Abstract
Abstract. Clouds are a key modulator of the Earth energy budget at the top of the atmosphere and at the surface. While the cloud top height is operationally retrieved with global coverage, only few methods have been proposed to determine cloud base height (zbase) from satellite measurements. This study presents a new approach to retrieve cloud base heights using the Multi-angle Imaging SpectroRadiometer (MISR) on the Terra satellite. It can be applied if some cloud gaps occur within the chosen distance of typically 10 km. The MISR cloud base height (MIBase) algorithm then determines zbase from the ensemble of all MISR cloud top heights retrieved at a 1.1 km horizontal resolution in this area. MIBase is first calibrated using 1 year of ceilometer data from more than 1500 sites within the continental United States of America. The 15th percentile of the cloud top height distribution within a circular area of 10 km radius provides the best agreement with the ground-based data. The thorough evaluation of the MIBase product zbase with further ceilometer data yields a correlation coefficient of about 0.66, demonstrating the feasibility of this approach to retrieve zbase. The impacts of the cloud scene structure and macrophysical cloud properties are discussed. For a 3-year period, the median zbase is generated globally on a 0.25∘ × 0.25∘ grid. Even though overcast cloud scenes and high clouds are excluded from the statistics, the median zbase retrievals yield plausible results, in particular over ocean as well as for seasonal differences. The potential of the full 16 years of MISR data is demonstrated for the southeast Pacific, revealing interannual variability in zbase in accordance with reanalysis data. The global cloud base data for the 3-year period (2007–2009) are available at https://doi.org/10.5880/CRC1211DB.19.
Highlights
As Boucher et al (2013) state in the IPCC Assessment Report 5, clouds and aerosols continue to contribute the largest uncertainty to estimates and interpretations of the Earth’s changing energy budget
In order to preliminarily assess how well clouds are represented in common reanalysis, we compare Multi-angle Imaging SpectroRadiometer (MISR)-derived zbase and ztop to cloud heights derived from ERA-Interim (Dee et al, 2011) which is provided by the European Centre for Medium-Range Weather Forecasts (ECMWF)
This is possibly due to the larger retrieval cell which is set up for the retrievals from MISR as opposed to the point measurements provided by the ceilometer
Summary
As Boucher et al (2013) state in the IPCC Assessment Report 5, clouds and aerosols continue to contribute the largest uncertainty to estimates and interpretations of the Earth’s changing energy budget. Passive measurements in the near-infrared exploiting spectral information have been proposed by Ferlay et al (2010) They suggest an approach to infer the cloud vertical extent from multi-angular POLDER (POLarization and Directionality of the Earth’s Reflectances) oxygen A-band measurements. Exploiting the different viewing angles provided by POLDER, Desmons et al (2013) apply this approach to infer the vertical position of clouds Their comparison to retrievals from the cloud profiling radar on CloudSat and CALIOP shows that this method works best for liquid clouds over ocean with a retrieval bias of 5 m and a standard deviation of the retrieval differences of 964 m. The cloud vertical extent is derived as the difference between ztop (radar) and zbase (ceilometer) from ground-based measurements.
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