Lothar Schüller

Radiative Properties of Boundary Layer Clouds: Droplet Effective Radius versus Number Concentration

The plane-parallel model for the parameterization of clouds in global climate models is examined in order to estimate the effects of the vertical profile of the microphysical parameters on radiative transfer calculations for extended boundary layer clouds. The vertically uniform model is thus compared to the adiabatic stratified one. The validation of the adiabatic model is based on simultaneous measurements of cloud microphysical parameters in situ and cloud radiative properties from above the cloud layer with a multispectral radiometer. In particular, the observations demonstrate that the dependency of cloud optical thickness on cloud geometrical thickness is larger than predicted with the vertically uniform model and that it is in agreement with the prediction of the adiabatic one. Numerical simulations of the radiative transfer have been performed to establish the equivalence between the two models in terms of the effective radius. They show that the equivalent effective radius of a vertically uniform model is between 80% and 100% of the effective radius at the top of an adiabatic stratified model. The relationship depends, in fact, upon the cloud geometrical thickness and droplet concentration. Remote sensing measurements of cloud radiances in the visible and near infrared are then examined at the scale of a cloud system for a marine case and the most polluted case sampled during the second Aerosol Characterization Experiment. The distributions of the measured values are significantly different between the two cases. This constitutes observational evidence of the aerosol indirect effect at the scale of a cloud system. Finally, the adiabatic stratified model is used to develop a procedure for the retrieval of cloud geometrical thickness and cloud droplet number concentration from the measurements of cloud radiances. It is applied to the marine and to the polluted cases. The retrieved values of droplet concentration are significantly underestimated with respect to the values measured in situ. Despite this discrepancy the procedure is efficient at distinguishing the difference between the two cases.

Entrainment-Mixing and Radiative Transfer Simulation in Boundary Layer Clouds

Abstract In general circulation models, clouds are parameterized and radiative transfer calculations are performed using the plane-parallel approximation over the cloudy fraction of each model grid. The albedo bias resulting from the plane-parallel representation of spatially heterogeneous clouds has been extensively studied, but the impact of entrainment-mixing processes on cloud microphysics has been neglected up to now. In this paper, this issue is examined by using large-eddy simulations of stratocumulus clouds and tridimensional calculations of radiative transfer in the visible and near-infrared ranges. Two extreme scenarios of mixing are tested: the homogeneous mixing scheme with constant concentration and reduced droplet sizes, against the inhomogeneous mixing scheme, with reduced concentration and constant droplet sizes. The tests reveal that entrainment-mixing effects at cloud top may substantially bias the simulated albedo. In the worse case, which corresponds to a fragmented and thin stratocumu...

An Algorithm for the Retrieval of Droplet Number Concentration and Geometrical Thickness of Stratiform Marine Boundary Layer Clouds Applied to MODIS Radiometric Observations

Algorithms are now currently used for the retrieval of cloud optical thickness and droplet effective radius from multispectral radiance measurements. This paper extends their application to the retrieval of cloud droplet number concentration, cloud geometrical thickness, and liquid water path in shallow convective clouds, using an algorithm that was previously tested with airborne measurements of cloud radiances and validated against in situ measurements of the same clouds. The retrieval is based on a stratified cloud model of liquid water content and droplet spectrum. Radiance measurements in visible and near-infrared channels of the Moderate Resolution Imaging Spectroradiometer (MODIS), which is operated from the NASA platforms Terra and Aqua, are analyzed. Because of uncertainties in the simulation of the continental surface reflectance, the algorithm is presently limited to the monitoring of the microphysical structure of boundary layer clouds over the ocean. Two MODIS scenes of extended cloud fields over the North Atlantic Ocean trade wind region are processed. A transport and dispersion model (the Hybrid Single-Particle Lagrangian Integrated Trajectory Model, HYSPLIT4) is also used to characterize the origin of the air masses and hence their aerosol regimes. One cloud field formed in an air mass that was advected from southern Europe and North Africa. It shows high values of the droplet concentration when compared with the second cloud system, which developed in a more pristine environment. The more pristine case also exhibits a higher geometrical thickness and, thus, liquid water path, which counterbalances the expected cloud albedo increase of the polluted case. Estimates of cloud liquid water path are then compared with retrievals from the Special Sensor Microwave Imager (SSM/I). SSM/I-derived liquid water paths are in good agreement with the MODIS-derived values.

Generating cloudmasks in spatial high-resolution observations of clouds using texture and radiance information

The detection of clouds in measurements taken by airborne and spaceborne remote sensing sensors in the visible and near-infrared is often difficult due to the high albedo of underlying surfaces such as snow- and ice-covered surfaces as well as sunglint regions of water surfaces. The authors show that the measured intensity of the reflected solar radiation together with texture information is effective in detecting clouds over water surfaces which are affected by sunglint. An automated cloud-masking technique for images measured by a compact airborne spectrographic imager ( casi ) during the ACE-2 CLOUDYCOLUMN experiment has been developed based on supervised learning of an artificial neural network. The neural network has been trained on radiances, texture features, and gradient-filtered radiances. The radiances were measured at a single wavelength but with high spatial resolution so that characteristic spatial features within an image can be used to discriminate clouds from sunglint, cloud shadow and ocean surface. The accuracy of the cloudmask-generating algorithm was investigated on the basis of the testing set for the neural network. Maximum errors of 3.4% and 1% occur for detecting cloudy and cloud-free pixels, respectively. The performance of the network was compared with a second network trained on radiances alone. The second network is up to 44% less efficient for cloud detection which demonstrates the improvement arising from the use of texture information together with spatial high-resolution observations.

