Temperature profiles from airborne pyrgeometer measurements of broadband terrestrial radiation

Profiles of broadband terrestrial radiation from airborne pyrgeometer measurements aboard research aircraft Polar 5 obtained during the VERDI campaign in 2012 were used to derive vertical temperature profiles. The retrievals were performed utilizing radiative transfer simulations by libRadtran (Mayer and Kylling, 2005). Manually changing the temperature of the input file for the simulations resulting calculated profiles of terrestrial irradiance were compared with measured profiles and iterated until best agreement. The selected test case shows the possibility of this technique and reveals several possible improvements. The algorithm has to be optimized to adapt the modelling temperature profile automatically using least-square error minimization between measured and modelled irradiance profiles. Additionally the vertical resolution has to be increased to consider small-scale variations. Using humidity and pressure profiles from ground-based observations and nearby radiosoundings significantly improves the retrieved temperature profiles.Vertikalprofile der breitbandigen terrestrischen Strahlung von flugzeuggetragenen Pyrgeometermessungen an Bord des Forschungsflugzeuges Polar 5 während der VERDI Kampagne in 2012 wurden genutzt, um Vertikalprofile der Temperatur abzuleiten. Das Retrieval erfolgte mit Hilfe von Strahlungstransfersimulationen unter der Verwendungung von libRadtran (Mayer and Kylling, 2005). Dazu wurde die Temperatur der Modellatmosphäre variert, bis eine bestmögliche Übereinstimmung von gemessenem und modellierten Irradianzprofil erzielt wurde. Der ausgesuchte Testfall zeigt das Potential dieser Technik und mögliche Verbessungsansätze. So muss der verwendete Algorithmus automatisiert werden, indem das modellierte Strahlungsprofil mit Hilfe eines Iterationsverfahrens und der Methode der kleinsten Fehlerquadrate an das gemessene Profil angepasst wird. Zusätzlich ist eine Erhöhung der vertikalen Auflösung von 50 m auf 10 m Schritte nötig, um kleinskalige Fluktuationen zu berücksichtigen. Die Verwendung von bodengebundenen Beobachtungen und nahegelegenen Radiosondenaufstiegen in der Modellatmosphäre führt zu einer weiteren signifikanten Verbesserung der abgeleiteten Temperaturprofile

Characterization and calibration of a Full Stokes polarization camera

Initially unpolarized solar radiation is polarized in the atmosphere due to scattering processes at molecules and aerosols. Therefore, the measurement of the polarization state of solar radiation is of vital importance in remote sensing. A SALSA Full Stokes polarization camera measuring the complete Stokes vectors in real time is characterized within this work. The main focus lies on the radiometric calibration as well as the determination and validation of the calibration matrix based on a Data Reduction method. One main issue is the temporal instability of the calibration matrix, which gives rise to the need of a thorough calibration process. In accordance with theoretical expectations and model simulations, the SALSA Full Stokes polarization camera provides reliable measurement results under the condition of Rayleigh scattering.Die beim Eintritt in die Atmosphäre unpolarisierte solare Strahlung wird durch Streuprozesse an Molekülen oder Aerosolpartikeln polarisiert. Die Messung des Polarisationszustandes der solaren Strahlung spielt deshalb in der Fernerkundung eine wichtige Rolle. Die vorliegende Arbeit charakterisiert eine SALSA Full Stokes Polarisationskamera, die den kompletten Stokes-Vektor in Echtzeit misst. Das Hauptaugenmerk liegt dabei auf der radiometrischen Kalibrierung sowie der Bestimmung und Validierung der Kalibrationsmatrix über die Methode der Datenreduktion. Die zeitliche Instabilität der Kalibrationsmatrix stellt ein großes Problem dar und stellt Anforderungen an den Umfang der Kalibrierung. Mit der SALSA Full Stokes Polarisationskamera sind zuverlässige Messungen unter einer rayleighstreuenden Atmosphäre möglich, die in Übereinstimmung mit den theoretischen Erwartungen und Modellsimulationen stehen

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

Simulated and observed horizontal inhomogeneities of optical thickness of Arctic stratus

