scholarly journals Influence of ice crystal shape on retrieval of cirrus optical thickness and effective radius: A case study

2009 ◽  
Vol 114 (D19) ◽  
Author(s):  
H. Eichler ◽  
A. Ehrlich ◽  
M. Wendisch ◽  
G. Mioche ◽  
J.-F. Gayet ◽  
...  
Keyword(s):  
Optical Thickness ◽  
Effective Radius ◽  
Crystal Shape ◽  
Ice Crystal

scholarly journals Application of ground-based hyperspectral imaging to retrieve ice crystal shape and fields of cirrus optical thickness

2013 ◽  
Vol 6 (1) ◽  
pp. 1201-1238 ◽  
Author(s):  
M. Schäfer ◽  
E. Bierwirth ◽  
A. Ehrlich ◽  
F. Heyner ◽  
M. Wendisch
Keyword(s):  
Hyperspectral Imaging ◽  
Optical Thickness ◽  
Surface Albedo ◽  
Effective Radius ◽  
Spectral Radiance ◽  
Trade Wind ◽  
Crystal Shape ◽  
Ice Crystal ◽  
Retrieval Method ◽  
Cloud Height

Abstract. A ground-based hyperspectral imaging spectrometer (AisaEAGLE) is applied to measure downward spectral radiance fields with high spatial (1024 spatial pixels within 36.7° field of view), spectral (488 spectral pixels, 400–970 nm, 1.25 nm full width at half maximum) and temporal (4–30 Hz) resolution. The calibration, measurement, and data evaluation procedures are introduced. A method is presented to retrieve the cirrus optical thickness τci using ground-based spectral radiance data collected by AisaEAGLE. On the basis of four measurement cases during the second campaign of the Cloud Aerosol Radiation and tuRbulence of trade wInd cumuli over BArbados (CARRIBA) project in 2011 the spatial inhomogeneity of the investigated cirrus is characterized by the standard deviation of the retrieved τci, as well as the width of the frequency distribution of the retrieved τci. By comparing measured and simulated downward solar radiance as a function of scattering angle, a first estimation of the detected cirrus ice crystal shape is given and used in the retrieval of the τci. The sensitivity of the retrieval method with respect to surface albedo, effective radius reff, cloud height, and ice crystal shape was characterized. Significant sensitivities of the retrieval method were found for the assumed surface albedo (up to 30%) and ice crystal shape (up to 90%). The sensitivity with regard to the effective radius (≤ 5%) and the cloud height (≤ 0.5%) is rather small and can be neglected.

scholarly journals Influence of local surface albedo variability and ice crystal shape on passive remote sensing of thin cirrus

2014 ◽  
Vol 14 (4) ◽  
pp. 1943-1958 ◽  
Author(s):  
C. Fricke ◽  
A. Ehrlich ◽  
E. Jäkel ◽  
B. Bohn ◽  
M. Wirth ◽  
...  
Keyword(s):  
Optical Thickness ◽  
Surface Albedo ◽  
Near Infrared ◽  
Effective Radius ◽  
High Spectral Resolution ◽  
Spectral Radiance ◽  
Crystal Shape ◽  
Frequency Distributions ◽  
Airborne Measurements ◽  
Moderate Resolution Imaging Spectroradiometer

Abstract. Airborne measurements of solar spectral radiance reflected by cirrus are performed with the HALO-Solar Radiation (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 variability on the retrieval of cirrus optical thickness and crystal effective radius. The applied retrieval of cirrus optical properties is based on a standard two-wavelength approach utilizing measured and simulated reflected radiance in the visible and near-infrared spectral region. Frequency distributions of the surface albedos from Moderate resolution Imaging Spectroradiometer (MODIS) satellite observations are used to compile surface-albedo-dependent lookup tables of reflected radiance. For each assumed surface albedo the cirrus optical thickness and effective crystal radius are retrieved as a function of the assumed surface albedo. The results for the cirrus optical thickness are compared to measurements from the High Spectral Resolution Lidar (HSRL). The uncertainty in cirrus optical thickness due to local variability of surface albedo in the specific case study investigated here is below 0.1 and thus less than that caused by the measurement uncertainty of both instruments. It is concluded that for the retrieval of cirrus optical thickness the surface albedo variability is negligible. However, for the retrieval of crystal effective radius, the surface albedo variability is of major importance, introducing uncertainties up to 50%. Furthermore, the influence of the bidirectional reflectance distribution function (BRDF) on the retrieval of crystal effective radius was investigated and quantified with uncertainties below 10%, which ranges below the uncertainty caused by the surface albedo variability. The comparison with the independent lidar data allowed for investigation of the role of the crystal shape in the retrieval. It is found that if assuming aggregate ice crystals, the HSRL observations fit best with the retrieved optical thickness from HALO-SR.

