A novel ice particle optical property model, referred to as the Lidar Ice Model (LIM), is developed for applications to active remote sensing in three channels centered at 355, 532, and 1064 nm. The theoretical lidar ratio and integrated attenuated backscatter (IAB) computed based on the LIM are consistent with the Cloud-Aerosol Lidar and Infrared Pathfinder in Space Observations (CALIPSO) counterparts due to a better treatment of backscattering properties in the present study. The LIM provides robust optical properties to facilitate the implementation of active remote sensing algorithms for inferring the microphysical and radiative properties of ice clouds.

Inferring cirrus cloud properties from spaceborne observations inevitably involves uncertainties from both forward models and measurements. These uncertainties are present in the retrieval products and propagate to downstream data-ingesting applications in atmospheric and climate modeling. Satellite reflective solar hyperspectral instruments are highly information dense and offer a means to reduce uncertainties in cirrus cloud property retrieval products. However, hyperspectral instruments contain a large number of channels imposing large computational expenses both with measurement data and radiative transfer modeling needed to perform cirrus cloud property retrievals. While using all the channels will maximize the information content, the computational expense may not be acceptable. The purpose of this study is to objectively identify channel subsets in the spectral coverage of the Climate Absolute Radiance and Refractivity Observatory Pathfinder (CLARREO-PF) instrument that, when used as reflectance inputs to a cirrus radiative and microphysical property retrieval, will minimize uncertainties in the retrieval products. An ensemble of 216 cloud/atmosphere cases are used with an information content iterative routine and a clustering algorithm. Water vapor channels are identified as highly informative for cirrus cloud optical depth and cloud top pressure, away from window regions normally used in reflective solar ice cloud property retrievals. The ice-absorbing channels are most informative for effective particle size. The extractable independent information content increases with the increasing number of CLARREO-PF channels that are added to the retrieval system. However, with each channel added to the retrieval selection, the independent information content of all the remaining unselected channels decreases.

Visible-near infrared (VIS-NIR) and thermal infrared (TIR) methods have long been used for ice cloud property retrievals based on satellite observations. Both retrieval methods are sensitive to the assumed ice particle models, which can significantly impact the accuracy of the retrieved microphysical and radiative properties of ice clouds. The Two-Habit model (THM) is considered a suitable ice particle model for passive remote sensing of global ice clouds, as confirmed by spectral consistency in cloud optical thickness (COT), which refers to agreement between VIS-NIR and TIR COT retrievals. However, the ratio of COTs retrieved from these two methods using the THM varies with scattering angle, indicating the potential influence of atmospheric and cloud properties on the spectral consistency of the inferred COT. The present study investigates potential factors affecting the angular dependence of the COT ratio based on observations made by the Advanced Baseline Imager (ABI) sensors on seventeenth Geostationary Operational Environmental Satellite (GOES-17). For optically thin clouds (COT<1) illuminated at a solar zenith angle (SZA)<20 degrees, the heterogeneous particle combination of mixed-phase clouds is the dominant factor that produces an angular-dependent negative bias in COT spectral consistency. At greater SZAs, cloud three-dimensional radiative effects are presumed to be a dominant factor. Negative biases in the cloud top temperature and sea surface temperature contribute to negative biases in the COT ratio. This study suggests that the angular dependence of the COT ratio could help identify mixed-phase clouds and estimate their impact on the spectral consistency in retrieving ice cloud COT.

We estimate top-of-the-atmosphere (TOA) cirrus cloud radiative forcing from satellite lidar observations. We describe the technical innovations necessary to conduct this experiment. Specifically, we highlight advances in broadband radiative transfer modeling, and the optical and physical parameterizations necessary to run them. We present two years of results, across two decades (2008 vs. 2018), highlighting seasonal and annual differences. The goal of this endeavor is to standardize mechanics for transitioning this work to operational Level 2 ground and satellite lidar products moving forward. However, the unique TOA forcing characteristics of cirrus clouds are also discussed, including specifically daytime characteristics than can oscillate between warming and cooling regionally.

Ice clouds and airborne dust are two important atmospheric constituents, as they substantially regulate the radiative energy budget in the earth-atmosphere system. The single-scattering properties of these atmospheric constituents are fundamental to achieving a better understanding of their roles from the radiation perspective, such as for applications to remote sensing technique implementations and radiative transfer simulations. Ice crystals and dust aerosol particles in the atmosphere are almost exclusively nonspherical particles with complex morphologies. In this paper, we report advanced modeling capabilities in computing the optical properties of these nonspherical dielectric particles in the atmosphere. In particular, we use an ensemble approach along with a combination of the invariant imbedding T-matrix method and the physical-geometric optics method to consider the effects of the overall nonspherical geometries and small-scale irregularities or surface roughness on the single-scattering properties of ice crystals and dust aerosols.

