ESA's Environmental Satellite Research Articles

A global climatology of total columnar water vapour from SSM/I and MERIS

Abstract. A global time series of total columnar water vapour from combined data of the Medium Resolution Imaging Spectrometer (MERIS) onboard ESA's Environmental Satellite (ENVISAT) and the Special Sensor Microwave/Imager (SSM/I) onboard the satellite series of the US Defense Meteorological Satellite Program (DMSP) is presented. The unique data set, generated in the framework of the ESA Data User Element (DUE) GlobVapour project, combines atmospheric water vapour observations over land and ocean, derived from measurements in the near-infrared and the microwave range, respectively. Daily composites and monthly means of total columnar water vapour are available as global maps on rectangular latitude–longitude grids with a spatial resolution of 0.05° × 0.05° over land and 0.5° × 0.5° over ocean for the years 2003 to 2008. The data are stored in NetCDF files and is fully compliant with the NetCDF Climate Forecast convention. Through the combination of high-quality microwave observations and near-infrared observations over ocean and land surfaces, respectively, the data set provides global coverage. The combination of both products is carried out such that the individual properties of the microwave and near-infrared products, in particular their uncertainties, are not modified by the merging process and are therefore well defined. Due to the global coverage and the provided uncertainty estimates this data set is potentially of high value for climate research. The SSM/I-MERIS TCWV data set is freely available via the GlobVapour project web page (www.globvapour.info) with associated doi:10.5676/DFE/WV_COMB/FP. In this paper, the details of the data set generation, i.e. the satellite data used, the retrieval techniques and merging approaches, are presented. The derived level 3 products are compared to global radiosonde data from the GCOS upper air network (GUAN), showing a high agreement with a root-mean-square deviation of roughly 4.4 kg m−2 and a small wet bias well below 1 kg m−2. Furthermore, the data set is shown to be free of seasonal biases. The consistency of the MERIS and SSM/I retrievals is demonstrated by applying the MERIS retrieval to sun glint areas over ocean.

Validation of MERIS Cloud-Top Pressure Using Airborne Lidar Measurements

Abstract The results of a validation of the European Space Agency’s (ESA) operational Medium-Resolution Imaging Spectrometer (MERIS) cloud-top pressure (CTP) product by airborne lidar measurements are presented. MERIS, mounted on the polar-orbiting ESA Environmental Satellite (ENVISAT), provides radiance measurements within the oxygen A absorption band around 761 nm. The exploitation of these data allows the retrieval of CTP. The validation flights were performed in the northeastern part of Germany between April and June 2004 and were temporally and spatially synchronized with the ENVISAT overpasses. The Cessna 207T of the Freie Universität Berlin was equipped with the portable lidar system (POLIS) of the Ludwig-Maximilians-Universität München and a GPS navigation system. The maximum flying altitude was around 3000 m; therefore, the validation measurements were limited to situations with low-level clouds only. The validation was done by comparing MERIS data and lidar data. The statistical analysis of the observations revealed a high accuracy of the MERIS CTP product for low-level clouds, apart from a slight systematic overestimation of cloud-top heights. The root-mean-square error was 249 m, with a bias of +232 m. In the average top height level of ∼2000 m, these values are commensurate to pressure values of 24 hPa (rmse) and −22 hPa (bias). Furthermore, this validation campaign revealed deficiencies of the MERIS cloud mask to detect small-scale broken clouds.

Vibrationally excited ozone in the middle atmosphere

Daytime mesospheric limb emission spectra of ozone in the 4.8 μ m regime are analyzed with respect to vibrational excitation and relaxation processes. The data, which was obtained by the MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) instrument on board ESA's Environmental Satellite, is simulated by means of a non-local thermodynamic equilibrium (non-LTE) model utilizing O 3 -abundance, temperature, and pressure data from simultaneous retrievals in other spectral regions. The vibrational states of ozone depart from LTE due to the absorption of radiation from the lower atmosphere and due to the production of excited states in the O + O 2 + M → O 3 ( v ) + M recombination reaction. The energy flow into the ozone molecule as well as the collisional relaxation are highly uncertain. Model calculations that assume ozone formation at energies larger than 5000 cm - 1 underestimate the measured radiances by a factor of 2–3 in the 50–75 km altitude regime, if the nominal relaxation scheme is assumed. Agreement between measured and modeled radiances is achieved, if the collisional rates for the transformation of hot band stretching to bending quanta are reduced by about a factor of three, or if the quasi-nascent distribution of ozone favors vibrational states in the 3000 cm - 1 region.