scholarly journals Potential of remote sensing of cirrus optical thickness by airborne spectral radiance measurements in different viewing angles and nadir geometry

2016 ◽  
Author(s):  
Kevin Wolf ◽  
André Ehrlich ◽  
Tilman Hüneke ◽  
Klaus Pfeilsticker ◽  
Frank Werner ◽  
...  
Keyword(s):  
Remote Sensing ◽  
Radiative Transfer ◽  
Optical Thickness ◽  
Convective Cloud ◽  
Sensitivity Study ◽  
Mean Value ◽  
Spectral Radiance ◽  
Crystal Shape ◽  
Ice Crystal ◽  
The North

Abstract. Spectral radiance measurements from 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 of cirrus optical thickness τ in nadir and off-nadir geometry. The comparison is based on a sensitivity study using radiative transfer simulations and on measurements during 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 cirrus optical thickness τ, effective radius τeff, viewing angle of the sensor σL, surface albedo α and ice crystal shape. From the calculations it is concluded that off-nadir measurements at wavelengths larger than λ = 900 nm significantly improve the ability to measure clouds of low optical thickness. The comparison of nadir and off-nadir retrievals of τ from mini-DOAS, SMART and independent estimates by the Water Vapour Lidar Experiment in Space (WALES) show general agreement within the range of measurement uncertainties. For the selected example case a mean optical thickness of 0.54±0.2 is derived by SMART and 0.49±0.2 by mini-DOAS nadir channels, while WALES obtained a mean value of 0.32 at 532 nm wavelength respectively. The mean of τ derived from the scanning mini-DOAS channels is 0.26. For the few simultaneous measurements, the scanning mini-DOAS measurements systematically underestimate (−17.6 %) the nadir observations from SMART and mini-DOAS, most likely due to the different probed scenes. The different values of τ derived by SMART, mini-DOAS and WALES can be potentially linked to spatial averages, ice crystal shape and the measurement strategies. The agreement of the simulations and retrievals indicate that off-nadir measurements are generally suited better to retrieve τ of thin clouds.

scholarly journals Retrieval of cirrus optical thickness and assessment of ice crystal shape from ground-based imaging spectrometry

2013 ◽  
Vol 6 (8) ◽  
pp. 1855-1868 ◽  
Author(s):  
M. Schäfer ◽  
E. Bierwirth ◽  
A. Ehrlich ◽  
F. Heyner ◽  
M. Wendisch
Keyword(s):  
Optical Thickness ◽  
Surface Albedo ◽  
Spatial Inhomogeneity ◽  
Spectral Radiance ◽  
Trade Wind ◽  
Crystal Shape ◽  
Ice Crystal ◽  
Enhanced Sensitivity ◽  
Cloud Height ◽  
Calibration Measurement

Abstract. A ground-based hyperspectral imaging spectrometer (AisaEAGLE, manufactured by Specim Ltd., Finland) is applied to measure downward spectral radiance fields with high spatial (1024 spatial pixels within 36.7° field of view), spectral (488 spectral pixels, 400–970 nm, 1.25 nm full width at half maximum), and temporal (4–30 Hz) resolution. The calibration, measurement and data evaluation procedures are introduced. A new method is presented to retrieve the cirrus optical thickness (τci) using the spectral radiance data collected by AisaEAGLE. The data were collected during the Cloud Aerosol Radiation and tuRbulence of trade wInd cumuli over BArbados (CARRIBA) project in 2011. The spatial inhomogeneity of the investigated cirrus is characterised by the standard deviation of the retrieved τci as well as the width of its frequency distribution. By comparing measured and simulated downward solar spectral radiance as a function of scattering angle, some evidence of the prevailing cirrus ice crystal shape can be obtained and subsequently used to substantiate the retrieval of τci. The sensitivity of the retrieval method with respect to surface albedo, effective radius (reff), cloud height and ice crystal shape is quantified. An enhanced sensitivity of the retrieved τci is found with respect to the surface albedo (up to 30%) and ice crystal shape (up to 90%). The sensitivity with regard to the effective ice crystal radius (≤ 5%) and the cloud height (≤ 0.5%) is rather small and can be neglected.

scholarly journals FAME-C: cloud property retrieval using synergistic AATSR and MERIS observations

