A New Three–Dimensional Vector Radiative Transfer Model and Applications to Saharan Dust Fields

In this thesis a new three–dimensional (3D) vector radiative transfer model, the Solver for Polarized Atmospheric Radiative Transfer Applications (SPARTA) is introduced, validated against benchmark results, and applied to scientific problems. SPARTA employs the statistical forward Monte Carlo technique for efficient column–response pixel–based radiance calculations comprising polarization for 3D inhomogeneous cloudless and cloudy atmospheres. By means of SPARTA, two scientific issues in the field of radiative transfer are investigated. A sensitivity study has been conducted to illustrate the errors introduced by neglecting the effects of polarization in radiation simulations. Two atmospheric scenarios have been considered: a pure one–dimensional (1D) Rayleigh atmosphere and two–dimensional (2D) realistic inhomogeneous dust fields. In case of a purely molecular atmosphere, these errors strongly depend on molecular anisotropy, isotropic reflection, and more importantly, on single scattering albedo and optical thickness (saturation occurs for τ close to 1). Overall errors in the reflected field range up to about 10.5%. On the other hand, for rather high optical thickness, the bias induced by ignoring polarization for realistic inhomogeneous atmospheres is negligible (less than 1%). In addition, solar radiative transfer simulations for LIDAR–measured fields of optical properties of Saharan dust have been performed in order to quantify the effects induced by neglecting the horizontal photon transport and internal inhomogeneities (3D radiative effects) in radiance simulations including polarization. Results are presented for two exemplary mineral dust fields constructed from LIDAR observations. For each case, three radiative calculations are investigated: a 1D calculation according to the plane–parallel (1D mode); an Independent Pixel Approximation (IPA mode); and the 2D mode. The differences in domain–averaged normalized radiances of reflection and transmission are insignificant between the 1D or IPA and 2D calculation modes. However, local differences were observed since extinction is hinge on horizontal spatial variability. In the areas with large spatial variability in optical thickness, the radiance fields of the 2D mode differ about ±20% for the first and second Stokes elements (I, Q) from the fields of the 1D mode. This work points to a brand–new field: the quantification of the sensitivity of polarization to 3D radiative effects

Three Dimensional Radiative Effects in Passive Millimeter/Sub-Millimeter All-sky Observations

This study was conducted to quantify the errors prompted by neglecting three-dimensional (3D) effects, i.e., beam-filling and horizontal photon transport effects, at millimeter/sub-millimeter wavelengths. This paper gives an overview of the 3D effects that impact ice cloud retrievals of both current and proposed (Ice Cloud Imager) satellite instruments operating at frequencies of approximate to 186.3 and approximate to 668 GHz. The 3D synthetic scenes were generated from two-dimensional (2D) CloudSat (Cloud Satellite) observations over the tropics and mid-latitudes using a stochastic approach. By means of the Atmospheric Radiative Transfer Simulator (ARTS), three radiative transfer simulations were carried out: one 3D, one independent beam approximation (IBA), and a one-dimensional (1D). The comparison between the 3D and IBA simulations revealed a small horizontal photon transport effect, with IBA simulations introducing mostly random errors and a slight overestimation (below 1 K). However, performing 1D radiative transfer simulations results in a significant beam-filling effect that increases primarily with frequency, and secondly, with footprint size. For a sensor footprint size of 15 km, the errors induced by neglecting domain heterogeneities yield root mean square errors of up to approximate to 4 K and approximate to 13 K at 186.3 GHz and 668 GHz, respectively. However, an instrument operating at the same frequencies, but with a much smaller footprint size, i.e., 6 km, is subject to smaller uncertainties, with a root mean square error of approximate to 2 K at 186.3 GHz and approximate to 7.1 K at 668 GHz. When designing future satellite instruments, this effect of footprint size on modeling uncertainties should be considered in the overall error budget. The smallest possible footprint size should be a priority for future sub-millimeter observations in light of these results

SPARTA – Solver for Polarized Atmospheric Radiative Transfer Applications: Introduction and application to Saharan dust fields