Infrared spectroscopy and the terrestrial atmosphere

Fourier transform infrared spectroscopy is an extremely powerful technique for atmospheric remote sensing. In particular, on board a satellite and working in emission in the limb-viewing mode, it allows a global coverage during day and night. Based on this method the MIPAS experiment has been recently launched on ESA's Environmental satellite (ENVISAT). However, accurate retrievals require high quality spectroscopic parameters. It is the goal of this paper to show on a few selected examples the importance of using for the retrievals high quality spectral parameters derived from the best possible experimental and theoretical laboratory methods. To cite this article: J.-M. Flaud, H. Oelhaf, C. R. Physique 5 (2004).

A modified k-distribution approach applied to narrow band water vapour and oxygen absorption estimates in the near infrared

We apply a modified version of the k-distribution method to derive expressions for the total and in-layer transmission of layered atmospheres in the H 2O and O 2 absorption bands around 900 and 762 nm, respectively. The calculations are exemplary performed for the channels settings of the forthcoming Medium Resolution Imaging Spectrometer (MERIS), to be launched onboard ESA's Environmental Satellite in 2000, but apply for other narrow band sensors in the visible and near infrared as well. The modifications of the k-distribution approach are (1) that we do not employ the correlated k-distribution approach to derive weights and averaged absorption coefficients in layered atmospheres. Instead, we define a mapping function on the considered spectral interval so that the relation between the approximated absorption coefficients and the associated wavelengths is explicitly fixed for all layers. (2) we derive a consistent concept of introducing the sensor's spectral response functions and the spectral variation of the solar constant in the derivation of the relative weights of the single terms of the k-distribution fit. We compare line-by-line transmissions with k-distribution results performed for 20 terms and show that the in-layer and at the same time the total atmospheric transmission in the O 2A-absorption band can be fitted with a relative accuracy of 1% and in the water vapour absorption band with a relative accuracy higher than 0.1%. We compare these results to the correlated k-distribution approach and find improvements of the accuracy of the fit of a factor of 5–10 for the O 2A-band as well as for the water vapour absorption band. Comparing idealised rectangular and actual spectral response functions, we find that errors in the estimation of the atmospheric transmission induced by an idealised rectangular filter function exceed 40% relative (10% absolute) for the narrow channel covering the O 2A-band (3.75 nm half-width) and 5% relative error (3% absolute) for the 10 nm channel located at 900 nm.

Aerosol remote sensing over oceans

A global detection of aerosol parameters is essential to improve our understanding of the role of aerosols in the earth climate. The new generation of earth observation satellites, such as the ESA Environmental Satellite, ENVISAT, which will have onboard the Medium Resolution Imaging Spectrometer, will enhance significantly the accuracy of the estimation of tropospheric aerosols from space. The main goals of this investigation are the determination of the aerosol optical thickness and the discrimination of different aerosol types by using multispectral radiance measurements. Radiative transfer calculations were used to simulate the three proposed channels of MERIS in the red and near infrared region (750–1040 nm) which are dedicated to aerosol retrieval. The radiative transfer model for a combined atmosphere ocean system is based on the Matrix-Operator-Method. The assumption of only one type of aerosol within the evaluation process leads to errors of a factor of two in the estimation of the aerosol optical thickness. In areas where several aerosols may be expected an investigation of the aerosol type is necessary. First results indicate that the aerosol type can be estimated quite well from MERIS measurements.