Search

The HT-FRTC can be used across the electromagnetic spectrum from the microwave through to the ultraviolet to calculate transmittance, radiance and flux spectra, which are represented by their principal components. The code uses monochromatic calculations at a small number of frequencies, which are selected by k-means clustering, to predict the principal component scores. It has been found that kernel regression yields more robust predictions than a linear regression. The principal components cover the spectrum at a very high spectral resolution, similar to that of conventional line-by-line models such that the individual spectral lines are resolved. This approach allows very fast line-by-line-like, hyperspectral and broadband simulations for satellite-based, airborne and ground-based sensors. The principal components are derived during a code training phase from monochromatic simulations for a diverse set of atmospheres and surfaces. They are sensor independent and no extra training is required for additional sensors. The HT-FRTC has been trained with all of the trace gases in the HIgh-resolution TRANsmission molecular absorption database (HITRAN) and on a large variety of surface emissivity/reflectance spectra. It can be run for any Lambertian or specular surface. The HT-FRTC is a plane-parallel one-dimensional model but some effects of the Earth’s sphericity are accounted for. Solar, lunar and cosmic microwave background sources have been included. Scattering by frozen and liquid cloud, precipitation particles and twenty different aerosol species has been included, as well as Rayleigh scattering which is significant in the short-wave. The scattering phase function can be fully accounted for by an integrated monochromatic version of the Edwards-Slingo spherical harmonics radiation code or approximately by a modification to the extinction (Chou scaling).

This article presents a generic flexible framework for an End-to-end Instrument Performance Simulation System (EIPS) for satellite atmospheric remote sensing instruments. A systematic process for developing an end-to-end simulation system based on Rodgers&rsquo, atmospheric observing system design process has been visualised. The EIPS has been developed to support the quantitative evaluation of new satellite instrument concepts in terms of performance simulations, design optimisation, and trade-off analysis. Important features of this framework include: fast radiative transfer simulation capabilities (fast computation and line-by-line like simulations), applicability across the whole electromagnetic (EM) spectrum and a number of integrated retrieval diagnostics. Because of its applicability across the whole EM spectrum, the framework can be usefully applied to synergistic atmospheric retrieval studies. The framework is continually developing and evolving, and finding applications to support and evaluate emerging instrument and mission concepts. To demonstrate the framework&rsquo, s flexibility in relation to advanced sensor technologies in the microwave range, a novel superconducting transition edge sensor (TES) -based multi-spectral microwave instrument has been presented as an example. As a case study, the performance of existing multi-spectral-type microwave instruments and a TES-technology-based multi-spectral microwave instrument has been simulated and compared using the developed end-to-end simulation framework.

The spectrum of reflected solar radiation emerging at the top of the atmosphere is rich with Earth system information. To identify spectral signatures in the reflected solar radiation and directly relate them to the underlying physical properties controlling their structure, over 90 000 solar reflectance spectra are computed over West Africa in 2010 using a fast radiation code employing the spectral characteristics of the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY). Cluster analysis applied to the computed spectra reveals spectral signatures related to distinct surface properties, and cloud regimes distinguished by their spectral shortwave cloud radiative effect (SWCRE). The cloud regimes exhibit a diverse variety of mean broadband SWCREs, and offer an alternative approach to define cloud type for SWCRE applications that does not require any prior assumptions. The direct link between spectral signatures and distinct physical properties extracted from clustering remains robust between spatial scales of 1, 20, and 240 km, and presents an excellent opportunity to understand the underlying properties controlling real spectral reflectance observations. Observed SCIAMACHY spectra are assigned to the calculated spectral clusters, showing that cloud regimes are most frequent during the active West African monsoon season of June–October in 2010, and all cloud regimes have a higher frequency of occurrence during the active monsoon season of 2003 compared with the inactive monsoon season of 2004. Overall, the distinct underlying physical properties controlling spectral signatures show great promise for monitoring evolution of the Earth system directly from solar spectral reflectance observations.

A number of methods exist for solving the problem of electromagnetic scattering by atmospheric ice crystals. Amongst these methods, only a few are used to generate “benchmark” results in the atmospheric science community. Most notably, the T-matrix method, Discrete Dipole Approximation, and the Finite-Difference Time-Domain method. The Boundary Element Method (BEM), however, has received considerably less attention in this community despite its extensive use and development in other areas of applied mathematics and engineering. Recently the group of Betcke et al. (2015 [1] ) at University College London has released a high performance open source boundary element library called BEM++. In this paper, we employ BEM++ to calculate the scattering properties of hexagonal ice columns of fixed orientation, as well as more complicated particles such as hollow columns and bullet-rosettes. The results for hexagonal columns are compared to those obtained using a highly accurate and well-established T-matrix method (Baran et al., 2001 [2] ) for a range of different wavelengths and size parameters. It is shown that the results are in excellent agreement and that BEM++ is a fast alternative to the T-matrix method and others for generating benchmark results. However, the large memory requirements of BEM++ cause it to be limited to size parameters ~15 on a standard desktop PC if an accuracy of roughly 1% is required. The main advantages of BEM++ over many other methods are its flexibility to be applied to homogeneous dielectric particles of arbitrarily complex shape, and its open availability. This flexibility is illustrated by the application of BEM++ to scattering by hollow columns with different cavity types, as well as bullet-rosettes with 2–6 branches.

