Manfred Wendisch - Profile on Academia.edu (original) (raw)

Papers by Manfred Wendisch

Bulletin of the American Meteorological Society, Oct 1, 2004

Andreas Macke, and many more participants. Special thanks go to Wim Hovius for the excellent tech... more Andreas Macke, and many more participants. Special thanks go to Wim Hovius for the excellent technical coordination of all of the measurements.

Spectral radiance measurements collected in nadir and sideward viewing directions by two airborne... more Spectral radiance measurements collected in nadir and sideward viewing directions by two airborne passive solar remote sensing instruments, the Spectral Modular Airborne Radiation measurement sysTem (SMART) and the Differential Optical Absorption Spectrometer (mini-DOAS), are used to compare the remote sensing results of cirrus optical thickness τ . The comparison is based on a sensitivity study using radiative transfer simulations (RTS) and on data obtained during three airborne field campaigns: the North Atlantic Rainfall VALidation (NARVAL) mission, the Mid-Latitude Cirrus Experiment (ML-CIRRUS) and the Aerosol, Cloud, Precipitation, and Radiation Interactions and Dynamics of Convective Cloud Systems (ACRIDICON) campaign. Radiative transfer simulations are used to quantify the sensitivity of measured upward radiance I with respect to τ , ice crystal effective radius r eff , viewing angle of the sensor θ V , spectral surface albedo α, and ice crystal shape. From the calculations it is concluded that sideward viewing measurements are generally better suited than radiance data from the nadir direction to retrieve τ of optically thin cirrus, especially at wavelengths larger than λ = 900 nm. Using sideward instead of nadir-directed spectral radiance measurements significantly improves the sensitivity and accuracy in retrieving τ , in particular for optically thin cirrus of τ ≤ 2. The comparison of retrievals of τ based on nadir and sideward viewing radiance measurements from SMART, mini-DOAS and independent estimates of τ from an additional ac-tive remote sensing instrument, the Water Vapor Lidar Experiment in Space (WALES), shows general agreement within the range of measurement uncertainties. For the selected example a mean τ of 0.54 ± 0.2 is derived from SMART, and 0.49 ± 0.2 by mini-DOAS nadir channels, while WALES obtained a mean value of τ = 0.32 ± 0.02 at 532 nm wavelength, respectively. The mean of τ derived from the sideward viewing mini-DOAS channels is 0.26 ± 0.2. For the few simultaneous measurements, the mini-DOAS sideward channel measurements systematically underestimate (-17.6 %) the nadir observations from SMART and mini-DOAS. The agreement between mini-DOAS sideward viewing channels and WALES is better, showing the advantage of using sideward viewing measurements for cloud remote sensing for τ ≤ 1. Therefore, we suggest sideward viewing measurements for retrievals of τ of thin cirrus because of the significantly enhanced capability of sideward viewing compared to nadir measurements.

Atmospheric Chemistry and Physics, Apr 16, 2009

During the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign, which was... more During the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign, which was conducted in March and April 2007, an optically thin ice cloud was observed south of Svalbard at around 3 km altitude. The microphysical and radiative properties of this particular subvisible midlevel cloud were investigated with complementary remote sensing and in situ instruments. Collocated airborne lidar remote sensing and spectral solar radiation measurements were performed at a flight altitude of 2300 m below the cloud base. Under almost stationary atmospheric conditions, the same subvisible midlevel cloud was probed with various in situ sensors roughly 30 min later. From individual ice crystal samples detected with the Cloud Particle Imager and the ensemble of particles measured with the Polar Nephelometer, microphysical properties were retrieved with a bi-modal inversion algorithm. The best agreement with the measurements was obtained for small ice spheres and deeply rough hexagonal ice crystals. Furthermore, the single-scattering albedo, the scattering phase function as well as the volume extinction coefficient and the effective diameter of the crystal population were determined. A lidar ratio of 21(±6) sr was deduced by three independent methods. These parameters in conjunction with the cloud optical thickness obtained from the lidar measurements were used to compute spectral and broadband radiances and irradiances with a radiative transfer code. The simulated results agreed with the observed spectral downwelling radiance

Atmospheric Measurement Techniques, Mar 1, 2013

The sensitivity of passive remote sensing measurements to retrieve microphysical parameters of co... more The sensitivity of passive remote sensing measurements to retrieve microphysical parameters of convective clouds, in particular their thermodynamic phase, is investigated by three-dimensional (3-D) radiative transfer simulations. The effects of different viewing geometries and vertical distributions of the cloud microphysical properties are investigated. Measurement examples of spectral solar radiance reflected by cloud sides (passive) in the near-infrared (NIR) spectral range are performed together with collocated lidar observations (active). The retrieval method to distinguish the cloud thermodynamic phase (liquid water or ice) exploits different slopes of cloud side reflectivity spectra of water and ice clouds in the NIR. The concurrent depolarization backscattering lidar provides geometry information about the cloud distance and height as well as the depolarization.

