Search

Central Europe has a vital and extensive meteorological research community comprising national weather services, universities, and research organizations and institutes. Nearly all of them are involved in the open scientific questions regarding clouds and precipitation processes. The research activities include observations (from in situ ground-based remote sensing radio soundings to satellite-based observations), model development on all scales (from direct numerical simulations to global climate models), and other activities. With Germany as an example our first objective is to show the large amount and the diversity of observations regarding clouds and precipitation. The goal is to give an overview of existing measurements and datasets to show the benefit of combining the different information from a variety of observations. Up to now the access to and the usage of these datasets from different sources was not straightforward, due to the issue of missing data and archiving standards for observational data. This then motivates our second objective, which is to introduce our solution for this issue—the novel Standardized Atmospheric Measurement Data archive (SAMD). SAMD is one of the outcomes of the German research initiative High Definition Clouds and Precipitation for Advancing Climate Prediction [HD(CP)2]. The goal of SAMD is an easy-to-use approach for both data producers and archive users. Therefore the archive provides observational data in the common Climate Forecast (CF) Conventions format and makes it available to the broader public. SAMD offers highly standardized quality-controlled data and metadata for a wide range of instruments, with open access, which makes this novel archive important for the research community.

A consortium of polar scientists combined observational forces in a field campaign of unprecedented complexity to uncover the secrets of clouds and their role in Arctic amplification. Two research aircraft, an icebreaker research vessel, an ice-floe camp including an instrumented tethered balloon, and a permanent ground-based measurement station were employed in this endeavour. Clouds play an important role in Arctic amplification. This term represents the recently observed enhanced warming of the Arctic relative to the global increase of near-surface air temperature. However, there are still important knowledge gaps regarding the interplay between Arctic clouds and aerosol particles, surface properties, as well as turbulent and radiative fluxes that inhibit accurate model simulations of clouds in the Arctic climate system. In an attempt to resolve this so-called Arctic cloud puzzle, two comprehensive and closely coordinated field studies were conducted: the ”Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD)” aircraft campaign, and the ”Physical feedbacks of Arctic boundary layer, Sea ice, Cloud and AerosoL (PASCAL)” ice breaker expedition. Both observational studies were performed in the framework of the German ”ArctiC Amplification:Climate Relevant Atmospheric and SurfaCe Processes, and Feedback Mechanisms (AC)3” project. They took place in the vicinity of Svalbard (Norway) in May and June 2017. ACLOUD and PASCAL explored four pieces of the Arctic cloud puzzle: Cloud properties, aerosol impact on clouds, atmospheric radiation, and turbulent-dynamical processes. The two instrumented Polar 5 and Polar 6 aircraft, the icebreaker research vessel (RV) Polarstern, an ice-floe camp including an instrumented tethered balloon, and the permanent, ground-based measurement station at Ny-Ålesund (Svalbard) were employed to observe Arctic low and mid-level, mixed-phase clouds, and to investigate related atmospheric and surface processes. The Polar 5 aircraft served as a remote sensing observatory examining the clouds from above by downward-looking sensors; the Polar 6 aircraft operated as a flying in-situ measurement laboratory sampling inside and below the clouds. Most of the collocated Polar 5/6 flights were conducted either above RV Polarstern or over the Ny-Ålesund station, both of which monitored the clouds from below using similar but upward-looking remote sensing techniques as the Polar 5 aircraft. Several of the flights were carried out underneath collocated satellite tracks. The paper motivates the scientific objectives of the ACLOUD/PASCAL observations and describes the measured quantities, retrieved parameters, and the applied complementary instrumentation. Furthermore, it discusses selected measurement results, and poses critical research questions to be answered in future papers analyzing the data from the two field campaigns.