Data from hyperspectral infrared sounders are routinely ingested worldwide by National Weather Centers (NWCs). The cloud-free fraction of this data is used for initializing forecasts which include profiles of temperature, water vapor, water cloud and ice cloud profiles on a global grid. Although the data from these sounders are sensitive to the vertical distribution of ice and liquid water in clouds, this information is not fully utilized. In the future, this information could be used for validating clouds in NWC models and for initializing forecasts. We evaluate how well the calculated radiances from hyperspectral Radiative Transfer Models (RTMs) compare to cloudy radiances observed by AIRS and to one another. Vertical profiles of the clouds, temperature and water vapor from ECMWF (European Center for Medium-range Weather Forecasting) were used as input for the RTMs. For non-frozen ocean day and night data, the histograms derived from the calculations by several RTMs at 900 cm(exp -1)have a better than 0.95 correlation with the histogram derived from the AIRS observations, with a bias relative to AIRS of typically less than 2 K. Differences in the cloud physics and cloud overlap assumptions result in little bias between the RTMs, but the standard deviation of the differences ranges from 6 to 12 K. Results at 2616 cm(exp -1) at night are reasonably consistent with results at 900 cm(exp -1). Except for RTMs which use full scattering calculations, the bias and histogram correlations at 2616 cm(exp -1) are inferior to those at 900 cm(exp -1) for daytime calculations.

This is the first paper to investigate the relationship between the shape of the scattering phase function of cirrus and the relative humidity with respect to ice (RHi, using space-based solar radiometric angle-dependent measurements. The relationship between RHi and the complexity of ice crystals has been previously studied using data from aircraft field campaigns and laboratory cloud chambers. However, to the best of our knowledge, there have been no studies to date that explore this relationship through the use of remotely sensed space-based angle-dependent solar radiometric measurements. In this paper, one case study of semi-transparent cirrus, which occurred on 25 January 2010 off the north-east coast of Scotland, is used to explore the possibility of such a relationship. Moreover, for the first time, RHi fields predicted by a high-resolution numerical weather prediction (NWP) model are combined with satellite retrievals of ice crystal complexity. The NWP model was initialised at midnight, on 25 January 2010, and the mid-latitude RHi field was extracted from the NWP model at 13:00 UTC. At about the same time, there was a PARASOL (Polarization and Anisotropy of Reflectance for Atmospheric science coupled with Observations from a Lidar) overpass, and the PARASOL swath covered the NWP-model-predicted RHi field. The cirrus case was located over Scotland and the North Sea. From the satellite channel based at 0.865 μm, the directionally averaged and directional spherical albedos were retrieved between the scattering angles of about 80 and 130°. An ensemble model of cirrus ice crystals is used to predict phase functions that vary between phase functions that exhibit optical features (referred to as pristine) and featureless phase functions. For each of the PARASOL pixels, the phase function that best minimised differences between the spherical albedos was selected. This paper reports, for this one case study, an association between the most featureless phase function model and the highest values of NWP-predicted RHi (i.e. when RHi > 1.0). For pixels associated with NWP-model-predicted RHi < 1, it was impossible to generally discriminate between phase function models at the 5% significance level. It is also shown that the NWP model prediction of the vertical profile of RHi is in good agreement with dropsonde, in situ measurements and independent aircraft-based physical retrievals of RHi. Furthermore, the NWP model prediction of the cirrus cloud-top height and its vertical extent is also found to be in good agreement with aircraft-based lidar measurements.

Typically referred to as brownout or whiteout, degraded visual environments (DVE) more accurately includes all forms of materially reduced visibility where pilots and operators may become disorientated. These conditions include very low night illumination levels, adverse meteorological conditions (clouds and precipitation) and obscurant particulates (dust storms and sea spray), which can all reduce photopic and thermal target contrasts with respect to their background environment. DVE operations are a significant and persistent safety concern for autonomous or piloted platforms at all points of their mission cycle including launch, transit, execution, egress and recovery. Impacted platforms of all sizes can include aircraft (from drones to hybrid airships), ground vehicles and maritime vessels (surface or submerged). This paper seeks to highlight the capabilities of the Met Office over both land and sea in accurately forecasting the presence of DVE through Numerical Weather Prediction (NWP). These forecasts can then be furthermore leveraged with Met Office expertise in environmental impacts to predict the likely effects on sensor acquisition ranges. These sensor impacts are robustly modelled through Tactical Decision Aids (TDAs) that can take account of platform range, elevation, sensor waveband, target characteristics and background environment, in addition to many other pertinent parameters. The Met Office is the United Kingdom’s National Meteorological Service (NMS) tasked to provide weather information and weather related warnings. Within the realm of defence engagement, the Met Office has provided key elements of geospatial intelligence capability for multinational operations worldwide for over a century. The Met Office supports its defence stakeholders, such as NATO and the USAF, by assimilating over 10 million daily global observations into its Unified Model (UM) for Numerical Weather Predication (NWP). The UM provides the input for the Havemann-Taylor Fast Radiative Transfer Code (HT-FRTC) to predict atmospheric impacts on sensor performance within the context of Tactical Decision Aids (TDAs), such as Neon for thermal contrast and MONIM for night illumination levels. This global UM forecast data, with high temporal-spatial resolution regional sub-models, enables further environmental impact prediction for atmospheric dispersion events such as volcanic ash and radiological incidents through the NAME capability (Numerical Atmospheric-dispersion Modelling Environment). In addition to capabilities in Space Weather, the Met Office also specializes in operational ocean monitoring and forecasting services to support safe operations in the marine environment, but have also evolved to cater for, amongst others, marine security, commercial operations, licensing for marine operations and environmental monitoring.