Based on airborne spectral imaging observations three-dimensional (3-D) radiative effects between... more Based on airborne spectral imaging observations three-dimensional (3-D) radiative effects between Arctic boundary layer clouds and ice floes have been identified and quantified. A method is presented to discriminate sea ice and open water in case of clouds from imaging radiance measurements. This separation simultaneously reveals

Atmospheric Chemistry and Physics, Mar 30, 2017

Atmospheric Measurement Techniques, Mar 13, 2019

In situ measurements of cloud droplet number concentration N are limited by the sampled cloud vol... more In situ measurements of cloud droplet number concentration N are limited by the sampled cloud volume. Satellite retrievals of N suffer from inherent uncertainties, spatial averaging, and retrieval problems arising from the commonly assumed strictly adiabatic vertical profiles of cloud properties. To improve retrievals of N it is suggested in this paper to use a synergetic combination of passive and active airborne remote sensing measurement, to reduce the uncertainty of N retrievals, and to bridge the gap between in situ cloud sampling and global averaging. For this purpose, spectral solar radiation measurements above shallow trade wind cumulus were combined with passive microwave and active radar and lidar observations carried out during the second Next Generation Remote Sensing for Validation Studies (NARVAL-II) campaign with the High Altitude and Long Range Research Aircraft (HALO) in August 2016. The common technique to retrieve N is refined by including combined measurements and retrievals of cloud optical thickness τ , liquid water path (LWP), cloud droplet effective radius r eff , and cloud base and top altitude. Three approaches are tested and applied to synthetic measurements and two cloud scenarios observed during NARVAL-II. Using the new combined retrieval technique, errors in N due to the adiabatic assumption have been reduced significantly.

Arctic boundary-layer clouds in the vicinity of Svalbard ( • N, 15 • E) were observed with airbor... more Arctic boundary-layer clouds in the vicinity of Svalbard ( • N, 15 • E) were observed with airborne remote sensing and in situ methods. The cloud optical thickness and the droplet effective radius are retrieved from spectral radiance data in nadir and and from hyperspectral radiances in a 40 • field of view. Two approaches are used for the spectral retrieval, combining the signal from either two or five wavelengths. Two wavelengths are found to be sufficient for an accurate retrieval of the cloud optical thickness, while the retrieval of droplet effective radius is more sensitive to the method applied. The comparison to in situ data cannot give a definite answer as to which method is better because of unavoidable time delays between the in situ measurements and the remotesensing observations.

Airborne measurements of solar spectral radiance reflected by cirrus are performed with the HALO-... more Airborne measurements of solar spectral radiance reflected by cirrus are performed with the HALO-SR instrument onboard the High Altitude and Long Range Research Aircraft (HALO) in November 2010. The data are used to quantify the influence of surface albedo heterogeneities on the retrieval of cirrus optical thickness and crystal effective radius. Based on radiative transfer calculations the cirrus properties are derived using a standard bispectral retrieval method. Frequency distributions of the surface albedos derived from MODIS satellite observations are used to create albedo dependend lookup tables of reflected radiance. For each albedo respectively lookup table, a corresponding result for the cirrus optical thickness and effective radius is retrieved. The retrieved cloud properties are analysed in a statistical manner to investigate the influence of surface albedo heterogeneities. The results for the cirrus optical thickness are compared to HSRL-lidar derived values which allows to investigate the role of ice crystal shape in addition. It is found that if assuming aggregate ice crystals the HSRLlidar observations fit best to the retrieved optical thickness using spectral radiance. The uncertainty in cirrus optical thickness due to uncertainties in the surface albedo is below 0.1 and thus below the instrument uncertainty. Therefor, it is concluded that for the retrieval of cirrus optical thickness the surface albedo heterogeneity is negligible. For the retrieval of cirrus effective radius, the surface albedo is of importance introducing uncertainties up to 50 %. However, it was be shown that the influence of the bidirectional reflectance distribution function (BRDF) is below 10 % and thus smaller than the uncertainty caused by the surface albedo.

