Richard Siddans

Direct measurement of tropospheric ozone distributions from space

The role of ozone in absorbing ultraviolet solar radiation is well known. Ozone also makes a significant contribution to the radiative balance of the upper troposphere and lower stratosphere, such that changes in the distribution of ozone in these atmospheric regions will affect the radiative forcing of climate,. Furthermore, tropospheric ozone is the source of the hydroxyl radical which controls the abundance and distribution of many atmospheric constituents, including greenhouse gases such as methane and hydrochlorofluorocarbons. Tropospheric ozone is produced photochemically in situ and is also transported down from the stratosphere, but the relative importance of these two sources to its global budget is poorly understood. High-quality tropospheric and lower-stratospheric ozone profile measurements are available from sondes and lidar techniques, but their geographical sampling is very limited. Complementary satellite measurements of the global ozone distribution in this height region are therefore required to quantify ozones tropospheric budget and its participation in climate-forcing and tropospheric chemistry. Here we present direct measurements of tropospheric ozone concentrations from space, made by the European Space Agencys Global Ozone Monitoring Experiment. These results demonstrate the potential of satellite measurements to provide self-consistent tropospheric and stratospheric ozone distributions on a global scale.

A blind test retrieval experiment for infrared limb emission spectrometry

The functionality and characteristics of six different data processors (i.e., retrieval codes in their actual software and hardware environment) for analysis of high-resolution limb emission infrar ...

Cloud retrievals from satellite data using optimal estimation: evaluation and application to ATSR

Clouds play an important role in balancing the Earth’s radiation budget. Hence, it is vital that cloud climatologies are produced that quantify cloud macro and micro physical parameters and the associated uncertainty. In this paper, we present an algorithm ORAC (Oxford-RAL retrieval of Aerosol and Cloud) which is based on fitting a physically consistent cloud model to satellite observations simultaneously from the visible to the mid-infrared, thereby ensuring that the resulting cloud properties provide both a good representation of the short-wave and long-wave radiative effects of the observed cloud. The advantages of the optimal estimation method are that it enables rigorous error propagation and the inclusion of all measurements and any a priori information and associated errors in a rigorous mathematical framework. The algorithm provides a measure of the consistency between retrieval representation of cloud and satellite radiances. The cloud parameters retrieved are the cloud top pressure, cloud optical depth, cloud effective radius, cloud fraction and cloud phase. The algorithm can be applied to most visible/infrared satellite instruments. In this paper, we demonstrate the applicability to the Along-Track Scanning Radiometers ATSR-2 and AATSR. Examples of applying the algorithm to ATSR-2 flight data are presented and the sensitivity of the retrievals assessed, in particular the algorithm is evaluated for a number of simulated single-layer and multi-layer conditions. The algorithm was found to perform well for single-layer cloud except when the cloud was very thin; i.e., less than 1 optical depths. For the multi-layer cloud, the algorithm was robust except when the upper ice cloud layer is less than five optical depths. In these cases the retrieved cloud top pressure and cloud effective radius become a weighted average of the 2 layers. The sum of optical depth of multi-layer cloud is retrieved well until the cloud becomes thick, greater than 50 optical depths, where the cloud begins to saturate. The cost proved a good indicator of multi-layer scenarios. Both the retrieval cost and the error need to be considered together in order to evaluate the quality of the retrieval. This algorithm in the configuration described here has been applied to both ATSR-2 and AATSR visible and infrared measurements in the context of the GRAPE (Global Retrieval and cloud Product Evaluation) project to produce a 14 yr consistent record for climate research.

