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)

A general database of hydrometeor single scattering properties at microwave and sub-millimetre wavelengths

A main limitation today in simulations and inversions of microwave observations of ice hydrometeors (cloud ice, snow, hail, etc.) is the lack of data describing the interaction between electromagnetic waves and the particles. To improve the situation, the development of a comprehensive dataset of such scattering properties has been started. The database aims at giving a broad coverage in both frequency (1 to 886 GHz) and temperature (190 to 270 K), to support both passive and active current and planned measurements, and to provide data corresponding to the full Stokes vector. This first version of the database is restricted to totally random particle orientation. Data for 34 particle sets, i.e. habits, have been generated. About 17 of the habits can be classified as single crystals, three habits can be seen as heavily rimed particles, and the remaining habits are aggregates of different types, e.g. snow and hail. The particle sizes considered vary between the habits, but maximum diameters of 10 and 20 mm are typical values for the largest single crystal and aggregate particles, respectively, and the number of sizes per habit is at least 30. Particles containing liquid water are also inside the scope of the database, but this phase of water is so far only represented by a liquid sphere habit. The database is built upon the netCDF4 file format. Interfaces to browse, extract and convert data for selected radiative transfer models are provided in MATLAB and Python. The database and associated tools are publicly available from Zenodo (https://doi.org/10.5281/zenodo.1175572, Ekelund et al., 2018b), and https://doi.org/10.5281/zenodo.1175588, Mendrok et al., 2018, respectively). Planned extensions include non-spherical raindrops, melting particles and a second orientation case that can be denoted as azimuthally random

Martian magnetism with orbiting sub-millimeter sensor:simulated retrieval system

A Mars-orbiting sub-millimeter sensor can be used to retrieve the magnetic field at low altitudes over large areas of significant planetary crustal magnetism of the surface of Mars from measurements of circularly polarized radiation emitted by the 368 GHz ground-state molecular oxygen absorption line. We design a full retrieval system for one example orbit to show the expected accuracies on the magnetic field components that one realization of such a Mars satellite mission could achieve. For one set of measurements around a tangent profile, we find that the two horizontal components of the magnetic field can be measured at about 200 nT error with a vertical resolution of around 4 km from 6 up to 70 km in tangent altitude. The error is similar regardless of the true strength of the magnetic field, and it can be reduced by repeated measurements over the same area. The method and some of its potential pitfalls are described and discussed

All-sky information content analysis for novel passive microwave instruments in the range from 23.8 to 874.4 GHz

We perform an all-sky information content analysis for channels in the millimetre and sub-millimetre wavelength with 24 channels in the region from 23.8 to 874.4 GHz. The employed set of channels corresponds to the instruments ISMAR and MARSS, which are available on the British FAAM research aircraft, and it is complemented by two precipitation channels at low frequencies from Deimos. The channels also cover ICI, which will be part of the MetOp-SG mission. We use simulated atmospheres from the ICON model as basis for the study and quantify the information content with the reduction of degrees of freedom (Delta DOF). The required Jacobians are calculated with the radiative transfer model ARTS. Specifically we focus on the dependence of the information content on the atmospheric composition. In general we find a high information content for the frozen hydrometeors, which mainly comes from the higher frequency channels beyond 183.31 GHz (on average 3.10 for cloud ice and 2.57 for snow). Considerable information about the microphysical properties, especially for cloud ice, can be gained. The information content about the liquid hydrometeors comes from the lower frequency channels. It is 1.69 for liquid cloud water and 1.08 for rain using the full set of channels. The Jacobians for a specific cloud hydrometeor strongly depend on the atmospheric composition. Especially for the liquid hydrometeors the Jacobians even change sign in some cases. However, the information content is robust across different atmospheric compositions. For liquid hydrometeors the information content decreases in the presence of any frozen hydrometeor, for the frozen hydrometeors it decreases slightly in the presence of the respective other frozen hydrometeor. Due to the lack of channels below 183 GHz liquid hydrometeors are hardly seen by ICI. However, the overall results with regard to the frozen hydrometeors also hold for the ICI sensor. This points to ICI\u27s great ability to observe ice clouds from space on a global scale with a good spatial coverage in unprecedented detail

ARTS, the Atmospheric Radiative Transfer Simulator - Version 2.2, the planetary toolbox edition