Spectral radiance measurements by a digital single-lens reflex camera were used to derive the bi-... more Spectral radiance measurements by a digital single-lens reflex camera were used to derive the bi-directional reflectivity of clouds and different surfaces in the Arctic. The camera has been calibrated radiometrically and spectrally to provide accurate radiance measurements with high angular resolution. A comparison with spectral radiance -1 agreed best with the measurements. For the cloud HDRF, the best agreement was obtained by a broad and weak fog bow simulated with a cloud droplet effective radius of R eff = 4 µm. This value agrees with the particle sizes from in situ measurements and retrieved from the spectral radiance of the SMART-Albedometer.

Atmospheric Measurement Techniques, Sep 4, 2017

The passive solar remote sensing of cloud properties over highly reflecting ground is challenging... more The passive solar remote sensing of cloud properties over highly reflecting ground is challenging, mostly due to the low contrast between the cloud reflectivity and that of the underlying surfaces (sea ice and snow). Uncertainties in the retrieved cloud optical thickness τ and cloud droplet effective radius r eff,C may arise from uncertainties in the assumed spectral surface albedo, which is mainly determined by the generally unknown effective snow grain size r eff,S . Therefore, in a first step the effects of the assumed snow grain size are systematically quantified for the conventional bispectral retrieval technique of τ and r eff,C for liquid water clouds. In general, the impact of uncertainties of r eff,S is largest for small snow grain sizes. While the uncertainties of retrieved τ are independent of the cloud optical thickness and solar zenith angle, the bias of retrieved r eff,C increases for optically thin clouds and high Sun. The largest deviations between the retrieved and true original values are found with 83 % for τ and 62 % for r eff,C . In the second part of the paper a retrieval method is presented that simultaneously derives all three parameters (τ , r eff,C , r eff,S ) and therefore accounts for changes in the snow grain size. Ratios of spectral cloud reflectivity measurements at the three wavelengths λ 1 = 1040 nm (sensitive to r eff,S ), λ 2 = 1650 nm (sensitive to τ ), and λ 3 = 2100 nm (sensitive to r eff,C ) are combined in a trispectral retrieval algorithm. In a feasibility study, spectral cloud reflectivity measurements collected by the Spectral Modular Airborne Radiation measurement sysTem (SMART) during the research campaign Vertical Distribution of Ice in Arctic Mixed-Phase Clouds (VERDI, April/May 2012) were used to test the retrieval procedure. Two cases of observations above the Canadian Beau-fort Sea, one with dense snow-covered sea ice and another with a distinct snow-covered sea ice edge are analysed. The retrieved values of τ , r eff,C , and r eff,S show a continuous transition of cloud properties across snow-covered sea ice and open water and are consistent with estimates based on satellite data. It is shown that the uncertainties of the trispectral retrieval increase for high values of τ , and low r eff,S but nevertheless allow the effective snow grain size in cloud-covered areas to be estimated.

Microphysical and radiation measurements were collected with the novel AIRcraft TOwed Sensor Shut... more Microphysical and radiation measurements were collected with the novel AIRcraft TOwed Sensor Shuttle (AIRTOSS) -Learjet tandem platform. The platform is a combination of an instrumented Learjet 35A research aircraft and an aerodynamic bird, which is detached from and retracted back to the aircraft during flight via a steel wire with a length of 4000 m. Both platforms are equipped with radiative, cloud microphysical, trace gas, and meteorological instruments. The purpose of the development of this tandem set-up is to study the inhomogeneity of cirrus as well as other stratiform clouds. Sophisticated numerical flow simulations were conducted in order to optimally integrate an axially asymmetric Cloud Combination Probe (CCP) inside AIRTOSS. The tandem platform was applied during measurements at altitudes up to 36 000 ft (10 970 m) in the framework of the AIRTOSS -Inhomogeneous Cirrus Experiment (AIRTOSS-ICE). Ten flights were performed above the North Sea and Baltic Sea to probe frontal and in situ formed cirrus, as well as anvil outflow cirrus. For one flight, cirrus microphysical and radiative properties displayed significant inhomogeneities resolved by both measurement platforms. The CCP data show that the maximum of the observed particle number size distributions shifts with decreasing altitude from 30 to 300 µm, which is typical for frontal, midlatitude cirrus. Theoretical considerations imply that cloud particle aggregation inside the studied cirrus is very unlikely. Consequently, diffusional growth was identified to be the