Clouds influence the shortwave (SW) and longwave (LW) radiative fluxes, thereby affecting the radiative budget by enhancing or diminishing the heat budget at the surface (SFC), at the top of the atmosphere (TOA), and through the atmosphere. In the Arctic, their complexity enhances due to their intrinsic interactions with several physical processes and feedback mechanisms.With the aim to further investigate the Arctic system, the project (AC)³ (Arctic Amplification: Climate Relevant Atmospheric and SurfaCe Processes and Feedback Mechanisms) established two major field campaigns in summer of 2017. Both performed in situ and remote sensing observations over the ocean with PS106 and in the air with ACLOUD (Macke and Flores, 2018, Wendisch et al., 2019). The observations collected during PS106 are considered to investigate the effects and influence of clouds in the radiation budget for the summer central Arctic.The PS106 expedition took place aboard the German research vessel Polarstern which was equipped with active and passive remote sensing instrumentation (Griesche et al., 2020). The synergistic operation of this instrumentation was used to derive macro and microphysical properties of clouds by applying the Cloudnet algorithm. These retrievals together with vertical profiles of temperature and relative humidity are used as input to the Rapid Radiative Transfer Model for GCM applications (RRTMG). The results of the broadband SW and LW radiative simulations along with hourly satellite products from Clouds and the Earth’s Radiant Energy System (CERES) Synoptic 1-degree Ed.4. are compared to ship-borne observations indicating a better agreement for single-level liquid clouds than for more challenging sky conditions. The results of the comparison bring sufficient information to discuss a radiative closure assessment for selected case studies and for the entire PS106 expedition. Based on these results the cloud radiative effect (CRE) is calculated indicating a net effect of -8.1 W/m².The study is extended by applying this methodology to the recent Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC). Preliminary results will be presented for the first leg which will allow a direct comparison of the contrasting properties of cloud radiative effects during summer and winter season.ReferencesGriesche, H. J., and coauthors. (2020): Application of the shipborne remote sensing supersite OCEANETfor profiling of Arctic aerosols and clouds during Polarstern cruise PS106, Atmos. Meas. Tech., 13,5335–5358, https://doi.org/10.5194/amt-13-5335-2020Macke, A. and Flores, H. (2018): The Expeditions PS106/1 and 2 of the Research Vessel POLARSTERNto the Arctic Ocean in 2017 , Berichte zur Polar- und Meeresforschung = Reports on polar and marineresearch, Bremerhaven, Alfred Wegener Institute for Polar and Marine Research, 719 , 171 p.http://hdl.handle.net/10013/epic.4ff2b0cd-1b2f-4444-a97f-0cd9f1d917abWendisch, M., and coauthors. (2019): The Arctic Cloud Puzzle: Using ACLOUD/PASCAL MultiplatformObservations to Unravel the Role of Clouds and Aerosol Particles in Arctic Amplification. Bull. Amer.Meteor. Soc., 100, 841–871, https://doi.org/10.1175/BAMS-D-18-0072.1

Air pollution, which kills an estimated 7 million people every year, is one of the greatest environmental health risks of our times. Finding solutions to this threat poses challenges to practitioners and policymakers alike. Increasing awareness on the benefits of transdisciplinary research in solution-oriented sustainable development projects has led to the establishment of the research project &ldquo, A Transdisciplinary Approach to Mitigate Emissions of Black Carbon&rdquo, (TAME-BC). This paper introduces the TAME-BC research setup that took place with Metro Manila, Philippines, case study. The approach integrates BC measurements with technological, socio-political, and health aspects to improve the scientific state of the art, policymaking, transport sector planning, and clinical studies related to air pollution health effects. The first pillar in the setup presents an (1) air quality assessment through aerosol measurements and instrumentation, complemented by a (2) description and assessment of the current policies, technologies, and practices of the transport sector that is responsible for pollution levels in the Philippines, as well as a (3) BC exposure and associated health impacts assessment. The fourth pillar is intercrossing, fostering (4) knowledge co-creation through stakeholder involvement across scales. We argue that this transdisciplinary approach is useful for research endeavors aiming for emission mitigation in rapidly urbanizing regions beyond Metro Manila.

As part of the High Definition Clouds and Precipitation for advancing Climate Prediction Observational Prototype Experiment (HOPE), a high-density network of 99 silicon photodiode pyranometers was set up around Jülich (10 km × 12 km area) from April to July 2013 to capture the small-scale variability of cloud-induced radiation fields at the surface. In this paper, we provide the details of this unique setup of the pyranometer network, data processing, quality control, and uncertainty assessment under variable conditions. Some exemplary days with clear, broken cloudy, and overcast skies were explored to assess the spatiotemporal observations from the network along with other collocated radiation and sky imager measurements available during the HOPE period.

A method is presented to analyze the cloud life cycle of frontal systems passing over European supersites. It combines information on the vertical profiles of cloud properties derived from ground-based observations with cloud products obtained from satellite-based observations, including their spatial variability. The Euler and Lagrange perspectives are adopted to consider the history of a cloud system that passes the supersites. The forward model known as RTTOV (Radiative Transfer for the Television and Infrared Observation Satellite Operational Vertical Sounder) and the ground-based “CloudNET” products are used to simulate synthetic satellite observations at the supersites, which are subsequently compared with the actual observations of the Meteosat Spinning Enhanced Visible and Infrared Imager (SEVIRI) instrument. Different metrics are considered to quantify and interpret the consistency of the synthetic and the observed satellite data: brightness temperatures at the thermal IR channels, the split-window channels, and trispectral combinations, as well as the outgoing longwave radiation. In this way, the uncertainties of the individual datasets are investigated. This knowledge provides the motivation to combine the disjunct cloud products from satellite with those from ground instruments to characterize the development of the passing cloud frontal systems. In addition, back trajectories started at different stages of the cloud system were used to analyze its history prior to the supersite overpass. The trajectories are used to study, for example, the life time of the cloud frontal system, changes of the cloud phase, and the evolution of cloud physics such as optical thickness, effective particle size, and water path. As a test bed, a case study with a cold front passing Lindenberg, Germany, is presented.

