Rasmus Lindstrot

Remote Sensing of Cloud-Top Pressure Using Moderately Resolved Measurements within the Oxygen A Band—A Sensitivity Study

Abstract Reflected solar radiation measured by the Medium Resolution Imaging Spectrometer (MERIS) on the Environmental Satellite (Envisat) is currently used within the European Space Agency’s ground segment for the retrieval of cloud-top pressure. The algorithm is based on the analysis of the gaseous absorption of solar radiation in the oxygen A band at 761 nm. The strength of absorption is directly related to the average photon pathlength, which is mainly determined by the cloud-top pressure. However, it additionally depends on surface and cloud properties, like cloud thickness and microphysics. The interpretation of the measurements is further complicated by the temperature dependence of the absorption line shapes and the sensitivity to the spectral properties of the spectrometer like spectral position and width. This paper is focused on results of sensitivity studies using the Matrix Operator Model (MOMO) radiative transfer model that examine the most important parameters affecting the measurements of ...

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 Universitat Berlin was equipped with the portable lidar system (POLIS) of the Ludwig-Maximilians-Universitat Munchen 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 ...

The Retrieval of Land Surface Pressure from MERIS Measurements in the Oxygen A Band

Abstract Measurements of the Medium-Resolution Imaging Spectrometer (MERIS) on the Environmental Satellite (Envisat) are used for the retrieval of surface pressure above land and ice surfaces. The algorithm is based on the exploitation of gaseous absorption in the oxygen A band at 762 nm. The strength of absorption is directly related to the average photon pathlength, which in clear-sky cases above bright surfaces is mainly determined by the surface pressure, with minor influences from scattering at aerosols. Sensitivity studies regarding the influences of aerosol optical thickness and scale height and the temperature profile on the measured radiances are presented. Additionally, the sensitivity of the retrieval to the accuracy of the spectral characterization of MERIS is quantified. The algorithm for the retrieval of surface pressure (SPFUB) is presented and validated against surface pressure maps constructed from ECMWF sea level pressure forecasts in combination with digital elevation model data. The ac...

Retrieval of daytime total columnar water vapour from MODIS measurements over land surfaces

A retrieval of total column water vapour (TCWV) from MODIS (Moderate-resolution Imaging Spectroradiometer) measurements is presented. The algorithm is adapted from a retrieval for MERIS (Medium Resolution Imaging Spectrometer) from Lindstrot et al. (2012). It obtains the TCWV for cloud-free scenes above land at a spatial resolution of 1 km× 1 km and provides uncertainties on a pixel-by-pixel basis. The algorithm has been extended by introducing empirical correction coefficients for the transmittance calculation within the forward operator. With that, a wet bias of the MODIS algorithm against ARM microwave radiometer data has been eliminated. The validation against other ground-based measurements (GNSS water vapour stations, GUAN radiosondes, and AERONET sun photometers) on a global scale reveals a bias between −0.8 and −1.6 mm and root mean square deviations between 0.9 and 2 mm. This is an improvement in comparison to the operational TCWV Level 2 product (bias between −1.9 and −3.2 mm and root mean square deviations between 1.9 and 3.4 mm). The comparison to MERIS TCWV for an example overpass exposes a systematic dry bias.

Empirical Correction of Stray Light within the MERIS Oxygen A-Band Channel

Abstract Spaceborne spectrometers like the Medium Resolution Imaging Spectrometer (MERIS) on board the Environmental Satellite (Envisat) are widely used for the remote sensing of atmospheric and oceanic properties and make an important contribution to the monitoring of the earth’s atmosphere system. To enable retrievals with high accuracy, the spectral and radiometric properties of the instruments have to be characterized with high precision. One of the main sources of radiometric errors is stray light caused by multiple reflections and scattering at the optical elements within the instruments. If not corrected for properly, the stray light–induced offsets of measured intensity can lead to significant errors in the derived parameters. The effect of stray light is particularly momentous in the case of measurements inside strong absorption bands like the oxygen A band at 0.76 μm or the ρστ absorption band of water vapor around 0.9 μm. For example, the retrieval of surface and cloud-top pressure from MERIS m...