AbstractNon-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 532nm 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

The sub-adiabatic model as a concept for evaluating the representation and radiative effects of low-level clouds in a high-resolution atmospheric model

The realistic representation of low-level clouds, including their radiative effects, in atmospheric models remains challenging. A sensitivity study is presented to establish a conceptual approach for the evaluation of low-level clouds and their radiative impact in a highly resolved atmospheric model. Considering simulations for six case days, the analysis supports the notion that the properties of clouds more closely match the assumptions of the sub-adiabatic rather than the vertically homogeneous cloud model, suggesting its use as the basis for evaluation. For the considered cases, 95.7% of the variance in cloud optical thickness is explained by the variance in the liquid water path, while the droplet number concentration and the sub-adiabatic fraction contribute only 3.5% and 0.2% to the total variance, respectively. A mean sub-adiabatic fraction of 0.45 is found, which exhibits strong inter-day variability. Applying a principal component analysis and subsequent varimax rotation to the considered set of nine properties, four dominating modes of variability are identified, which explain 97.7% of the total variance. The first and second components correspond to the cloud base and top height, and to liquid water path, optical thickness, and cloud geometrical extent, respectively, while the cloud droplet number concentration and the sub-adiabatic fraction are the strongest contributors to the third and fourth components. Using idealized offline radiative transfer calculations, it is confirmed that the shortwave and longwave cloud radiative effects exhibit little sensitivity to the vertical structure of clouds. This reconfirms, based on an unprecedented large set of highly resolved vertical cloud profiles, that the cloud optical thickness and the cloud top and bottom heights are the main factors dominating the shortwave and longwave radiative effect of clouds and should be evaluated together with radiative fluxes using observations to attribute model deficiencies in the radiative fluxes to deficiencies in the representation of clouds. Considering the different representations of cloud microphysical processes in atmospheric models, the analysis has been further extended and the deviations between the radiative impact of the single- and double-moment schemes are assessed. Contrasting the shortwave cloud radiative effect obtained from the double-moment scheme to that of a single-moment scheme, differences of about similar to 40 Wm(-2) and significant scatter are observed. The differences are attributable to a higher cloud albedo resulting from the high values of droplet number concentration in particular in the boundary layer predicted by the double-moment scheme, which reach median values of around similar to 600 cm(-3)

Introducing hydrometeor orientation into all-sky microwave and submillimeter assimilation

Numerical weather prediction systems still employ many simplifications when assimilating microwave radiances under all-sky conditions (clear sky, cloudy, and precipitation). For example, the orientation of ice hydrometeors is ignored, along with the polarization that this causes. We present a simple approach for approximating hydrometeor orientation, requiring minor adaption of software and no additional calculation burden. The approach is introduced in the RTTOV (Radiative Transfer for TOVS) forward operator and tested in the Integrated Forecast System (IFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF). For the first time within a data assimilation (DA) context, this represents the ice-induced brightness temperature differences between vertical (V) and horizontal (H) polarization-the polarization difference (PD). The discrepancies in PD between observations and simulations decrease by an order of magnitude at 166.5 GHz, with maximum reductions of 10-15 K. The error distributions, which were previously highly skewed and therefore problematic for DA, are now roughly symmetrical. The approach is based on rescaling the extinction in V and H channels, which is quantified by the polarization ratio. Using dual-polarization observations from the Global Precipitation Mission microwave imager (GMI), suitable values for were found to be 1.5 and 1.4 at 89.0 and 166.5 GHz, respectively. The scheme was used for all the conical scanners assimilated at ECMWF, with a broadly neutral impact on the forecast but with an increased physical consistency between instruments that employ different polarizations. This opens the way towards representing hydrometeor orientation for cross-track sounders and at frequencies above 183.0 GHz where the polarization can be even stronger