The Edition 2 (Ed2) cloud property retrieval algorithm system was upgraded and applied to the MODerate-resolution Imaging Spectroradiometer (MODIS) data for the Clouds and the Earth's Radiant Energy System (CERES) Edition 4 (Ed4) products. New calibrations for solar channels and the use of the 1.24-μm channel for cloud optical depth (COD) over snow improve the daytime consistency between Terra and Aqua MODIS retrievals. Use of additional spectral channels and revised logic enhanced the cloud-top phase retrieval accuracy. A new ice crystal reflectance model and a CO2-channel algorithm retrieved higher ice clouds, while a new regional lapse rate technique produced more accurate water cloud heights than in Ed2. Ice cloud base heights are more accurate due to a new cloud thickness parameterization. Overall, CODs increased, especially over the polar (PO) regions. The mean particle sizes increased slightly for water clouds, but more so for ice clouds in the PO areas. New experimental parameters introduced in Ed4 are limited in utility, but will be revised for the next CERES edition. As part of the Ed4 retrieval evaluation, the average properties are compared with those from other algorithms and the differences between individual reference data and matched Ed4 retrievals are explored. Part II of this article provides a comprehensive, objective evaluation of selected parameters. More accurate interpretation of the CERES radiation measurements has resulted from the use of the Ed4 cloud properties.

The MODIS Level-2 cloud product (Earth Science Data Set names MOD06 and MYD06 for Terra and Aqua MODIS, respectively) provides pixel-level retrievals of cloud-top properties (day and night pressure, temperature, and height) and cloud optical properties (optical thickness, effective particle radius, and water path for both liquid water and ice cloud thermodynamic phases-daytime only). Collection 6 (C6) reprocessing of the product was completed in May 2014 and March 2015 for MODIS Aqua and Terra, respectively. Here we provide an overview of major C6 optical property algorithm changes relative to the previous Collection 5 (C5) product. Notable C6 optical and microphysical algorithm changes include: (i) new ice cloud optical property models and a more extensive cloud radiative transfer code lookup table (LUT) approach, (ii) improvement in the skill of the shortwave-derived cloud thermodynamic phase, (iii) separate cloud effective radius retrieval datasets for each spectral combination used in previous collections, (iv) separate retrievals for partly cloudy pixels and those associated with cloud edges, (v) failure metrics that provide diagnostic information for pixels having observations that fall outside the LUT solution space, and (vi) enhanced pixel-level retrieval uncertainty calculations. The C6 algorithm changes collectively can result in significant changes relative to C5, though the magnitude depends on the dataset and the pixel's retrieval location in the cloud parameter space. Example Level-2 granule and Level-3 gridded dataset differences between the two collections are shown. While the emphasis is on the suite of cloud optical property datasets, other MODIS cloud datasets are discussed when relevant.

Atmospheric ice particles can be rimed and contaminated (e.g., by soot attachments). Previous optical property calculations usually assume rimed particles such as graupel and hailstones to be homogeneous spheres with fixed densities. The relevant dielectric constants are estimated with the effective medium approximation (EMA), although such particles are predominately nonspherical, porous, and contain small interior grains. This paper assesses the effects of nonsphericity, density, and inhomogeneity of graupel and hailstones on their optical properties. The bicontinuous medium approximation (BMA) is employed to simulate the particle internal structure. Conical shapes are compared with spherical and spheroidal shapes to assess the effect of nonsphericity. At frequencies lower than 89 GHz, the optical properties are more sensitive to particle's mass density than to overall particle shape, and the internal structure plays an insignificant role when the particle effective diameter (a quantity involving the particle size distribution) is smaller than approximately 10 mm, and the internal grain size is smaller than 0.2 mm. With a small grain size, the BMA phase function converges to the EMA phase function with an effective refractive index calculated with the Bruggeman formulation. Simulated top of atmosphere radiances at three microwave frequencies, 18.7, 36.5, and 89 GHz, are quite sensitive to ice particle effective diameter between 1 and 5 mm, ice fraction between 0.1 and 0.9, and ice water path between 1 and 5 kg/m2. Thus, these frequencies are suitable for retrieving the microphysical properties.

Asian summer monsoon and atmospheric aerosol simultaneously influence the climate in the East Asian region. However, substantial uncertainties exist in the current understanding of the interactions between monsoon and aerosol and their combined effects. Previous studies have shown that aerosols influence the strength of monsoon and monsoon-related water cycles; however, monsoon strongly regulates the aerosol spatial distribution. This letter investigates the radiative flux response at the top of the atmosphere to the Asian summer monsoon by using observations made by the Clouds and Earth's Radiant Energy System and the Moderate Resolution Imaging Spectroradiometer. In comparison with the ten-year (2002-2011) mean climatology, the aerosol radiative effect is estimated over two eastern Asia regions for the months of July in 2002 and 2003, corresponding to a weak and a strong summer monsoon event, respectively. The dramatically different influences show the aerosol radiative forcing over land to be strongly responsive to Asian summer monsoon. Furthermore, the reanalysis-based estimate of the aerosol radiative effect is consistent with its observation-only counterpart.

