Peter N. Francis

Optical properties and direct radiative effect of Saharan dust: A case study of two Saharan dust outbreaks using aircraft data

The radiative effects of Saharan dust are measured during two flights by the Met Office C-130 aircraft off the west coast of Africa. Data from the broadband radiometers suggests that the perturbation to the top of the atmosphere net solar irradiance is as strong as -60 W m -2 -!-5 W m -2 during the dust events. In situ measurements with the nephelometer and particle soot absorption photometer suggest that the single scattering albedo is approximately 0.87 at a wavelength of 0.55 Ixm. This is in agreement with the optical parameters calculated from independent measurements of the particle size distributions combined with suitable refractive indices and Mie-scattering theory. The wavelength dependence of the extinction coefficient derived from measurements of the scattering coefficient by the nephelometer is also in excellent agreement with the calculations. Independent surface-based measurements from Cape Verde suggest that the wavelength dependence of the aerosol optical depth appears reasonable. Calculations of the downward solar irradiances within the aerosol layer are generally in good agreement with the measurements demonstrating consistency between the measurements and the modeling efforts. The terrestrial radiative effect is not detectable by the current instrumentation, though it cannot be considered negligible. These measurements suggest that satellite retrieval algorithms may misclassify the aerosol outbreak as cloud because the aerosol optical depth at 0.55 Ixm is as high as 1.15, which is in excess of the thresholds used in some cloud detection algorithms. The measurements demonstrate that this method could be used to provide an accurate benchmark for satellite-based estimates of the radiative effect of aerosols.

Comparison of observed and modeled direct aerosol forcing during TARFOX

Aircraft measurements have been made of the downward and upward solar irradiance under cloud-free conditions over a range of aerosol loadings in the summer haze plume off the East Coast of the United States during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX). Optical properties calculated from measured aerosol microphysical and chemical properties have been used as input to a shortwave radiative transfer model. This model was used to calculate the diurnally averaged direct aerosol forcing and to compare with values deduced from the aircraft radiative measurements. The modeled and observed forcings agree well when the aerosol has a significant absorbing component.

The Role of Ice Particle Shapes and Size Distributions in the Single Scattering Properties of Cirrus Clouds

Abstract The roles of ice particle size distributions (SDs) and particle shapes in cirrus cloud solar radiative transfer are investigated by analyzing SDs obtained from optical array probe measurements (particle sizes larger than 20–40 μm) during intensive field observations of the International Cirrus Experiment, the European Cloud and Radiation Experiment, the First ISCCP Regional Experiment, and the Central Equatorial Pacific Experiment. It is found that the cloud volume extinction coefficient is more strongly correlated with the total number density than with the effective particle size. Distribution-averaged mean single scattering properties are calculated for hexagonal columns, hexagonal plates, and polycrystals at a nonabsorbing (0.5 μm), moderately absorbing (1.6 μm), and strongly absorbing (3.0 μm) wavelength. At 0.5 μm (1.6 μm) (3.0 μm), the spread in the resulting mean asymmetry parameters due to different SDs is smaller than (comparable to) (smaller than) the difference caused by applying diff...

Aircraft measurements of the solar and infrared radiative properties of cirrus and their dependence on ice crystal shape

We present aircraft measurements of the radiative transfer properties of thin cirrus cloud sampled off the east coast of Scotland on November 9, 1995. Downwelling radiances were measured from below the cirrus at 0.87, 1.61, 3.7, 8.55, and 11.0 μm, thereby covering a large range of size parameter and ice refractive index and enabling information on cirrus optical thickness, effective crystal size, and scattering phase function to be deduced. The sensitivity of these quantities to the ice crystal shape assumed in the calculations is examined, and the results are compared with in situ data. We find that a randomized polycrystal shape produces effective sizes that are consistent with the in situ data across all wavelengths considered and performs better in this respect than the other crystal shapes analyzed. However, the optical thicknesses retrieved from the 0.87 μm radiances for this shape are considerably less than those derived from the 11.0 μm data, implying that the phase function at solar wavelengths is in error for this shape over a significant portion of the full scattering angle range. An empirical phase function derived from laboratory measurements produces optical thicknesses which are more consistent with the 11.0 μm and in situ data and matches the angular distribution of scattered radiance more accurately than that calculated using any of the model crystal shapes. The anomalous diffraction approximation is found to produce good agreement with the measurements at 8.55 and 11.0 μm for the crystal sizes relevant to the present case study.