Implementation of polarization into a 3D Monte Carlo Radiative Transfer Model

Non-spherical particles scatter and polarize solar radiation depending on their shape, size, chemical composition and orientation. In addition, such information is crucial in radiative transfer modeling. Therefore, in this study, the implementation of polarization into a three-dimensional radiative transfer model is introduced and its validation through benchmark results. The model is based on the statistical Monte Carlo method (in the forward scheme) and takes into account multiple scattering and the polarization states of the monochromatic radiation. It calculates column-response pixel-based polarized radiative densities for 3D inhomogeneous cloudy atmospheres and is hence best suited for use in remote sensing applications. To this end, the model can be used to explore the potential of remote sensing techniques which distinguish between spherical and non-spherical particles on the one side and coarse mode dust particles and ice particles on the other side

On the accuracy of RTTOV-SCATT for radiative transfer at all-sky microwave and submillimeter frequencies

With the new generation of microwave instruments and, especially, the Ice Cloud Imager covering submillimeter frequencies, it is necessary to evaluate the performance of the operational Radiative Transfer model for TOVS (RTTOV). Thus, an intercomparison study has been conducted between RTTOV and the reference model ARTS (Atmospheric Radiative Transfer Simulator), with an emphasis on cloudy and precipitating conditions, covering frequencies between ≈53.6 and ≈664.0 GHz. Overall a rather good agreement is found between the δ-Eddington solution embedded in the scattering solver of RTTOV, RTTOV-SCATT, and the discrete ordinate solution embedded in ARTS. Under clear-sky conditions, given a consistent spectroscopy, the agreement is within 0.4 K over all frequencies considered. When idealized, homogeneous cloudy conditions are employed, the agreement is mostly \ub12 K; this range is exceeded only at high scattering conditions. However, the following weaknesses are identified: the δ-Eddington solution fails to produce deep enough brightness temperature depressions at increasingly high scattering conditions and is not sufficient to capture the phase function structures at size parameters above 2–3; conditions typically found at around 664.0 GHz. When realistic hydrometeor profiles are employed, δ-Eddington leads to a root mean squared error of 1 K, with individual errors between 0 and 4 K. Infrequently, and in localized areas, larger discrepancies are identified, exceeding 10 K. However, these inaccuracies stemming from the simplified physics of RTTOV-SCATT were found at least an order of magnitude smaller than the cloud and precipitation representation errors assigned in data assimilation. Thus, we support the use of RTTOV-SCATT at submillimeter frequencies for operational purposes

On the accuracy of RTTOV-SCATT for radiative transfer at all-sky microwave and submillimeter frequencies

With the new generation of microwave instruments and, especially, the Ice Cloud Imager covering submillimeter frequencies, it is necessary to evaluate the performance of the operational Radiative Transfer model for TOVS (RTTOV). Thus, an intercomparison study has been conducted between RTTOV and the reference model ARTS (Atmospheric Radiative Transfer Simulator), with an emphasis on cloudy and precipitating conditions, covering frequencies between ≈53.6 and ≈664.0 GHz. Overall a rather good agreement is found between the δ-Eddington solution embedded in the scattering solver of RTTOV, RTTOV-SCATT, and the discrete ordinate solution embedded in ARTS. Under clear-sky conditions, given a consistent spectroscopy, the agreement is within 0.4 K over all frequencies considered. When idealized, homogeneous cloudy conditions are employed, the agreement is mostly ±2 K; this range is exceeded only at high scattering conditions. However, the following weaknesses are identified: the δ-Eddington solution fails to produce deep enough brightness temperature depressions at increasingly high scattering conditions and is not sufficient to capture the phase function structures at size parameters above 2–3; conditions typically found at around 664.0 GHz. When realistic hydrometeor profiles are employed, δ-Eddington leads to a root mean squared error of 1 K, with individual errors between 0 and 4 K. Infrequently, and in localized areas, larger discrepancies are identified, exceeding 10 K. However, these inaccuracies stemming from the simplified physics of RTTOV-SCATT were found at least an order of magnitude smaller than the cloud and precipitation representation errors assigned in data assimilation. Thus, we support the use of RTTOV-SCATT at submillimeter frequencies for operational purposes.Fil: Barlakas, Vasileios. Chalmers University of Technology; SueciaFil: Galligani, Victoria Sol. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; Argentina. Instituto Franco-Argentino sobre Estudios del Clima y sus Impactos; ArgentinaFil: Geer, Alan J.. European Centre For Medium-range Weather Forecasts; Reino UnidoFil: Eriksson, Patrick. Chalmers University of Technology; Sueci