Measuring volcanic plume and ash properties from space

Abstract The remote sensing of volcanic ash plumes from space can provide a warning of an aviation hazard and knowledge on eruption processes and radiative effects. In this paper new algorithms are presented to provide volcanic plume properties from measurements by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), the Advanced Along Track Scanning Radiometer (AATSR) and the Spinning Enhanced Visible and Infrared Imager (SEVIRI). A challenge of remote sensing is to provide near-real-time methods to identify, and so warn of, the presence of volcanic ash. To achieve this, a singular vector decomposition method has been developed for the MIPAS instrument on board the Environmental Satellite. This method was applied to observations of the ash clouds from the eruptions of Nabro and the Puyehue–Cordón Caulle in 2011 and led to a sensitive volcanic signal flag which was capable of tracking changes in the volcanic signal spectra as the plume evolved. A second challenge for remote sensing is to identify the ash plume height. This is a critical parameter for the initialization of algorithms that numerically model the evolution and transport of a volcanic plume. As MIPAS is a limb sounder, the identification of ash also provides an estimate of height provided the plume is above about 6 km. This is complemented by a new algorithm, Stereo Ash Plume Height Retrieval Algorithm, that identifies plume height using the parallax between images provided by Along Track Scanning Radiometer-type instruments. The algorithm was tested on an image taken at 14:01 GMT on 6 June 2011 of the Puyehue–Cordón Caulle eruption plume and gave a height of 11.9±1.4 km, which agreed with the value derived from the location of the plume shadow (12.7±1.8 km). This plume height was similar to the height observed by MIPAS (12 ± 1.5 km) at 02:56 GMT on 6 June. The quantitative use of satellite imagery and the full exploitation of high-resolution spectral measurements of ash depends upon knowing the optical properties of the observed ash. Laboratory measurements of ash from the 1993 eruption of Mt Aso, Japan have been used to determine the refractive indices from 1 to 20 µm. These preliminary measurements have spectral features similar to ash values that have been used to date, albeit with slightly different positions and strengths of the absorption bands. The refractive indices have been used to retrieve ash properties (plume height, optical depth and ash effective radius) from AATSR and SEVIRI instruments using two versions of Oxford-RAL Retrieval of Aerosol and Cloud (ORAC) algorithm. For AATSR a new ash cloud type was used in ORAC for the analysis of the plume from the 2011 Eyjafjallajökull eruption. For the first c. 500 km of the plume ORAC gave values for plume height of 2.5–6.5 km, optical depth 1–2.5 and effective radius 3–7 µm, which are in agreement with other observations. A weakness of the algorithm occurs when underlying cloud invalidates the assumption of a single cloud layer. This is rectified in a modified version of ORAC applied to SEVIRI measurements. In this case an extra model of a cloud underlying the ash plume was included in the range of applied models. In cases where the plume overlay cloud, this new model worked well, showing good agreement with correlative Cloud–Aerosol Lidar with Orthogonal Polarization observations.

Evaluation of Global Ozone Monitoring Experiment (GOME) ozone profiles from nine different algorithms

An evaluation is made of ozone profiles retrieved from measurements of the nadir-viewing Global Ozone Monitoring Experiment (GOME) instrument. Currently four different approaches are used to retrieve ozone profile information from GOME measurements, which differ in the use of external information and a priori constraints. In total nine different algorithms will be evaluated exploiting the Optimal Estimation (Royal Netherlands Meteorological Institute, Rutherford Appleton Laboratory, University of Bremen, National Oceanic and Atmospheric Administration, Smithsonian Astrophysical Observatory), Phillips-Tikhonov Regularization (Space Research Organization Netherlands), Neural Network (Center for Solar Energy and Hydrogen Research, Tor Vergata University), and Data Assimilation (German Aerospace Center) approaches. Analysis tools are used to interpret data sets that provide averaging kernels. In the interpretation of these data, the focus is on the vertical resolution, the indicative altitude of the retrieved value, and the fraction of a priori information. The evaluation is completed with a comparison of the results to lidar data from the NDSC (Network for Detection of Stratospheric Change) stations in Andoya (Norway), Observatoire Haute Provence (France), Mauna Loa (USA), Lauder (New Zealand) and Dumont d’Urville (Antarctic) for the years 1997–1999. In total the comparison involves nearly 1000 ozone profiles, and allows the analysis of GOME data measured in different global regions and hence observational circumstances. The main conclusion of this paper is that unambiguous information on the ozone profile can at best be retrieved in the altitude range 15–48 km with a vertical resolution of 10 to 15 km, precision of 5–10%, and a bias up to 5% or 20% depending on the success of recalibration of the input spectra. The sensitivity of retrievals to ozone at lower altitudes varies from scheme to scheme and includes significant influence from a priori assumptions.

Oxford-RAL Aerosol and Cloud (ORAC): aerosol retrievals from satellite radiometers

This chapter describes an optimal estimation retrieval scheme for the derivation of the properties of atmospheric aerosol from top-of-atmosphere (TOA) radiances measured by satellite-borne visible-IR radiometers. The algorithm makes up part of the Oxford-RAL Aerosol and Cloud (ORAC) retrieval scheme (the other part of the algorithm performs cloud retrievals and is described in detail elsewhere [by Watts et al.] [37]).

MARSCHALS: development of an airborne millimeter-wave limb sounder

MARSCHALS (Millimeter-wave Airborne Receivers for Spectroscopic CHaracterization in Atmospheric Limb Sounding) is being developed with funding from the European Space Agency as a simulator of MASTER (Millimeter-wave Acquisitions for Stratosphere Troposphere Exchange Research), a limb sounding instrument in a proposed future ESA Earth Explorer Core Mission. The principal and most innovative objective of MARSCHALS is to simulate MASTERs capability for sounding O3, H2O and CO at high vertical resolution in the upper troposphere (UT) using millimeter wave receivers at 300, 325, and 345 GHz. Spectra are recorded in these bands with 200 MHz resolution. As such, MARSCHALs is the first limb-sounder to be explicitly designed and built for the purpose of sounding the composition of the UT, in addition to the Lower Stratosphere (LS) where HNO3, N2O and additional trace gases will also be measured. A particular attribute of millimeter-wave measurements is their comparative insensitivity to ice clouds. However, to assess the impact on the measurements of cirrus in the UT, MARSCHALs has a near-IR digital video camera aligned in azimuth with the 235 mm limb-scanning antenna. In addition to UT and LS aircraft measurements, MARSCHALs is capable of making mid-stratospheric measurements from a balloon platform when fitted with a 400 mm antenna. Provision has been made to add further receiver channels and a high resolution spectrometer.