Two-dimensional horizontal fields of cloud optical thickness τ derived from airborne measurements of solar spectral, cloud-reflected radiance are compared with semi-idealized large eddy simulations (LESs) of Arctic stratus performed with the Consortium for Small-scale Modeling (COSMO) atmospheric model. The measurements were collected during the Vertical Distribution of Ice in Arctic Clouds (VERDI) campaign carried out in Inuvik, Canada, in April/May 2012. The input for the LESs is obtained from collocated airborne dropsonde observations of a persistent Arctic stratus above the sea-ice-free Beaufort Sea. Simulations are performed for spatial resolutions of 50m (1.6km × 1.6km domain) and 100m (6.4km × 6.4km domain). Macrophysical cloud properties, such as cloud top altitude and vertical extent, are well captured by the COSMO simulations. However, COSMO produces rather homogeneous clouds compared to the measurements, in particular for the simulations with coarser spatial resolution. For both spatial resolutions, the directional structure of the cloud inhomogeneity is well represented by the model. Differences between the individual cases are mainly associated with the wind shear near cloud top and the vertical structure of the atmospheric boundary layer. A sensitivity study changing the wind velocity in COSMO by a vertically constant scaling factor shows that the directional, small-scale cloud inhomogeneity structures can range from 250 to 800m, depending on the mean wind speed, if the simulated domain is large enough to capture also large-scale structures, which then influence the small-scale structures. For those cases, a threshold wind velocity is identified, which determines when the cloud inhomogeneity stops increasing with increasing wind velocity

Impact of aerosol particles on measured and simulated polarized solar radiation

Solar radiation scattered within the atmosphere by atmospheric particles and aerosol particles has been investigated with regard to their state of polarization. Therefore measurements are performed with the COmpact RAdiation measurement System CORAS to analyze the individual components of the Stokes vector describing the measured radiation. For this purpose new optical inlets including a polarization filter have been developed. In parallel, radiative transfer simulations are conducted to interpret the measurements. For this purpose, two different radiative transfer solvers (SCIATRAN and polRadtran) were used. The simulations have been compared with the measurements to characterize the aerosol optical thickness and the predominant aerosol type

Airborne remote sensing of Arctic boundary-layer mixed-phase clouds

This article gives an overview on the investigations on Artic boundary-layer mixed-phase clouds conducted within the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) in spring 2007. In particular the horizontal and vertical disribution of ice crystals within the clouds was determined by three independent airborne instruments (lidar, in situ and solar radiation measurements). Spectral measurements of cloud top reflectivity have been utilized to retrieve information on the ice phase by analyzing the spectral pattern of the cloud top reflectance in the wavelength range dominated by liquid water and ice absorption (1400-1700 nm). A new algorithm to derive an ice index which distinguishes pure ice, liquid water, and mixed-phase clouds was developed. The horizontal distribution of the ice index, observed during ASTAR 2007, agrees with airborne lidar and in situ measurements showing patches of glaciated clouds at an air mass transition zone within the investigated mixed-phase cloud fields. Information on the vertical distribution of ice crystals in mixed-phase clouds was derived by comparing the measured cloud top reflectivity in the wavelength band 1400-1700 nm to radiative transfer simulations. To interpret the data, the vertical weighting of the measurements was calculated. In the investigated wavelength range the weightings differ according to the spectral absorption of ice and liquid water. From the observed spectral cloud reflectivity with low values in the ice absorption maximum (1400 nm) and higher values at the liquid water absorption maximum (1700 nm) it was concluded that ice crystals were present in the otherwise liquid dominated cloud top layer. Although in situ measurements (limited due to vertical resolution and detection limits) did confirm these findings only in certain limits, the retrieved vertical structure is in agreement with published ground based remote sensing measurements

Case study of a humidity layer above Arctic stratocumulus and potential turbulent coupling with the cloud top

Specific humidity inversions (SHIs) above low-level cloud layers have been frequently observed in the Arctic. The formation of these SHIs is usually associated with large-scale advection of humid air masses. However, the potential coupling of SHIs with cloud layers by turbulent processes is not fully understood. In this study, we analyze a 3 d period of a persistent layer of increased specific humidity above a stratocumulus cloud observed during an Arctic field campaign in June 2017. The tethered balloon system BELUGA (Balloon-bornE moduLar Utility for profilinG the lower Atmosphere) recorded vertical profile data of meteorological, turbulence, and radiation parameters in the atmospheric boundary layer. An in-depth discussion of the problems associated with humidity measurements in cloudy environments leads to the conclusion that the observed SHIs do not result from measurement artifacts. We analyze two different scenarios for the SHI in relation to the cloud top capped by a temperature inversion: (i) the SHI coincides with the cloud top, and (ii) the SHI is vertically separated from the lowered cloud top. In the first case, the SHI and the cloud layer are coupled by turbulence that extends over the cloud top and connects the two layers by turbulent mixing. Several profiles reveal downward virtual sensible and latent heat fluxes at the cloud top, indicating entrainment of humid air supplied by the SHI into the cloud layer. For the second case, a downward moisture transport at the base of the SHI and an upward moisture flux at the cloud top is observed. Therefore, the area between the cloud top and SHI is supplied with moisture from both sides. Finally, large-eddy simulations (LESs) complement the observations by modeling a case of the first scenario. The simulations reproduce the observed downward turbulent fluxes of heat and moisture at the cloud top. The LES realizations suggest that in the presence of a SHI, the cloud layer remains thicker and the temperature inversion height is elevated

