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Kuo-Nan Liou, S. C. S. Ou, and P. J. Lu

Abstract

A one-dimensional climate model with an interactive cloud formation program is developed to investigate its effects on temperature perturbations due to various radiative forcings including doubling of CO2, a 2% increase of the solar constant and the increase of the cirrus IR emissivity. By virtue of the K-theory for turbulence transfer of sensible and latent heat flux we demonstrate that the model may be described by a set of partial differential equations governing the thermodynamic energy balance, water vapor transport, vertical velocity in the cloudy region and cloud cover. In particular, we illustrate that the climatic temperature perturbation experiment may be carried out as a boundary value problem. Moreover, in order to effectively incorporate interactive cloud formation and radiative transfer programs in the model, we have designed a cloud compaction scheme based on statistical and stochastic procedures for the estimate of cloud covers, thicknesses, heights and positions for high, middle and low clouds. We show that, overall, the interactive cloud formation program reduces the sensitivity of temperature increases caused by positive radiative forcings and therefore generates a negative feedback in reference to the fixed cloud program. Low and high cloud formations lead to negative feedbacks as a result of the increased low cloud cover and thickness and decreased high cloud cover and thickness caused by temperature increases. The former strengthens the solar albedo effects, whereas the latter weakens the IR greenhouse effects. On the other hand, the middle cloud formation exhibits a positive feedback because of the reduction in cloud cover and thickness. Further, we show that the cloud cover variation alone will produce a larger reduction in temperature increases. In light of these experimental results, it appears physically appropriate to conclude that an interactive cloud formation program with radiative transfer coupling in the context of a model setting will lead to a negative feedback in temperature sensitivity analyses.

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S. C. Ou, K. N. Liou, and J. F. King

Abstract

We have explored the applicability of the differential inversion (DI) method to temperature retrievals in both clear and cloudy atmospheres using red satellite data. The main theme of the DI is that the local Planck intensity can be exactly expressed by a linear combination of the derivatives of radiances in the logarithmic pressure coordinate. The inversion coefficients are obtained by fitting the weighting function to a generalized form. The higher-order derivatives of radiances are determined from polynomial fittings. The satellite dataset used in this work contains collocated brightness temperatures and radiosonde data that have been collected during the period of Baseline Upper Atmospheric Network (BUAN) experiments. These data include both cloudy and clear cases. A multispectral cloud-removal method using the principle of the N * method has been developed. This method uses radiances of High-Resolution Infrared Radiation Sounder channels 6, 7, and 8 to estimate clear radiances of these channels and the surface temperature simultaneously based on radiative transfer simulations. Subsequently, the quantity N * (the ratio of effective cloud cover over adjacent pixels) and the clear radiances of the rest of the channels are evaluated.

Retrieval results are presented in terms of rms temperature differences between retrieved and sounding profiles. Considering all clear and partly cloudy cases, the rms differences in temperature of approximately 2 K for retrievals using the DI are comparable to those using the minimum-variance scheme. The rms differences in temperature for retrievals using the multispectral cloud-removal scheme are slightly larger than those using the BUAN cloud-removal scheme by approximately 0.5 K. Finally, the rms temperature differences are much smaller than those for the first guess of the minimum-variance scheme. These results indicate fire that the DJ can achieve acceptable performance without first-guess or error covariance matrices; second, that the proposed multispectral cloud-removal method is also capable of generating reasonable cloud-removed clear radiances; and finally that the DI can be used as a tool to obtain first guesses in the current operational method and to perform large-volume temperature retrievals for climate studies.

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S. C. Ou, K. N. Liou, and B. A. Baum

Abstract

A numerical scheme has been developed to identify multilayer cirrus cloud systems using Advanced Very Higher Resolution Radiometer (AVHRR) data. It is based on the physical properties of the AVHRR channels 1–2 reflectance ratios, the brightness temperature differences between channels 4 and 5, and the channel 4 brightness temperatures. In this scheme, clear pixels are first separated from cloudy pixels, which are then classified into three types: cirrus, cirrus/low cloud, and low clouds. The authors have applied this scheme to the satellite data collected over the FIRE II IFO [First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment II intensive field observations area during nine overseas within seven observation dates. Determination of the threshold values used in the detection scheme are based on statistical analysts of these satellite data. The authors have validated the detection results against the cloudy conditions inferred from the collocated and coincident ground-based lidar and radar images, balloonborne replicator data, and National Center for Atmospheric Research CLASS (Cross-chain Loran Atmospheric Sounding System) humidity soundings on a case-by-case basis. In every case, the satellite detection results are consistent with the cloudy conditions inferred from these independent and complementary measurement. The present scheme is well suited for the detection of midlatitude, multilayer cirrus cloud systems and tropical anvils.