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

The recently increasing development of whole sky imagers enables temporal and spatial high-resolution sky observations. One application already performed in most cases is the estimation of fractional sky cover. A distinction between different cloud types, however, is still in progress. Here, an automatic cloud classification algorithm is presented, based on a set of mainly statistical features describing the color as well as the texture of an image. The k-nearest-neighbour classifier is used due to its high performance in solving complex issues, simplicity of implementation and low computational complexity. Seven different sky conditions are distinguished: high thin clouds (cirrus and cirrostratus), high patched cumuliform clouds (cirrocumulus and altocumulus), stratocumulus clouds, low cumuliform clouds, thick clouds (cumulonimbus and nimbostratus), stratiform clouds and clear sky. Based on the Leave-One-Out Cross-Validation the algorithm achieves an accuracy of about 97%, outperforming previous algorithms with accuracies of at most 62%. An additional test run of random images is presented, still yielding a success rate of about 75%, or up to 88% if only "serious" errors with respect to radiation impact are considered. Reasons for the decrement in accuracy are discussed, and ideas to further improve the classification results, especially in problematic cases, are investigated.

The influence of the external and internal structure of clouds on the incoming solar radiation cannot yet be included in parameterizations used in numerical models. Based on numerical simulations, SCHEWSKI and MACKE (2003) (Schewski-parameterization) have shown that a robust link exists between the domain averaged cloud and the domain averaged solar broadband radiation fluxes, despite the 3d nature of the cloud fields involved. The present work revisits this approach with observed cloud (cloud cover and liquid water path) and radiation (downwelling shortwave radiative flux) properties obtained from the Richard Assmann Observatory (RAO) of the German Weather Service in Lindenberg. Applying the original (model based) cloud-radiation parameterization by SCHEWSKI and MACKE (2001) to observed domain averaged cloud fields yields an overall good correlation between observed and parameterized downwelling solar radiation fluxes. However, the parameterized fluxes strongly underestimate the observations. The Schewski parameterization has been modified by removing the bias and re-adjusting the parameterization coefficients to match the observed cloud and radiation correlation. Furthermore, the empirical parameterization by ZILLMAN (1972) has been implemented for describing the clear conditions. Applying the new parameterization to an independent data set provides significant improvements. However, the accuracy remains in the order of previously used one-or two-parameter empirical cloud-radiation parameterizations. We conclude that cloud cover and liquid water path, i.e. those data that are available from large scale climate models, cannot be regarded as sufficient to describe the cloud radiative effect at the surface.

Cloud cover estimation is an important part of routine meteorological observations. Cloudiness measurements are used in climate model evaluation, nowcasting solar radiation, parameterizing the fluctuations of sea surface insolation, and building energy transfer models of the atmosphere. Currently, the most widespread ground-based method to measure cloudiness is based on analyzing the unpolarized intensity and color distribution of the sky obtained by digital cameras. As a new approach, we propose that cloud detection can be aided by the additional use of skylight polarization measured by 180° field-of-view imaging polarimetry. In the fall of 2010, we tested such a novel polarimetric cloud detector aboard the research vessel Polarstern during expedition ANT-XXVII/1. One of our goals was to test the durability of the measurement hardware under the extreme conditions of a trans-Atlantic cruise. Here, we describe the instrument and compare the results of several different cloud detection algorithms, some conventional and some newly developed. We also discuss the weaknesses of our design and its possible improvements. The comparison with cloud detection algorithms developed for traditional nonpolarimetric full-sky imagers allowed us to evaluate the added value of polarimetric quantities. We found that (1) neural-network-based algorithms perform the best among the investigated schemes and (2) global information (the mean and variance of intensity), nonoptical information (e.g., sun-view geometry), and polarimetric information (e.g., the degree of polarization) improve the accuracy of cloud detection, albeit slightly.

In this modelling study the effects of cloud inhomogeneities on the observations of the passive microwave radiometer SSM/I (Special Sensor Microwave/Imager) are investigated. Brightness temperatures in the radiometer's field-of-view are calculated by using the results of a numerical cloud model as input for a 3-dimensional radiative transfer model. These brightness temperatures are compared with those obtained by the assumption of a plane parallel homogeneous cloud with the equivalent mean liquid water path (LWP). Differences between the two temperatures (the so-called 'beam-filling effect') are related to the mean LWP and to cloud inhomogeneity. It was found that not only the strength of the beam-filling effect but also its dependencies on mean LWP and its inhomogeneity are different for the different microwave frequencies. For 19 and 22 GHz the beam-filling effect increases monotonically with the mean LWP and inhomogeneity, while the beam-filling effect decreases with increased LWP at 37 and 85 GHz at large LWP. In general the beam-filling effect is more pronounced in the horizontal polarisation. Its maximum is found for 37 GHz with differences up to -60 Kelvin compared to the homogeneous case. The effect of the lowered brightness temperatures of inhomogeneous clouds in a LWP algorithm (WENG and GRODY, 1994) is analysed. The largest effect arises from decreased 37 GHz brightness temperature. The mean underestimation of LWP is 0.0179 kg/m 2 and the maximum amounts to 0.7 kg/m 2 .