The predictive quality of an ensemble model of cirrus ice crystals to model passive and active measurements of ice cloud, from the ultraviolet (UV) to the microwave, is tested. The ensemble model predicts m ∝ D2, where D is the maximum dimension of the ice crystal, and m is its mass. This predicted m-D relationship is applied to a moment estimation parametrization of the particle size distribution (PSD), to estimate the PSD shape, given ice water content (IWC) and in-cloud temperature. The same microphysics is applied across the electromagnetic spectrum to model UV, infrared, microwave and radar observations. The short-wave measurements consist of airborne UV backscatter lidar (light detection and ranging) estimates of the volume extinction coefficient, total solar optical depth, and space-based multi-directional spherical albedo retrievals, at 0.865 µm, between the scattering angles 85° and 125°. The airborne long-wave measurements consist of high-resolution interferometer upwelling brightness temperatures, obtained between the wavelengths of about 3.45 µm and 4.1 µm, and 8.0 µm to 12.0 µm. The low-frequency measurements consist of ground-based Chilbolton 35 GHz radar reflectivity measurements and space-based upwelling 190 GHz brightness temperature measurements. The predictive quality of the ensemble model is demonstrated to be generally within the experimental uncertainty of the lidar backscatter estimates of the volume extinction coefficient and total solar optical depth. The ensemble model prediction of the high-resolution brightness temperature measurements is generally within ±2 K and ±1 K at solar and infrared wavelengths, respectively. The 35 GHz radar reflectivity and 190 GHz brightness temperatures are generally simulated to within ±2 dBZe, and ±2 K, respectively. The directional spherical albedo observations suggest that the scattering phase function of the most randomized ensemble model gives the best fit to the measurements (generally within ±3%). This article demonstrates that the ensemble model, assuming the same microphysics, is physically consistent across the electromagnetic spectrum.

The Havemann-Taylor Fast Radiative Transfer Code (HT-FRTC) represents transmittances, radiances and fluxes by principal components that cover the spectra at very high resolution, allowing fast highly-resolved pseudo line-by-line, hyperspectral and broadband simulations across the electromagnetic spectrum form the microwave to the ultraviolet for satellite-based, airborne and ground-based sensors. HT-FRTC models clear atmospheres and those containing clouds and aerosols, as well as any surface (land/sea/man-made). The HT-FRTC has been used operationally in the NEON Tactical Decision Aid (TDA) since 2008. The TDA combines the HT-FRTC with a thermal contrast model and an NWP model forecast data feed to predict the apparent thermal contrast between different surfaces and ground-based targets in the thermal and short-wave IR. The new objective here is to predict the optical contrast of air-borne targets under realistic night-time scenarios in the Photopic and NVG parts of the spectrum. This requires the inclusion of all the relevant radiation sources, which include twilight, moonlight, starlight, airglow and cultural light. A completely new exact scattering code has been developed which allows the straight-forward addition of any number of direct and diffuse sources anywhere in the atmosphere. The new code solves the radiative transfer equation iteratively and is faster than the previous solution. Simulations of scenarios with different light levels, from situations during a full moon to a moonless night with very low light levels and a situation with cultural light from a town are presented. The impact of surface reflectance and target reflectance is investigated.

The HT-FRTC is a principal component based fast radiative transfer code that can be used across the electromagnetic spectrum from the microwave through to the ultraviolet to calculate transmittance, radiance and flux spectra. The principal components cover the spectrum at a very high spectral resolution, which allows very fast line-by-line, hyperspectral and broadband simulations for satellite-based, airborne and ground-based sensors. The principal components are derived during a code training phase from line-by-line simulations for a diverse set of atmosphere and surface conditions. The derived principal components are sensor independent, i.e. no extra training is required to include additional sensors. During the training phase we also derive the predictors which are required by the fast radiative transfer code to determine the principal component scores from the monochromatic radiances (or fluxes, transmittances). These predictors are calculated for each training profile at a small number of frequencies, which are selected by a k-means cluster algorithm during the training phase. Until recently the predictors were calculated using a linear regression. However, during a recent rewrite of the code the linear regression was replaced by a Gaussian Process (GP) regression which resulted in a significant increase in accuracy when compared to the linear regression. The HT-FRTC has been trained with a large variety of gases, surface properties and scatterers. Rayleigh scattering as well as scattering by frozen/liquid clouds, hydrometeors and aerosols have all been included. The scattering phase function can be fully accounted for by an integrated line-by-line version of the Edwards-Slingo spherical harmonics radiation code or approximately by a modification to the extinction (Chou scaling).

A method to approximate azimuthally resolved light scattering patterns and phase functions due to diffraction and external reflection by strongly absorbing facetted particles is demonstrated for a cube and compared with results from an exact method, T-matrix. A phase function averaged over a range of orientations of a strongly absorbing hexagonal column of aspect ratio unity has been calculated and tested against Discrete Dipole Approximation (DDA) results for a size parameter of 50., Final Accepted Version

Here we demonstrate the feasibility of applying a fast radiative transfer code, based on empirical orthogonal functions, in conjunction with a 1D-Var physical retrieval system to hyperspectral data taken from space/airborne radiance measurements in order to retrieve the emissivity spectra of the underlying surface. This approach was successfully tested using hyperspectral data obtained during the JAIVEx campaign in 2007 with the Airborne Research Interferometer Evaluation System (ARIES) on board of the UK Atmospheric Research aircraft. Using ARIES has the advantage that, during low-level flights, the surface emissivities can be derived directly from the hyperspectral data. Thus ARIES is capable of providing both the hyperspectral radiance data and the ‘true’ surface emissivity which has obvious advantages for validating the retrieval of the surface emissivity spectra. © Crown Copyright 2009. Published by John Wiley & Sons, Ltd.

A new parametrization of the single scattering properties of ice crystals suitable for use in general circulation models of the atmosphere is presented. The parametrization is developed from a recent treatment of the scattering properties of an ensemble of ice crystals, which has been validated against observational data and is based on a more rigorous treatment of ice optics than has been possible until recently. The new parametrization is compared with existing schemes in idealized test cases and its impact in a climate model is assessed.

