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Bastiaan van Diedenhoven
and
Brian Cairns

Abstract

We provide a parameterization of the extinction efficiency, single-scattering albedo, and asymmetry parameter of single ice crystals with any combination of particle volume, projected area, component aspect ratio, and crystal distortion at any wavelength between 0.2 and 100 μm. The parameterization is an extension of the one previously published by van Diedenhoven et al. In addition, the parameterized optical properties are integrated over size distributions yielding bulk extinction efficiencies, single-scattering albedos, and asymmetry parameters for large ranges of effective radii, particle component aspect ratios, and crystal distortion values. The parameterization of single-particle optical properties is evaluated with a reference database. The bulk optical properties are evaluated against the ice model selected for the Moderate Resolution Imaging Spectroradiometer (MODIS) collection 6 products, for which accurate optical properties are available. Mean absolute errors in parameterized extinction efficiency, asymmetry parameter, and single-scattering albedo are shown to be 0.0272, 0.008 90, and 0.004 68, respectively, for shortwave wavelengths, while they are 0.0641, 0.0368, and 0.0200 in the longwave. Shortwave and longwave asymmetry parameters and single-scattering albedos are shown to vary strongly with particle component aspect ratio and distortion, resulting in substantial variation in shortwave fluxes, but relatively small variations in longwave cloud emissivity. The parameterization and bulk optical properties are made publicly available.

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Jacek Chowdhary
,
Brian Cairns
, and
Larry D. Travis

Abstract

To evaluate the global effects of aerosols on the direct radiative balance, tropospheric chemistry, and cloud properties of the earth's atmosphere requires high-precision remote sensing that is sensitive to the aerosol optical thickness, size distribution, refractive index, and number density. This study uses the multiangle 0.41-, 0.55-, 0.865-, and 2.25-μm channel data from the airborne Research Scanning Polarimeter to retrieve aerosol properties over the Pacific Ocean. It is shown that such photopolarimetric data are highly sensitive to the size distribution and refractive index of aerosol particles, which reduces the nonuniqueness in aerosol retrievals using such data as compared with less comprehensive datasets. Moreover, it is found that polarized reflectances obtained at the shorter wavelengths (0.41 and 0.55 μm) are significantly less sensitive to the contribution of the ocean's upwelling light than total reflectance measurements, providing a natural tool for the separation between the estimation of oceanic and atmospheric scattering properties.

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William B. Rossow
,
Carl Delo
, and
Brian Cairns

Abstract

The effect of small spatial-scale cloud variations on radiative transfer in cloudy atmospheres currently receives a lot of research attention, but the available studies are not very clear about which spatial scales are important and report a very large range of estimates of the magnitude of the effects. Also, there have been no systematic investigations of how to measure and represent these cloud variations. The cloud climatology produced by the International Satellite Cloud Climatology Project (ISCCP) is exploited to 1) define and test different methods of representing cloud variation statistics; 2) investigate the range of spatial scales that should be included; 3) characterize cloud variations over a range of time- and space scales covering mesoscale (30–300 km, 3–12 h) into part of the lower part of the synoptic scale (300–3000 km, 1–30 days); 4) obtain a climatology of the optical thickness, emissivity, and cloud-top temperature variability of clouds that can be used in weather and climate GCMs, together with the parameterization proposed by Cairns et al., to account for the effects of small-scale cloud variations on radiative fluxes; and 5) evaluate the effect of observed cloud variations on the earth's radiation budget. These results lead to the formulation of a revised conceptual model of clouds for use in radiative transfer calculations in GCMs. The complete variability climatology can be obtained from the ISCCP Web site at http://isccp.giss.nasa.gov.

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Bastiaan van Diedenhoven
,
Andrew S. Ackerman
,
Brian Cairns
, and
Ann M. Fridlind

Abstract

A parameterization is presented that provides extinction cross section σ e , single-scattering albedo ω, and asymmetry parameter g of ice crystals for any combination of volume, projected area, aspect ratio, and crystal distortion at any wavelength in the shortwave. Similar to previous parameterizations, the scheme makes use of geometric optics approximations and the observation that optical properties of complex, aggregated ice crystals can be well approximated by those of single hexagonal crystals with varying size, aspect ratio, and distortion levels. In the standard geometric optics implementation used here, σ e is always twice the particle projected area. It is shown that ω is largely determined by the newly defined absorption size parameter and the particle aspect ratio. These dependences are parameterized using a combination of exponential, lognormal, and polynomial functions. The variation of g with aspect ratio and crystal distortion is parameterized for one reference wavelength using a combination of several polynomials. The dependences of g on refractive index and ω are investigated and factors are determined to scale the parameterized g to provide values appropriate for other wavelengths. The parameterization scheme consists of only 88 coefficients. The scheme is tested for a large variety of hexagonal crystals in several wavelength bands from 0.2 to 4 μm, revealing absolute differences with reference calculations of ω and g that are both generally below 0.015. Over a large variety of cloud conditions, the resulting root-mean-squared differences with reference calculations of cloud reflectance, transmittance, and absorptance are 1.4%, 1.1%, and 3.4%, respectively. Some practical applications of the parameterization in atmospheric models are highlighted.