The aim of this paper is to study the dependence of reflection, absorption, and transmission of light by optically thick, weakly absorbing disperse media on the shape of constituent particles. Single scattering by nonspherical particles is calculated in the framework of the Rayleigh and geometrical optics approximations. The ratio of the average volume to the average surface of particles and their volumetric concentration are kept constant. Thus, the extinction coefficient and the optical thickness of different media under consideration are the same. Calculations are performed for ice particles at wavelengths of 0.645, 1.24, 1.64, and 2.13 μm. The respective refractive indices range from 1.3082–i1.325×10−8 (weak absorption) to 1.2674–i5.65×10−4 (moderate absorption). The radiative characteristics are calculated in the framework of approximate analytical formulae valid for optically thick, weakly absorbing media.

Sensitivities of cirrus cloud radiative forcing as well as solar albedo and infrared emittances to ice crystal size spectrum and ice crystal shape were examined using a coupled cloud-radiation model. The single- and bi-modal crystal size distribution were considered and simulated based on field measurements. Optical parameters of ice crystals shaped as hexagonal columns and random polycrystals (being frequently found in cirrus clouds) were calculated with a ray-tracing method. Both solar and infrared cirrus radiative forcing are influenced by the pattern of crystal size spectra. The net radiative forcing is lower for bi-modal than for single-modal spectra. The solar radiative forcing of cirrus cloud is reduced by nonspherical ice crystals, due to larger albedo effects of nonspherical crystals compared to those of equivalent spherical crystals. Moreover, this reduction in solar radiative forcing by random polycrystals is even larger than that by hexagonal column crystals. The cloud radiative forcing, solar albedo and infrared emittance are changed significantly as the mean crystal size approaches the smaller size end. Furthermore, net cloud radiative forcing is positive in most cirrus cases. Exceptions are cirrus clouds with a large number (>107 m−3) of small (mean maximum dimension

The roles of ice particle size distributions (SDs) and particle shapes in cirrus cloud solar radiative transfer are investigated by analyzing SDs obtained from optical array probe measurements (particle sizes larger than 20–40 μm) during intensive field observations of the International Cirrus Experiment, the European Cloud and Radiation Experiment, the First ISCCP Regional Experiment, and the Central Equatorial Pacific Experiment. It is found that the cloud volume extinction coefficient is more strongly correlated with the total number density than with the effective particle size. Distribution-averaged mean single scattering properties are calculated for hexagonal columns, hexagonal plates, and polycrystals at a nonabsorbing (0.5 μm), moderately absorbing (1.6 μm), and strongly absorbing (3.0 μm) wavelength. At 0.5 μm (1.6 μm) (3.0 μm), the spread in the resulting mean asymmetry parameters due to different SDs is smaller than (comparable to) (smaller than) the difference caused by applying diff...

The single scattering properties of nonspherical raindrops have been calculated by means of the geometric optics approximation to ascertain the usefulness of lidar remote sensing of rainrates. Based on the theoretical hydrodynamical studies of Chuang and Beard (J. Atmos. Sci., 1990, 47, 1374Ð1389), a Chebyshe¤-series of shape coe¦cients has been selected to account for the size dependent particle nonsphericity. The single scattering calculations for randomly oriented raindrops with particle radii ranging from 0.5 to 4.5mm exhibit a very pronounced dependence of the phase matrix on particle shape. However, most of these changes are not monotonic with increasing size, which complicates correlations between rainrates and the radiative properties of the raindrops. A comparison of ray tracing results by Chebyshe¤-type particles and axis-ratio equivalent spheroids shows signiÞcant di¤erences for particles with radii larger than 1 mm. Backscattering intensity as well as linear and circular depolarization ratios for horizontally oriented raindrops show a non-monotonic increase with particle size. The size distribution averaged backscattering properties are poorly correlated with rainrates. We conclude that lidar remote sensing of rainrates does not seem to be a promising attempt. However, this conclusion may be subject to changes if raindrop oscillations, which have not been considered in this study, a¤ect the size distribution averaged backscattering properties.