Specular reflection from horizontally oriented plates (HOPs) has significant effects on lidar backscatter. The intensity of specular reflection from HOPs is high in warm mixed-phase clouds and low in cold ice clouds. The theoretical simulations of lidar backscatter and depolarization ratio are consistent with spaceborne measurements for optically thick mixed-phase and ice clouds if an equivalent percentage of HOPs of 0.08%-0.3% is assumed. Based on the joint probability density function of the attenuated backscatter and depolarization ratio observed by the Cloud-Aerosol Lidar with Orthogonal Polarization aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations platform, it is estimated that HOPs exist in approximately 60% of optically thick (τ > 3) ice and mixed-phase cloud layers. The cloud-layer temperature is the primary factor affecting the distribution of HOPs. Specifically, HOPs exist in approximately 88% of optically thick ice and mixed-phase cloud layers warmer than -30°C, in approximately 84% of ice and mixed-phase cloud layers between -30°C and -45°C, and in approximately 29% of cold ice cloud layers below -45°C.

The effect of thin cirrus clouds in retrieving the dust optical depth from MODIS observations is investigated by using a simplified aerosol retrieval algorithm based on the principles of the Deep Blue aerosol property retrieval method. Specifically, the errors of the retrieved dust optical depth due to thin cirrus contamination are quantified through the comparison of two retrievals by assuming dust-only atmospheres and the counterparts with overlapping mineral dust and thin cirrus clouds. To account for the effect of the polarization state of radiation field on radiance simulation, a vector radiative transfer model is used to generate the lookup tables. In the forward radiative transfer simulations involved in generating the lookup tables, the Rayleigh scattering by atmospheric gaseous molecules and the reflection of the surface assumed to be Lambertian are fully taken into account. Additionally, the spheroid model is utilized to account for the nonsphericity of dust particles in computing their optical properties. For simplicity, the single-scattering albedo, scattering phase matrix, and optical depth are specified a priori for thin cirrus clouds assumed to consist of droxtal ice crystals. The present results indicate that the errors in the retrieved dust optical depths due to the contamination of thin cirrus clouds depend on the scattering angle, underlying surface reflectance, and dust optical depth. Under heavy dusty conditions, the absolute errors are comparable to the predescribed optical depths of thin cirrus clouds.

The National Aeronautics and Space Administration's Clouds and the Earth's Radiant Energy System (CERES) Project was designed to improve our understanding of the relationship between clouds and solar and longwave radiation. This is achieved using satellite broad-band instruments to map the top-of-atmosphere radiation fields with coincident data from satellite narrow-band imagers employed to retrieve the properties of clouds associated with those fields. This paper documents the CERES Edition-2 cloud property retrieval system used to analyze data from the Tropical Rainfall Measuring Mission Visible and Infrared Scanner and by the MODerate-resolution Imaging Spectrometer instruments on board the Terra and Aqua satellites covering the period 1998 through 2007. Two daytime retrieval methods are explained: the Visible Infrared Shortwave-infrared Split-window Technique for snow-free surfaces and the Shortwave-infrared Infrared Near-infrared Technique for snow or ice-covered surfaces. The Shortwave-infrared Infrared Split-window Technique is used for all surfaces at night. These methods, along with the ancillary data and empirical parameterizations of cloud thickness, are used to derive cloud boundaries, phase, optical depth, effective particle size, and condensed/frozen water path at both pixel and CERES footprint levels. Additional information is presented, detailing the potential effects of satellite calibration differences, highlighting methods to compensate for spectral differences and correct for atmospheric absorption and emissivity, and discussing known errors in the code. Because a consistent set of algorithms, auxiliary input, and calibrations across platforms are used, instrument and algorithm-induced changes in the data record are minimized. This facilitates the use of the CERES data products for studying climate-scale trends.

Ultraspectral resolution infrared (IR) radiances obtained from nadir observations provide information about the atmosphere, surface, aerosols, and clouds. Surface spectral emissivity (SSE) and surface skin temperature from current and future operational satellites can and will reveal critical information about the Earth's ecosystem and land-surface-type properties, which might be utilized as a means of long-term monitoring of the Earth's environment and global climate change. In this study, fast radiative transfer models applied to the atmosphere under all weather conditions are used for atmospheric profile and surface or cloud parameter retrieval from ultraspectral and/or hyperspectral spaceborne IR soundings. An inversion scheme, dealing with cloudy as well as cloud-free radiances observed with ultraspectral IR sounders, has been developed to simultaneously retrieve atmospheric thermodynamic and surface or cloud microphysical parameters. This inversion scheme has been applied to the Infrared Atmospheric Sounding Interferometer (IASI). Rapidly produced SSE is initially evaluated through quality control checks on the retrievals of other impacted surface and atmospheric parameters. Initial validation of retrieved emissivity spectra is conducted with Namib and Kalahari desert laboratory measurements. Seasonal products of global land SSE and surface skin temperature retrieved with IASI are presented to demonstrate seasonal variation of SSE.