The Havemann-Taylor Fast Radiative Transfer Code (HT-FRTC) is a core component of the Met Office NEON Tactical Decision Aid (TDA). Within NEON, the HT-FRTC has for a number of years been used to predict the infrared apparent thermal contrasts between different surface types as observed by an airborne sensor. To achieve this, the HT-FRTC is supplied with the inherent temperatures and spectral properties of these surfaces (i.e. ground target(s) and backgrounds). A key strength of the HT-FRTC is its ability to take into account the detailed properties of the atmosphere, which in the context of NEON tend to be provided by a Numerical Weather Prediction (NWP) forecast model. While water vapour and ozone are generally the most important gases, additional trace gases are now being incorporated into the HT-FRTC. The HT-FRTC also includes an exact treatment of atmospheric scattering based on spherical harmonics. This allows for the treatment of several different aerosol species and of liquid and ice clouds. Recent developments can even account for rain and falling snow. The HT-FRTC works in Principal Component (PC) space and is trained on a wide variety of atmospheric and surface conditions, which significantly reduces the computational requirements regarding memory and processing time. One clear-sky simulation takes approximately one millisecond at the time of writing. Recent developments allow the training of HT-FRTC to be both completely generalised and sensor independent. This is significant as the user of the code can add new sensors and new surfaces/targets by supplying extra files which contain their (possibly classified) spectral properties. The HT-FRTC has been extended to cover the spectral range of Photopic and NVG sensors. One aim here is to give guidance on the expected, directionally resolved sky brightness, especially at night, again taking the actual or forecast atmospheric conditions into account. Recent developments include light level predictions during the period of twilight.

The Havemann-Taylor Fast Radiative Transfer Code (HT-FRTC) is a component of the Met Office NEON Tactical Decision Aid (TDA). Within NEON, the HT-FRTC has for a number of years been used to predict the IR apparent thermal contrasts between different surface types as observed by an airborne sensor. To achieve this, the HT-FRTC is supplied with the inherent temperatures and spectral properties of these surfaces (i.e. ground target(s) and background). A key strength of the HT-FRTC is its ability to take into account the detailed properties of the atmosphere, which in the context of NEON tend to be provided by a Numerical Weather Prediction (NWP) forecast model. While water vapour and ozone are generally the most important gases, additional trace gases are now being incorporated into the HT-FRTC. The HT-FRTC also includes an exact treatment of atmospheric scattering based on spherical harmonics. This allows the treatment of several different aerosol species and of liquid and ice clouds. Recent developments can even account for rain and falling snow. The HT-FRTC works in Principal Component (PC) space and is trained on a wide variety of atmospheric and surface conditions, which significantly reduces the computational requirements regarding memory and time. One clear-sky simulation takes approximately one millisecond. Recent developments allow the training to be completely general and sensor independent. This is significant as the user of the code can add new sensors and new surfaces/targets by simply supplying extra files which contain their (possibly classified) spectral properties. The HT-FRTC has been extended to cover the spectral range of Photopic and NVG sensors. One aim here is to give guidance on the expected, directionally resolved sky brightness, especially at night, again taking the actual or forecast atmospheric conditions into account. Recent developments include light level predictions during the period of twilight.

The HT-FRTC is a principal component based fast radiative transfer code that can be used across the electromagnetic spectrum from the microwave through to the ultraviolet to calculate transmittance, radiance and flux spectra. The principal components cover the spectrum at a very high spectral resolution, which allows very fast line-by-line, hyperspectral and broadband simulations for satellite-based, airborne and ground-based sensors. The principal components are derived during a code training phase from line-by-line simulations for a diverse set of atmosphere and surface conditions. The derived principal components are sensor independent, i.e. no extra training is required to include additional sensors. During the training phase we also derive the predictors which are required by the fast radiative transfer code to determine the principal component scores from the monochromatic radiances (or fluxes, transmittances). These predictors are calculated for each training profile at a small number of frequencies, which are selected by a k-means cluster algorithm during the training phase. Until recently the predictors were calculated using a linear regression. However, during a recent rewrite of the code the linear regression was replaced by a kernel regression which resulted in a significant increase in accuracy when compared to the linear regression. The HT-FRTC has been trained with a large variety of gases, surface properties and scatteres. Rayleigh scattering as well as scattering by frozen/liquid clouds, hydrometeors and aerosols have all been included. The scattering phase function can be fully accounted for by an integrated line-by-line version of the Edwards-Slingo spherical harmonics radiation code or approximately by a modification to the extinction (Chou scaling). Typically the simulation of a whole clear-sky radiance spectrum (3600000 monochromatic frequencies) takes less than one millisecond.

We have developed a new algorithm for the simultaneous retrieval of the atmospheric profiles (temperature, humidity, ozone and aerosol) and the surface reflectance from hyperspectral radiance measurements obtained from air/space-borne, hyperspectral imagers such as Hyperion EO-1. The new scheme, proposed here, consists of a fast radiative transfer code, based on empirical orthogonal functions (EOFs), in conjunction with a 1D-Var retrieval scheme. The inclusion of an ’exact’ scattering code based on spherical harmonics, allows for an accurate treatment of Rayleigh scattering and scattering by aerosols, water droplets and ice-crystals, thus making it possible to also retrieve cloud and aerosol optical properties, although here we will concentrate on non-cloudy scenes. We successfully tested this new approach using hyperspectral images taken by Hyperion EO-1, an experimental pushbroom imaging spectrometer operated by NASA.