We present details of a scheme for retrieving cirrus cloud optical thickness and effective particle size from nadir-viewing reflectance measurements. Two near-infrared wavelengths are used, one with negligible ice absorption (1.04 μm) and one with significant ice absorption (1.55 μm). Four ice crystal shapes are used in the scheme: ice spheres, hexagonal columns, hexagonal plates and randomized polycrystals. We highlight the sensitivity of the retrievals to the shape of the phase function, which follows from the particular ice crystal shape assumed. Five aircraft-based retrievals are presented, all cases occurring during the 1993 intensive field campaign of the European Cloud Radiation Experiment (EUCREX'93). The retrieved effective sizes are compared with the in situ measurements taken from the same cases. In these comparisons, account has to be taken of the fact that (a) in situ replicator data indicate that the presence of small (

Simulations of scattering and polarization properties for randomly oriented polyhedral ice crystals are presented based on the geometric optics and the far-field diffraction approximation. Particle shapes range from various hexagonal symmetric particles to highly complex shaped deterministic and random fractals. All calculations are performed at a wavelength of 0.55 µm. Hexagonal symmetric particles show several narrow scattering peaks besides the well known 22° and 46° halos. Column-like ice crystals provide neutral points (NP) at larger scattering angles than plate-like ice crystals. The ranges of NPs for column-like and plate-like crystals are separated at a scattering angle of about 156°, which may allow a polarimetric distinction between these two crystal types. The effects of particle size are studied by applying observationally derived aspect-ratio parameterizations to the individual particle types. Differences in the asymmetry parameter versus size relations for column-like particle types...

A new radiation scheme, suitable for two-stream radiation transfer models, was developed for cirrus clouds. Analytical expressions were derived for the extinction and absorption coefficients and the asymmetry parameter. These are functions of the ice particle size distribution parameters, ice particle shapes, and wavelength. The ice particle shapes considered were hexagonal plates and columns, bullet rosettes, and planar polycrystals. These appear to be the principal crystal types found in cirrus clouds. The formulation of radiative properties accounts for the size distribution projected area and the distance radiation travels through ice particles. For absorption, refraction and internal reflection of radiation were parameterized. By assuming an idealized cirrus cloud, the dependence of the single scatter albedo, reflectance, and emissivity on wavelength, ice particle shape, and size distribution was demonstrated. Reflectance and emissivity exhibited a strong dependence on ice particle shape, wi...

We examine the sensitivity of cirrus cloud albedo and bidirectional reflection function to particle shape using the phase functions of liquid water spheres, regular hexagonal ice crystals, and random-fractal ice particles calculated at a nonabsorbing visible wavelength of 0.63 μm. Accurate multiple-scattering calculations for plane-parallel clouds show that hexagonal ice crystal clouds have systematically larger planetary and global albedos than liquid water clouds of the same optical thickness. There is accumulating evidence that the idealized phase function of regular hexagonal crystals, which causes pronounced halos, is not necessarily the best representation of the range of reflectance characteristics of the majority of ice clouds. A more typical representation of the scattering phase function for ice clouds that are composed of a complex set of crystal shapes and sizes may be obtained using a model of randomly shaped irregular particles. Even larger cloud albedos are obtained for the random-fractal particle model because of its smaller asymmetry parameter. Our computations also show that a larger planetary albedo does not always imply a larger reflectance and that the relative brightness of ice versus liquid water clouds is highly scattering-geometry dependent. Use of the wrong particle shape model (crystal instead of water droplet and vice versa) in retrieving cloud optical thickness from bidirectional reflectance measurements can result in an underestimation or overestimation of the true optical thickness by a factor that can exceed 3. At some scattering geometries, use of the wrong model can give an unrealistically large optical thickness or no solution at all. Overall, bidirectional reflectance differences between random-fractal and regular hexagonal particle shapes are significantly smaller than those between either ice crystal and liquid water spheres, except at the back scattering direction.

In this case study the development of cloud properties (cloud optical depth, effective radius and cloud top height) during the life-cycle of a convective cloud system over Europe was analyzed. To retrieve the properties we developed a retrieval scheme based on the radiative transfer code MOMO and an optimal estimation procedure. Input data are the visible to short-wavelength infrared channels from SEVIRI. In contrast to many other retrieval schemes we used 4 channels simultaneously. Especially the 3,9μm channel provides additional information due to the fact that it measures solar reflectance and thermal emission and allows the inclusion of cloud top height into the retrieval. By using a time series of SEVIRI measurements we want to provide and examine the microphysical development of the cloud over life-time. We monitored the growth of the system and found the most active parts of the convection with the highest water content and optical depth in those regions where the cloud top height is largest, too. The effective radius of the cloud particles is largest in older regions of the cloud system, where the cloud is already decaying.

Radiative fields of three-dimensional inhomogeneous Saharan dust clouds have been calculated at solar wavelength (0.6 μm) by means of a Monte Carlo radiative transfer model. Scattering properties are taken from measurements in the SAMUM campaigns, from light scattering calculations for spheroids based on the MIESCHKA code, from Mie theory for spheres and from the geometric optics method assuming irregular shaped particles. Optical properties of different projected area equivalent shapes are compared. Large differences in optical properties are found especially in the phase functions. Results of radiative transfer calculations based on the Monte Carlo method are shown exemplarily for one dust cloud simulated by the cloud resolving atmospheric circulation model LM-MUSCAT-DES. Shape-induced differences in the radiation fluxes are pronounced, for example, the domain averaged normalized radiance is about 30% lower in the case of a dust plume consisting of spheroids or irregular particles compared to spheres. The effect of net horizontal photon transport (3-D effect) on the reflected radiance fields is only notable at the largest gradients in optical thickness. For example, the reflectance at low sun position differs locally about 15% when horizontal photon transport is accounted for. ‘Sharp edges' due to 1-D calculations are smoothed out in the 3-D case. DOI: 10.1111/j.1600-0889.2011.00560.x