In support of the Ice, Cloud, and land Elevation Satellite (ICESat)-II mission, this paper studies the bias in surface-elevation measurements caused by undetected thin clouds. The ICESat-II satellite may only have a 1064-nm single-channel lidar onboard. Less sensitive to clouds than the 532-nm channel, the 1064-nm channel tends to miss thin clouds. Previous studies have demonstrated that scattering by cloud particles increases the photon-path length, thus resulting in biases in ice-sheet-elevation measurements from spaceborne lidars. This effect is referred to as atmospheric path delay. This paper complements previous studies in the following ways: First, atmospheric path delay is estimated over the ice sheets based on cloud statistics from the Geoscience Laser Altimeter System onboard ICESat and the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard Terra and Aqua. Second, the effect of cloud particle size and shape is studied with the state-of-the-art phase functions developed for MODIS cirrus-cloud microphysical model. Third, the contribution of various orders of scattering events to the path delay is studied, and an analytical model of the first-order scattering contribution is developed. This paper focuses on the path delay as a function of telescope field of view (FOV). The results show that reducing telescope FOV can significantly reduce the expected path delay. As an example, the average path delays forFOV= 167 ¿rad (a 100-m-diameter circle on the surface) caused by thin undetected clouds by the 1064-nm channel over Greenland and East Antarctica are illustrated.

Light scattering by horizontally oriented platelike particles under normal incidence, such as ice plates or tree leaves under spaceborne lidar or radar waves, needs to be investigated for remote sensing of cirrus clouds or vegetation canopies. The solutions from the conventional geometrical ray tracing method for the scattering of electromagnetic waves by these particles are quite inaccurate because of the singularity problem that is inherent to this method. The scattering properties of large horizontally oriented platelike particles are usually approximated by using physical optics or electromagnetic wave theory while ignoring the side-face effect of the plates. In this paper, to examine the effect of side faces on light scattering by platelike particles, a 2-D finite-difference time-domain technique is applied to calculate light scattering by horizontally oriented ice and leaf strips under normal or quasi-normal incidence. It is found that for moderate-sized strips, the side faces of the particles scatter a significant amount of energy, resulting in strong maxima in the scattering phase function at certain scattering angles. By ignoring the effect of side faces, the scattering phase functions derived from electromagnetic wave theory have significant errors for small or moderate-sized strips. However, the ratio of the amount of energy scattered by the side faces to the total scattered energy decreases with the increase of strip width. When the size parameter of the strip is in the limit of geometric optics, the side-face effect is reduced to a negligible amount. However, even in this case, the polarization degrees from the approximation solutions of physical optics or electromagnetic wave theory ignoring the side-face effect still have large errors.

The simplified ray-tracing technique reported in Part I of this paper is employed to compute the single-scattering properties of hexagonal columns with maximum dimensions ranging from 2 to 3500 mum with a size-bin resolution of 2 mum at wavelengths of 0.86 and 2.13 mum. For small ice crystals, the current treatment of surface roughness may not be adequate because the applicability of the principles of geometric optics breaks down for small roughness scale. However, for ice crystals smaller than 40 mum, the aspect ratios of these particles are close to one, and the effect of surface roughness is quite small. In this paper, the diffraction is accounted for in the same way as in the case of smooth particles. It is essentially unfeasible to incorporate the effect of surface roughness into the numerical computation of the diffraction contribution. The scattering properties of individual ice crystals are then averaged over 18 particle size distributions whose effective particle radii (re) range from 5 to 90 mum. The single-scattering properties of ice clouds are strongly sensitive to surface roughness condition. Lookup tables that are built for the correlation between the bidirectional reflectances at wavelengths of 0.86 and 2.13 mum with different roughness conditions are used to retrieve ice cloud optical thickness and effective particle size over oceans. Pronounced differences are noticed for the retrieved cirrus cloud optical thickness and effective particle sizes in conjunction with different surface roughness conditions. The values of the retrieved cirrus cloud optical thickness in the case of the rough surface are generally smaller than their counterparts associated with smooth surface conditions. The effect of surface roughness on the retrieved effective particle radii is not pronounced for slight and moderate roughness conditions. However, when the surfaces of ice crystals are substantially rough, the retrieved effective radii associated with roughened particles are larger and smaller than their smooth surface counterparts forlarge (re>50 mum) and small (ree<50 mum). In general, the dominant effect of surface roughness on cloud property retrievals is to decrease the retrieved optical thickness and to increase the retrieved effective particle size in comparison with their counterparts in the case of smooth ice particles.

