Clouds and aerosol in infrared radiative transfer calculations for the analysis of satellite observations

Beschreibung

High-altitude clouds play an important role in the Earth’s atmosphere. Polar stratospheric clouds are involved in processes leading to polar ozone destruction. The radiative impact of cirrus clouds is one of the least understood processes affecting the climate. Aside from this, clouds and aerosols affect the measured spectra of remote sensing instruments such as the Michelson Interferometer for Passive Atmospheric Sounding Envisat (MIPAS). In this thesis, the JUelich RApid Spectral SImulation Code (JURASSIC), which is a spectrally averaging forward model, was substantially extended by implementing a scattering module to account for scattering on aerosols and clouds. The new code was carefully cross-checked at a high spectral resolution with the well-established line-by-line models Reference Forward Model (RFM) and the Karlsruhe Optimized and Precise Radiative transfer Algorithm (KOPRA). The comparison shows that JURASSIC agrees with RFM within the MIPAS noise equivalent radiance. At several line centres, larger differences were found up to a factor of 7 of the noise equivalent radiance at the 792 cm−1 CO2 line centre. The difference between RFM and KOPRA was as much as a factor of 55 of the noise equivalent radiance at this line centre. This may have implications for temperature retrievals in the troposphere. The microphysical properties of stratospheric and tropospheric aerosol and cloud particles required for the scattering calculations are collected. The single scattering properties and their effect on simulated spectra are studied. Because of the large variability of the tropospheric aerosol it is found that tropospheric aerosol extinction coefficients in the infrared generally should not be scaled from one wavelength to another. Instead it is suggested that the extinction coefficient for every wavelength should be calculated. Due to the lack of information on the vertical profile of aerosol log-normal parameters, which are of great value not only for remote sensing applications, but also for climate modelling and cloud simulations, a new climatology is compiled from published lognormal distributions. A first study on the detectability and discrimination of measured volcanic ash particles of the Eyjafjallaj¨okull eruption with MIPAS demonstrates that MIPAS is able to detect and discriminate volcanic ash. The MIPAS measurements are used to assess the quality of the predictions of the location of the ash cloud by the EURopean Air pollution Dispersion model (EURAD). It was found that the model predicts ash cloud filaments where no particles are detected in MIPAS. Further, for several profiles the model overestimates the altitude of the ash layer by up to 3 km. The spectra of profiles in which clouds are detected, however, resemble ice cloud spectra. No unambiguous ash cloud spectrum could be identified in MIPAS observations.

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Sabine Grießbach

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