S. C. Ou

Remote sensing of cloud properties using MODIS airborne simulator imagery during SUCCESS: 1. Data and models

We investigate methods to infer cloud properties such as cloud optical thickness, thermodynamic phase, cloud particle size, and cloud overlap by comparing cloud and clear-sky radiative transfer computations to measurements provided by the Moderate Resolution Imaging Spectroradiometer (MODIS) airborne simulator (MAS). The MAS scanning spectroradiometer was flown on the NASA ER-2 during the Subsonic Aircraft Contrail and Cloud Effects Special Study (SUCCESS) field campaign during April and May 1996. The MAS bands chosen for this study correspond to wavelengths of 0.65, 1.63, 1.90, 2.15, 3.82, 8.52, 11, and 12 μm. Clear-sky absorption due to water vapor, ozone, and other trace gases is calculated using a set of correlated k-distribution routines developed specifically for these MAS bands. Scattering properties (phase function, single-scattering albedo, and extinction cross section) are derived for water droplet clouds using Mie theory. Scattering properties for ice-phase clouds are incorporated for seven cirrus models: cirrostratus, cirrus uncinus, cold cirrus, warm cirrus, and cirrus at temperatures of T = −20°C, −40°C, and −60°C. The cirrus are composed of four crystal types: hexagonal plates, two-dimensional bullet rosettes, hollow columns, and aggregates. Results from comparison of MAS data from a liquid water cloud with theoretical calculations indicate that estimates of optical thickness and particle size are reasonably consistent with one another no matter which spectral bands are used in the analysis. However, comparison of MAS data from a cirrus cloud with theoretical calculations shows consistency in optical thickness but not with particle size among the various band combinations used in the analysis. The methods described in this paper are used in two companion papers to explore techniques to infer cloud thermodynamic phase and cloud overlap.

Remote sensing of cirrus cloud parameters using advanced very-high-resolution radiometer 3.7- and 10.9-μm channels

We develop a retrieval scheme by using advanced very-high-resolution radiometer (AVHRR) 3.7- and 10.9-microm data to compute simultaneously the temperature, optical depth, and mean effective ice-crystal size for cirrus clouds. The methodology involves the numerical solution of a set of nonlinear algebraic equations derived from the theory of radiative transfer. The solution requires the correlation of emissivities of two channels in terms of the effective extinction ratio. The dependence of this ratio on ice-crystal size distribution is examined by using an adding-doubling radiative transfer program. Investigation of the effects of cirrus parameters on upwelling radiances reveals that the brightnesstemperature difference between the two channels becomes larger for colder cirrus and smaller ice-crystal sizes. We apply the current retrieval scheme to satellite data collected at 0930 UTC, 28 October 1986, over the region of the First International Satellite Cloud Climatology Project Regional Experiment CirrusIntesive Field Observation. We select the data over an area (~ 44 degrees N, 92 degrees W) near Fort McCoy, Wisconsin, for analysis. The retrieved cirrus heights compare reasonably well with lidar measurements taken at Fort McCoy 2 h after a satellite overpass at the target region. The retrieved mean effective crystal size is close to that derived from in situ aircraft measurements over Madison, Wisconsin, six hours after a satellite overpass.

Remote Sensing of Cirrus Cloud Parameters Based on a 0.63 - 3.7 µm Radiance Correlation Technique Applied to AVHRR Data

Using the data gathered from the Advanced Very High Resolution Radiometer (AVHRR) 0.63 and 3.7 µm channels, an algorithm for the inference of cirrus cloud optical depth and mean effective size has been developed for the first time. This scheme is based on the correlation between the 3.7 µm (total) and 0.63 µm radiances that is constructed from radiative transfer calculations involving ice crystal clouds. Application of the algorithm to AVHRR channels has been performed by using data sets that were collected during the First ISCCP Regional Experiment, Phase II, Cirrus Intensive Field Observation (FIRE-II-IFO; November- December 1991) at Coffeyville, Kansas. For validation, the in-situ data collected by the balloon-borne replicator and airborne 2-D probes that were collocated with AVHRR pixels were carefully analyzed for five cases involving single and multiple cirrus cloud layers. We demonstrate that the retrieved cirrus cloud optical depths and mean effective sizes compare reasonably well with those determined from the in- situ analyses.

Remote sensing of cirrus cloud optical thickness and effective particle size for the National Polar-orbiting Operational Environmental Satellite System Visible/infrared imager Radiometer Suite: sensitivity to instrument noise and uncertainties in environmental parameters

We describe sensitivity studies on the remote sensing of cirrus cloud optical thickness and effective particle size using the National Polar-orbiting Operational Environmental Satellite System Visible/Infrared Imager Radiometer Suite 0.67-, 1.24-, 1.61-, and 2.25-microm reflectances and thermal IR 3.70- and 10.76-microm radiances. To investigate the accuracy and precision of the solar and IR retrieval methods subject to instrument noise and uncertainties in environmental parameters, we carried out signal-to-noise ratio tests as well as the error budget study, where we used the University of California at Los Angeles line-by-line equivalent radiative transfer model to generate radiance tables for synthetic retrievals. The methodology and results of these error analyses are discussed.

Surface aerosol radiative forcing derived from collocated ground-based radiometric observations during PRIDE, SAFARI, and ACE-Asia

An approach is presented to estimate the surface aerosol radiative forcing by use of collocated cloud-screened narrowband spectral and thermal-offset-corrected radiometric observations during the Puerto Rico Dust Experiment 2000, South African Fire Atmosphere Research Initiative (SAFARI) 2000, and Aerosol Characterization Experiment-Asia 2001. We show that aerosol optical depths from the Multiple-Filter Rotating Shadowband Radiometer data match closely with those from the Cimel sunphotometer data for two SAFARI-2000 dates. The observed aerosol radiative forcings were interpreted on the basis of results from the Fu-Liou radiative transfer model, and, in some cases, cross checked with satellite-derived forcing parameters. Values of the aerosol radiative forcing and forcing efficiency, which quantifies the sensitivity of the surface fluxes to the aerosol optical depth, were generated on the basis of a differential technique for all three campaigns, and their scientific significance is discussed.