Fast Radiative Transfer Approximating Ice Hydrometeor Orientation and its Implication on IWP Retrievals

The accurate simulation of microwave observations of clouds and precipitation are com-putationally challenging. A common simplification is the assumption of totally random orientation (TRO); however, studies have revealed that TRO occurs relatively infrequently in reality. A more appropriate assumption is that of azimuthally random orientation (ARO), but so far it has been a com-putationally expensive task. Recently a fast approximate approach was introduced that incorporates hydrometeor orientation into the assimilation of data from microwave conically scanning instruments. The approach scales the extinction in vertical (V) and horizontal (H) polarised channels to approximate ARO. In this study, the application of the approach was extended to a more basic radiative transfer perspective using the Atmospheric Radiative Transfer Simulator and the high-frequency channels of the Global Precipitation Measurement Microwave Imager (GMI). The comparison of forward simulations and GMI observations showed that with a random selection of scaling factors from a uniform distribution between 1 and 1.4–1.5, it is possible to mimic the full distribution of observed polarisation differences at 166 GHz over land and water. The applicability of this model at 660 GHz was also successfully demonstrated by means of existing airborne data. As a complement, a statistical model for polarised snow emissivity between 160 and 190 GHz was also developed. Combining the two models made it possible to reproduce the polarisation signals that were observed over all surface types, including snow and sea ice. Further, we also investigated the impact of orientation on the ice water path (IWP) retrievals. It has been shown that ignoring hydrometeor orientation has a significant negative impact (∼20% in the tropics) on retrieval accuracy. The retrieval with GMI observations produced highly realistic IWP distributions. A significant highlight was the retrieval over snow covered regions, which have been neglected in previous retrieval studies. These results provide increased confidence in the performance of passive microwave observation simulations and mark an essential step towards developing the retrievals of ice hydrometeor properties based on data from GMI, the Ice Cloud Imager (ICI) and other conically scanning instruments

Bulk hydrometeor optical properties for microwave and sub-millimetre radiative transfer in RTTOV-SCATT v13.0

Satellite observations of radiation in the microwave and sub-millimetre spectral regions (broadly from 1 to 1000 GHz) can have strong sensitivity to cloud and precipitation particles in the atmosphere. These particles (known as hydrometeors) scatter, absorb, and emit radiation according to their mass, composition, shape, internal structure, and orientation. Hence, microwave and sub-millimetre observations have applications including weather forecasting, geophysical retrievals and model validation. To simulate these observations requires a scattering-capable radiative transfer model and an estimate of the bulk optical properties of the hydrometeors. This article describes the module used to integrate single-particle optical properties over a particle size distribution (PSD) to provide bulk optical properties for the Radiative Transfer for TOVS microwave and sub-millimetre scattering code, RTTOV-SCATT, a widely used fast model. Bulk optical properties can be derived from a range of particle models including Mie spheres (liquid and frozen) and non-spherical ice habits from the Liu and Atmospheric Radiative Transfer Simulator (ARTS) databases, which include pristine crystals, aggregates, and hail. The effects of different PSD and particle options on simulated brightness temperatures are explored, based on an analytical two-stream solution for a homogeneous cloud slab. The hydrometeor scattering "spectrum" below 1000 GHz is described, along with its sensitivities to particle composition (liquid or ice), size and shape. The optical behaviour of frozen particles changes in the frequencies above 200 GHz, moving towards an optically thick and emission-dominated regime more familiar from the infrared. This region is little explored but will soon be covered by the Ice Cloud Imager (ICI)