Surface roughness of ice crystals is a morphological parameter important to the scattering characteristics of these particles. The intent of this paper, reported in two parts (hereafter, Parts I and II), is to investigate the accuracy associated with some simplifications in calculating the single-scattering properties of roughened ice crystals and to quantify the effect of surface roughness on the retrieval of the optical and microphysical properties of ice clouds from satellite observations. In Part I, two ray-tracing schemes, a rigorous algorithm and an approximate algorithm with a simplified treatment of surface roughness, are employed to calculate the single-scattering properties of randomly oriented hexagonal ice crystals with size parameters in the geometric optics regime. With the rigorous approach, it requires substantial computational effort to accurately account for the multiple external reflections between various roughness facets and the reentries of outgoing rays into the particles in the ray-tracing computation. With the simplified ray-tracing scheme, the ray-tracing calculation for roughened particles is similar to that for smooth particles except that, in the former case, the normal of the particle surface is statistically perturbed for each reflection-refraction event. The simplified ray-tracing scheme can account for most the effects of surface roughness on particle single-scattering properties without incurring substantial demand on computational resources and, thus, provides an efficient way to compute the single-scattering properties of roughened particles. The effect of ice-crystal surface roughness on the retrieval of the optical thicknesses and effective particle sizes of cirrus clouds is reported in Part II.

An algorithm has been developed to infer ice cloud optical depth from the isolated cirrus reflectance derived from Moderate Resolution Imaging Spectroradiometer (MODIS) observations. The present method is a modification of a previous study, which, due to limitations in the assumed ice particle habit distribution, limited ice cloud optical depth retrieval to the tropics. Here, the bulk scattering properties of ice clouds are updated, utilizing the new ice crystal size and habit distributions developed for the latest MODIS collection 5 operational cloud retrieval algorithm. The cirrus reflectance parameter, which is derived from reflectance measurements in the 0.66- and 1.375-mum spectral bands, is highly sensitive to ice cloud optical depth, allowing for optical depth retrieval. In the retrieval, an effective particle diameter of 50 mum is used, representing the peak of the global distribution of the effective particle sizes from the MODIS operational cloud products. The applicability of the algorithm is illustrated using daily cirrus reflectance data from the MODIS level-3 data set. The present method, which derives the contribution to optical depth by ice clouds only, is complementary to the operational MODIS ice cloud retrieval, which provides a total column optical depth.

Most satellite-based ice cloud retrieval algorithms rely on precomputed lookup libraries for inferring the ice cloud optical thickness (tau) and effective particle size ( De). However, this retrieval methodology does not account for the case where cloud geometrical thickness may vary by several kilometers. In this paper, we investigate the effect of the ice cloud geometrical thickness on the retrieval of tau and Defor algorithms using the Moderate Resolution Imaging Spectroradiometer infrared (IR) bands at 8.5 and 11 mum (or 12 mum) or solar bands at 0.65 and 1.64 mum (or 2.13 mum). We use a rigorous radiative transfer package to simulate the IR brightness temperatures and solar reflectances, assuming that the ice cloud top height is fixed at 12 or 15 km with a variation of cloud geometrical thickness from 0.5 to 5 km. The simulated brightness temperatures and reflectances are then used to investigate the errors of cloud tau and Deinferred from the precomputed lookup tables developed with a specific geometrical thickness. It is found that the retrieval errors in tau and Deincrease with increasing tau for the IR and solar methods. In both cases, cloud tau and Demay be underestimated and overestimated, respectively, if the effect of the cloud geometrical thickness is not taken into account. The effect of the cloud geometrical thickness on the retrieval of cloud optical and microphysical properties is much larger for the IR algorithm than for the solar-band-based algorithm. This paper demonstrates that the inclusion of the information about the cloud geometrical thickness may improve the accuracy of the retrieval of the cloud properties on the basis of the precomputed lookup libraries

This paper reports on the comparison of two latest versions (collections 4 and 5) of ice cloud products derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) measurements. The differences between the bulk optical properties of ice clouds used in collections 4 and 5 and the relevant impact on simulating the correlation of the bidirectional reflection functions at two MODIS bands centered at 0.65 (or 0.86) and 2.13 mum are investigated. The level-3 MODIS ice cloud properties (specifically, ice cloud fraction, optical thickness, and effective particle size in this paper) from the collection 4 and 5 datasets are compared for a tropical belt of 30deg S-30deg N. Furthermore, the impact of the differences between the MODIS collection 4 and 5 ice cloud products on the simulation of the radiative forcing of these clouds is investigated. Over the tropics, the averaged ice cloud fraction from collection 5 is 1.1% more than the collection 4 counterpart, the averaged optical thickness from collection 5 is 1.2 larger than the collection 4 counterpart, and the averaged effective particle radius from collection 5 is 1.8 mum smaller than the collection 4 counterpart. Moreover, the magnitude of the differences between collection 5 and 4 ice cloud properties also depends on the surface characteristics, i.e., over land or over ocean. The differences of these two datasets (collections 4 and 5) of cloud properties can have a significant impact on the simulation of the radiative forcing of ice clouds. In terms of total (longwave plus shortwave) cloud radiative forcing, the differences between the collection 5 and 4 results are distributed primarily between -60 and 20 W ldr m-2but peak at 0 W ldr m-2.