This article describes the latest stable release (version 2.2) of the Atmospheric Radiative Transfer Simulator (ARTS), a public domain software for radiative transfer simulations in the thermal spectral range (microwave to infrared). The main feature of this release is a planetary toolbox that allows simulations for the planets Venus, Mars, and Jupiter, in addition to Earth. This required considerable model adaptations, most notably in the area of gaseous absorption calculations. Other new features are also described, notably radio link budgets (including the effect of Faraday rotation that changes the polarization state) and the treatment of Zeeman splitting for oxygen spectral lines. The latter is relevant, for example, for the various operational microwave satellite temperature sensors of the Advanced Microwave Sounding Unit (AMSU) family

Simulation study for the Stratospheric Inferred Winds (SIW) sub-millimeter limb sounder

Stratospheric Inferred Winds (SIW) is a Swedish mini sub-millimeter limb sounder selected for the 2nd InnoSat platform, with launch planned for around 2022. It is intended to fill the altitude gap between 30 and 70 km in atmospheric wind measurements and also aims at pursuing the limb observations of temperature and key atmospheric constituents between 10 and 90 km when current satellite missions will probably come to an end. Line-of-sight winds are retrieved from the Doppler shift of molecular emission lines introduced by the wind field. Observations will be performed with two antennas pointing toward the limb in perpendicular directions in order to reconstruct the 2-D horizontal wind vector. Each antenna has a vertical field of view (FOV) of 5 km. The chosen spectral band, near 655 GHz, contains a dense group of strong O3 lines suitable for exploiting the small amount of wind information in stratospheric spectra. Using both sidebands of the heterodyne receiver, a large number of chemical species will be measured, including O3 isotopologues, H2O, HDO, HCl, ClO, N2O, HNO3, NO, NO2, HCN, CH3CN and HO2. This paper presents a simulation study that assesses measurement performance. The line-of-sight winds are retrieved between 30 and 90 km with the best sensitivity between 35 and 70 km, where the precision (1σ) is 5-10 mĝ€†sĝ\u271 for a single scan. Similar performance can be obtained during day and night conditions except in the lower mesosphere, where the photo-dissociation of O3 in daytime reduces the sensitivity by 50 % near 70 km. Profiles of O3, H2O and temperature are retrieved with high precision up to 50 km ( < 1 %, < 2 %, 1 K, respectively). Systematic errors due to uncertainties in spectroscopic parameters, in the radiometer sideband ratio and in the radiance calibration process are investigated. A large wind retrieval bias of 10-30 mĝ€†sĝ\u271 between 30 and 40 km could be induced by the air-broadening parameter uncertainties of O3 lines. This highlights the need for good knowledge of these parameters and for studying methods to mitigate the retrieval bias.

Das Modell SARTre für Strahlungstransfersimulationen in sphärischen Atmosphären und seine Anwendung zur Ableitung von Zirruswolkeneigenschaften