Some Aircraft Observations of the Scattering Properties of Ice Crystals

Abstract Aircraft–borne radiometer measurements made within three cirrus cloud layers are presented together with associated in situ cloud microphysical data. The angular variation of the observed shortwave radiances is simulated using a Monte Carlo multiple-scattering model, making use of a number of scattering phase functions, both theoretical and experimental. It is found that, in general, the laboratory-measured phase functions yield the closest agreement with the aircraft radiances.

A case study of the radiative forcing of persistent contrails evolving into contrail-induced cirrus

over 50,000 km 2 . The shortwave (SW) and longwave (LW) radiative forcing of the contrail-induced cirrus is estimated using a combination of geostationary satellite instruments, numerical weather prediction models, and surface observation sites. As expected, the net radiative effect is a relatively small residual of the much stronger but opposing SW and LW effects, locally totaling around 10 W m �2 during daylight hours and 30 W m �2 during nighttime. A simple estimate indicates that this single localized event may have generated a global-mean radiative forcing of around 7% of recent estimates of the persistent contrail radiative forcing due to the entire global aircraft fleet on a diurnally averaged basis. A single aircraft operating in conditions favorable for persistent contrail formation appears to exert a contrail-induced radiative forcing some 5000 times greater (in W m � 2 km �1 ) than recent estimates of the average persistent contrail radiative forcing from the entire civil aviation fleet. This study emphasizes the need to establish whether similar events are common or highly unusual for a confident assessment of the total climate effect of aviation to be made.

Testing the coherence of cirrus microphysical and bulk properties retrieved from dual‐viewing multispectral satellite radiance measurements

In this paper the coherence of retrieved cirrus microphysical and bulk properties using data from a satellite-based dual-viewing and multispectral instrument is tested using different ice crystal models. Radiance data from the dual-viewing Along Track Scanning Radiometer (ATSR-2) instrument is used to show that coherent retrievals are possible between nonabsorbing (visible) and infrared wavelengths if an appropriate ice crystal model is employed. The dominating crystal habit is estimated by finding the ice crystal model that best fits the dual-view 0.87 μm reflectance data. The ice crystal models tested are hexagonal plates, hexagonal columns, bullet-rosettes (six branched), and randomized polycrystals, all of which are assumed to be randomly oriented in space. Given the best fit crystal shape other cirrus properties, such as optical depth, crystal maximum dimension, and an estimate of ice water path, are retrieved by contrasting reflectance data at the wavelengths of 0.55 and 1.6 μm. To demonstrate probable retrieval errors in terms of optical depth and crystal maximur dimension, if the wrong crystal habit is applied, a tropical convective case in the western Pacific Ocean is used as a typical example. It is found that the more complex particles as represented by the bullet-rosette and randomized polycrystal best fit the ATSR-2 radiance data, while the pristine geometries represented by the hexagonal plate and column do not. These results indicate that phase functions that are relatively flat at backscattering angles should be employed in satellite remote sensing of cirrus. Moreover, if hexagonal plates or columns were assumed as the habit in the radiative transfer model, then this would lead to retrieval errors of about a factor of 2 for optical depth(overestimate) and crystal maximum dimension(underestimate). To test if the retrieved optical depth at the wavelength of 0.55 μm is coherent, the extinction optical depth at the wavelength of 10.8 μm is also retrieved to test for a one-to-one relationship between the two wavelengths. To validate this procedure the same test is applied to a near coincident aircraft and ATSR-2 midlatitude case study.