Cloud microphysical and optical properties are inferred from the bidirectional reflectances simulated for a single-layered cloud consisting of an external mixture of ice particles and liquid droplets. The reflectances are calculated with a rigorous discrete ordinates radiative transfer model and are functions of the cloud effective particle size, the cloud optical thickness, and the values of the ice fraction in the cloud (i.e., the ratio of ice water content to total water content). In the present light scattering and radiative transfer simulations, the ice fraction is assumed to be vertically homogeneous; the habit (shape) percentage as a function of ice particle size is consistent with that used for the Moderate Resolution Imaging Spectroradiometer (MODIS) operational (Collection 4 and earlier) cloud products; and the surface is assumed to be Lambertian with an albedo of 0.03. Furthermore, error analyses are performed that pertain to the inference of the mixed-phase cloud effective particle size and optical thickness. Errors are calculated with respect to the assumption of a cloud containing solely liquid or ice phase particles. The analyses suggest that the effective particle size inferred for a mixed-phase cloud can be underestimated (or overestimated) if pure liquid phase (or pure ice phase) is assumed for the cloud, whereas the corresponding cloud optical thickness can be overestimated (or underestimated)

The ground-based infrared radiance measurements acquired on July 14, 20, and 28, 2002 during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers Florida Area Cirrus Experiment (CRYSTAL-FACE) campaign have been used for simultaneously retrieving the optical thickness and effective particle size on the basis of the retrieval algorithm reported in the preceding counterpart of this paper. The corresponding ice water path is derived from the retrieved optical thickness and effective particle size. Specifically, the data used for the retrieval include: 1) the infrared radiance spectrum observed by an atmospheric emitted radiance interferometer at the surface; 2) the sky condition and cloud height determined from a sky imager and a micropulse lidar; and 3) the sounding data for the profiles of temperature, pressure, and relative humidity. For these three case studies, the retrieved cirrus optical thickness, effective particle size, and ice water path are in the range of 0.2-1.5, 18-42 μm, and 2-15 g /spl middot/ m/sup -2/, respectively. Furthermore, error analyses show that the retrieval uncertainties of the optical thickness and effective particle size are less than 15% if the uncertainty of water vapor vertical profile is within 5%. The retrieval errors are within 10% if the uncertainty of cloud temperature is within 7 K.

The surface radiance spectrum within the terrestrial infrared window (i.e., wavelengths between 8-12 μm or wavenumbers between 833-1250 cm/sup -1/) is sensitive to the optical and microphysical properties of cirrus clouds. Numerous microwindows where atmospheric absorption is minimum exist in the spectral regions of 820-960 cm/sup -1/ and 1100-1240 cm/sup -1/. The minimum radiances at the microwindows in these two spectral regions can be fitted by using two linear lines. The slope of the fitting line for the spectral region of 820-960 cm/sup -1/ is sensitive to the effective size of ice crystals within cirrus clouds, whereas the intercept of the fitting line for the spectral region of 1100-1240 cm/sup -1/ is sensitive to the optical thickness of the clouds. Based on this spectral feature, a new retrieval method has been developed for simultaneously retrieving cirrus optical thickness and the effective particle size of ice crystals. Furthermore, the ice water path of cirrus clouds can be estimated from the retrieved values of cloud optical thickness and effective particle size.

Techniques for retrieving cloud optical properties, i.e., the optical depths and particle size distributions, using atmospheric "window" channels in the visible and near-infrared spectral regions are well established. For partially transparent thin cirrus clouds, these "window" channels receive solar radiances scattered by the surface and lower level water clouds. Accurate retrieval of optical properties of thin cirrus clouds requires proper modeling of the effects from the surface and the lower level water clouds. In this paper, we describe a new concept using two strong water vapor absorption channels near 1.38 and 1.88 /spl mu/m, together with one window channel, for remote sensing of cirrus optical properties. Both the 1.38- and 1.88-/spl mu/m channels are highly sensitive in detecting the upper level cirrus clouds. Both channels receive little scattered solar radiances from the surface and lower level water clouds because of the strong water vapor absorption below cirrus. The 1.88-/spl mu/m channel is quite sensitive to changes in ice particle size distributions, while the 1.38-/spl mu/m channel is less sensitive. These properties allow for simultaneous retrievals of optical depths and particle size distributions of cirrus clouds with minimal contaminations from the surface and lower level water clouds. Preliminary tests of this new concept are made using hyperspectral imaging data collected with the Airborne Visible Infrared Imaging Spectrometer. The addition of a channel near 1.88 /spl mu/m to future multichannel meteorological satellite sensors would improve our ability in global remote sensing of cirrus optical properties.