Atmospheric Chemistry and Physics, Nov 9, 2020

Clouds play a potentially important role in Arctic climate change but are poorly represented in c... more Clouds play a potentially important role in Arctic climate change but are poorly represented in current atmospheric models across scales. To improve the representation of Arctic clouds in models, it is necessary to compare models to observations to consequently reduce this uncertainty. This study compares aircraft observations from the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) campaign around Svalbard, Norway, in May-June 2017 and simulations using the ICON (ICOsahedral Non-hydrostatic) model in its numerical weather prediction (NWP) setup at 1.2 km horizontal resolution. By comparing measurements of solar and terrestrial irradiances during ACLOUD flights to the respective properties in ICON, we showed that the model systematically overestimates the transmissivity of the mostly liquid clouds during the campaign. This model bias is traced back to the way cloud condensation nuclei (CCN) get activated into cloud droplets in the two-moment bulk microphysical scheme used in this study. This process is parameterized as a function of gridscale vertical velocity in the microphysical scheme used, but in-cloud turbulence cannot be sufficiently resolved at 1.2 km horizontal resolution in Arctic clouds. By parameterizing subgrid-scale vertical motion as a function of turbulent kinetic energy, we are able to achieve a more realistic CCN activation into cloud droplets. Additionally, we showed that by scaling the presently used CCN activation profile, the hydrometeor number concentration could be modified to be in better agreement with ACLOUD observations in our revised CCN activation parameterization. This consequently results in an improved representation of cloud optical properties in our ICON simulations.

Due to their potential to either warm or cool the surface, liquid-phase clouds and their interact... more Due to their potential to either warm or cool the surface, liquid-phase clouds and their interaction with the ice-free and sea-ice-covered ocean largely determine the energy budget and surface temperature in the Arctic. Here, we use airborne measurements of solar spectral cloud reflectivity obtained during the ACLOUD campaign in summer 2017 and the AFLUX campaign in spring 2019 in the vicinity of Svalbard to retrieve microphysical properties of liquid-phase clouds. The retrieval was tailored to provide consistent results over sea-ice and open ocean surfaces. Clouds including ice crystals that significantly bias the retrieval results were filtered from the analysis. A comparison with in-situ measurements shows a good agreement with the retrieved effective radii and an overestimation of the liquid water path and a reduced agreement for boundary-layer clouds with varying fractions of ice water content. Considering these limitations, retrieved microphysical properties of clouds observed over ice-free ocean and sea-ice in spring and early summer in the Arctic are compared. In early summer, the liquidphase clouds have a larger median effective radius (9.5 µm), optical thickness (11.8) and liquid water path (72.3 g m -2 ) compared to spring conditions (8.7 µm, 8.3, 51.8 g m -2 , respectively). The results show larger cloud droplets over the ice-free Arctic Ocean compared to sea-ice in spring and early summer caused mainly by the temperature differences of the surfaces and related convection processes. Due to their larger droplet sizes the liquid clouds over the ice-free ocean have slightly reduced optical thicknesses and lower liquid water contents compared to the sea-ice surface conditions. The comprehensive data set on microphysical properties of Arctic liquid-phase clouds is publicly available and could, e.g., help to constrain models or be used to investigate effects of liquid-phase clouds on the radiation budget.

Spectral airborne upward and downward irradiance measurements are used to derive the area-average... more Spectral airborne upward and downward irradiance measurements are used to derive the area-averaged surface albedo. Real surfaces are not homogeneous in their reflectivity. Therefore, this work studies the effects of the heterogeneity of surface reflectivity on the area-averaged surface albedo to quantify how well aircraft measurements can resolve the small-scale variability of the local surface albedo. For that purpose spatially heterogeneous surface albedo maps were input into a 3-dimensional (3-D) Monte Carlo radiative transfer model to simulate 3-D irradiance fields. The calculated up-and downward irradiances in altitudes between 0.1 km and 5 km are used to derive the area-averaged surface albedo using an iterative retrieval method that removes the effects due to atmospheric scattering and absorption within the layer beneath the considered level. For the case of adjacent land and sea surfaces a parametrization is presented which quantifies the horizontal distance to the coastline that is required to reduce surface heterogeneity effects on the area-averaged surface albedo to a given limit. The parametrization which is a function of altitude, aerosol optical depth, and the ratio of local land and sea albedo was applied for airborne spectral measurements. In addition, the deviation between area-averaged and local surface albedo is determined for more complex surface albedo maps. For moderate aerosol conditions (optical depth less than 0.4) and the visible wavelength range, the altitude and the heterogeneity of the surface albedo are the dominant factors determining the mean deviation between local and area-averaged surface albedo. A parametrization of the mean deviation is applied to an albedo map that was derived from a Landsat image of an area in East Anglia (UK). Parametrization and direct comparison of local and area-averaged surface albedo show similar mean deviations (20 % vs. 25 %) over land.