The single-scattering phase functions of polyhedral-shaped ice particles are calculated by means of geometric optics and the diffraction theory. Particle orientation is assumed to be random in space. Particle shapes are taken both from ice-crystal classifications and from in situ measurements. The effects of particle concavity on the scattering signature are discussed in detail. A common feature is the pronounced forward-scattering peak, as well as different halo peaks that are due to a minimum deviation at corresponding ice prisms. An unusual halo phenomena, which results from a minimum deviation in a double-prism configuration, is found and verified. The comparison of different particle types shows that backscattering is a sensitive indicator for the identification of types of ice-crystal. Aggregate particles, like bullet rosettes, basically show the scattering characteristics of their individual components.

We report, for the f irst time to our knowledge, comparisons of light-scattering computations for large, randomly oriented, moderately absorbing spheroids based on the geometric-optics approximation and the exact T-matrix method. We show that in most cases the geometric-optics approximation is (much) more accurate for spheroids than for surface-equivalent spheres and can be used in phase function computations (but not in polarization computations) for nonspherical particles with size parameters as small as 60. Differences in the single-scattering albedo between geometric-optics and T-matrix results are surprisingly small, even for small size parameters.

It has been hypothesized that the frequent lack of halos in observations of cirrus and contrails and laboratory measurements is caused by small ice crystal sizes that put the particles outside the geometrical optics domain of size parameters. We test this hypothesis by exploiting a strong similarity of ray tracing phase functions for finite hexagonal and circular ice cylinders and using T-matrix computations of electromagnetic scattering by circular cylinders with size parameters up to 180 in the visible. We conclude that well-defined halos should be observable for ice crystal size parameters of the order of 100 and larger and discuss remote-sensing implications of this result.

The single and multiple scattering and absorption properties of hexagonal ice columns with different degrees of particle orientation are modeled in the solar spectral range by means of a ray-tracing single-scattering code and a Monte Carlo radiative-transfer code. The scattering properties are most sensitive to particle orientation for the solar zenith angles of 50 degrees (asymmetry parameter) and 90 degrees (single-scattering albedo). Provided that the ice columns are horizontally oriented, the usual assumption of random orientation leads to an overestimation (underestimation) of the reflected (transmitted) solar broadband radiation at high Sun elevation and to an underestimation (overestimation) at medium solar zenith angles. The orientation effect is more (less) pronounced in scattering and transmission (absorption) for smaller ice crystals.

The solar radiative properties of cirrus clouds depend on ice particle shape, size, and orientation, as well as on the spatial cloud structure. Radiation schemes in atmospheric circulation models rely on estimates of cloud optical thickness only. In the present work, a Monte Carlo radiative transfer code is applied to various cirrus cloud scenarios to obtain the radiative response of uncertainties in the above-mentioned microphysical and spatial cloud properties (except orientation). First, plane-parallel homogeneous (0D) clouds with different crystal shapes (hexagonal columns, irregular polycrystals) and 114 different size distributions have been considered. The resulting variabilities in the solar radiative fluxes are in the order of a few percent for the reflected and about 1% for the diffusely transmitted fluxes. Largest variabilities in the order of 10% to 30% are found for the solar broadband absorptance. However, these variabilities are smaller than the flux differences caused by the choice of ice particle geometries.The influence of cloud inhomogeneities on the radiative fluxes has been examined with the help of time series of Raman lidar extinction coefficient profiles as input for the radiative transfer calculations. Significant differences between results for inhomogeneous and plane-parallel clouds were found. These differences are in the same order of magnitude as those arising from using extremely different crystal shapes for the radiative transfer calculations. From this sensitivity study, the ranking of cirrus cloud properties according to their importance in solar broadband radiative transfer is optical thickness, ice crystal shape, ice particle size, and spatial structure.

In order to investigate the accuracy of simplification in modeling the radiative transfer in those solar spectral regions with major impacts on bio-organisms, i.e., the UVA (0.32–0.4 μm), the UVB (0.28–0.32 μm), and the photosynthetically active radiation (PAR, 0.4–0.7 μm), radiative transfer calculations with varying treatments of cloud geometries (plane-parallel homogeneous (PPHOM), independent column approximation (ICA), and three-dimensional (3-D) inhomogeneous) have been performed. The complete sets of atmospheric information for 133 cloud realizations are taken from the three-dimensional nonhydrostatic mesoscale atmospheric model (GESIMA). A Monte Carlo radiative transfer model (GRIMALDI) has been developed that simulates scattering and absorption for arbitrarily three-dimensional distributions of cloud hydrometeors, air molecules, and water vapor. Results are shown for domain-averaged direct and total transmission (and so, implicitly, diffuse transmission) at the ground surface. In the UVA the PPHOM assumption leads to an underestimation in direct (total) downward flux by as much as 43 (28) W m−2, which is about 49% (32%) of the incoming irradiation, whereas results based on the ICA are almost identical to the 3-D case, except for convective clouds where the error in the UVA for direct (total) downward flux reaches 5 (2) W m−2, or 6% (2%) of the incoming solar irradiation.

