Martin Glew

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

[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;

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.

Cirrus Measurements during the EAQUATE Campaign

Ice clouds have a significant impact on the Earths water and radiation budgets. Here we present remote sensing and in-situ measurements of frontal cirrus, using the FAAM aircraft during the EAQUATE intercomparison campaign with AQUA.

Validation of ATSR-2 visible channel reflectances and cloud parameter retrievals using Meteorological Office Research Flight SATE 2 data

Near coincident data from satellite and aircraft overpasses of stratocumulus in the South Atlantic have been analyzed. The satellite data are from the Along Track Scanning Radiometer-2 (ATSR-2) onboard the European Remote Sensing satellite-2 (ERS- 2) and are measurements of reflectance at wavelengths of 0.87 micrometer and 1.6 micrometer. From these, retrieval algorithms estimate the cloud optical depth and effective drop size using well established principles. The aircraft data consist of reflectance measurements made using the Scanning Airborne Filter Radiometer (SAFIRE) from a transit made just above the cloud, and liquid water content (LWC) and effective drop size measurement made using an FSSP probe from a transit made approximately 100 m below cloud top. The aircraft flights were made as part of the South Atlantic Tropical Experiment-2 which took place off the Namibian coast in October 1995 and which was designed to coincide with ERS-2 overpasses where possible. One particular flight, A423, has been studied here as offering the best chance of intercomparison of the respective systems measurements. Two flight legs have been analyzed, one for validation of reflectance measurements and one for validation of cloud effective radius. Poor correlation in initial comparisons based on the geolocation of the two instruments was improved greatly by allowing for, and estimating, the advection of the cloud deck by the local wind. Other adjustments included compensating a small error in the ATSR geolocation and allowing for differences in the respective instrument view angles. Following these adjustments, good agreement is shown for the 11 micrometer brightness temperatures and for the 0.87 micrometer reflectances. Large biases in the 1.6 micrometer reflectances confirm calibration errors that were already suspected for both instruments. Using the same wind advection the cloud effective radius retrievals were compared for the previous flight leg. Agreement is shown to be within 0.5 micrometer for measurements within 5 minutes of the exact collocation time. This is a remarkable result considering the sensitivity of the ATSR retrievals to 1.6 micrometer calibration errors.