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Brian Cairns
,
Andrew A. Lacis
, and
Barbara E. Carlson

Abstract

The effect on absorption in clouds of having an inhomogeneous distribution of droplets is shown to depend on whether one replaces a homogeneous cloud by an inhomogeneous cloud that has the same mean optical thickness, or one that has the same spherical albedo. For the purposes of general circulation models (GCMs), the more appropriate comparison is between homogeneous and inhomogeneous clouds that have the same spherical albedo, so that the radiation balance of the planet with space is maintained. In this case it is found, using Monte Carlo and independent pixel approximation calculations, that inhomogeneous clouds can absorb more than homogeneous clouds. It is also found that because of the different effects of cloud inhomogeneity on absorption and on the transmission of the direct beam the absorption efficiency of an inhomogeneous cloud may be either greater (for low and high optical depths) or lesser (for intermediate optical depths) than that for a homogeneous cloud of the same mean optical depth. This effect is relevant both to in-cloud absorption and to absorption below clouds. In order to include these effects in GCMs a simple renormalization of the single-scattering parameters of radiative transfer theory is derived that allows the effects of cloud inhomogeneities to be included in plane-parallel calculations. This renormalization method is shown to give reasonable results when compared with Monte Carlo calculations, has the appropiate limits for conservative and completely absorbing cases, and provides a simple interpretation of the effects of cloud inhomogeneities that could readily be incorporated in a GCM.

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Bastiaan van Diedenhoven
,
Ann M. Fridlind
,
Andrew S. Ackerman
, and
Brian Cairns

Abstract

Satellite measurements are used to evaluate the glaciation, particle shape, and effective radius in cloud-resolving model simulations of tropical deep convection. Multidirectional polarized reflectances constrain the ice crystal geometry and the thermodynamic phase of the cloud tops, which in turn are used to calculate near-infrared reflectances so as to constrain the simulated ice effective radius, thereby avoiding inconsistencies between retrieval algorithms and model simulations. Liquid index values derived from Polarization and Directionality of the Earth’s Reflectances (POLDER) measurements indicate only ice-topped clouds at brightness temperatures (BTs) lower than −40°C, only liquid clouds at BT > −20°C, and both phases occurring at temperatures in between. Liquid index values calculated from model simulations generally reveal too many ice-topped clouds at BT > −20°C. The model assumption of platelike ice crystals with an aspect ratio of 0.7 is found consistent with POLDER measurements for BT < −40°C when very rough ice crystals are assumed, leading to an asymmetry parameter of 0.74, whereas measurements indicate more extreme aspect ratios of ~0.15 at higher temperatures, yielding an asymmetry parameter of 0.84. MODIS-retrieved ice effective radii are found to be 18–28 μm at BT < −40°C, but biased low by about 5 μm owing primarily to the assumption of pristine crystals in the retrieval. Simulated 2.13-μm reflectances at BT < −40°C are found to be about 0.05–0.1 too large compared to measurements, suggesting that model-simulated effective radii are 7–15 μm too small. Two simulations with contrasting ice nucleation schemes showed little difference in simulated effective radii at BT < −40°C, indicating that homogeneous nucleation is dominating in the simulations. Changes around −40°C in satellite observations suggest a change in cloud-top ice shape and/or size in natural deep convection possibly related to a change in the freezing mechanism.

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Bastiaan van Diedenhoven
,
Andrew S. Ackerman
,
Ann M. Fridlind
, and
Brian Cairns

Abstract

The use of ensemble-average values of aspect ratio and distortion parameter of hexagonal ice prisms for the estimation of ensemble-average scattering asymmetry parameters is evaluated. Using crystal aspect ratios greater than unity generally leads to ensemble-average values of aspect ratio that are inconsistent with the ensemble-average asymmetry parameters. When a definition of aspect ratio is used that limits the aspect ratio to below unity for both hexagonal plates and columns, the effective asymmetry parameters calculated using ensemble-average aspect ratios are generally consistent with ensemble-average asymmetry parameters, especially if aspect ratios are geometrically averaged. Ensemble-average distortion parameters generally also yield effective asymmetry parameters that are largely consistent with ensemble-average asymmetry parameters. In the case of mixtures of plates and columns, it is recommended to geometrically average the aspect ratios and to subsequently calculate the effective asymmetry parameter using a column or plate geometry when the contribution by columns to a given mixture’s total projected area is greater or less than 50%, respectively. In addition, it is shown that ensemble-average aspect ratios, distortion parameters, and asymmetry parameters can generally be retrieved accurately from simulated multidirectional polarization measurements based on mixtures of varying columns and plates. However, such retrievals tend to be somewhat biased toward yielding columnlike aspect ratios. Furthermore, generally large retrieval errors can occur for mixtures with approximately equal contributions of columns and plates and for ensembles with strong contributions of thin plates.