Title and Contents 1 Introduction 1 2 Fundamentals of Radiative Transfer 5 2.1 The Radiative Transfer Equation 5 2.2 Radiative Transfer of Solar and Terrestrial Radiation 7 2.3 Radiative Properties of Matter 8 2.4 Composition and Structure of the Earth's Atmosphere 16 3 The Radiative Transfer Model SARTre 23 3.1 Basic Principles 23 3.2 Calculating Properties of Molecular Atmospheric Constituents 30 3.3 Particle Properties 31 3.4 Deriving the Multiple Scattering Contribution from DISORT 32 3.5 Observational Geometry in Spherical Atmospheres 37 3.6 Remarks and Outlook 39 4 Verification - Intercomparison of Terrestrial Radiative Transfer 41 4.1 Models in the Intercomparison 41 4.2 Intercomparison Setup 44 4.3 Case I - Clear-Sky Intercomparison 48 4.4 Case II - Thermal Emission of Clouds 51 4.5 Scattering in SARTre - Internal Tests 54 4.6 Case III - Scattering Intercomparison 57 4.7 Summary 65 5 Verification - Intercomparison of Solar Radiative Transfer 67 5.1 Models in the Intercomparison 68 5.2 Intercomparison Setup of Atmosphere and Geometry 69 5.3 The Intercomparison Cases - Results and Interpretation 71 5.4 Summary 76 6 Validation - Derivation of Cirrus Cloud Properties from MIPAS data 79 6.1 The MIPAS Data Set 79 6.2 Observation and Simulation of High Altitude Ice Clouds 80 6.3 Setup of Atmosphere, Observation Geometry and Sensor 86 6.4 Sensitivity Study 89 6.5 Retrieval Results 104 7 Summary 109 Acronyms, Symbols and Indices 111 Bibliography 119 Acknowledgements 127The aim of the work presented here has been the development of a consistent radiative transfer model, which is able to consider emitted and scattered radiation from solar and terrestrial sources. To our knowledge, SARTre, the [Approximate] Spherical Atmospheric Radiative Transfer model, is the first model, that is capable of limb modeling in ultraviolet, visible, near to far infrared, and microwave spectral region. Within this thesis, the structure and principles of the radiative transfer model SARTre have been described. In particular, the assumption of a locally plane-parallel atmosphere used for the calculation of multiple scattering contributions is addressed. Verification and Validation of the model SARTre are described and their results are discussed. The verification is done by model intercomparison, separately for terrestrial and solar radiative transfer. The verification of terrestrial radiative transfer includes the calculation of limb spectra under clear-sky and cloudy conditions. Results are compared to diverse models. The validity of the local planarity assumption in case of multiple scattering is evaluated. For the verification of solar radiative transfer, limb scattering radiances have been compared in the ultraviolet spectral range, which is characterized by strong Rayleigh scattering and - in part - by strong ozone absorption. As reference, a Monte Carlo model has been used. In general it has been found, that SARTre performs well, when absorption dominates. The pseudo- spherical approximation for the calculation of the multiple scattering term reaches its limit at high tangent altitudes in strongly scattering atmospheres. SARTre has been used to study effects of cirrus clouds on infrared limb emission spectra. Sensitivity concerning a number of cloud parameters, e.g. cloud optical and geometrical thickness, cloud altitude, ice water path, particle size and shape, has been evaluated. Based on the sensitivity study, cirrus cloud properties have been derived from a single MIPAS limb sequence taken over the Arabian peninsula. Spectra modeled by SARTre have been fitted to MIPAS spectra simultaneously for two subsequent limb measurements over three microwindows located in the atmospheric window around 10 μm. Even without a sophisticated retrieval algorithm, it was possible to fit simulation and measurement close to measurement accuracy in the continuum signal as well as for the molecular absorption lines, e.g. of H2O. A direct validation of SARTre retrieved cloud properties has not been possible due to the lack of proper in-situ data. Cirrus cloud properties have not been acquired yet by in-situ measurements in parallel to MIPAS overpasses. But, the plausibility of derived cloud properties has been discussed in relation to MODIS and MERIS data. Furthermore, the good agreement of model results and measurements over a wider spectral range suggests, that the model works correctly and hence, can be taken as validated.In der vorliegenden Arbeit wird das Strahlungstransfermodell SARTre ([Approximate] Spherical Atmospheric Radiative Transfer model) vorgestellt. SARTre wurde entwickelt mit dem Ziel, ein konsistentes Modell zur Simulation von Strahlungstransfer unter Einbeziehung emittierter und gestreuter Strahlung aus terrestrischen und solaren Quellen zur Verfügung zu stellen. Soweit bekannt, ist es das erste Modell, welches fähig ist, Limb-Beobachtungen im Bereich ultravioletter, sichtbarer und infraroter Strahlung sowie im Mikrowellenbereich zu modellieren. Das Modell wird in seiner Struktur beschrieben und die wesentlichen Ansätze zur Lösung des Strahlungstransferproblems in sphärischer Atmosphäre dargelegt. Insbesondere wird auf die Annahme lokaler Planparallelität der Erdatmosphäre eingegangen, welche die Grundlage zur Berechnung des Beitrages aus Mehrfachstreuung darstellt. Verifikation und Validierung des Modells SARTre werden beschrieben und die Ergebnisse diskutiert. Die Verifikation erfolgt getrennt für die Bereiche des terrestrischen und solaren Strahlungstransfers jeweils durch Modellvergleich. Infrarot-Limb-Spektren wurden sowohl unter Bedingungen einer klaren, wolkenfreien Atmosphäre sowie für das Auftreten von Eiswolken im Sichtfeld simuliert und mit Modellierungsergebnissen verschiedener anderer Modelle verglichen. Die Gültigkeit der Annahme einer lokal planparallelen Atmosphäre im Fall von Mehrfachstreuung wurde geprüft. Die Verifikation des Modells hinsichtlich solarem Strahlungstransfer erfolgt durch Vergleich simulierter Limb-Beobachtungen im ultravioletten Spektralbereich, der durch starke Rayleigh-Streuung sowie z.T. starke Ozonabsorption gekennzeichnet ist. Als Referenz wird ein Monte Carlo Modell herangezogen. Es wurde festgestellt, dass die pseudo-sphärische Näherung in der Berechnung des Mehrfachstreubeitrags bei der Modellierung von Limb-Beobachtungen in größeren Tangentenhöhen an ihre Grenzen stößt, sobald Streuung in der Atmosphäre dominiert. Die Validierung von SARTre baut auf einer Untersuchung des Effekts von Zirruswolken auf Limb-Emissions-Spektren auf. Simulierte Spektren werden mit MIPAS-Limbmessungen verglichen. Über den simultanen Abgleich von modellierten und gemessenen Spektren zweier aufeinanderfolgender Messungen unterschiedlicher Tangentenhöhe in drei ausgewählten Spektralintervallen innerhalb des atmosphärischen Fensters bei 10 μm werden Eigenschaften des Zirrus abgeleitet. Es wird demonstriert, dass die von SARTre modellierten Spektren die MIPAS-Messungen gut, d.h. bis in den Bereich der Messgenauigkeit, nachvollziehen können. Dies gilt sowohl für die Kontinuumsbereiche als auch bezüglich der Gasabsorptionslinien. Die Plausibilität der gefundenen Wolkeneigenschaften wird u.a. im Vergleich zu MODIS- und MERIS-Daten diskutiert. Da momentan keine parallel zu MIPAS-Überflügen gemessenen Parameter von Eiswolken existieren, ist die Validierung lediglich indirekt möglich. Die gute Übereinstimmung von Modell und Messung in den über einen breiten Spektralbereich verteilten Fenstern legt jedoch die Korrektheit des Modells nahe, das damit als validiert gelten kann