Combining atmospheric and snow radiative transfer models to assess the solar radiative effects of black carbon in the Arctic

The magnitude of solar radiative effects (cooling or warming) of black carbon (BC) particles embedded in the Arctic atmosphere and surface snow layer was explored on the basis of case studies. For this purpose, combined at- mospheric and snow radiative transfer simulations were per- formed for cloudless and cloudy conditions on the basis of BC mass concentrations measured in pristine early summer and more polluted early spring conditions. The area of inter- est is the remote sea-ice-covered Arctic Ocean in the vicin- ity of Spitsbergen, northern Greenland, and northern Alaska typically not affected by local pollution. To account for the radiative interactions between the black-carbon-containing snow surface layer and the atmosphere, an atmospheric and snow radiative transfer model were coupled iteratively. For pristine summer conditions (no atmospheric BC, minimum solar zenith angles of 55◦) and a representative BC particle mass concentration of 5 ng g−1 in the surface snow layer, a positive daily mean solar radiative forcing of +0.2Wm−2 was calculated for the surface radiative budget. A higher load of atmospheric BC representing early springtime conditions results in a slightly negative mean radiative forcing at the surface of about −0.05 W m−2, even when the low BC mass concentration measured in the pristine early summer condi- tions was embedded in the surface snow layer. The total net surface radiative forcing combining the effects of BC em- bedded in the atmosphere and in the snow layer strongly de- pends on the snow optical properties (snow specific surface area and snow density). For the conditions over the Arctic Ocean analyzed in the simulations, it was found that the at- mospheric heating rate by water vapor or clouds is 1 to 2 or-ders of magnitude larger than that by atmospheric BC. Sim- ilarly, the daily mean total heating rate (6 K d−1) within a snowpack due to absorption by the ice was more than 1 order of magnitude larger than that of atmospheric BC (0.2 K d−1). Also, it was shown that the cooling by atmospheric BC of the near-surface air and the warming effect by BC embedded in snow are reduced in the presence of clouds

Water vapor retrieval in the upper troposphere and lower stratosphere using airborne measurements of spectral solar irradiance

Airborne measurements of the downward spectral solar irradiance were analyzed with differential optical absorption spectroscopy for the integrated water vapor (IWV) in the atmospheric column above the aircraft. The measurements are obtained from two campaigns in 2016, during which the High Altitude and Long Range Research Aircraft (HALO) took measurements of the downward solar irradiance within the upper troposphere and lower stratosphere (UTLS). The feasibility and limitations of the presented method are discussed for the dry conditions, which are typical for the high altitudes of the UTLS and above. Considering the uncertainties encountered in the irradiance measurements and the high sensitivity of the retrieval, the method was unable to provide sound results for the stratosphere, but provided reasonable results in the troposphere.Flugzeuggetragene Messungen der abwärtsgerichteten spektralen solaren Irradianz wurden mit der Methode der differenziellen optischen Absorptionsspektroskopie auf den integrierten Wasserdampf (IWV) in der Atmosphäre oberhalb des Flugzeuges untersucht. Im Rahmen zweier Messkampagnen im Jahr 2016 führte das High Altitude and Long Range Research Aircraft (HALO) Messungen der spektralen solaren Irradianz in dem Höhenbereich der oberen Troposphäre und unteren Stratosphäre (UTLS) durch. Hier werden das Potential und die Limitierungen eines solchen Verfahrens untersucht, um Wasserdampf in den trockenen Gegebenheiten abzuleiten, die in und oberhalb der UTLS herrschen. Angesichts der Messunsicherheiten und der hohen Sensitivität des Verfahrens, konnten in der Statosphäre nicht aussagekräftige Ergebnisse erreicht werden, aber in der Troposphäre konnte das Verfahren zuverlässige Ergebnisse liefern