International audience; Various in situ measurements of the light-scattering diagram in ice clouds were performed with a new nephelometer during several airborne campaigns. These measurements were favorably compared with a theoretical scattering model called Inhomogeneous Hexagonal Monocrystal (IHM) model. This model consists in computing the scattering of light by an ensemble of randomly oriented hexagonal ice crystals containing spherical impurities of soot and air bubbles. It is achieved by using a combination of ray tracing, Mie theory, and Monte Carlo techniques and enables to retrieve the six independent elements of the scattering matrix. This good agreement between nephelometer measurements and IHM model provides an opportunity to use this model in order to analyze ADEOS-POLDER total and polarized reflectance measurements over ice clouds. POLDER uses an original concept to observe ice cloud properties, enabling to measure reflectances and polarized reflectances, for a given scene, under several (up to 14) viewing directions. A first analysis of ice cloud spherical albedoes over the terrestrial globe for November 10, 1996, and April 23, 1997, shows a rather good agreement between measurements and modeling. Moreover, polarized reflectances are also calculated and show a satisfactory agreement with measurements.

The standard geometric optics (GO) technique predicts that the phase function for large nonspherical particles with parallel plane facets (e.g., hexagonal ice crystals) should have an infinitesimally narrow δ-function transmission peak caused by rays twice transmitted (refracted) in exactly the forward scattering direction. However, exact T-matrix computations and physical considerations based on the Kirchhoff approximation suggest that this peak is an artifact of GO completely ignoring physical optics effects and must be convolved with the Fraunhofer pattern, thereby producing a phase function component with an angular profile similar to the standard diffraction component. This convolution can be performed with a simple procedure which supplements the standard ray-tracing code and makes the computation of the phase function and its Legendre expansion both more physically realistic and more accurate.

We show that the use of a matrix inversion scheme based on a special lower triangular-upper triangular factorization rather than on the standard Gaussian elimination significantly improves the numerical stability of T-matrix computations for nonabsorbing and weakly absorbing nonspherical particles. As a result, the maximum convergent size parameter for particles with small or zero absorption can increase by a factor of several and can exceed 100. We describe an improved scheme for evaluating Clebsch-Gordon coefficients with large quantum numbers, which allowed us to extend the analytical orientational averaging method developed by Mishchenko [J. Opt. Soc. Am. A 8, 871 (1991)] to larger size parameters. Comparisons of T-matrix and geometrical optics computations for large, randomly oriented spheroids and finite circular cylinders show that the applicability range of the ray-tracing approximation depends on the imaginary part of the refractive index and is different for different elements of the scattering matrix.

The single-scattering properties of concave fractal polyhedra are investigated, with particle size parameters ranging from the Rayleigh to geometric-optics regimes. Two fractal shape parameters, irregularity and aspect ratio, are used to iteratively construct "generations" of irregular fractal particles. The pseudospectral time-domain (PSTD) method and the improved geometric-optics method (IGOM) are combined to compute the single-scattering properties of fractal particles over the range of size parameters. The effects of fractal generation, irregularity, and aspect ratio on the single-scattering properties of fractals are investigated. The extinction efficiency, absorption efficiency, and asymmetry factor, calculated by the PSTD method for fractal particles, with small-to-moderate size parameters, smoothly bridges the gap between those size parameters and size parameters for which solutions given by the IGOM may be used. Somewhat surprisingly, excellent agreement between values of the phase function of randomly oriented fractal particles calculated by the two numerical methods is found, not only for large particles, but in fact extends as far down in equivalent-projected-area size parameters as 25. The agreement in the case of other nonzero phase matrix elements is not as good at so small a size. Furthermore, the numerical results of ensemble-averaged phase matrix elements of a single fractal realization are compared with dust particle measurements, and good agreement is found by using the fractal particle model to represent data from a study of feldspar aerosols.

The scattering phase functions of arbitrary shaped ice particles are calculated by means of geometrical optics and diffraction theory. Their orientation is assumed to be random in space. Particle shapes are taken both from ice crystal classifications and from in-situ measurements made during the International Cirrus Experiment ICE, 1989. The spectral variability is discussed for hexagonal ice crystal columns in the range from 0.2 µm to 16.0 µm with medium spectral resolution (Δλ ~ 0.5µm), and from 2.5µm to 3µm with high spectral resolution (Δλ ~ 0.02µm). In principal, this variability reflects the different absorption mechanisms within the individual regions. The comparison of different particle types shows that backscattering is a sensitive indicator for ice crystal type identification. Aggregated particles like bullet rosettes show basically the scattering characteristics of their individual components. A qualitative estimate of the scattering behaviour of highly complex structured particles is demonstrated by scattering calculations at two dimensional fractal particles.

