Earth Observation Data Group

Abstract:

An inter-comparison study of the aerosol optical thickness (AOT) at 0.55 mu m retrieved using different satellite instruments and algorithms based on the analysis of backscattered solar light is presented for a single scene over central Europe on October 13th, 2005. For the first time comparisons have been performed for as many as six instruments on multiple satellite platforms. Ten different algorithms are briefly discussed and inter-compared. It was found that on the scale of a single pixel there can be large differences in AOT retrieved over land using different retrieval techniques and instruments. However, these differences are not as pronounced for the average AOT over land. For instance, the average AOT at 0.55 mu m for the area 7-12E, 49-53N was equal to 0.14 for MISR, NASA MODIS and POLDER algorithms. It is smaller by 0.01 for the ESA MERIS aerosol product and larger by 0.04 for the MERIS BAER algorithm. AOT as derived using AATSR gives on average larger values as compared to all other instruments, while SCIAMACHY retrievals underestimate the aerosol loading. These discrepancies are explained by uncertainties in a priori assumptions used in the different algorithms and differences in the sensor characteristics. Validation against AERONET shows that MERIS provides the most accurate AOT retrievals for this scene. (C) 2007 Elsevier B.V. All rights reserved.

Abstract:

The differing path lengths of the forward and nadir views of the Advanced Along-Track Scanning Radiometer (AATSR) are used to separate the contributions from aerosol scattering and surface reflectance in the observed top-of-atmosphere (TOA) radiance. This poster presents an effort to extend the Oxford-RAL retrieval of Aerosols and Clouds (ORAC) scheme used by the GRAPE and GlobAEROSOL projects [1], which currently uses data from the nadir viewing geometry only, to take advantage of the dual-view capabilities AATSR offers. The new algorithm uses optimal estimation to retrieve aerosol optical depth at 550 nm, effective radius and surface albedo at 550 nm for both forward and nadir viewing geometries (with the spectral shape of the surface constrained by a model based on Cox and Munk statistics [2,3] for the sea, and MODIS data [4] for the land).

Abstract:

A case-study is presented comparing the Oxford-RAL retrieval of Aerosol and Cloud (ORAC) algorithm, applied to AATSR and Meteosat-8 SEVIRI data, and the dual-view AATSR aerosol retrieval developed at TNO. The study compares data from an AATSR overpass of the Northern Adriatic and Po Valley region on 4 September 2004, during which time there were two AERONET sunphotometer stations operating in the Venice region as part of the ADRIEX campaign.We present the results of a comparison of the optical depth determined from the two AATSR retrievals and the SEVIRI retrieval at the time of the AATSR overpass. The comparison shows that the satellite retrievals consistently overestimate the aerosol optical depth compared to the AERONET site. A possible reason for this over sea is an inability of the algorithms at present to take into account the ocean colour of coastal waters. Future improvements to the algorithms are suggested. Copyright (C) 2007 Royal Meteorological Society.

Abstract:

The deuterium content of water vapour in a given air mass is sensitive to its temperature and condensation history. Isotopic measurements therefore have the potential to shed light on the transport of air and water vapour into the stratosphere. Previous measurements of the isotopic composition in the upper troposphere and stratosphere have been sparse in terms of both spatial and temporal coverage. Presented here are retrievals of the deuterium content of water vapour (HDO or delta D) from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) satellite instrument. These retrievals offer the first global scale coverage of the isotopic composition of water vapour in this altitude region and span a time period of almost two years. The spatial coverage and the time span of the dataset offer previously unattainable insight into the mean seasonal and spatial distributions of the isotopic composition of water vapour in the upper troposphere and stratosphere. Measurements of HDO are extremely challenging due to low sensitivity in the spectra at low temperatures and water vapour amounts. Nonetheless, the data show a number of interesting results. Zonal mean profiles show the greatest depletion in delta D in the tropical upper troposphere, with decreasing depletion with altitude in the stratosphere due to the influence of methane oxidation. Seasonal zonal means also show a strong depletion in the southern polar spring at around 300 mbar, which is thought to be related to the occurrence of polar stratospheric clouds and dehydration events in the polar vortex. Geographically, the regions and time periods where the greatest depletions are observed in the tropical upper troposphere are those associated with strong convective activity. Results confirm that temporal variability is central to the transport of water vapour into the tropical tropopause layer (TTL) and stratosphere. The data presented here show an annual cycle in delta D in the TTL which is tied to temperature and suggests that this signature propagates upwards into the stratosphere. The data also show a number of points where extremely dry air is associated with relatively enriched delta D values. We postulate that these points are evidence of ice lofting. Copyright (C) 2007 Royal Meteorological Society.