An objective method is developed to monitor the stability of spaceborne instruments, aimed at distinguishing climate trend from instrument drift in satellite-based climate observation records. This method is based on four-years of Clouds and the Earth's Radiant Energy System (CERES) broadband observations of deep convective cloud systems with cloud-top temperature lower than 205 K and with large optical depths. The implementation of this method to the CERES instrument stability analysis reveals that the monthly albedo distributions are practically the same for deep convective clouds with CERES measurements acquired from both the Tropical Rainfall Measuring Mission and Terra satellite platforms, indicating that CERES instruments are well calibrated and stable during both missions. Furthermore, with a nonlinear regression neural network narrowband-broadband conversion, this instrument-stability monitoring method can also be applied to narrowband instruments such as the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Visible Infrared Scanner (VIRS). The results show that the drifts associated with both VIRS and MODIS instruments are less than 1% during a four-year period. Since the CERES albedo measurements are highly accurate, the absorptance of these opaque clouds can be reliably estimated. The absorptions of these clouds from observations are around 25%, whereas the absorptions from theory can be as low as 18%, depending on ice cloud microphysics.

The theoretical basis is explored for inferring the microphysical properties of ice clouds from high spectral resolution infrared (IR) observations. Extensive radiative transfer simulations are carried out to address relevant issues. The single-scattering properties of individual ice crystals are computed from state-of-the-art light scattering computational methods and are subsequently averaged for 30 in situ particle size distributions and for four additional analytical Gamma size distributions. The nonsphericity of ice crystals is shown to have a significant impact on the radiative signatures in the IR spectrum. Furthermore, the errors associated with the use of the Henyey-Greenstein phase function can be larger than 1 K in terms of brightness temperature for large particle effective sizes (/spl sim/80 /spl mu/m) at wavenumbers where the scattering of the IR radiation by ice crystals is not negligible. The simulations undertaken in this paper show that the slope of the IR brightness temperature spectrum between 790-960 cm/sup -1/ is sensitive to the effective particle size. Furthermore, a strong sensitivity of the IR brightness temperature to cloud optical thickness is noted within the 1050-1250-cm/sup -1/ region. Based on these spectral features, a technique is presented for the simultaneous retrieval of the visible optical thickness and effective particle size from high spectral resolution IR data for ice clouds. An error analysis shows that the uncertainties of the retrieved optical thickness and effective particle size have a small range of variation. The error for retrieving particle size in conjunction with an uncertainty of 5 K in cloud temperature, or a surface temperature uncertainty of 2.5 K, is less than 15%. The corresponding errors in the uncertainty of optical thickness are within 5% to 20%, depending on the value of cloud optical thickness. The applicability of the present retrieval technique is demonstrated using airborne high-resolution IR measurements obtained during two field campaigns.

In this paper, we introduce a method to retrieve the optical thickness of tropical cirrus clouds using the isolated visible cirrus reflectance (without atmospheric and surface effects). The isolated cirrus reflectance is inferred from level 1b calibrated 0.66- and 1.375-/spl mu/m Moderate Resolution Imaging Spectroradiometer (MODIS) data. We created an optical properties database and optical thickness lookup library using previously calculated single-scattering data in conjunction with the discrete ordinates radiative transfer (DISORT) code. An algorithm was constructed based on this lookup library to infer the optical thickness of tropical cirrus clouds for each pixel in a MODIS image. We demonstrate the applicability of this algorithm using several independent MODIS images from the Terra satellite. The present method is complimentary to the MODIS operational cloud retrieval algorithm for the case of cirrus clouds.

An approach is developed to infer the optical thickness of semitransparent ice clouds (when optical thickness is less than 5) from Atmospheric Infrared Sounder (AIRS) high spectral resolution radiances. A fast cloud radiance model is developed and coupled with an AIRS clear-sky radiative transfer model for simulating AIRS radiances when ice clouds are present. Compared with more accurate calculations based on the discrete ordinates radiative transfer model, the accuracy of the fast cloud radiance model is within 0.5 K (root mean square) in terms of brightness temperature (BT) and runs three orders of magnitude faster. We investigate the sensitivity of AIRS spectral BTs and brightness temperature difference (BTD) values between pairs of wavenumbers to the cloud optical thickness. The spectral BTs for the atmospheric window channels within the region 1070-1135 cm/sup -1/ are sensitive to the ice cloud optical thickness, as is the BTD between 900.562 cm/sup -1/ (located in an atmospheric window) and 1558.692 cm/sup -1/ (located in a strong water vapor absorption band). Similarly, the BTD between a moderate absorption channel (1587.495 cm/sup -1/) and the strong water absorption channel (1558.692 cm/sup -1/) is sensitive to ice cloud optical thickness. Neither of the aforementioned BTDs is sensitive to the effective particle size. Thus, the optical thickness of semitransparent ice clouds can be retrieved reliably. We have developed a spectrum-based approach and a BTD-based method to retrieve the optical thickness of semitransparent ice clouds. The present retrieval methods are applied to a granule of AIRS data. The ice cloud optical thicknesses derived from the AIRS measurements are compared with those retrieved from the Moderate Resolution Imaging Spectroradiometer (MODIS) 1.38and 0.645-/spl mu/m bands. The optical thicknesses inferred from the MODIS measurements are collocated and degraded to the AIRS spatial resolution. Results from the MODIS and AIRS retrievals are in reasonable agreement over a wide range of optical thicknesses.