This paper investigates the single-scattering properties of elongated hexagonal ice columns in random orientation. The calculation of these properties becomes more challenging with electromagnetic methods as the aspect ratio and/or the size parameter of the particles increases. The results presented in this paper have been obtained with a new version of a T-matrix implementation which uses only variables of data type real. This re-formulation makes it possible to run the code on a DEC Alpha workstation in quadruple precision. With the increased numerical precision aspect ratios of 10 are obtained at moderate size parameters. The single-scattering properties of elongated hexagonal columns are compared against those of various other particles. Circular cylinders of the same aspect ratio and surface area have quite similar single-scattering properties. They agree well in all phase matrix elements and the extinction efficiency and asymmetry parameter are within 25% and 5%, respectively. The single-scattering properties of elongated hexagonal columns are very different to those with equal surface-area and aspect ratios around unity. The extinction efficiencies of the more compact particles are several times larger than those of the elongated columns and the phase matrix elements are very different. The spheres of equal volume and equal surface area do not mimic the single-scattering properties of elongated hexagonal columns either. The agreement with the smaller sphere of equal volume-to-surface-area ratio is reasonable. It is found that for hexagonal columns with increasing aspect ratios, all phase matrix elements except the phase function converge to the Rayleigh solution. The results of this paper imply that such scattering behaviour, previously demonstrated for elongated spheroids, is more general and does also apply to faceted objects like hexagonal columns.

The dependence of retrieved ice-crystal effective dimension on assumed ice-crystal shape and size-distribution function is investigated at solar wavelengths (0.87, 1.6 and 3.7 µm). The dual viewing Along Track Scanning Radiometer is used to retrieve ice-crystal effective dimension using a method of Optimal Estimation over semi-transparent cirrus located in the tropics and mid-latitudes. The ice-crystal effective dimension is defined in terms of the effective diameter and it is retrieved assuming roughened hexagonal ice aggregates, pristine hexagonal ice columns, and four-branched bullet rosettes. It is shown that if a phase function that well represents the scattering properties of cirrus is applied to each crystal type then the retrieved ice-crystal effective diameter is only weakly dependent on ice-crystal shape and size-distribution function. Absolute differences between retrieved ice-crystal effective diameter assuming hexagonal ice columns and ice aggregates combined with a representative phase function are generally well within ±5 µm. The findings of this paper have the important implication that, in climate models, simulation of cirrus cloudy radiances at solar wavelengths might be made to be independent of assumed ice-crystal shape and size-distribution function. © Crown copyright, 2004.

In this paper a consistent set of single-scattering properties is presented for radiative transfer calculations and remote sensing of cirrus cloud. The single-scattering properties consist of the extinction coefficient, single-scattering albedo and phase function. A randomly oriented randomized hexagonal ice aggregate is assumed to derive the extinction coefficient and single-scattering albedo. The phase function is an extension of the Henyey–Greenstein model called the “analytic” phase function, which is generated from the asymmetry parameter at non-absorbing and absorbing wavelengths. The satellite-based dual-view along track scanning radiometer (ATSR-2) instrument is utilized to test the single-scattering properties for consistency at scattering angles between about 60° and 170°, using a method of Optimal Estimation. Optimal Estimation is applied to a set of cloud parameters and radiance measurements, which are made simultaneously at the wavelengths of 0.87, 1.6, 3.7, 11.0 and 12.0 μm , over cases of cirrus cloud located in the tropics and mid-latitudes. If the single-scattering properties and assumed model parameters were a perfect representation of the radiative properties of cirrus then the measurement residuals (i.e., differences between measurements and simulated measurements) would be identically equal to zero at each of the wavelengths for all scattering angles. It is found that the randomized ice aggregate combined with the analytic phase function minimizes the measurement residuals to generally well within ±1% (reflectance) and ±1 K (brightness temperature) at 0.87, 11.0 and 12.0 μm and to within ±3% and ±3 K at 1.6 and 3.7 μm , respectively. This compares to measurement residuals of about 8% and 10 K if the single-scattering properties are based on the randomly oriented hexagonal ice column. It is recommended that single-scattering properties based on the randomized ice aggregate combined with the analytic phase function (or a very similar phase function) should be applied to remote sensing and radiative transfer studies of cirrus cloud.

This paper describes a series of improvements to an already existing T -matrix implementation for calculating the single-scattering properties of non-axisymmetric particles at large size parameters. The improvements aim to use the computational resources in terms of memory and time as efficiently as possible by taking into account various simplifications of the T -matrix formulation. In this paper, the randomly oriented hexagonal ice crystal is considered and the orientational averaging is done analytically. Within the framework of the T -matrix approach, symmetries of the hexagonal cylinders allow the formulation of eight independent subproblems, for each of which the memory demand is reduced by a factor of 144 compared to the general formulation. The memory demand strongly increases as a function of size parameter. For non-axisymmetric particles, memory demand rises approximately as the fourth power of the size parameter. In order to have more memory available, the T -matrix code was parallelized and implemented on a multi-processor CRAY T3E supercomputer. Extended precision is software emulated. With these new developments the single-scattering properties of randomly oriented hexagonal columns can be calculated for size parameters up to about 40. This size parameter reaches into the region of improved geometric optics (IGO), which is applicable down to size parameters of around 20. Comparisons of IGO and T -matrix results for the single-scattering albedo and the extinction efficiency show that at a size parameter of around 30, the single-scattering albedos are generally within about 2%. IGO underpredicts the extinction efficiency by 4% at λ=3.775 μm and by 11% at the more absorbing wavelength of λ=4.9 μm . This underprediction may be due to the version of IGO used since this does not take into account the inhomogeneity of the refracted wave in an absorbing medium.