Multiple-scattering correction factors for cirrus particle extinction coefficients measured with Raman and high spectral resolution lidars are calculated with a radiative-transfer model. Cirrus particle-ensemble phase functions are computed from single-crystal phase functions derived in a geometrical-optics approximation. Seven crystal types are considered. In cirrus clouds with height-independent particle extinction coefficients the general pattern of the multiple-scattering parameters has a steep onset at cloud base with values of 0.5-0.7 followed by a gradual and monotonic decrease to 0.1-0.2 at cloud top. The larger the scattering particles are, the more gradual is the rate of decrease. Multiple-scattering parameters of complex crystals and of imperfect hexagonal columns and plates can be well approximated by those of projected-area equivalent ice spheres, whereas perfect hexagonal crystals show values as much as 70% higher than those of spheres. The dependencies of the multiple-scattering parameters on cirrus particle spectrum, base height, and geometric depth and on the lidar parameters laser wavelength and receiver field of view, are discussed, and a set of multiple-scattering parameter profiles for the correction of extinction measurements in homogeneous cirrus is provided.

We obtain and analyze simple analytical formulas for asymmetry parameters and absorption cross sections of large, nonspherical particles. The formulas are based on the asymptotic properties of these characteristics at strong and weak absorption of radiation inside particles. The absorption cross section depends on parameter phi, which determines the value of the light-absorption cross section for weakly absorbing particles. It is larger for nonspherical scatterers. The asymmetry parameter depends on two parameters. The first is the asymmetry parameter g(0) of a nonspherical, transparent particle with the same shape as an absorbing one. The second parameter, beta, determines the strength of the influence of light absorption on the value of the asymmetry parameter. Parameter beta is larger for nonspherical particles. One can find these three parameters (phi, g(0), and beta) using a ray-tracing code (RTC) for nonabsorbing and weakly absorbing particles. The RTC can then be used to check the accuracy of the equations at any absorption for hexagonal cylinders and spheroids. It is found that the error of computing the absorption cross section and 1 - g (g is the asymmetry parameter) is less than 20% at the refractive index of particles n = 1.333. Values for asymmetry parameters of large, nonabsorbing, spheroidal particles with different aspect ratios are tabulated for the first time to our knowledge. They do not depend on the size of particles and can serve as an independent check of the accuracy of T-matrix codes for large parameters.

We ascertain the usefulness of simple ice particle geometries for modeling the intensity distribution of light scattering by atmospheric ice particles. To this end, similarities and differences in light scattering by axis-equivalent, regular and distorted hexagonal cylindric, ellipsoidal, and circular cylindric ice particles are reported. All the results pertain to particles with sizes much larger than a wavelength and are based on a geometrical optics approximation. At a nonabsorbing wavelength of 0.55 µm, ellipsoids (circular cylinders) have a much (slightly) larger asymmetry parameter g than regular hexagonal cylinders. However, our computations show that only random distortion of the crystal shape leads to a closer agreement with g values as small as 0.7 as derived from some remote-sensing data analysis. This may suggest that scattering by regular particle shapes is not necessarily representative of real atmospheric ice crystals at nonabsorbing wavelengths. On the other hand, if real ice particles happen to be hexagonal, they may be approximated by circular cylinders at absorbing wavelengths.

Non-spherical particles in the atmosphere absorb and scatter solar radiation. They change the polarization state of solar radiation depending on their shape, size, chemical composition and orientation. To quantify polarization effects, a new three-dimensional (3D) vector radiative transfer model, SPARTA (Solver for Polarized Atmospheric Radiative Transfer Applications) is introduced and validated against benchmark results. SPARTA employs the statistical forward Monte Carlo technique for efficient column-response pixel-based radiance calculations including polarization for 3D inhomogeneous cloudless and cloudy atmospheres. A sensitivity study has been carried out and exemplarily results are presented for two lidar-based mineral dust fields. The scattering and absorption properties of the dust particles have been computed for spheroids and irregular shaped particles. Polarized radiance fields in two-dimensional (2D) and one-dimensional (1D) inhomogeneous Saharan dust fields have been calculated at 532 nm wavelength. The domain-averaged results of the normalized reflected radiance are almost identical for the 1D and 2D modes. In the areas with large spatial gradient in optical thickness with expected significant horizontal photon transport, the radiance fields of the 2D mode differ by about ±12% for the first Stokes component (radiance, I) and ±8% for the second Stokes component (linear polarization, Q) from the fields of the 1D mode.