater perturbations in radiance, thus biasing results to a higher success rate for H2O cloud retrievals. Application of the method to MRO/MCS data with a full-optimal estimation retrieval tool such as NemesisMCS will be the topic of a future study.

Stratospheric ozone trends and variability as seen by SCIAMACHY from 2002 to 2012

Abstract. Vertical profiles of the rate of linear change (trend) in the altitude range 15–50 km are determined from decadal O3 time series obtained from SCIAMACHY1/ENVISAT2 measurements in limb-viewing geometry. The trends are calculated by using a multivariate linear regression. Seasonal variations, the quasi-biennial oscillation, signatures of the solar cycle and the El Niño–Southern Oscillation are accounted for in the regression. The time range of trend calculation is August 2002–April 2012. A focus for analysis are the zonal bands of 20° N–20° S (tropics), 60–50° N, and 50–60° S (midlatitudes). In the tropics, positive trends of up to 5% per decade between 20 and 30 km and negative trends of up to 10% per decade between 30 and 38 km are identified. Positive O3 trends of around 5% per decade are found in the upper stratosphere in the tropics and at midlatitudes. Comparisons between SCIAMACHY and EOS MLS3 show reasonable agreement both in the tropics and at midlatitudes for most altitudes. In the tropics, measurements from OSIRIS4/Odin and SHADOZ5 are also analysed. These yield rates of linear change of O3 similar to those from SCIAMACHY. However, the trends from SCIAMACHY near 34 km in the tropics are larger than MLS and OSIRIS by a factor of around two. 1 SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY 2 European environmental research satellite 3 Earth Observing System (EOS) Microwave Limb Sounder (MLS) 4 Optical Spectrograph and InfraRed Imager System 5 Southern Hemisphere ADditional OZonesondes

Scientific Assessment of the SWIFT Instrument Design

Abstract The Stratospheric Wind Interferometer for Transport Studies (SWIFT) is a proposed satellite instrument. SWIFT is an imaging field-widened Doppler Michelson interferometer. It observes a thermal IR atmospheric emission line in a limb-viewing geometry in order to measure stratospheric winds and stratospheric ozone concentration profiles with global coverage during both day and night. SWIFT has the capability of improving the knowledge of the dynamics of the stratosphere and global distribution of and global transport of ozone. The target wind and ozone accuracies are 3 m s−1 and 5%–10%, respectively. The instrument is a follow up to the highly successful Canada–France Wind-Imaging Interferometer (WINDII) instrument on NASA's Upper Atmosphere Research Satellite (UARS). To assess the suitability of the method of Doppler imaging Michelson interferometry for the measurement of stratospheric wind and ozone using the SWIFT instrument, a scientific assessment of the instrument performance was undertaken using forward and inverse modeling and error analyses. This paper is aimed at determining the technical and scientific feasibility of the SWIFT instrument and its ability to meet the science requirements. This paper also briefly describes the SWIFT experiment, the data retrieval algorithms, and technical challenges in stratospheric wind measurements. Meeting the wind accuracy requirement imposes tight requirements on instrument thermal stability, filter monitoring, and determination of reference phase calibration. The SWIFT instrument design shows a strong level of dependence on the knowledge of atmospheric N2O concentration. The presence of N2O as an interfering species degrades the SWIFT performance at all altitudes with the largest impact especially for altitudes below 30 km.

Phase-Corrected Near-Field Measurements of the TELIS Telescope at 637 GHz

Results are presented for the telescope optics verification of the TELIS instrument which is developed and constructed at the Deutsches Zentrum fU¿r Luft- und Raumfahrt (DLR), Germany. TELIS is a balloon-borne limb-sounding radiometer which measures pressure-broadened emission lines of various species of the earth's atmosphere. Due to the strong brightness gradient in the limb-viewing geometry the antenna beam of the instrument needs to be well-defined in order to ensure proper operation. Near-field measurements made at 637 GHz of the TELIS telescope were used to derive the far-field pattern. Error compensation included correction of the phase error of the near-field data due to cable flexing. The phase error due to cable flexing was measured using a technique based on heterodyne reception of the signal reflected at the end of the cable. The results are compared to an optics model based on physical optics. Discrepancies between measurements and the model are resolved by misaligning the mirrors in the optics model while minimizing the deviations from theory and experiment.