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S. C. Ou, K. N. Liou, and T. R. Caudill

Abstract

Surface observations show that multilayer clouds frequently occur in frontal areas where cirrus clouds overlie boundary layer convective clouds or stratus clouds. In this paper, an algorithm is presented for the retrieval of cirrus cloud optical depths and ice crystal sizes in multilayer cloud systems based on the theory of radiative transfer and parameterizations. For the validation of the retrieval program, AVHRR data is analyzed for two dates during FIRE-II-IFO in which cirrus clouds overlie a layer of low stratus cloud. It is shown that the domain-averaged retrieved cloud temperatures are within the boundaries of cirrus clouds determined from the collocated replicator, radar, and lidar data. The retrieved ice crystal mean effective sizes and optical depths are also in general agreement with the values determined from the balloon-borne replicator and 2D probe data.

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K. N. Liou, S. C. Ou, Y. Takano, and Q. Liu

Abstract

The delta-four-stream polarized (vector) thermal radiative transfer has been formulated and numerically tested specifically for application to satellite data assimilation in cloudy atmospheres. It is shown that for thermal emission in the earth’s atmosphere, the [I, Q] component of the Stokes vector can be decoupled from the [U, V] component and that the solution of the vector equation set involving the four-stream approximation can be expressed in an analytic form similar to the scalar case. Thus, the computer time requirement can be optimized for the simulation of forward radiances and their derivatives. Computations have been carried out to illustrate the accuracy and efficiency of this method by comparing radiance and polarization results to those computed from the exact doubling method for radiative transfer for a number of thermal infrared and microwave frequencies. Excellent agreement within 1% is shown for the radiance results for all satellite viewing angles and cloud optical depths. For polarization, differences between the two are less than 5% if brightness temperature is used in the analysis. On balance of the computational speed and accuracy, the four-stream approximation for radiative transfer appears to be an attractive means for the simulation of cloudy radiances and polarization for research and data assimilation purposes.

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N. X. Rao, S. C. Ou, and K. N. Liou

Abstract

A numerical scheme has been developed to remove the solar component in the Advanced Very High Resolution Radiometer (AVHRR) 3.7-µm channel for the retrieval of cirrus parameters during daytime. This method uses a number of prescribed threshold values for AVHRR channels 1 (0.63 µm), 2 (0.8 µm), 3 (3.7 µm), 4 (10.9 µm), and 5 (12 µm) to separate clear and cloudy pixels. A look-up table relating channels 1 and 3 solar reflectances is subsequently constructed based on the prescribed mean effective ice crystal sizes and satellite geometric parameters. An adding–doubling radiative transfer program has been used to generate numerical values in the construction of the look-up table. Removal of the channel 3 solar component is accomplished by using the look-up table and the measured channel 1 reflectance. The cloud retrieval scheme described in Ou et al. has been modified in connection with the removal program. The authors have applied the removal–retrieval scheme to the AVHRR global area coverage daytime data, collected during the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment cirrus intensive field observation (FIRE IFO) at 2100 UTC 28 October 1986 over the Wisconsin area. Distributions of the retrieved cloud heights and optical depths are comparable to those determined from Geostationary Operational Environmental Satellite visible and IR channels data reported by Minnis et al. Morwver, verifications of the retrieved cirrus temperature and height against lidar data have been carried out using results reported from three FIRE IFO nations. The retrieved cloud heights are within 0.5 km of the measured lidar values.

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R. A. Hansell, S. C. Tsay, Q. Ji, N. C. Hsu, M. J. Jeong, S. H. Wang, J. S. Reid, K. N. Liou, and S. C. Ou