A new treatment predicting the extinction and absorption properties of ice particles is evaluated in this study using laboratory measurements of the extinction efficiency, Q ext . In this treatment, the degree of ‘photon tunneling’ for ice crystals is unspecified, and laboratory measurements of Q ext were used in conjunction with this scheme to quantify the significance of this process by determining a tunneling factor, denoted t f . The term tunneling here refers to the interaction of a particle with radiation outside its area cross-section. A t f of 1.0 corresponds to tunneling exhibited by ice spheres as predicted by Mie theory, while a t f of 0 indicates no tunneling. The laboratory work entailed Fourier transform infrared spectroscopy (FTIR) for optical depth measurements in an ice cloud grown in a chamber, over a wavelength range of 2– 18 μm . From these measurements, the extinction efficiency Q ext as a function of wavelength was determined. Ice particle size spectra were measured in the cloud chamber, and were used to predict Q ext using the radiation scheme noted above and also using a new implementation of T-matrix, which is based on the exact geometry of a ‘pristine’ hexagonal ice crystal, without approximating the crystal as a spheroid. Results show that t f values determined from the laboratory measurements and the new radiation scheme are qualitatively in agreement with t f values based on fundamental theory. Mean Q ext errors (relative to measured Q ext ) over all wavelengths sampled were ⩽3.0% when using a constant optimized t f in the radiation scheme, and ⩽2.3% when using a t f scheme based on complex angular momentum theory. Moreover, Q ext as predicted from T-matrix over the wavelength interval 8– 12 μm is also in excellent agreement with the measured Q ext . A single wavelength calculation at 14 μm was performed using the finite difference time domain (FDTD) and T-matrix methods, both of which agreed precisely with the measured Q ext value. This validates the integrity of T-matrix, FDTD, the new radiation scheme, and the laboratory measurements for the corresponding range of wavelengths and size parameters. Collectively, these results indicate the tunneling contributions predicted for solid hexagonal columns are realistic.

Absorption and extinction properties of the finite hexagonal ice column and hexagonal ice plate in random and preferred orientation are studied at the wavelength of 80 μm using a new implementation of exact T-matrix theory. For the case of random orientation at size parameters around two, it is shown that the hexagonal ice column and hexagonal ice plate absorption resonances are diminished relative to Mie theory, and the same behaviour is also noted for an aggregate particle consisting of eight hexagonal elements. The absorption properties of the aggregate particle have been calculated using the finite-difference time-domain method. It is also shown that extinction and absorption solutions for the hexagonal ice column and hexagonal ice plate can differ significantly if incidence occurs perpendicular or parallel to the cylindrical axis of the hexagon. For the case of perpendicular incidence on the edge of the hexagon, absorption solutions can exceed those of Mie theory, and for the case of parallel incidence, behaviour of the extinction solutions for hexagonal ice columns and hexagonal ice plates is shown to be similar to previously published work based on the prolate and oblate spheroid. Interference structure, associated with surface waves, is resolved on the hexagonal column extinction solution and the hexagonal plate absorption solution, thereby demonstrating that surface waves can exist on a non-axisymmetric geometry. The usefulness of assuming the hexagonal ice column in retrieval of ice crystal effective size is also investigated using aircraft based radiometric observations of semi-transparent cirrus at the wavelengths of 8.5 and 11 μm .

The sensitivity of microwave radiative transfer through the atmosphere with respect to three different raindrop geometries (spheres, oblate spheroids and oblate raindrop shapes realistically approximated by a series of Chebyshev polynomials) is investigated with a one-dimensional numerical model. All rotational symmetric particles are aligned with their axis of rotation along the vertical. Single-scattering calculations are performed with an extended boundary condition T-matrix code provided by Mishchenko and the Discretized Mie Formalism. In contrast to the former code the latter allows to model the scattering by the Chebyshev-shaped raindrops. The change of brightness temperature when using one of the oblate particle shapes instead of the widely used spherical assumption reaches up to −5 K for 85 GHz at upward directions. The calculated microwave signatures are most sensitive to the shape of liquid water drops at frequencies around 20 GHz, while the results at 37 and 85 GHz show larger impact by ice particles. Polarization differences exhibit large changes when using oblate particle shapes instead of spheres, leading to changes of +6 K at upward and −15 K at downward directions. The slight variation of geometry when using Chebyshev shapes gives additional changes of −2 and +4 K at the same directions of emerging radiation. Calculations with an ice particle layer above the rain layer prove that the scattering signature of the underlying rain layer will not be screened by the ice particle scattering. This result shows the importance of correct treatment of raindrop shape even in the presence of large ice particles.

Asymptotic approximations to the Mie absorption and extinction efficiency factors have previously been derived from complex-angular-momentum theory. In this paper adjustments are made to the complex-angular-momentum absorption efficiency, based on the equivalent volume-to-area sphere, so that the asymptotic approximation can be applied to other particle shapes. Results are presented for randomly oriented finite-hexagonal ice columns in the infrared, comparing adjusted complex-angular-momentum theory to absorption solutions obtained from ray-tracing and Discretised-Mie-Formalism. The adjusted complex-angular-momentum absorption efficiencies and single scattering albedos are within 3% of the ray-tracing results. In the resonance region the adjusted complex-angular-momentum absorption efficiency is within 6% of the Discretised-Mie-Formalism results. The advantages of adjusted complex-angular-momentum theory are its speed compared to Discretised-Mie-Formalism and the ability to directly compare terms with geometric optics. The study of adjusted complex-angular-momentum suggests that phenomena often referred to as “tunnelling” are dependent on morphology as well as refractive index and size parameter. The dependence on morphology can be used to estimate the absorption efficiency at other infrared wavelengths. The analysis is also used to show the limits over which the anomalous diffraction approximation, often used in remote sensing retrievals of ice cloud properties, is a valid assumption.

This paper demonstrates that an ensemble model of cirrus ice crystals that follows observed mass-dimensional power laws can predict the scattering properties of cirrus across the electromagnetic spectrum, without the need for tailor made scattering models for particular regions of the spectrum. The ensemble model predicts a mass-dimensional power law of the following form, mass ∝ D2 (where D is the maximum dimension of the ice crystal). This same mass-dimensional power law is applied across the spectrum to predict the particle size distribution (PSD) using a moment estimation parameterization of the PSD. The PSD parameterization predicts the original PSD, using in-situ estimates (bulk measurements) of the ice water content (IWC) and measurements of the in-cloud temperature; the measurements were obtained from a number of mid-latitude cirrus cases, which occurred over the U.K. during the winter and spring of 2010. It is demonstrated that the ensemble model predicts lidar backscatter estimates, at 0.355 μm,...