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Mikhail D. Alexandrov
,
Alexander Marshak
,
Brian Cairns
, and
Andrew S. Ackerman

Abstract

We present a generalization of the binary-value Markovian model previously used for statistical characterization of cloud masks to a continuous-value model describing 1D fields of cloud optical thickness (COT). This model has simple functional expressions and is specified by four parameters: the cloud fraction, the autocorrelation (scale) length, and the two parameters of the normalized probability density function of (nonzero) COT values (this PDF is assumed to have gamma-distribution form). Cloud masks derived from this model by separation between the values above and below some threshold in COT appear to have the same statistical properties as in binary-value model described in our previous publications. We demonstrate the ability of our model to generate examples of various cloud-field types by using it to statistically imitate actual cloud observations made by the Research Scanning Polarimeter (RSP) during two field experiments.

Free access
Igor V. Geogdzhayev
,
Michael I. Mishchenko
,
William B. Rossow
,
Brian Cairns
, and
Andrew A. Lacis

Abstract

Described is an improved algorithm that uses channel 1 and 2 radiances of the Advanced Very High Resolution Radiometer (AVHRR) to retrieve the aerosol optical thickness and Ångström exponent over the ocean. Specifically discussed are recent changes in the algorithm as well as the results of a sensitivity study analyzing the effect of several sources of retrieval errors not addressed previously. Uncertainties in the AVHRR radiance calibration (particularly in the deep-space count value) may be among the major factors potentially limiting the retrieval accuracy. A change by one digital count may lead to a 50% change in the aerosol optical thickness and a change of 0.4 in the Ångström exponent. On the other hand, the performance of two-channel algorithms weakly depends on a specific choice of the aerosol size distribution function with less than 10% changes in the optical thickness resulting from replacing a power law with a bimodal modified lognormal distribution. The updated algorithm is applied to a 10-yr period of observations (Jul 1983–Aug 1994), which includes data from NOAA-7, NOAA-9 (Feb 1985–Nov 1988), and NOAA-11 satellites. (The results are posted online at http://gacp.giss.nasa.gov/retrievals.)

The NOAA-9 record reveals a seasonal cycle with maxima occurring around January–February and minima in June–July in the globally averaged aerosol optical thickness. The NOAA-7 data appear to show a residual effect of the El Chichón eruption (Mar 1982) as increased optical thickness values in the beginning of the record. The June 1991 eruption of Mt. Pinatubo resulted in a sharp increase in the aerosol load to more than double its normal value. The NOAA-9 record shows no discernible long-term trends in the global and hemisphere averages of the optical thickness and Ångström exponent. On the other hand, there is a discontinuity in the Ångström exponent values derived from NOAA-9 and NOAA-11 data and a significant temporal trend in the NOAA-11 record. The latter is unlikely to be related to the Mt. Pinatubo eruption and may be indicative of a serious calibration problem.

The NOAA-9 record shows that the Northern Hemisphere mean optical thickness systematically exceeds that averaged over the Southern Hemisphere. Zonal means of the optical thickness exhibit an increase in the tropical regions of the Northern Hemisphere associated with annual desert dust outbursts and a springtime increase at middle latitudes of the Northern Hemisphere. Increased aerosol loads observed at middle latitudes of the Southern Hemisphere are probably associated with higher sea salt particle concentrations. Reliable extension of the retrieval record beyond the NOAA-9 lifetime will help to corroborate these findings.

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Mikhail D. Alexandrov
,
Andrew A. Lacis
,
Barbara E. Carlson
, and
Brian Cairns

Abstract

A retrieval algorithm for processing multifilter rotating shadowband radiometer (MFRSR) data from clear and partially cloudy days is described and validated. This method, while complementary to the Langley approach, uses consistency between the direct normal and diffuse horizontal measurements combined with a regression technique to simultaneously retrieve daily time series of column mean aerosol particle size, aerosol optical depth, NO2, and ozone amounts along with the instrument's calibration constants. Comparison with the traditional Langley calibration method demonstrates two advantages of the approach described here: greater calibration stability and a decreased sensitivity of retrievals to calibration errors.

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