During the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign, which was... more During the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign, which was conducted in March and April 2007, an optically thin ice cloud was observed at around 3 km altitude south of Svalbard. The microphysical and radiative properties of this particular subvisible midlevel cloud were investigated with complementary remote sensing and in-situ instruments. Collocated airborne lidar remotesensing and spectral solar radiation measurements were performed at a flight altitude of 2300 m below the cloud base. Under almost stationary atmospheric conditions, the same subvisible midlevel cloud was probed with various in-situ sensors roughly 30 min later. From individual ice crystal samples detected with the Cloud Particle Imager and the ensemble of particles measured with the Polar Nephelometer, we retrieved the single-scattering albedo, the scattering phase function as well as the volume extinction coefficient and the effective diameter of the crystal population. Furthermore, a lidar ratio of 21 (±6) sr was deduced by two independent methods. These parameters in conjunction with the cloud optical thickness obtained from the lidar measurements were used to compute spectral and broadband radiances and irradiances with a radiative transfer code. The simulated results agreed with the observed spectral downwelling radiance within the range given by the measurement uncertainty. Furthermore, the broadband radiative simulations estimated a net (solar plus thermal infrared) radiative forcing of the subvisible midlevel ice cloud of -0.4 W m -2 (-3.2 W m -2 in the solar and +2.8 W m -2 in the thermal infrared wavelength range).

Atmospheric Chemistry and Physics, Apr 11, 2012

Spectral radiance measurements by a digital single-lens reflex camera were used to derive the dir... more Spectral radiance measurements by a digital single-lens reflex camera were used to derive the directional reflectivity of clouds and different surfaces in the Arctic. The camera has been calibrated radiometrically and spectrally to provide accurate radiance measurements with high angular resolution. A comparison with spectral radiance measurements with the Spectral Modular Airborne Radiation measurement sysTem (SMART-Albedometer) showed an agreement within the uncertainties of both instruments (6 % for both). The directional reflectivity in terms of the hemispherical directional reflectance factor (HDRF) was obtained for sea ice, ice-free ocean and clouds. The sea ice, with an albedo of ρ = 0.96 (at 530 nm wavelength), showed an almost isotropic HDRF, while sun glint was observed for the ocean HDRF (ρ = 0.12). For the cloud observations with ρ = 0.62, the cloudbow -a backscatter feature typically for scattering by liquid water droplets -was covered by the camera. For measurements above heterogeneous stratocumulus clouds, the required number of images to obtain a mean HDRF that clearly exhibits the cloudbow has been estimated at about 50 images (10 min flight time). A representation of the HDRF as a function of the scattering angle only reduces the image number to about 10 (2 min flight time). The measured cloud and ocean HDRF have been compared to radiative transfer simulations. The ocean HDRF simulated with the observed surface wind speed of 9 m s -1 agreed best with the measurements. For the cloud HDRF, the best agreement was obtained by a broad and weak cloudbow simulated with a cloud droplet effective radius of R eff = 4 µm. This value agrees with the particle sizes derived from in situ measurements and retrieved from the spectral radiance of the SMART-Albedometer.

Atmospheric Chemistry and Physics, Dec 16, 2008

Arctic boundary-layer clouds were investigated with remote sensing and in situ instruments during... more Arctic boundary-layer clouds were investigated with remote sensing and in situ instruments during the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign in March and April 2007. The clouds formed in a cold air outbreak over the open Greenland Sea. Beside the predominant mixed-phase clouds pure liquid water and ice clouds were observed. Utilizing measurements of solar radiation reflected by the clouds three methods to retrieve the thermodynamic phase of the cloud are introduced and compared. Two ice indices I S and I P were obtained by analyzing the spectral pattern of the cloud top reflectance in the near infrared (1500-1800 nm wavelength) spectral range which is characterized by ice and water absorption. While I S analyzes the spectral slope of the reflectance in this wavelength range, I P utilizes a principle component analysis (PCA) of the spectral reflectance. A third ice index I A is based on the different side scattering of spherical liquid water particles and nonspherical ice crystals which was recorded in simultaneous measurements of spectral cloud albedo and reflectance. Radiative transfer simulations show that I S , I P and I A range between 5 to 80, 0 to 8 and 1 to 1.25 respectively with lowest values indicating pure liquid water clouds and highest values pure ice clouds. The spectral slope ice index I S and the PCA ice index I P are found to be strongly sensitive to the effective diameter of the ice crystals present in the cloud. Therefore, the identification of mixed-phase

