Cloud top pressure (CTP) retrievals from measurements within the O2 A-band of MERIS (Medium Resolution Imaging Spectrometer on board ENVISAT) and of OLCI (Ocean and Land Color Imager onboard Sentinel-3a/b) have been developed including the additional O2 A-band channels of OLCI.

The MERIS retrieval have been used to generate global time series of cloud top pressure. We assessed the global MERIS CTP product during its 10 years of operation to detect regional cloud anomalies, the results will be discussed.

The advantage of the 3 O2 A-band channels to detect cloud properties will be demonstrated. The sensitivity of the retrievals by OLCI with respect to cloud top pressure, cloud geometrical thickness and profile of cloud extinction will be discussed.

METimage is a cross-purpose medium resolution, multi-spectral optical imager/radiometer serving operational meteorology and climate applications, capable of measuring thermal radiance emitted by the Earth and solar backscattered radiation. It will be on board the Second Generation EUMETSAT Polar System (EPS-SG) operational meteorological satellites. The primary objective of the METimage mission is to provide high quality imagery data for global and regional Numerical Weather Prediction and Nowcasting.

Clouds determine the amount of solar radiation scattered back into space and block the terrestrial radiation from the Earth’s surface; thus, cloud properties (such as the cloud top height) serve global circulation model and make climate change prediction more realistic.

As part of the EPS-SG/METimage L2 cloud product, EUMETSAT will disseminate a Cloud Top Pressure product retrieved from observations in the O2 A-band absorption channel and nearby visible window channels. The algorithm is based on the optimal estimation method, it assumes mono-layer clouds and makes use of CloudSat/Calipso climatology in order to vary the vertical structure (both extinction profile and cloud geometrical thickness) of the cloud as a function of the optical thickness and cloud top pressure in the forward model. We will present the algorithm, the error budget, and the results obtained when testing the algorithm using A-train data: we perform the retrieval on Parasol data and compare the cloud top pressure to CloudSat/Caliop measurements.

Departure of actual cloud vertical structure from our model is an important source of error for the retrieved CTP. In this context, multi-layer situations are particularly problematic. Additional measurement must be used (on top of O2 A-band absorption channel) to detect such situations. We will present results obtained when adding short wave infrared water vapor absorption channel(s) to the retrieval scheme.

The algorithm is primarily developed for the future EPS-SG/METimage instrument. It can be adapted to instruments with similar spectral characteristics (e.g. MERIS, PARASOL/POLDER, Sentinel3/OLCI, 3MI).

The determination of effective cloud properties like cloud fraction and cloud height is important for the correction of cloud effects on tropospheric trace gas products derived from satellite observations. Moreover, information on cloud properties is interesting in itself, because clouds are important in many aspects of the atmosphere-earth system.
Information on cloud height is usually retrieved from UV/vis satellite observations based on the shielding effect of clouds on atmospheric absorptions of the oxygen molecule (O2) or dimer (O4). Alternatively, information on cloud height can also be obtained from measurements of the strength of the so-called Ring effect, the filling-in of solar Fraunhofer lines due to rotational Raman scattering on air molecules. Algorithms based on the Ring effect have the advantage that they are much less affected by changes of the effective albedo (caused by surface and cloud effects) than the absorptions of O2 and O4. Also the wavelength at which the Ring effect is analysed is much closer to the wavelength ranges of most atmospheric trace gas analyses.
However, in spite of the rather strong spectral signature of the Ring effect, the analysis of the Ring effect is complicated by possible spectral interference with spectrograph straylight. Also the wavelength dependence of the Ring effect has to be considered. For TROPOMI observations, a further problem occurs, since the boundaries between the different spectral channels are optimised for trace gas analyses, but are not the optimum choice for the analysis of the Ring effect.
In this study we present how these problems can be successfully addressed. We also present first results of effective cloud height retrieved from TROPOMI and how they depend on the observational geometry and surface type. Finally the new cloud height results are compared with other cloud height algorithms.

It is now well established that clouds and to a lesser extent aerosols constitutes the largest uncertainty to estimate the Earth’s energy budget (IPCC 2017). In this context the impact of high clouds, which has been shown to be of significant importance, is still poorly quantified. This is mainly due to the complexity of their microphysics (number density, shape and size distribution) and the incomplete knowledge of their macrophysical properties such as altitude, geometrical extent together with their water content.

Recently, the FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) experiment has been selected in phase A to be considered as one option for the Earth Explorer mission EE-9. The FORUM instrument will be a Far-Infrared Spectrometer able to measure the top of atmosphere radiance in a spectral region ranging from 100 to 1600 cm-1 (6.25 to 100 μm) with an expected spectral resolution of 0.3 cm-1. Such an instrument has never been launched before and will allow a better quantification and characterization of several climate forcing variables that contribute to the outgoing long wave radiation on a global scale.