Cloud-top pressure retrieval using the oxygen A-band in the IRS-3 MOS instrument

Backscattered solar radiation as measured by MOS (Modular Optoelectronical Scanner) on the IRS3 (Indian Remote sensing Satellite 3) has been used in an algorithm to retrieve cloud top pressure. The algorithm uses a radiance ratio between absorbing channels in the Oxygen‐A absorption band at 761 nm and a window channel at 750 nm. The ratios are directly related to the average photon path length, which is mainly determined by the cloud top pressure. This paper presents the principles of the retrieval scheme, results of a sensitivity study and a first validation using radiosondes.

Microphysical and radiative properties of stratocumulus clouds: the EUCREX mission 206 case study

In this conclusion paper, remote sensing retrievals of cloud optical thickness performed during the EUCREX mission 206 are analyzed. The comparison with estimates derived from in situ measurements demonstrates that the adiabatic model of cloud microphysics is more realistic than the vertically uniform plane parallel model (VUPPM) for parameterization of optical thickness. The analysis of the frequency distributions of optical thickness in the cloud layer then shows that the adiabatic model provides a good prediction when the cloud layer is thick and homogeneous, while it overestimates significantly the optical thickness when the layer is thin and broken. Finally, it is shown that the effective optical thickness over the whole sampled cloud is smaller than the adiabatic prediction based on the mean geometrical thickness of the cloud layer. The high sensitivity of the optical thickness on cloud geometrical thickness suggests that the effect of aerosol and droplet concentration on precipitation efficiency, and therefore on cloud extent and lifetime, is likely to be more significant than the Twomey effect.

Calibration of high resolution remote sensing instruments in the visible and near infrared

Abstract Measurements of the reflected solar radiation with high spectral resolution airborne instruments are usually used to develop new remote sensing techniques. The observed spectral features in the signals provide the possibility to define useful band settings for future satellite instruments. A precise wavelength and radiometric calibration is a prerequisite for such tasks. In this paper, a calibration procedure for the airborne spectrometer OVID is presented. The Optical Visible and near Infrared Detector consists of two similar detector systems, (600 – 1100 nm = VIS and 900 – 1700 nm = NIR). The spectral resolution is ≈1.7 nm for the VIS-system and ≈6 nm for the IR-system. This instrument is applied for the retrieval of water vapour content, aerosol and cloud properties. Besides the spectral and intensity calibration, also corrections for the dark current signals and for defective pixels have been performed. An indirect verification of the calibration procedure by the comparison of OVID measurements in cloudy and cloud free atmospheres with radiative transfer simulations is discussed in this paper. The used radiation transfer model MOMO is based on the matrix operator method.

Retrieval of cloud optical and microphysical properties from multispectral radiances

This paper describes measurements made with a high spectral resolution radiometer (OVID) during the EUCREX mission 206, for the retrieval of optical and microphysical cloud properties. The retrieval algorithm is based on the calculation of the radiative transfer through a cloudy atmosphere with a numerical model based on the Matrix Operator method. The simulation of the spectral radiances at the flight level in 942 spectral channels fits precisely the actual spectra measured with OVID, with a good description of the absorption and scattering features. A number of 1486 simulations, based on a plane parallel model of clouds with various combinations of cloud optical thickness and droplet effective radius, form the lookup table of reflectance values that are used to retrieve these parameters from the measured spectra. The OVID measurements performed along five legs flown by the DLR-F20 aircraft during the EUCREX mission 206 have been processed. The retrieved values of optical thickness and effective radius are compared in the EUCREX summary paper to estimates derived from in situ measurements and values retrieved from airborne lidar and radiometric measurements as well as from satellite radiometric measurements.

Microphysical and radiative properties of stratocumulus

Abstract During the Cloudy Column project of the Aerosol Characterisation Experiment (ACE2) stratocumulus clouds formed in air masses of different origins have been sampled. The paper presents results from in-situ measurements and the comparison of the derived radiative properties with those remotely sensed. A simple parametrisation of the optical thickness of the adiabatic cloud regions is presented. The cloud optical thickness is much more sensitive to the cloud geometrical thickness ( H 5 3 ) than to the droplet concentration ( N 1 3 ).

Multispectral flight measurements of atmospheric water vapour bands

Abstract A number of aircraft flights have been performed with the airborne Optical Visible and near Infrared Detector during the European Cloud and Radiation Experiment in April 94. The OVID-Spectrometer consists of two similar detector systems (600 – 1100nm = VIS and 900 – 1700nm = IR). We have developed a data processing algorithm for the new instrument which includes an unprecedented high resolution wavelength- and radiance calibration, a requisite for analysing absorption processes within the atmosphere. The resulting spectral resolution is ∼ 1.7nm for the VIS-system and ∼ 6nm for the IR-system. A series of spectra was recorded over the Atlantic ocean under fair weather conditions. These spectra are analysed with special emphasis on the water vapour absorption bands. A precise knowledge of all these bands is necessary to predict the absorption of radiation within clouds. Together with model calculations such investigations will help us to develop new remote sensing techniques, such as for the retrieval of water vapour and cloud properties.