Abstract

In September 2006, NASA Goddard’s mobile ground-based laboratories were deployed to Sal Island in Cape Verde (16.73°N, 22.93°W) to support the NASA African Monsoon Multidisciplinary Analysis (NAMMA) field study. The Atmospheric Emitted Radiance Interferometer (AERI), a key instrument for spectrally characterizing the thermal IR, was used to retrieve the dust IR aerosol optical depths (AOTs) in order to examine the diurnal variability of airborne dust with emphasis on three separate dust events. AERI retrievals of dust AOT are compared with those from the coincident/collocated multifilter rotating shadowband radiometer (MFRSR), micropulse lidar (MPL), and NASA Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) sensors. The retrieved AOTs are then inputted into the Fu–Liou 1D radiative transfer model to evaluate local instantaneous direct longwave radiative effects (DRELW) of dust at the surface in cloud-free atmospheres and its sensitivity to dust microphysical parameters. The top-of-atmosphere DRELW and longwave heating rate profiles are also evaluated. Instantaneous surface DRELW ranges from 2 to 10 W m−2 and exhibits a strong linear dependence with dust AOT yielding a DRELW of 16 W m−2 per unit dust AOT. The DRELW is estimated to be ∼42% of the diurnally averaged direct shortwave radiative effect at the surface but of opposite sign, partly compensating for the shortwave losses. Certainly nonnegligible, the authors conclude that DRELW can significantly impact the atmospheric energetics, representing an important component in the study of regional climate variation.

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K. N. Liou, S. C. Ou, Y. Takano, F. P. J. Valero, and T. P. Ackerman

Abstract

A dual-channel retrieval technique involving the water vapor band at 6.5 μm and the window region at 10.5 gm has been developed to infer the temperature and emissivity of tropical anvils. This technique has been applied to data obtained from the ER-2 narrow field-of-view radiometers during two flights in the field observation of the Stratosphere-Troposphere Exchange Project (STEP) near Damn, Australia, January-February 1987. The retrieved cloud temperatures are between 190 and 240 K, while the cloud emissivities derived from the retrieval algorithm range from about 0.2 to 1. Moreover, the visible optical depths have been obtained from the cloud emissivity through a theoretical parameterization with values of 0.5-10. A significant portion of tropical cirrus clouds are found to have optical depths greater than about 6. Because of the parameterization, the present technique is unable to precisely determine the optical depth values for optically thick cirrus clouds.

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Qing Yue, K. N. Liou, S. C. Ou, B. H. Kahn, P. Yang, and G. G. Mace

Abstract

A thin cirrus cloud thermal infrared radiative transfer model has been developed for application to cloudy satellite data assimilation. This radiation model was constructed by combining the Optical Path Transmittance (OPTRAN) model, developed for the speedy calculation of transmittances in clear atmospheres, and a thin cirrus cloud parameterization using a number of observed ice crystal size and shape distributions. Numerical simulations show that cirrus cloudy radiances in the 800–1130-cm−1 thermal infrared window are sufficiently sensitive to variations in cirrus optical depth and ice crystal size as well as in ice crystal shape if appropriate habit distribution models are selected a priori for analysis. The parameterization model has been applied to the Atmospheric Infrared Sounder (AIRS) on board the Aqua satellite to interpret clear and thin cirrus spectra observed in the thermal infrared window. Five clear and 29 thin cirrus cases at nighttime over and near the Atmospheric Radiation Measurement program (ARM) tropical western Pacific (TWP) Manus Island and Nauru Island sites have been chosen for this study. A χ2-minimization program was employed to infer the cirrus optical depth and ice crystal size and shape from the observed AIRS spectra. Independent validation shows that the AIRS-inferred cloud parameters are consistent with those determined from collocated ground-based millimeter-wave cloud radar measurements. The coupled thin cirrus radiative transfer parameterization and OPTRAN, if combined with a reliable thin cirrus detection scheme, can be effectively used to enhance the AIRS data volume for data assimilation in numerical weather prediction models.

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S. C. Ou, K. N. Liou, Y. Takano, and R. L. Slonaker

Abstract

This paper presents a conceptual approach toward the remote sensing of cirrus cloud particle size and optical depth using the degree of polarization and polarized reflectance associated with the first three Stokes parameters, I, Q, and U, for the 0.865- and 2.25-μm wavelengths. A vector line-by-line equivalent radiative transfer program including the full Stokes parameters based on the adding method was developed. The retrieval algorithm employs the steepest-descent method in the form of a series of numerical iteration procedures to search for the simulated polarization parameters that best match the measured values. Sensitivity studies were performed to investigate the behavior of phase-matrix elements as functions of scattering angles for three ice crystal size–shape combinations. Overall, each phase-matrix element shows some sensitivity toward ice crystal shape, size, and surface roughness due to the various optical effects. Synthetic analysis reveals that the retrieval algorithm is highly accurate, while polarimetric and radiometric error sources cause very small retrieval errors. Finally, an illustrative example of applying the retrieval algorithm to airborne Polarization and Directionality of the Earth’s Reflectances (POLDER) data during the European Cloud and Radiation Experiment (EUCREX) is presented.

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