Inter-comparison of MERIS, MODIS and MISR cloud top heights Catherine Naud (1) , Bryan Baum (2) , Ralf Bennarzt (3) , Juergen Fischer (4) , Richard Frey (3) , Paul Menzel (3) , Jan- Peter Muller (1) , Rene Preusker (4) , and Hong Zhang (3) (1) University College London, Dept. Geomatic Eng., Gower Str, London WC1E 6BT, United Kingdom (2) NASA Langley/CIMSS, Madison, WI 53706, United States (3) University of Wisconsin-Madison, Madison, WI 53706, United States (4) Free University Berlin, 12165 Berlin, Germany ABSTRACT The objective of this study was to assess the accuracy of MERIS cloud top pressures through an inter-comparison with CO 2 -slicing MODIS cloud top pressures and stereo MISR cloud top heights, both instruments of which are onboard the NASA-TERRA platform. For ground-based “truth” measurements, millimetre wavelength cloud radars data were used at the ARM SGP and UK Chilbolton sites. Cloud top pressures were transformed into heights using ECMWF Operational analysis profiles. An excellent agreement was found between MERIS, MISR and the radar cloud top heights when single opaque clouds were present. When clouds were optically thin, MERIS cloud top pressure retrievals were not available and when more than one cloud layer was present, MERIS CTH often referred to the lowest cloud top or some altitude between cloud layers. MISR cloud top heights referred to low clouds in multiple layers conditions, due to their higher contrast, and MODIS CTHs were found too low for low clouds, slightly too high for mid-level clouds and within the cloud extent or near the top of high clouds. 1 INTRODUCTION A new series of polar orbiters has recently been launched with a new generation of instruments onboard that use different ways of observing the Earth and different methods of observing clouds in particular. The ENVISAT satellite was launched on March 1 st 2002 with the Medium Resolution Imaging Spectrometer (MERIS) onboard. MERIS observes the Earth with 15 very narrow spectral channels in the visible and near-infrared. Cloud top pressures (CTPs) are estimated using a channel in the Oxygen absorption A-band centred at 0.76µm [1]. In order to assess the accuracy of this product, we compared MERIS CTPs with other ground-based and satellite instruments over two locations: the Atmospheric Radiation Measurement (ARM) program Southern Great Plain (SGP) site near Lamont, OK (36.62°N- 97.5°W) and the Chilbolton Facility for Atmospheric and Radio Research (CFARR) site in Southern England (51.15°N, 1.43°W). At both sites, vertically pointing MilliMeter wavelength Cloud Radars (MMCR) are continuously and automatically operated to monitor clouds. The CFARR 94GHz and the SGP 35GHz MMCR data are both processed using a code developed by [2] and a continuous cloud mask indicates the presence of hydrometeors every 10s at a 75m vertical resolution, giving a ground “truth” value of cloud top height (CTH) at the time of the ENVISAT overpass. This assessment of MERIS cloud top pressure accuracy was also conducted by comparing them with retrievals from the Multi-angle Imaging SpectroRadiometer (MISR) and the MODerate resolution Imaging Spectrometer (MODIS). Both instruments are onboard TERRA, which was launched in December 1999. The instruments and techniques used to retrieve and compare cloud top heights are presented in the next section. Section 3 presents the results for both sites, and section 4 summarises our conclusions. 2 INSTRUMENTS AND PRODUCTS The MMCR instruments give vertical profiles of cloud presence every 10s and cloud boundaries (masks) can be obtained with a 75m accuracy. The cloud mask can identify hydrometeors and can partly distinguish them from clutter such as vegetation debris or insects. Because MMCRs are detecting precipitation as well as cloud, cloud bases are detected with a vertically pointing lidar as these instruments are not as sensitive to rain droplets below cloud base. The radar cloud top height time series were sampled over 5, 10, 20 and 40 minutes centred on MERIS and MODIS start times, in order to get a full picture of the CTH variability over time in situations of broken clouds. The median CTH was then calculated over these time periods, as well as the frequency of cloud occurrence. MERIS is a pushbroom imaging spectrometer onboard ENVISAT that measures the solar radiation reflected by the Earth, in 15 spectral bands, programmable in width and position, in the visible and near infra-red. The algorithm used to estimate the cloud top pressure uses reflected solar radiation within the Oxygen A-band absorption band centred at 0.76μm. The algorithm is based on the assumption that the mean photon path length of the reflected solar radiation is related to the amount of absorption measured in the O 2 A-band. In a cloudy atmosphere the mean photon path length is _________________________________________________________________ Proceedings MERIS User Workshop, Frascati, Italy 10 – 13 November 2003 (ESA SP-549, May 2004)