The seasonal variations of water vapor and cirrus clouds over the Tibetan Plateau are investigated using the recently available Level 3 monthly-mean atmospheric data products with a 1/spl deg/ /spl times/ 1/spl deg/ latitude-longitude grid. The data products are derived from the multichannel imaging data acquired with the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra Spacecraft. It is shown that the water vapor concentration over the Tibetan Plateau is normally low, whereas high clouds (mainly cirrus clouds) over the Plateau occur quite frequently. On an annual scale, the water vapor concentration reaches its maximum in July and its minimum in January. During the summer season, the southeastern part of Tibetan Plateau, which can be affected by moistures originating from the Bay of Bengal and southeastern Asia, is slightly moister than the other parts of the Plateau. This observation is in agreement with the previous surface meteorological measurements by Chinese scientists from the 1950s to mid-1970s. The mean high-cloud reflectance over the Plateau reaches its maximum in April and minimum in November. This feature of high clouds over the Plateau has not been reported previously. The special channel centered at 1.375-/spl mu/m on the MODIS instrument has allowed the observation. We present a plausible mechanism to explain the seasonal variations of high clouds over the Plateau. We expect that the water vapor and high-cloud measurements with MODIS can be used to improve the model initialization and validation for climate models involving the Tibetan Plateau and the nearby regions in Asia.

Identification of clouds over Earth's polar regions is difficult from satellite radiometric measurements in the visible and infrared (IR) atmospheric window regions because of the high albedos of snow- and ice-covered surfaces in the visible and the low-temperature contrast between the surface and the troposphere in the IR. The Moderate Resolution Imaging SpectroRadiometer (MODIS) on the Terra Spacecraft has a near-IR channel located within the strong water vapor absorption regions close to 1.38 /spl mu/m. This channel was originally designed for remote sensing of high-altitude clouds in the tropical and mid-latitude regions. In this paper, we report that this channel is also quite useful for detecting high clouds in polar regions during the daytime. Comparisons with IR emission techniques for polar cloud detections are also presented.

The backscattering of radiation at 0.532 and 1.064 /spl mu/m wavelengths by quasi-horizontally oriented hexagonal ice plates is investigated. The geometric optics ray tracing method is not applicable to the scattering problem associated with oriented ice crystals (in particular, in backscattering direction) because of the singularity of the ray-tracing technique. In the present study, we solve the scattered field of quasi-horizontally oriented ice plates using an approach based on electromagnetic wave theory. The effect of side faces of a plate on the internal field inside the particle is ignored. This is a reasonable approximation under the condition that the ratio of particle diameter to its length is large or the tilt of the particle symmetric axis from the zenith is small. Preliminary results show very strong resonant oscillations of backscattering cross section versus size parameter. The bulk backscattering intensity has been calculated by including size distribution and a random tilt of particle symmetric axis within a small angular region (2/spl deg/).

The Moderate Resolution Imaging Spectro-Radiometer (MODIS) on the Terra spacecraft has a channel near 1.38 /spl mu/m for remote sensing of high clouds from space. The implementation of this channel on MODIS was primarily based on previous analysis of hyperspectral imaging data collected with the Airborne Visible Infrared Imaging Spectrometer (AVIRIS). We describe an algorithm to retrieve cirrus bidirectional reflectance using channels near 0.66 and 1.38 /spl mu/m. It is shown that the apparent reflectance of the 1.38-/spl mu/m channel is essentially the bidirectional reflectance of cirrus clouds attenuated by the absorption of water vapor above cirrus clouds. A practical algorithm based on the scatterplot of 1.38-/spl mu/m channel apparent reflectance versus 0.66-/spl mu/m channel apparent reflectance has been developed to scale the effect of water vapor absorption so that the true cirrus reflectance in the visible spectral region can be obtained. To illustrate the applicability of the present algorithm, results for cirrus reflectance retrievals from AVIRIS and MODIS data are shown. The derived cirrus reflectance in the spectral region of 0.4-1 /spl mu/m can be used to remove cirrus contamination in a satellite image obtained at a visible channel. An example of such an application is shown. The spatially averaged cirrus reflectances derived from MODIS data can be used to establish global cirrus climatology, as is demonstrated by a sample global cirrus reflectance image.