New cryogenic heterodyne techniques applied in TELIS: the balloon-borne THz and submillimeter limb sounder for atmospheric research

We present a design concept for a new state-of-the-art balloon borne atmospheric monitor that will allow enhanced limb sounding of the Earths atmosphere within the submillimeter and far-infrared wavelength spectral range: TELIS, TErahertz and submm LImb Sounder. The instrument is being developed by a consortium of major European institutes that includes the Space Research Organization of the Netherlands (SRON), the Rutherford Appleton Laboratory (RAL) will utilize state-of-the-art superconducting heterodyne technology and is designed to be a compact, lightweight instrument cpaable of providing broad spectral coverage, high spectral resolution and long flight duration (~24 hours duration during a single flight campaign). The combination of high sensitivity and extensive flight duration will allow evaluation of the diurnal variation of key atmospheric constitutenets sucyh as OH, HO2, ClO, BrO togehter will onger lived constituents such as O3, HCL and N2O. Furthermore, TELIS will share a common balloon platform to that of the MIPAS-B Fourier Transform Spectrometer, developed by the Institute of Meteorology and Climate research of the over an extended spectral range. The combination of the TELIS and MIPAS instruments will provide atmospheric scientists with a very powerful observational tool. TELIS will serve as a testbed for new cryogenic heterodyne detection techniques, and as such it will act as a prelude to future spaceborne instruments planned by the European Space Agency (ESA).

MARSCHALS: airborne simulator of a future space instrument to observe millimeter-wave limb emission from the upper troposphere and lower stratosphere

MARSCHALS is the airborne simulator of a proposed future satellite instrument to measure millimetre-wave limb emission from O3, H2O, CO and other trace gases in the upper troposphere and lower stratosphere. To achieve comparatively high vertical resolution and pointing stability, MARSCHALS scans the atmospheric limb in 1km vertical steps using a 235mm diameter antenna controlled by a dedicated inertial measurement unit. A quasi-optical network directs radiation from the antenna or an ambient (~300K) or cold (~90K) calibration target into three front-end receivers and suppresses each unwanted side-band by >30dB using multi-layer frequency selective surfaces. Each receiver comprises a waveguide mixer pumped subharmonically by a phase-locked LO and a wideband IF preamplifier. The IF outputs are directed to channeliser spectrometers of 200MHz resolution which instantaneously and contiguously cover 12GHz wide (RF) frequency bands centred near 300, 325 and 345GHz. To identify clouds, images of near-IR sunlight scattered into the limb direction are recorded concurrently by an 850nm wavelength camera. MARSCHALS has been built under ESA contract by a consortium led by Rutherford Appleton Laboratory in the UK, and had its first flights on the Russian Geophysica (M55) aircraft during 2005, culminating in a deployment during the SCOUT-O3 campaign based in Darwin, Australia. This paper describes the MARSCHALS instrument and an initial assessment of its performance, determined on ground and during flight.

Millimeter-wave tomographic limb-sounding of the UTLS: investigation of retrieval resolution

The Millimetre-wave Acquisitions for Stratosphere-Troposphere Exchange Research (MASTER) instrument is intended to sound the gaseous composition of the upper troposphere and lower stratosphere (UTLS) in a future ESA space mission. A significant and inherent advantage of operation at millimetre and sub-millimetre wavelengths in comparison to limb-sounders operating at infra-red and shorter wavelengths is low sensitivity to cirrus clouds. MASTER will employ relatively small vertical and horizontal spacings between limb views, in order to over-sample the atmosphere in the orbit plane. By viewing each air mass from different directions, and including this information in the retrieval, horizontal as well as vertical structure of atmospheric fields may be captured. In order to examine this tomographic limb-sounding approach for MASTER, a state-of-the-art 2-D radiative transfer model and retrieval model have been developed and used in simulation experiments. A linear analysis has been performed to establish achievable horizontal and vertical retrieval resolution for target species and to identify additional parameters to include in the state vector in order to reduce error sensitivities. A realistic mid-latitude scenario and appropriate instrument and model errors have been considered. By accurately modelling radiative transfer in two dimensions within the orbit plane, and using multiple limb-sequences simultaneously in a 2-D retrieval, a horizontal resolution better than 200 km can be achieved, together with ~2 km vertical resolution for retrievals of water vapour, ozone and other trace gases in the UTLS.