Peter Albert

Assessment of the potential of MERIS near‐infrared water vapour products to correct ASAR interferometric measurements

Atmospheric water vapour is a major limitation for high precision Interferometric Synthetic Aperture Radar (InSAR) applications due to its significant impact on microwave signals. We propose a statistical criterion to test whether an independent water vapour product can reduce water vapour effects on InSAR interferograms, and assess the potential of the Medium Resolution Imaging Spectrometer (MERIS) near‐infrared water vapour products for correcting Advanced SAR (ASAR) data. Spatio‐temporal comparisons show c. 1.1 mm agreement between MERIS and GPS/radiosonde water vapour products in terms of standard deviations. One major limitation with the use of MERIS water vapour products is the frequency of cloud free conditions. Our analysis indicates that in spite of the low global cloud free conditions (∼25%), the frequency can be much higher for certain areas such as Eastern Tibet (∼38%) and Southern California (∼48%). This suggests that MERIS water vapour products show potential for correcting ASAR interferometric measurements in certain regions.

Remote Sensing of Atmospheric Water Vapor Using the Moderate Resolution Imaging Spectroradiometer

Abstract This paper presents first validation results for an algorithm developed for the retrieval of integrated columnar water vapor from measurements of the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on board the polar-orbiting Terra and Aqua platforms. The algorithm is based on the absorption of reflected solar radiation by atmospheric water vapor and allows the retrieval of integrated water vapor above cloud-free land surfaces. A comparison of the retrieved water vapor with measurements of the Microwave Water Radiometer at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site for a 10-month period in 2002 showed an rms deviation of 1.7 kg m−2 and a bias of 0.6 kg m−2. A comparison with radio soundings in central Europe from July 2002 to April 2003 showed an rms deviation of 2 kg m−2 and a bias of −0.8 kg m−2.

Generating cloudmasks in spatial high-resolution observations of clouds using texture and radiance information

The detection of clouds in measurements taken by airborne and spaceborne remote sensing sensors in the visible and near-infrared is often difficult due to the high albedo of underlying surfaces such as snow- and ice-covered surfaces as well as sunglint regions of water surfaces. The authors show that the measured intensity of the reflected solar radiation together with texture information is effective in detecting clouds over water surfaces which are affected by sunglint. An automated cloud-masking technique for images measured by a compact airborne spectrographic imager ( casi ) during the ACE-2 CLOUDYCOLUMN experiment has been developed based on supervised learning of an artificial neural network. The neural network has been trained on radiances, texture features, and gradient-filtered radiances. The radiances were measured at a single wavelength but with high spatial resolution so that characteristic spatial features within an image can be used to discriminate clouds from sunglint, cloud shadow and ocean surface. The accuracy of the cloudmask-generating algorithm was investigated on the basis of the testing set for the neural network. Maximum errors of 3.4% and 1% occur for detecting cloudy and cloud-free pixels, respectively. The performance of the network was compared with a second network trained on radiances alone. The second network is up to 44% less efficient for cloud detection which demonstrates the improvement arising from the use of texture information together with spatial high-resolution observations.

Cloud-top pressure retrieval using the oxygen A-band in the IRS-3 MOS instrument

Backscattered solar radiation as measured by MOS (Modular Optoelectronical Scanner) on the IRS3 (Indian Remote sensing Satellite 3) has been used in an algorithm to retrieve cloud top pressure. The algorithm uses a radiance ratio between absorbing channels in the Oxygen‐A absorption band at 761 nm and a window channel at 750 nm. The ratios are directly related to the average photon path length, which is mainly determined by the cloud top pressure. This paper presents the principles of the retrieval scheme, results of a sensitivity study and a first validation using radiosondes.

Comparison between ATSR-2 stereo, MOS O2-A band and ground-based cloud top heights

A new method to retrieve cloud top heights stereoscopically using the dual‐view facility of the Along Track Scanning Radiometer 2 (ATSR‐2) instrument is assessed. This assessment is performed through a comparison of the cloud top heights obtained from ATSR‐2 stereo and those derived from a 94‐GHz radar, radiosonde profiles and independently from the Modular Optoelectronic Scanner (MOS) using the O2‐A band. The data for this study were collected over the United Kingdom from September 1998–March 1999. The results show that the accuracy of the ATSR‐2 stereo heights is generally as predicted on theoretical grounds, with the errors in the 1.6 µm and 0.65 µm stereo heights rarely exceeding 2 km. Case study periods with disagreements between the ATSR‐2 heights and the ground‐based retrievals are often due to the lack of precise match‐ups between the ground‐based and satellite scenes, while the MOS O2‐A band is shown sometimes to miss the tops of high clouds. Evidence that the 11 µm channel is more sensitive to high clouds than originally thought is given and a future application of multi‐spectral stereo cloud top heights is proposed.

Satellite- and ground-based observations of atmospheric water vapor absorption in the region

Abstract Ground-based measurements of direct absorption of solar radiation between 9000 and 13,000 cm −1 (770– 1100 nm ) with a spectral resolution of 0.05 cm −1 are compared with line-by-line simulations of atmospheric absorption based on different molecular databases (HITRAN 2000, HITRAN 99, HITRAN 96 and ESA-WVR). Differences between measurements and simulations can be reduced to a great amount by scaling the individual line intensities with spectral and database dependent scaling factors. Scaling factors are calculated for the selected databases using a Marquardt non-linear least-squares fit together with a forward model for 100 cm −1 wide intervals between 10,150 and 11,250 cm −1 as well as for the water vapor absorption channels of the Medium Resolution Imaging Spectrometer (MERIS) onboard the European Space Agencys (ESA) ENVISAT platform and the Modular Optoelectronic Scanner (MOS) on the Indian IRSP-3 platform, developed by the German Aerospace Centre (DLR). For the latter, the scaling coefficients are converted into correction factors for retrieved total columnar water vapor content and used for a comparison of MOS-based retrievals of total columnar atmospheric water vapor above cloud-free land surfaces with radio soundings. The scaling factors determined for 100 cm −1 wide intervals range from 0.85 for the ESA-WVR molecular database to 1.15 for HITRAN 96. The best agreement between measurements and simulations is achieved with HITRAN 99 and HITRAN 2000, respectively, using scaling factors between 0.9 and 1. The effects on the satellite-based retrievals of columnar atmospheric water vapor range from 2% (HITRAN 2000) to 12% (ESA-WVR).