Satellite-pointing retrieval from atmospheric limb-scattering of solar UV-B radiation

We present a new algorithm for tangent height retrievals from limb-scattering observations. These observations are performed by satellite-based spectrometers operating in the UV, visible, and short-wave IR spectral ranges. They record the solar radiation scattered in Earth's atmosphere in limb-viewing geometry and aim at vertically resolved retrievals of the atmospheric composition with global coverage. Inaccuracies in the knowledge of the instrument's pointing frequently dominate the error budgets of the atmospheric composition products. Therefore, additional information on satellite pointing is crucial for the quality of all products derived from limb-scattering observations. The information is commonly expressed in terms of the tangent heights. The presented algorithm determines the tangent heights directly from the observed limb radiances by analyzing the shapes of the so-called knee in several UV-B radiance profiles. All vertical pointing information contained in a UV-B fit window is exploited by simultaneously retrieving the ozone profile. The algorithm has been implemented in the toolbox SCIARAYS and named Tangent height Retrieval by UV-B Exploitation (TRUE) knee method. We have applied it to five orbits of SCIAMACHY's limb observations. It achieves a precision of about 200 m when applied to an individual limb scan. The broadband structure of the observations can be reproduced within 1% RMS. A comparison of the retrieved tangent heights with the engineering ones delivered by ESA reveals that the engineering tangent heights exhibit a systematic error, which varies with an amplitude of about 3 km. Its origin is traced to the on-board orbit model of Envisat. PACS Nos.: 94.80.+g, 42.68.Ay, 92.60.Ta, 42.68.Wt, 07.87.+v, 92.70.Cp, and 07.60.Rd

A Microwave Occultation Observing System Optimized to Characterize Atmospheric Water, Temperature, and Geopotential via Absorption

A new remote sensing concept extrapolated from the GPS occultation concept is presented in which the signal frequencies are chosen to determine atmospheric water, temperature, and the geopotential of atmospheric pressure surfaces. Using frequencies near the 22- and 183-GHz water lines allows not only the speed of light to be derived as a GPS occultation but also derivation of profiles of absorption caused by atmospheric water. Given the additional water information, moisture and temperature as well as the geopotential of pressure surfaces can be separated and solved for. Error covariance results indicate that the accuracies of individual water profiles will be 0.5%‐3% extending from roughly 1‐75-km altitude. Temperature accuracies of individual profiles will be sub-Kelvin from ;1- to 70-km altitude depending on latitude and season. Accuracies of geopotential heights of pressure will be 10‐20 m from the surface to 60-km altitude. These errors are random such that climatological averages derived from this data will be significantly more accurate. Owing to the limb-viewing geometry, the along-track resolution is comparable to the 200‐300 km of the GPS occultation observations, but the shorter 22- and 183-GHz wavelengths improve the diffraction-limited vertical resolution to 100‐300 m. The technique can be also used to determine profiles of other atmospheric constituents such as upper-tropospheric and stratospheric ozone by using frequencies near strong lines of that constituent. The combined dynamic range, accuracy, vertical resolution, and ability to penetrate clouds far surpass that of any present or planned satellite sensors. A constellation of such sensors would provide an all-weather, global remote sensing capability including full sampling of the diurnal cycle for process studies related to water, climate research, and weather prediction in general.

A comparison of water vapor derived from GPS occultations and global weather analyses

Despite its fundamental importance in radiative transfer, atmospheric dynamics, and the hydrological cycle, atmospheric water is inadequately characterized particularly at a global scale. Occultation measurements from the Global Positioning System (GPS) should improve upon this situation. Individual occultations yield profiles of specific humidity accurate to 0.2 to 0.5 g/kg providing sensitive measurements of lower and middle tropospheric water vapor with global coverage in a unique, all‐weather, limb‐viewing geometry with several hundred meters to a kilometer vertical resolution. We have derived water vapor profiles from June 21 to July 4, 1995, using GPS occultation data combined with global temperature analyses from the European Center for Medium‐Range Weather Forecasts (ECMWF) and reanalyses from the National Centers for Environmental Prediction (NCEP). The zonal mean structure of the profiles exhibits basic climatological features of tropospheric moisture. Specific humidity biases between the GPS results and the ECMWF global humidity analyses in the middle to upper troposphere are ∼0.1 g/kg or less. Occultation results below 6 km altitude are generally drier than those of ECMWF with the bias generally increasing toward warmer temperatures. Near the height of the trade wind inversion, the ECMWF analyses are significantly moister than the occultation results due to vertical smoothing and overextension of the boundary layer top in the analyses. Overall, the occultation results are drier than the NCEP reanalyses with a marked exception near the Intertropical Convergence Zone (ITCZ) where occultation results are wetter by more than 10%. The occultation results are significantly wetter near the ITCZ and drier in the subtropics than the classical moisture climatology of Peixoto and Oort. Similarities between the NCEP and the Peixoto and Oort near‐ITCZ differences suggest that a common analysis/model problem may be responsible. The generally wetter Peixoto and Oort results in the subtropics are due in part to moist radiosonde biases. Discrepancies between these data sets are significant and limit our ability to resolve uncertainties in moisture control and feedbacks in a changing climate.