The high‐resolution infrared radiation sounder (HIRS) has been used to retrieve sulphuric acid aerosol from the June 15, 1991, eruption of Mount Pinatubo (15.1°N, 120.4°E). The aerosol climatology retrieved includes latitudinal distributions of column density and monthly global mass loadings from May 1991 to Autumn 1993. The method of retrieval utilizes the sensitivity of one of the HIRS channels situated at 8.3 μm to the aerosol. The retrieved transmission at 8.3 μm is related to the column density via the specific absorption coefficient. A second channel at 12.5 μm, which is relatively insensitive to the aerosol, is used to remove other variations in the signal at 8.3 μm. In July 1991, latitudinal distributions of column density show the aerosol confined to the tropics with the bulk of the aerosol mass located south of the equator. By May 1993 the aerosol has returned to a preemption level in the tropics. The transport of aerosol to southern latitudes is shown to be more rapid than to northern latitudes. Monthly global mass loadings are estimated to be 15 Mt (megatons) in July 1991, rising to 21 Mt by September 1991, later dropping to 16 Mt by July 1992, and returning to preemption values by October 1993. The results derived in this paper should prove useful for climate models attempting to study the global impact of the Pinatubo eruption.

SUMMARYAircraft measurements of broad-band infrared and solar irradiances together with radiance measurements at discrete angles and wavelengths are used, combined with microphysical observations, to investigate the optical properties of cirrus cloud.Results are found to be consistent with approximations appropriate to large particles with strong absorption in the thermal infrared. The observed angular scattering pattern is compared with other limited observational results and found to be consistent with them. Implications for predicting cloud albedo are discussed. An effective radius for the ice particles is also defined to enable solar absorption within the cloud to be predicted. Observations in the near-infrared are shown to be consistent with this definition although the agreement is dependent on a number of assumptions.

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Abstract.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/zm, 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/zm radiances for this shape are considerably less than those derived from the 11.0/zm 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/zm 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/zm for the crystal sizes relevant to the present case study.

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