A melting snow layer on Arctic sea ice, as a composition of ice, liquid water, and air, supplies ... more A melting snow layer on Arctic sea ice, as a composition of ice, liquid water, and air, supplies meltwater that may trigger the formation of melt ponds. As a result, surface reflection properties are altered during the melting season and thereby may change the surface energy budget. To study these processes, sea ice surface reflection properties were derived from airborne measurements using imaging spectrometers. The data were collected over the closed and marginal Arctic sea ice zone north of Svalbard in May/June 2017. A retrieval approach based on different absorption indices of pure ice and liquid water in the near-infrared spectral range was applied to the campaign data. The technique enables to retrieve the spatial distribution of the liquid water fraction of a snow layer and the effective radius of snow grains. For observations from three research flights liquid water fractions between 8.7 % and 15.6 % and snow grain sizes between 115 µm and 378 µm were derived. In addition, the melt pond depth was retrieved based on an existing approach that isolates the dependence of a melt pond reflectance spectrum on the pond depth by eliminating the reflection contribution of the pond ice bottom. The application of the approach to several case studies revealed a high variability of melt pond depth with maximum depths of 0.33 m. The results were discussed considering uncertainties arising from the reflectance measurements, the setup of radiative transfer simulations, and the retrieval method itself. Overall, the presented retrieval methods show the potential and the limitations of airborne measurements with imaging spectrometers to map the transition phase of the Arctic sea ice surface, examining the snow layer composition and melt pond depth. Compared to the globe, the Arctic experiences an enhanced warming, which is referred to as Arctic amplification . The snow-ice-surface-albedo feedback is one of the most important mechanisms driving Arctic amplification Wendisch et al., 2023). The Arctic sea ice albedo depends on wavelength, solar zenith angle, snow grain size, and shape as well as snow layer morphology, impurities, and liquid water fraction. Therefore, the sea ice albedo is strongly altered by melting processes

Bulletin of the American Meteorological Society, Oct 1, 2004

Atmospheric Chemistry and Physics, Apr 16, 2009

Atmospheric Measurement Techniques, Mar 1, 2013

Atmospheric Chemistry and Physics, Mar 30, 2017

Atmospheric Measurement Techniques, Mar 13, 2019

Atmospheric Measurement Techniques, Sep 4, 2017

Atmospheric Chemistry and Physics, Nov 9, 2020

During the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign, which was... more During the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign, which was conducted in March and April 2007, an optically thin ice cloud was observed at around 3 km altitude south of Svalbard. The microphysical and radiative properties of this particular subvisible midlevel cloud were investigated with complementary remote sensing and in-situ instruments. Collocated airborne lidar remotesensing and spectral solar radiation measurements were performed at a flight altitude of 2300 m below the cloud base. Under almost stationary atmospheric conditions, the same subvisible midlevel cloud was probed with various in-situ sensors roughly 30 min later. From individual ice crystal samples detected with the Cloud Particle Imager and the ensemble of particles measured with the Polar Nephelometer, we retrieved the single-scattering albedo, the scattering phase function as well as the volume extinction coefficient and the effective diameter of the crystal population. Furthermore, a lidar ratio of 21 (±6) sr was deduced by two independent methods. These parameters in conjunction with the cloud optical thickness obtained from the lidar measurements were used to compute spectral and broadband radiances and irradiances with a radiative transfer code. The simulated results agreed with the observed spectral downwelling radiance within the range given by the measurement uncertainty. Furthermore, the broadband radiative simulations estimated a net (solar plus thermal infrared) radiative forcing of the subvisible midlevel ice cloud of -0.4 W m -2 (-3.2 W m -2 in the solar and +2.8 W m -2 in the thermal infrared wavelength range).

Atmospheric Chemistry and Physics, Apr 11, 2012

Atmospheric Chemistry and Physics, Dec 16, 2008