Estimating cirrus cloud properties from MIPAS data

High resolution mid-infrared limb emission spectra observed by the spaceborne Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) showing evidence of cloud interference are analyzed. Using the new line-by-line multiple scattering [Approximate] Spherical Atmospheric Radiative Transfer code (SARTre), a sensitivity study with respect to cirrus cloud parameters, e.g., optical thickness and particle size distribution, is performed. Cirrus properties are estimated by fitting spectra in three distinct microwindows between 8 and 12 μm. For a cirrus with extremely low ice water path (IWP = 0.1 g/m2) and small effective particle size (D e = 10 μm) simulated spectra are in close agreement with observations in broadband signal and fine structures. We show that a multi-microwindow technique enhances reliability of MIPAS cirrus retrievals compared to single microwindow methods

Hochauflösende Infrarot-Modelle im Virtuellen Strahlungstransfer-Labor

Die Modellierung des Strahlungstransports im Ultraviolett-, Infrarot- und Mikrowellen-Spektralbereich (UV-IR-MW) ist eine wesentiche Komponente der Fernerkundung der Atmosphäre. Die höhere Genauigkeit und Auflösung gegenwärtiger Fernerkundungs-Sensoren wie auch die gestiegenen Rechner-Leistungen haben die Entwicklung komplexer, rechenintensiver Strahlungs-Transport-Modelle im MW-IR (Linie-für-Linie Modelle) und im UV (Vielfach-Streuung) ermöglicht, den Aufwand bzw. die "Bedienbarkeit" der Modelle jedoch nicht unbedingt vereinfacht. In diesem Beitrag werden einige Strahlungstransport-Modelle diskutiert, insbesondere "MIRART", ein hochauflösendes Linie-für-Linie (LiL) Modell fü Infrarot-Strahldichte und Transportmission. Schließlich wird das DLR-Virtual-Lab vorgestellt, welches eine webbasierte graphische Oberfläche für Strahlungstransportrechnungen ermöglicht. Neben einer Schnittstelle zum kompletten MIRART-Strahlungstransfer sowie einigen seiner Module bietet das "Virtuelle-Strahlungstransfer-Labor" auch Zugang zu einigen weiteren UV- und IR-Programmen