2014 ◽  
Vol 7 (11) ◽  
pp. 3873-3890 ◽  
Author(s):  
C. K. Carbajal Henken ◽  
R. Lindstrot ◽  
R. Preusker ◽  
J. Fischer
Keyword(s):  
Optical Thickness ◽  
Single Layer ◽  
Cloud Water ◽  
Effective Radius ◽  
Cloud Property ◽  
Microphysical Properties ◽  
Microphysical Property ◽  
Resolution Imaging ◽  
Cloud Optical Thickness ◽  
Cloud Water Path

Abstract. A newly developed daytime cloud property retrieval algorithm, FAME-C (Freie Universität Berlin AATSR MERIS Cloud), is presented. Synergistic observations from the Advanced Along-Track Scanning Radiometer (AATSR) and the Medium Resolution Imaging Spectrometer (MERIS), both mounted on the polar-orbiting Environmental Satellite (Envisat), are used for cloud screening. For cloudy pixels two main steps are carried out in a sequential form. First, a cloud optical and microphysical property retrieval is performed using an AATSR near-infrared and visible channel. Cloud phase, cloud optical thickness, and effective radius are retrieved, and subsequently cloud water path is computed. Second, two cloud top height products are retrieved based on independent techniques. For cloud top temperature, measurements in the AATSR infrared channels are used, while for cloud top pressure, measurements in the MERIS oxygen-A absorption channel are used. Results from the cloud optical and microphysical property retrieval serve as input for the two cloud top height retrievals. Introduced here are the AATSR and MERIS forward models and auxiliary data needed in FAME-C. Also, the optimal estimation method, which provides uncertainty estimates of the retrieved property on a pixel basis, is presented. Within the frame of the European Space Agency (ESA) Climate Change Initiative (CCI) project, the first global cloud property retrievals have been conducted for the years 2007–2009. For this time period, verification efforts are presented, comparing, for four selected regions around the globe, FAME-C cloud optical and microphysical properties to cloud optical and microphysical properties derived from measurements of the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra satellite. The results show a reasonable agreement between the cloud optical and microphysical property retrievals. Biases are generally smallest for marine stratocumulus clouds: −0.28, 0.41 μm and −0.18 g m−2 for cloud optical thickness, effective radius and cloud water path, respectively. This is also true for the root-mean-square deviation. Furthermore, both cloud top height products are compared to cloud top heights derived from ground-based cloud radars located at several Atmospheric Radiation Measurement (ARM) sites. FAME-C mostly shows an underestimation of cloud top heights when compared to radar observations. The lowest bias of −0.3 km is found for AATSR cloud top heights for single-layer clouds, while the highest bias of −3.0 km is found for AATSR cloud top heights for multilayer clouds. Variability is low for MERIS cloud top heights for low-level clouds, and high for MERIS cloud top heights for mid-level and high-level single-layer clouds, as well as for both AATSR and MERIS cloud top heights for multilayer clouds.

scholarly journals Quantifying the Dependence of Satellite Cloud Retrievals on Instrument Uncertainty

2017 ◽  
Vol 30 (17) ◽  
pp. 6959-6976 ◽  
Author(s):  
Yolanda L. Shea ◽  
Bruce A. Wielicki ◽  
Sunny Sun-Mack ◽  
Patrick Minnis
Keyword(s):  
Climate Change ◽  
Optical Thickness ◽  
Indirect Effect ◽  
Effective Temperature ◽  
Radiative Forcing ◽  
Cloud Fraction ◽  
Effective Radius ◽  
Aerosol Indirect Effect ◽  
Sources Of Uncertainty ◽  
Water Cloud

Cloud response to Earth’s changing climate is one of the largest sources of uncertainty among global climate model (GCM) projections. Two of the largest sources of uncertainty are the spread in equilibrium climate sensitivity (ECS) and uncertainty in radiative forcing due to uncertainty in the aerosol indirect effect. Satellite instruments with sufficient accuracy and on-orbit stability to detect climate change–scale trends in cloud properties will improve confidence in the understanding of the relationship between observed climate change and cloud property trends, thus providing information to better constrain ECS and radiative forcing. This study applies a climate change uncertainty framework to quantify the impact of measurement uncertainty on trend detection times for cloud fraction, effective temperature, optical thickness, and water cloud effective radius. Although GCMs generally agree that the total cloud feedback is positive, disagreement remains on its magnitude. With the climate uncertainty framework, it is demonstrated how stringent measurement uncertainty requirements for reflected solar and infrared satellite measurements enable improved constraint of SW and LW cloud feedbacks and the ECS by significantly reducing trend uncertainties for cloud fraction, optical thickness, and effective temperature. The authors also demonstrate improved constraint on uncertainty in the aerosol indirect effect by reducing water cloud effective radius trend uncertainty.

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