In this work we will focus on high clouds and will explore the capability of the FORUM instrument in characterizing their properties and hence their representation at a global scale. Of particular interest will be the ability of FORUM to retrieve the total amount of ice inside the cloud, as well as its altitude and geometrical extent. A large database, provided by ECMWF reanalysis over one year, representative of cloud column configuration in terms of water content profile (e.g. single and multi-layer cloud) and geographical location, will be used to test the FORUM performance. The retrieval algorithm will make use of the RTTOV forward model in an optimal estimation framework together with a Levenberg-Marquardt minimisation method to retrieve ice clouds properties. A comparison with other sensor such as IASI-NG will also be presented.

Cloud optical and microphysical properties, specifically cloud optical thickness (τ) and effective radius (re), are routinely retrieved from a range of polar and geostationary orbiting passive imager observations. When a visible and a shortwave-infrared channel are available, the most common approaches use the “Nakajima-King” principle, whereby the former channel is mainly sensitive to τ and the latter to re. While this principle is straightforward, challenges remain under specific illumination and viewing geometries, including the cloud glory and cloud bow, where the retrieval becomes very sensitive to underlying assumptions. These conditions often occur for geostationary satellites, adversely affecting the plausibility and quality of relevant cloud products.

The Spinning Enhanced Visible and Infrared Imager (SEVIRI), on board the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) Meteosat Second Generation (MSG) geostationary satellites, provides measurements in twelve spectral channels from the visible to the thermal infrared every fifteen minutes. This high temporal resolution allows the detection of diurnally resolved cloud properties. However, these are susceptible to the aforementioned issues, leading to irregularities in the diurnal data record.

In the present study, these issues and their effects on liquid cloud τ and re were analyzed using the Satellite Application Facility for Climate Monitoring (CM SAF) Cloud Physical Properties (CPP) retrieval algorithm, which is a Nakajima-King-based scheme. Quantification of the detected irregularities was based on the synergistic use of SEVIRI reflectances and cloud products from MSG-1 and -3, which are located over the Indian and Atlantic Ocean, respectively, and cover an extensive common area under different viewing angles. The analysis included areas with different underlying surfaces (ocean/land) and used different spectral combinations of visible and shortwave-infrared channels, which led to variations in the retrieval algorithm behavior. It was found that irregularities appearing close to cloud bow geometries are associated with characteristics inherent to the single scattering phase functions, whereas those related to the cloud glory are sensitive to the width of the assumed droplet size distribution, expressed as the effective variance (νe). Retrievals were repeated using a wide range of νe values, instead of a fixed value usually assumed. Consequent differences in the smoothness of the diurnal variation of τ and re allude to a potential method for retrieval of νe when specific cloud glory conditions are met. This potential was examined in various areas with uniform cloud cover, leading to promising results when compared with in situ measurements.

One of the major sources of uncertainty in climate projection is the cloud radiative forcing (CRF). A better understanding of the cloud vertical distribution (CVD) and its relationship to anthropogenic forcing is one crucial point on the way towards a better representation of CRF in climate models.
The moderate resolution imaging spectroradiometer (MODIS) aboard NASAs Terra and Aqua satellites, provides consistent well established data-sets of cloud physical and optical properties from 1999 to date on a high temporal and spatial resolution (on climate scales).
In this study, we used 14 years (September 2002-September 2016) of Aqua MODIS L2 (collection 6) cloud products to analyze cloud variability in the tropical Atlantic ocean and linking the satellite observations to large-scale dynamics. We focused on temporal changes and regional differences of cloud top height (CTH) and cloud fraction (CF). The latter was subdivided into low cloud fraction (LCF), middle cloud fraction (MCF) and high cloud fraction (HCF).
The analysis of in total 168 monthly means revealed, despite of a significant increase in AMSR-E level-3 domain-averaged SST (June 2002-October 2011), no significant trend in the domain-averaged CTH and CVD over the investigated period. A negative correlation between El Niño Southern Oscillation (ENSO) and the high cloud amount was found. From a La Niña to an El Niño event, the high cloud amount decreases, resulting in lower mean CTH. This observed relationship in the regional mean, was found to be strongest in regions with large-scale vertical upward movements. We related these observations to changes in the large- scale vertical upward movements, possibly due to an east-ward shift and weakening of the Walker (and the Atlantic Hadley) Cell during the transition from La Niña to El Niño conditions (e.g. Klein et al., 1999; Wang, 2004). As climate projection show similar changes in tropical Atlantic circulation in a warming climate (e.g., Vecchi and Soden, 2007; Bayr et al., 2014; Hu et al., 2018), large ENSO
events could be used as large ”experiment” for possible effects of the global warming on the ocean-atmosphere dynamics and the CRF in the tropical Atlantic.