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.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JD900319

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

An airborne albedometer including a low-cost, precise, and fast sensor head horizontal stabilization system was developed to measure spectral down- and upward irradiances between 400- and 1000-nm wavelength. It is installed on a small research aircraft (type Partenavia P68-B), but it can easily be mounted on other aircraft as well. The stabilization unit keeps the two radiation sensor heads (up- and downward looking) of the albedometer in a horizontal position during the flight with an accuracy of better than ±0.2° over a range of pitch and roll angles of ±6°. The system works properly for angular velocities up to 3° s-1 with a response time of the horizontal adjustment of 43 ms. Thus it can be applied even under turbulent atmospheric conditions. The limitations of the stabilization have been determined by laboratory and in-flight performance tests. As a result it is found that the new horizontal stabilization system ensures that misalignment-related uncertainties of the measured irradiances are ...

/pdf/an-airborne-spectral-albedometer-with-active-horizontal-tomm8nffv8.pdf

An Airborne Spectral Albedometer with Active Horizontal Stabilization

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.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98JD02021

Comparison of observed and modeled direct aerosol forcing during TARFOX

An airborne-spectral-scanning radiometer (aureolemeter) and corresponding algorithm for the analysis were developed to evaluate the optical stratification of aerosols from airborne measurements. Since the extinction of solar radiation is too small to be detected by a sunphotometer above the haze layer, our aureolemeter was designed to measure both intensities of direct and circumsolar radiations at the 330, 369, 500, 675, 776, 862, and 938 nm wavelengths. The radiometer was also designed to scan the solar almucanter in the range of scattering angles from 5° to 60° with a lapse of 3 seconds, by automatically detecting the sun’s position. The algorithm for data analysis involves iterations of the following two steps, i. e., a step that subtracts multiply scattered radiation by means of an accurate radiative transfer code for calculation of the solar aureole, and a step that inverts the volume spectrum of aerosols from the forward part of the phase function.

Airborne measurements of the optical stratification of aerosols in turbid atmospheres.

Aircraft measurements of the short-wave albedo of a marine stratus cloud are described and compared with results from radiative transfer models of plane-parallel clouds. the albedos are measured in the visible and near-infrared parts of the solar spectrum using clear dome and red dome pyranometers. Using all the available liquid water content data and the measured microphysical properties, two model clouds are constructed to represent the maximum and minimum liquid water paths encountered during the experiment. Model albedo calculations based on these clouds are compared with the range of albedos measured just above cloud top. the natural variation of liquid water path at the cloud edge is exploited as a further qualitative assessment of the models' behaviour.

C.J. Readings

Papers

The measurement of diffuse solar radiation from an aircraft