Daniel M. Peters

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.

Composition of Smoke Generated by Landing Aircraft

A combination of techniques has been used to examine the composition of smoke generated by landing aircraft. A sample of dust from the undercarriage from several commercial airliners was examined with SEM/EDX (Scanning Electron Microscope/Energy Dispersive X-ray) to determine its elemental composition and also with an aerosizer/aerodisperser in order to measure the particle size spectrum. The observed size spectrum was bimodal with equal numbers of particles at peaks of aerodynamic diameter ∼10 μm and ∼50 μm. The EDX analysis suggested that the former peak is carbonaceous, while the latter consists of elements typical of an asphalt concrete runway. In the field, a scanning Lidar, in combination with optical and condensation particle counters, was deployed to obtain limits to the number concentration and size of such particles. Most of the (strong) Lidar signal probably arose from the coarser 50 μm aerosol, while respirable aerosol was too sparse to be detected by the optical particle counters.

Measurements of the complex refractive index of volcanic ash at 450, 546.7, and 650 nm

The detection and quantification of volcanic ash is extremely important to the aviation industry, civil defense organizations, and those in peril from volcanic ashfall. To exploit the remote sensing techniques that are used to monitor a volcanic cloud and return information on its properties, the effective complex refractive index of the volcanic ash is required. This paper presents the complex refractive index determined in the laboratory at 450.0 nm, 546.7 nm, and 650.0 nm for volcanic ash samples from eruptions of Aso (Japan), Grimsvotn (Iceland), Chaiten (Chile), Etna (Italy), Eyjafjallajokull (Iceland), Tongariro (New Zealand), Askja (Iceland), Nisyros (Greece), Okmok (Alaska), Augustine (Alaska), and Spurr (Alaska). The Becke line method was used to measure the real part of the refractive index with an accuracy of 0.01. The values measured differed between eruptions and were in the range 1.51–1.63 at 450.0 nm, 1.50–1.61 at 546.7 nm, and 1.50–1.59 at 650.0 nm. A novel method is introduced to derive the imaginary part of the refractive index from the attenuation of light by ash. The method has a precision in the range 10−3–10−4. The values for the ash imaginary refractive index ranged 0.22–1.70 × 10−3 at 450.0 nm, 0.16–1.93 × 10−3 at 546.7 nm, and 0.15–2.08 × 10−3 at 650.0 nm. The accuracy of Becke and attenuation methods was assessed by measuring the complex refractive index of Hoya neutral density glass and found to have an accuracy of <0.01 and <2 × 10−5 for the real and imaginary parts of the refractive index, respectively.

Estimation of a lidar’s overlap function and its calibration by nonlinear regression

The overlap function of a Raman channel for a lidar system is retrieved by nonlinear regression using an analytic description of the optical system and a simple model for the extinction profile, constrained by aerosol optical thickness. Considering simulated data, the scheme is successful even where the aerosol profile deviates significantly from the simple model assumed. Application to real data is found to reduce by a factor of 1.4-2.0 the root-mean-square difference between the attenuated backscatter coefficient as measured by the calibrated instrument and a commercial instrument.

HIRDLS instrument radiometric calibration black body targets

The pre-launch calibration of the HIRDLS instrument took place in a dedicated facility at the University of Oxford. One aspect of this calibration was the determination of the response of the instrument to black body radiation. This was achieved with the use of purpose built full aperture black body targets which were mounted in the vacuum chamber together with all of the calibration equipment. Special attention was placed on the absolute knowledge of the emission from these targets. This was done through a combination of thermometric sensor calibration traceable to the International Temperature Standard (ITS-90), surface emission measurements, cavity design and modeling and controlling the stray light sources in the vacuum chamber. This paper describes the design requirements, implementation and performance achieved.

Prelaunch calibration of the NASA AURA HIRDLS instrument

The High Resolution Dynamics Limb Sounder (HIRDLS) instrument is scheduled for launch on the NASA AURA satellite in January 2004; it is a joint project between the UK and USA. HIRDLS is a mid-infrared limb emission sounder which will measure the concentration of trace species and aerosol, and temperature and pressure variations in the Earths atmosphere between about 8 and 100 km altitude on a finer spatial scale than has been achieved before. This will depend upon both a high quality of instrument build, and very precise pre-launch calibration. Proto Flight Model calibration was performed in a purpose-built laboratory at Oxford University during an 13-week period in 2002. The tests were made in vacuum under cryogenic conditions close to the space environment. The measurements were divided into spectral, spatial and radiometric, with the HIRDLS pointing capability being used to control which item of test equipment was viewed. A large degree of automation was achieved, and this combined with 24-hour/7-day working enabled a large quantity of information to be obtained.