A study of the absorption and extinction properties of hexagonal ice columns and plates in random and preferred orientation, using exact T-matrix theory and aircraft observations of cirrus

Abstract Absorption and extinction properties of the finite hexagonal ice column and hexagonal ice plate in random and preferred orientation are studied at the wavelength of 80 μm using a new implementation of exact T-matrix theory. For the case of random orientation at size parameters around two, it is shown that the hexagonal ice column and hexagonal ice plate absorption resonances are diminished relative to Mie theory, and the same behaviour is also noted for an aggregate particle consisting of eight hexagonal elements. The absorption properties of the aggregate particle have been calculated using the finite-difference time-domain method. It is also shown that extinction and absorption solutions for the hexagonal ice column and hexagonal ice plate can differ significantly if incidence occurs perpendicular or parallel to the cylindrical axis of the hexagon. For the case of perpendicular incidence on the edge of the hexagon, absorption solutions can exceed those of Mie theory, and for the case of parallel incidence, behaviour of the extinction solutions for hexagonal ice columns and hexagonal ice plates is shown to be similar to previously published work based on the prolate and oblate spheroid. Interference structure, associated with surface waves, is resolved on the hexagonal column extinction solution and the hexagonal plate absorption solution, thereby demonstrating that surface waves can exist on a non-axisymmetric geometry. The usefulness of assuming the hexagonal ice column in retrieval of ice crystal effective size is also investigated using aircraft based radiometric observations of semi-transparent cirrus at the wavelengths of 8.5 and 11 μm .

A consistent set of single-scattering properties for cirrus cloud: tests using radiance measurements from a dual-viewing multi-wavelength satellite-based instrument

Abstract In this paper a consistent set of single-scattering properties is presented for radiative transfer calculations and remote sensing of cirrus cloud. The single-scattering properties consist of the extinction coefficient, single-scattering albedo and phase function. A randomly oriented randomized hexagonal ice aggregate is assumed to derive the extinction coefficient and single-scattering albedo. The phase function is an extension of the Henyey–Greenstein model called the “analytic” phase function, which is generated from the asymmetry parameter at non-absorbing and absorbing wavelengths. The satellite-based dual-view along track scanning radiometer (ATSR-2) instrument is utilized to test the single-scattering properties for consistency at scattering angles between about 60° and 170°, using a method of Optimal Estimation. Optimal Estimation is applied to a set of cloud parameters and radiance measurements, which are made simultaneously at the wavelengths of 0.87, 1.6, 3.7, 11.0 and 12.0 μm , over cases of cirrus cloud located in the tropics and mid-latitudes. If the single-scattering properties and assumed model parameters were a perfect representation of the radiative properties of cirrus then the measurement residuals (i.e., differences between measurements and simulated measurements) would be identically equal to zero at each of the wavelengths for all scattering angles. It is found that the randomized ice aggregate combined with the analytic phase function minimizes the measurement residuals to generally well within ±1% (reflectance) and ±1 K (brightness temperature) at 0.87, 11.0 and 12.0 μm and to within ±3% and ±3 K at 1.6 and 3.7 μm , respectively. This compares to measurement residuals of about 8% and 10 K if the single-scattering properties are based on the randomly oriented hexagonal ice column. It is recommended that single-scattering properties based on the randomized ice aggregate combined with the analytic phase function (or a very similar phase function) should be applied to remote sensing and radiative transfer studies of cirrus cloud.

Aircraft observations and modeling of sky radiance distributions from aerosol during TARFOX

We present data from three cases studied during TARFOX, the Tropospheric Aerosol Radiative Forcing Observational Experiment. Aircraft measurements of the aerosol chemical composition, size distribution, and hygroscopic growth are used to construct a detailed picture of the aerosol microphysical characteristics. The distribution of the sky radiance field is then calculated at several wavelengths in the visible and near-infrared region of the spectrum from these data. The simulations are compared with observed radiance distributions made from one of the aircraft. Reasonable agreement is obtained for two of the cases considered, but significant differences exist for the third, which we are unable to explain at the present time.