Laser transmission through thin cirrus clouds

A near-infrared airborne-laser transmission model for thin cirrus clouds has been developed on the basis of the successive-order-of-scattering approach to account for multiple scattering by randomly and horizontally oriented ice crystals associated with an aircraft-target system. Direct transmission and transmission due to multiple scattering are formulated specifically for this geometric system, in which scattering and absorption associated with aerosols, water vapor, and air are accounted for. A number of sensitivity experiments have been performed for investigation of the effect of aircraft-target position, cirrus cloud optical depth, and ice crystal size on laser transmission for tactical applications. We show that transmission contributions produced by orders of scattering higher than 1 are small and can be neglected. The possibility of horizontal orientation of ice crystals can enhance transmission of laser beams in the aircraft-target geometry. Transmitted energy is strongly dependent on the horizontal distance between the aircraft and the target and on the cloud optical depth as well as on whether the cloud is above or below the aircraft.

Laser transmission-backscattering through inhomogeneous cirrus clouds

We have developed a two-dimensional (2D) model for inhomogeneous cirrus clouds in plane-parallel and spherical geometries for the analysis of the transmission and backscattering of high-energy laser beams. The 2D extinction-coefficient and mean effective ice-crystal size fields for cirrus clouds can be determined from a combination of the remote sensing of cirrus clouds by use of the Advanced Very High Resolution Radiometer on board National Oceanic and Atmospheric Administration satellites and the vertical profiling of ice-crystal size distributions available from limited measurements. We demonstrate that satellite remote sensing of the position and the composition of high cirrus can be incorporated directly in the computer model developed for the transmission and backscattering of high-energy laser beams in realistic atmospheres. The results of laser direct transmission, forward scattering, and backscattering are analyzed carefully with respect to aircraft height, cirrus cloud optical depth, and ice-crystal size and orientation. Uncertainty in laser transmission that is due to errors in the retrieved ice-crystal size is negligible. But uncertainty of the order of 2% can be produced if the retrieved optical depth has errors of +/-0.05. With both the aircraft and the target near the cloud top, the direct transmission decreases, owing to the propagation of the laser beam through the curved portion of the cloud top. This effect becomes more pronounced as the horizontal distance between the aircraft and the target increases.

Satellite remote sensing of dust aerosol indirect effects on cloud formation over Eastern Asia

The dust aerosol indirect effect of the first kind on ice and liquid water cloud formation has been investigated using available MODIS cloud and aerosol products on the basis of correlation analysis. The variability in the correlation between cloud parameters, including optical depth, effective particle size, cloud water path and cloud particle number concentration, and aerosol variables, including optical depth and number concentration, over Eastern Asia has been studied. Three MODIS scenes that contain a significant presence of local and transported dust and clouds have been selected for comprehensive analysis. For all cases studied, we demonstrate that there is a negative trend regarding the correlation between cloud particle size and aerosol optical depth, which is statistically significant. These results represent a strong evidence of dust and cloud interactions that are consistent with the hypothesis of the Twomey effect for clouds.

Comparison of the University of California at Los Angeles Line-by-Line Equivalent Radiative Transfer Model and the Moderate-Resolution Transmission Model for accuracy assessment of the National Polar-Orbiting Operational Environmental Satellite System’s Visible–Infrared Imager–Radiometer Suite cloud algorithms

To support the verification and implementation of the National Polar-Orbiting Operational Environmental Satellite Systems Visible-Infrared Imaging-Radiometric Suite (VIIRS) algorithms used for inferring cloud environmental data records, an intercomparison effort has been carried out to assess the consistency between the simulated cloudy radiances-reflectances from the University of California at Los Angeles Line-by-Line Equivalent Radiative Transfer Model and those from the Moderate-Resolution Transmission Model (MODTRAN) with the 16 stream Discrete Ordinate Radiative Transfer Model (DISORT) incorporated. For typical ice and water cloud optical depths and particle sizes, we found discrepancies in the visible and near-infrared reflectances from the two models, which presumably are due to the difference in phase function (nonspherical versus Henyey-Greenstein), different numbers of phase function expansion terms (16 versus 200 terms), and different treatment of forward peak truncation in each model. Using the MODTRAN4, we also found substantial differences in the infrared radiances for optically thick clouds. These differences led to the discovery by MODTRAN4 developers of an inconsistency in the MODTRAN4-DISORT interface. MODTRAN4 developers corrected the inconsistency, which provided dramatic reductions in the differences between the two radiative transfer models. The comparison not only affects the prospective test plan for the VIIRS cloud algorithms but also should lead to improvements in future MODTRAN releases.

Correction to “Simultaneous detection/separation of mineral dust and cirrus clouds using MODIS thermal infrared window data”

[1] In the paper ‘‘Simultaneous detection/separation of mineral dust and cirrus clouds using MODIS thermal infrared window data’’ by R. A. Hansell et al. (Geophysical Research Letters, 34, L11808, doi:10.1029/2007GL029388, 2007), the authors inadvertently omitted the following acknowledgment. [2] This research was also supported in part by NGST contract 97904DDM3S managed by A. Dybdahl and M. Mussetto. GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L13802, doi:10.1029/2007GL031035, 2007