Peter N. Francis

Bio: Peter N. Francis is an academic researcher from Met Office. The author has contributed to research in topics: Radiative transfer & Aerosol. The author has an hindex of 30, co-authored 59 publications receiving 3297 citations. Previous affiliations of Peter N. Francis include Defence Research Agency.

Improved Measurements of the Ice Water Content in Cirrus Using a Total-Water Probe

Abstract: This note describes an improved method for the measurement of the ice water content (IWC) of cirrus cloud using a total water content probe. A previous version of this technique assumed that the air in cloud-containing regions was saturated with respect to ice. This assumption has now been replaced with measurements of the water vapor content from a fast-response Lyman-α fluorescence water vapor sensor. The improved measurement of the vapor phase resolves anomalies in the earlier measurements that were due to the assumption of saturation with respect to ice everywhere within cloud. The comparison of IWC measurements made by this new method with those from a 2D optical array probe is greatly improved. The new measurements may now be used to provide much more stringent tests of the algorithms used for the derivation of crystal mass from measured size in 2D probe data.

Radiative properties and direct radiative effect of Saharan dust measured by the C-130 aircraft during SHADE: 1. Solar spectrum

Abstract: [1] The physical and optical properties of Saharan dust aerosol measured by the Met Office C-130 during the Saharan Dust Experiment (SHADE) are presented. Additional radiation measurements enable the determination of the aerosol optical depth, taerl, and the direct radiative effect (DRE) of the mineral dust. The results suggest that the absorption by Saharan dust is significantly overestimated in the solar spectrum if standard refractive indices are used. Our measurements suggest an imaginary part of the refractive index of 0.0015i is appropriate at a wavelength l of 0.55 mm. Different methods for determining taerl=0.55 are presented, and the accuracy of each retrieval method is assessed. The value taerl=0.55 is estimated as 1.48 ± 0.05 during the period of heaviest dust loading, which is derived from an instantaneous DRE of approximately � 129 ± 5 Wm � 2 or an enhancement of the local planetary albedo over ocean of a factor of 2.7 ± 0.1. A comparison of the DRE derived from the C-130 instrumentation and from the Clouds and the Earth’s Radiant Energy System (CERES) instrument on the Tropical Rainfall Measuring Mission (TRMM) satellite is presented; the results generally showing agreement to within a factor of 1.2. The results suggest that Saharan dust aerosol exerts the largest local and global DRE of all aerosol species and should be considered explicitly in global radiation budget studies. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 1640 Global Change: Remote sensing; 3359 Meteorology and Atmospheric Dynamics: Radiative processes;

The mean physical and optical properties of regional haze dominated by biomass burning aerosol measured from the C-130 aircraft during SAFARI 2000

Abstract: Original article can be found at: http://www.agu.org/journals/jd/ Copyright American Geophysical Union DOI: 10.1029/2002JD002226 [Full text of this article is not available in the UHRA]

Measurement and modeling of the Saharan dust radiative impact: Overview of the Saharan Dust Experiment (SHADE)

Abstract: [1] Aerosols are known to be important in determining Earth’s radiative balance. Dust aerosols are of particular interest since, in addition to their scattering and absorbing properties that affect the solar radiation, they also perturb the terrestrial radiation. Recent studies have shown that a significant proportion of mineral dust in the atmosphere may be of anthropogenic origin, and therefore they may have an important role in climate change by exerting a significant radiative forcing. However, the optical and radiative properties of dust are not yet very well-determined, and even the sign of the resulting forcing is still questionable. The Saharan Dust Experiment (SHADE) was designed to better determine the parameters that are relevant for computing the direct radiative effect. Two aircraft combining in situ and remote sensing instruments were coordinated with satellite overpasses and ground-based observations during the experiment, which was based in the Cape Verde area during the period 19–29 September 2000. These in situ and remotely sensed data provide new valuable information on the microphysical, optical properties, and radiative effects of a large mineral dust outbreak. In addition, a global chemical transport model was used for assessing the radiative impact of these events, which are shown to be important on regional and global scales. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 3359 Meteorology and Atmospheric Dynamics: Radiative processes; 3360 Meteorology and Atmospheric Dynamics: Remote sensing; KEYWORDS: Saharan dust, physical and optical properties, dust direct radiative forcing

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

Abstract: 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.

Global and regional climate changes due to black carbon

Abstract: Black carbon in soot is an efficient absorbing agent of solar irradiation that is preferentially emitted in the tropics and can form atmospheric brown clouds in mixture with other aerosols. These factors combine to make black carbon emissions the second most important contribution to anthropogenic climate warming, after carbon dioxide emissions.

Variability of Absorption and Optical Properties of Key Aerosol Types Observed in Worldwide Locations

Abstract: Aerosol radiative forcing is a critical, though variable and uncertain, component of the global climate. Yet climate models rely on sparse information of the aerosol optical properties. In situ measurements, though important in many respects, seldom provide measurements of the undisturbed aerosol in the entire atmospheric column. Here, 8 yr of worldwide distributed data from the AERONET network of ground-based radiometers were used to remotely sense the aerosol absorption and other optical properties in several key locations. Established procedures for maintaining and calibrating the global network of radiometers, cloud screening, and inversion techniques allow for a consistent retrieval of the optical properties of aerosol in locations with varying emission sources and conditions. The multiyear, multi-instrument observations show robust differentiation in both the magnitude and spectral dependence of the absorption—a property driving aerosol climate forcing, for desert dust, biomass burning, urban‐industrial, and marine aerosols. Moreover, significant variability of the absorption for the same aerosol type appearing due to different meteorological and source characteristics as well as different emission characteristics are observed. It is expected that this aerosol characterization will help refine aerosol optical models and reduce uncertainties in satellite observations of the global aerosol and in modeling aerosol impacts on climate.

Light Absorption by Carbonaceous Particles: An Investigative Review

Abstract: The optical properties of the light-absorbing, carbonaceous substance often called “soot,” “black carbon,” or “carbon black" have been the subject of some debate. These properties are necessary to model how aerosols affect climate, and our review is targeted specifically for that application. We recommend the term light-absorbing carbon to avoid conflict with operationally based definitions. Absorptive properties depend on molecular form, particularly the size of sp 2-bonded clusters. Freshly-generated particles should be represented as aggregates, and their absorption is like that of particles small relative to the wavelength. Previous compendia have yielded a wide range of values for both refractive indices and absorption cross section. The absorptive properties of light-absorbing carbon are not as variable as is commonly believed. Our tabulation suggests a mass-normalized absorption cross section of 7.5 ± 1.2 m2/g at 550 nm for uncoated particles. We recommend a narrow range of refractive indices for s...

Advances in understanding clouds from ISCCP

Abstract: This progress report on the International Satellite Cloud Climatology Project (ISCCP) describes changes made to produce new cloud data products (D data), examines the evidence that these changes are improvements over the previous version (C data), summarizes some results, and discusses plans for the ISCCP through 2005. By late 1999 all datasets will be available for the period from July 1983 through December 1997. The most significant changes in the new D-series cloud datasets are 1) revised radiance calibrations to remove spurious changes in the long-term record, 2) increased cirrus detection sensitivity over land, 3) increased low-level cloud detection sensitivity in polar regions, 4) reduced biases in cirrus cloud properties using an ice crystal microphysics model in place of a liquid droplet microphysics model, and 5) increased detail about the variations of cloud properties. The ISCCP calibrations are now the most complete and self-consistent set of calibrations available for all the weather...