Prelaunch Radiometric Calibration of the HIRDLS Flight Instrument: Results and Use in On-Orbit Data Processing

Results from the prelaunch radiometric calibration of the 21-channel High Resolution Dynamics Limb Sounder (HIRDLS) flight instrument are presented. The calibration was carried out in the Department of Physics of Oxford University. Two large aperture external blackbody cavities were used to generate stable radiances at target temperatures between ~90 and ~320 K. These blackbodies were located, along with the HIRDLS instrument, inside a large vacuum chamber. Data were taken at three different focal-plane temperatures (~61, ~66, and ~71 K). To complicate matters beyond the initial scope of the prelaunch calibration, a failure of some contamination close-out material (Kapton) that lined the inner fore-optics cavity occurred during launch, which made the original in-flight radiometric calibration procedure impossible. Accordingly, the radiometric conversion algorithm had to be changed, requiring more information from prelaunch calibration to be used than first envisioned. This paper discusses a variety of details, such as data-taking procedures, analysis methodology, associated error analyses, and necessary changes to the radiometric conversion algorithm needed for inflight data processing.

The improvement of lidar analysis through non-linear regression

Lidars are ideally placed to investigate the effects of aerosol and cloud on the climate system due to their unprecedented vertical and temporal resolution. Dozens of techniques have been developed in recent decades to retrieve the extinction and backscatter of atmospheric particulates in a variety of conditions. These methods, though often very successful, are fairly ad hoc in their construction, utilising a wide variety of approximations and assumptions that makes comparing the resulting data products with independent measurements difficult and their implementation in climate modelling virtually impossible. As with its application to satellite retrievals, the methods of non-linear regression can improve this situation by providing a mathematical framework in which the various approximations, estimates of experimental error, and any additional knowledge of the atmosphere can be clearly defined and included in a mathematically ‘optimal’ retrieval method, providing rigorously derived error estimates. In addition to making it easier for scientists outside of the lidar field to understand and utilise lidar data, it also simplifies the process of moving beyond extinction and backscatter coefficients and retrieving microphysical properties of aerosols and cloud particles. Such methods have been applied to a prototype Raman lidar system. A technique to estimate the lidar’s overlap function using an analytic model of the optical system and a simple extinction profile has been developed. This is used to calibrate the system such that a retrieval of the profile extinction and backscatter coefficients can be performed using the elastic and nitrogen Raman backscatter signals.

Stable Space‐Based Encapsulated Platinum Resistance Thermometry over the Range −9 °C to 50 °C

The High Resolution Dynamic Limb Sounder (HIRDLS) is a space‐based filter radiometer for atmospheric monitoring between the upper troposphere and the mesosphere. Retrievals of trace chemicals, temperature and geopotential height gradients will be obtained on a global scale. This radiometer uses the two point calibration algorithm in which the radiometer views a cold “zero radiance” space view, and a high‐emissivity warm blackbody target, of known temperature. To achieve a high radiometric performance, an accurate thermometry system is used to measure cavity temperature over the range −9 °C to +50 °C allowing the instrument’s gain to be measured as required (nominally every 66 seconds). The thermometry error budget for the cavity allows a temperature sensor stability of 25 mK over the mission lifetime of five years in‐flight and two years before instrument launch. This high thermometry performance is achieved using two redundant sets of three platinum resistance thermometers (PRTs) to sense the cavity temp...

Radiometric calibration of the HIRDLS flight instrument from prelaunch calibration data

Results from a pre-launch radiometric calibration of the 21-channel HIRDLS instrument will be presented. These data were obtained during a pre-launch calibration of HIRDLS at Oxford University (Fall 2002). Two external blackbody cavities were used to generate temperatures between ~90 K to ~320 K. These blackbodies were located, along with HIRDLS, inside a large vacuum chamber. Data were taken at three different focal-plane temperatures (61 K, 66 K, and 71 K). This paper will cover a variety of details; such as, data--taking procedures, analysis methodology, and the resulting linearity analyses.