CO2 Retrieval over Clouds from the OCO Mission: Model Simulations and Error Analysis

Abstract Spectral characteristics of the future Orbiting Carbon Observatory (OCO) sensor, which will be launched in January 2009, were used to infer the carbon dioxide column-averaged mixing ratio over liquid water clouds over ocean by means of radiative transfer simulations and an inversion process based on optimal estimation theory. Before retrieving the carbon dioxide column-averaged mixing ratio over clouds, cloud properties such as cloud optical depth, cloud effective radius, and cloud-top pressure must be known. Cloud properties were not included in the prior in the inversion but are retrieved within the algorithm. The high spectral resolution of the OCO bands in the oxygen absorption spectral region around 0.76 μm, the weak CO2 absorption band around 1.61 μm, and the strong CO2 absorption band around 2.06 μm were used. The retrieval of all parameters relied on an optimal estimation technique that allows an objective selection of the channels needed to reach OCO’s requirement accuracy. The errors du...

Fast reconstruction of hyperspectral radiative transfer simulations by using small spectral subsets: application to the oxygen A band

Hyperspectral radiative transfer simulations are a versatile tool in remote sensing but can pose a major computational burden. We describe a simple method to construct hyperspectral simulation results by using only a small spectral subsample of the simulated wavelength range, thus leading to major speedups in such simulations. This is achieved by computing principal components for a small number of representative hyperspectral spectra and then deriving a reconstruction matrix for a specific spectral subset of channels to compute the hyperspectral data. The method is applied and discussed in detail using the example of top-of-atmosphere radiances in the oxygen A band, leading to speedups in the range of one to two orders of magnitude when compared to radiative transfer simulations at full spectral resolution.

FAME-C: Retrieval of cloud top pressure with vertically inhomogeneous cloud profiles

A synergistic FAME-C (Freie Universitat Berlin AATSR-MERIS Cloud Retrieval) algorithm is developed within the frame of the ESA CCI Cloud project. Within FAME-C the ratio of two MERIS measurements (the Oxygen-A absorption channel and a window channel) is used to retrieve cloud top pressure. In case of high, extended clouds the retrieved cloud top pressure is generally too high. This can be understood as an overestimation of extinction in upper cloud layers due to the assumption of vertical homogeneous clouds in the radiative transfer simulations. To include more realistic cloud vertical profiles, one year of data from the Cloud Profiling Radar (CPR) onboard CloudSat has been used to determine average normalized cloud vertical extinction profiles with a fixed pressure thickness for nine cloud types. The nine cloud types are based on the ISCCP COT-CTP classification table. The retrieved cloud top pressure, now using CloudSat cloud profiles in the forward model, is compared to CPR reflectivities as well as the retrieved cloud top pressure using vertically homogeneous cloud profiles. In the first number of cases under examination the overestimation of cloud top pressure, and therefore the bias, is reduced by a large amount when using CloudSat vertical cloud profiles. Another advantage is that no assumption about the cloud geometrical thickness has to be made in the new retrieval. It should be noted that comparisons between FAME-C products and A-train products can only be made at high latitudes where A-train and ENVISAT have overlapping overflights.

The Impact of Subscale Inhomogeneity on Oxygen A Band Cloud-Top Pressure Estimates: Using ESA's MERIS as a Proxy for DSCOVR-EPIC

Author to whom correspondence should be addressed; E-Mail: rasmus.lindstrot@wew.fu-berlin.de;Tel.: +49-30-838-56657; Fax: +49-30-838-56664.Received: 9 May 2012; in revised form: 25 June 2012 / Accepted: 26 June 2012 /Published: 29 June 2012Abstract: Medium Spectral Resolution Imaging Spectrometer (MERIS) oxygen A bandmeasurements were used as a proxy for the Earth Polychromatic Imaging Camera (EPIC),to be launched on NASA’s Deep Space Climate Observatory (DSCOVR). The high spatialresolution of MERIS (1 1 km

Remote Sensing of Multilayer Cloud-Top Pressure Using Combined Measurements of MERIS and AATSR on board Envisat

Abstract A novel and unique algorithm for the retrieval of multilayer cloud-top pressure is presented, relying on synergetic observations of the Medium Resolution Imaging Spectrometer (MERIS) and Advanced Along Track Scanning Radiometer (AATSR) on board the Environmental Satellite (Envisat). The retrieval is based on the exploitation of the differing signals observed in the thermal infrared spectral region (AATSR) and the oxygen A band at 0.76 μm (MERIS). Past studies have shown that the cloud-top pressure retrieved from MERIS measurements is highly accurate in the case of low single-layered clouds. In contrast, in the presence of multilayered clouds like cirrus overlying water clouds, the derived cloud height is biased. In this framework, an optimal estimation algorithm for the correction of the measured O2 A transmission for the influence of the upper cloud layer was developed. The algorithm is best applicable in cases of optically thin cirrus (1 ≤ τ ≤ 5) above optically thick water clouds (τ > 5), as f...