Here, we present a new prototype algorithm for the simultaneous retrieval of the atmospheric profiles (temperature, humidity, ozone and aerosol) and the surface reflectance from hyperspectral radiance measurements obtained from air/space borne, hyperspectral imagers. The new scheme, proposed here, consists of a fast radiative transfer code, based on empirical orthogonal functions (EOFs), in conjunction with a 1D-Var retrieval scheme. The inclusion of an 'exact' scattering code based on spherical harmonics, allows for an accurate treatment of Rayleigh scattering and scattering by aerosols, water droplets and ice-crystals, thus making it possible to also retrieve cloud and aerosol optical properties, although here we will concentrate on non-cloudy scenes.

Here, we present a new prototype algorithm for the simultaneous retrieval of the atmospheric profiles (temperature, humidity, ozone and aerosol) and the surface reflectance from hyperspectral radiance measurements obtained from air/space-borne, hyperspectral imagers such as the 'Airborne Visible/Infrared Imager (AVIRIS) or Hyperion on board of the Earth Observatory 1. The new scheme, proposed here, consists of a fast radiative transfer code, based on empirical orthogonal functions (EOFs), in conjunction with a 1D-Var retrieval scheme. The inclusion of an 'exact' scattering code based on spherical harmonics, allows for an accurate treatment of Rayleigh scattering and scattering by aerosols, water droplets and ice-crystals, thus making it possible to also retrieve cloud and aerosol optical properties, although here we will concentrate on non-cloudy scenes. We successfully tested this new approach using two hyperspectral images taken by AVIRIS, a whiskbroom imaging spectrometer operated by the NASA Jet Propulsion Laboratory.

[1] An extensive set of airborne and satellite observations of volcanic ash from the Eyjafjallajokull Icelandic eruption are analyzed for a case study on 17 May 2010. Data collected from particle scattering probes and backscatter lidar on the Facility for Airborne Atmospheric Measurements (FAAM) BAe 146 aircraft allow estimates of ash concentration to be derived. Using radiative transfer simulations we show that airborne and satellite infrared radiances can be accurately modeled based on the in situ measured size distribution and a mineral dust refractive index. Furthermore, airborne irradiance measurements in the 0.3–1.7 μm range are well modeled with these properties. Retrievals of ash mass column loading using Infrared Atmospheric Sounding Interferometer (IASI) observations are shown to be in accord with lidar-derived mass estimates, giving for the first time an independent verification of a hyperspectral ash variational retrieval method. The agreement of the observed and modeled solar and terrestrial irradiances suggests a reasonable degree of radiative closure implying that the physical and optical properties of volcanic ash can be relatively well constrained using data from state-of-the-science airborne platforms such as the FAAM BAe 146 aircraft. Comparisons with IASI measurements during recent Grimsvotn and Puyehue volcanic eruptions demonstrate the importance of accurately specifying the refractive index when modeling the observed spectra.

We demonstrate the feasibility of retrieving the reflectance spectra from hyperspectral imagery at speeds comparable to AC schemes by using a fast scattering radiative transfer code in conjunction with a 1D-Var scheme.

Here we demonstrate the feasibility of applying a fast radiative transfer code, based on empirical orthogonal functions, in conjunction with a 1D‐Var physical retrieval system to hyperspectral data taken from space/airborne radiance measurements in order to retrieve the emissivity spectra of the underlying surface. This approach was successfully tested using hyperspectral data obtained during the JAIVEX campaign in 2007 with the Airborne Research Interferometer Evaluation System (ARIES) on board of the UK Atmospheric Research aircraft. Using ARIES has the advantage that, during low‐level flights, the surface emissivities can be derived directly from the hyperspectral data. Thus ARIES is capable of providing both the hyperspectral radiance data and the ’true’ surface emissivity which has obvious advantages for validating the retrieval of the surface emissivity spectra.

We calculated the scattering and absorption properties of randomly oriented hexagonal ice columns using T-matrix theory, employing analytic orientation averaging, and the finite-difference time-domain method, which uses a numerical procedure to simulate random orientation. The total optical properties calculated are the extinction efficiency, absorption efficiency, single-scattering albedo, and the asymmetry parameter. The optical properties are calculated at the wavelengths of 0.66, 8.5, and 12 mum, up to a size parameter of 20 at 0.66 mum and 15 at the two other wavelengths. The phase-matrix elements P11, P12, and P22 are also calculated and compared, up to a size parameter of 20 at 0.66 mum and 15 at 12.0 mum. The scattering and absorption solutions obtained from the two independent electromagnetic methods are compared and contrasted, as well as the central processing unit time and memory load for each size parameter. It is found that the total optical properties calculated by the two methods are well within 3% of each other for all three wavelengths and size parameters. In terms of the phase-matrix elements it is found that there are some differences between the T-matrix and the finite-difference time-domain methods appearing in all three elements. Differences between the two methods for the P11 element are seen particularly at scattering angles from approximately 120 degrees to 180 degrees ; and at the scattering angle of 180 degrees , relative differences are less than 16%. At scattering angles less than 100 degrees , agreement is generally within a few percent. Similar results are also found for the P12 and P22 elements of the phase matrix. The validity of approximating randomly oriented hexagonal ice columns by randomly oriented equal surface area circular cylinders is also investigated in terms of the linear depolarization ratio.

The applicability of various approximations for computing the absorption efficiency and single-scattering albedo of a randomly oriented hexagonal column is tested versus electromagnetic theory. To calculate the absorption efficiency and single-scattering albedo of the hexagonal column from electromagnetic theory we used a generalization to the separation-of-variables method, which enables continuous calculation of optical properties up to size parameters of 86. We found that the asymptotic absorption efficiency is independent of particle shape, and that, as the size parameter increases, the hexagonal column tends to its asymptotic absorption value more quickly than Mie theory. The asymptotic absorption limit of the hexagonal column is calculated accurately (to within 1%) and rapidly by use of the complex-angular-momentum approximation, indicating that this approximation could be used to calculate the absorption limit of nonspherical particles. The equal-volume sphere best approximates the hexagonal column single-scattering albedo at a strongly absorbing wavelength (e.g., 11.9 microm for an ice particle). However, in the resonance region (e.g., 80 microm for an ice particle) Mie theory fails to approximate the single-scattering albedo of the hexagonal column, but as the size parameter exceeds 10 the error in the sphere approximation reduces to within 2%. At 80-microm wavelength there is a characteristic ripple structure superimposed on the hexagonal column absorption efficiency solutions between size parameters from approximately 1 to 4. The ripple structure is indicative of surface-wave interference and is similar to the sphere but less pronounced on the hexagonal column. We investigated the applicability of ray tracing for calculating the single-scattering albedo at absorbing wavelengths relevant to remote sensing of ice particles in the atmosphere and found it to be within 4% for size parameters between 3 and 42 at 3.7-microm wavelength. At mid-infrared wavelengths (e.g., 8.5 and 11.9 microm) ray tracing is within 5% of electromagnetic theory for size parameters exceeding 10. We also tested the Bryant and Latimer absorption approximation to anomalous diffraction theory by using the separation-of-variables method.

Light scattering on finite dielectric cylinders having noncircular cross sections has become of growing importance in remote-sensing applications. For analyzing their scattering characteristics at moderate size parameters, i.e., at a region where ray-tracing techniques fail, a few methods have been developed, among which an approximation based on the generalized separation-of-variables method has become very successful. This approach reveals two interesting features, which we discuss, that reduce the numerical effort drastically if applied to hexagonal (in general, 2n-periodic) boundary surfaces. Finally, some results for the phase function of hexagonal ice columns are given.

International audience; In this paper we present microphysical properties of natural clouds retrieved from an airborne "Polar Nephelometer" measurements. The retrieval was done by means of an iterative inversion method, which is based on the bi-component (water droplets and ice crystals) representation of ice and mixed phase cloud composition. The present study shows that experimental scattering phase functions of ice crystals are characterized by high information content with respect to the aspect ratio of ice crystals which can be estimated in addition to their effective size distributions.

For the present investigation a new method for the calculation of the single scattering and absorption properties of hexagonal ice crystals called the Discretized Mie Formalism (DMF) is used together with a radiative transfer model in order to determine the influence of particle size and shape on the brightness temperature difference (BTD) between the two NOAA AVHRR infrared window channels 4 and 5. Comparing the results to those of optically equivalent spheres it is shown that particle shape has an influence on the observed BTD which is decreasing with increasing particle size.

In this contribution, the procedure of deriving a.n approximate solution for scattering on a finite cylinder from that of an infinite one by application of Huygens' principle has been extended to the case of non-circular cross-sections. To the best of our knowledge, this has been performed for the first time. Within our approximation, we are able to calculate total cross-sections and the albedo consistently, which are important for practical applications. The basic problem of the infinite cylinder we treated with the Discretized Mie Formalism (DMF). We proved that this technique can be successfully applied up to size-parameters of at least 100. Results for circular cylinders by our approximation and by the DMF are presented. Keywords: Hexagonal Ice Crystals, Cirrus, Nonspherical Hydrometeors, Discretized Mie Formalism (DMF), Huygens' principle

We present the results of radiative transfer calculations in the microwave region focussing on the effects of multiple scattering by nonspherical hydrometeors. The use of microwave frequencies, for example 37 GHz, leads for raindrops, typical with 4 mm diameter, to a size parameter of 1.5 and requires an exact scattering solution rather than the Rayleigh approximation. The distortion of raindrop shapes from the spherical geometry then becomes significant and has to be taken into account when the scattering properties are calculated. The model developed uses the full Stokes vector to include the effects of cross-polarization terms between all four Stokes components which are important when the scattering of nonspherical particles is considered. In this case the third and fourth Stokes component do not decouple from the first and second components. The one-dimensional microwave radiative transfer model is based on the successive order of scattering method and assumes azimuthal symmetry. The shape of the hydrometeors is approximated by rotational symmetric ellipsoids with a size dependent aspect ratio. These particles have a fixed orientation with their rotational axis aligned along the vertical. Results for nonspherical and spherical scattering will be presented. The differences between both methods, showing up to 15 Kelvin change in the polarization difference, depending on rain rate, frequency and viewing angle, will be discussed.

The Havemann‐Taylor Fast Radiative Transfer Code (HT‐FRTC) has been developed for the simulation of highly spectrally resolved measurements from satellite based (i.e. Infrared Atmospheric Sounding Interferometer (IASI), Atmospheric Infrared Sounder (AIRS)) and airborne (i.e. Atmospheric Research Interferometer Evaluation System (ARIES)) instruments. The use of principle components enables the calculation of a complete spectrum in less than a second. The principal compoents are derived from a diverse training set of atmospheres and surfaces and contain their spectral characteristics in a highly compressed form. For any given atmosphere/surface, the HT‐FRTC calculates the weightings (also called scores) of a few hundred principal components based on selected monochromatic radiative transfer calculations, which is far cheaper than thousands of channel radiance calculations. By intercomparison with line‐by‐line and other fast models the HT‐FRTC has been shown to be accurate. The HT‐FRTC has been successfully ...