The plane-parallel model for the parameterization of clouds in global climate models is examined in order to estimate the effects of the vertical profile of the microphysical parameters on radiative transfer calculations for extended boundary layer clouds. The vertically uniform model is thus compared to the adiabatic stratified one. The validation of the adiabatic model is based on simultaneous measurements of cloud microphysical parameters in situ and cloud radiative properties from above the cloud layer with a multispectral radiometer. In particular, the observations demonstrate that the dependency of cloud optical thickness on cloud geometrical thickness is larger than predicted with the vertically uniform model and that it is in agreement with the prediction of the adiabatic one. Numerical simulations of the radiative transfer have been performed to establish the equivalence between the two models in terms of the effective radius. They show that the equivalent effective radius of a vertically uniform model is between 80% and 100% of the effective radius at the top of an adiabatic stratified model. The relationship depends, in fact, upon the cloud geometrical thickness and droplet concentration. Remote sensing measurements of cloud radiances in the visible and near infrared are then examined at the scale of a cloud system for a marine case and the most polluted case sampled during the second Aerosol Characterization Experiment. The distributions of the measured values are significantly different between the two cases. This constitutes observational evidence of the aerosol indirect effect at the scale of a cloud system. Finally, the adiabatic stratified model is used to develop a procedure for the retrieval of cloud geometrical thickness and cloud droplet number concentration from the measurements of cloud radiances. It is applied to the marine and to the polluted cases. The retrieved values of droplet concentration are significantly underestimated with respect to the values measured in situ. Despite this discrepancy the procedure is efficient at distinguishing the difference between the two cases.

[1] The spatial variability of the microphysical fields in stratocumulus clouds is documented in this paper with statistics of droplet number concentration, droplet mean volume radius, and liquid water content for eight cases of the second Aerosol Characterization Experiment. Statistics are calculated in five sublayers, from cloud base to cloud top, and they are utilized for deriving estimates of cloud optical thickness and liquid water path, by assuming either random or maximum overlap. The resulting in situ frequency distributions of optical thickness and liquid water path are validated against distributions of these two parameters retrieved from independent remote sensing measurements of cloud radiances. They are also used for testing parameterizations of optical thickness based on liquid water path and either the droplet effective radius or the cloud droplet number concentration. This unique data set of extensive, concomitant, and independent measurements of cloud microphysical and radiative properties is finally used for assessing the detectability of the aerosol indirect effect through examination of the correlation between cloud optical thickness and droplet effective radius. If only cases of comparable values of geometrical thickness are considered, the correlation between optical thickness and effective radius is negative, as anticipated by Twomey [1977]. However, if the most polluted cases are also accounted for, the trend suggests a positive correlation. In fact, the most polluted cloud systems sampled during ACE-2 were slightly drier, hence thinner, than the marine and intermediate cases, hence producing a positive correlation between optical thickness and droplet effective radius. This study demonstrates that the monitoring of the aerosol indirect effect with satellite observations requires an independent retrieval of the liquid water path together with the cloud optical thickness and droplet effective radius.

Abstract. Long-term gridded precipitation products are crucial for several applications in hydrology, agriculture and climate sciences. Currently available precipitation products suffer from space and time inconsistency due to the non-uniform density of ground networks and the difficulties in merging multiple satellite sensors. The recent “bottom-up” approach that exploits satellite soil moisture observations for estimating rainfall through the SM2RAIN (Soil Moisture to Rain) algorithm is suited to build a consistent rainfall data record as a single polar orbiting satellite sensor is used. Here we exploit the Advanced SCATterometer (ASCAT) on board three Meteorological Operational (MetOp) satellites, launched in 2006, 2012, and 2018, as part of the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) Polar System programme. The continuity of the scatterometer sensor is ensured until the mid-2040s through the MetOp Second Generation Programme. Therefore, by applying the SM2RAIN algorithm to ASCAT soil moisture observations, a long-term rainfall data record will be obtained, starting in 2007 and lasting until the mid-2040s. The paper describes the recent improvements in data pre-processing, SM2RAIN algorithm formulation, and data post-processing for obtaining the SM2RAIN–ASCAT quasi-global (only over land) daily rainfall data record at a 12.5 km spatial sampling from 2007 to 2018. The quality of the SM2RAIN–ASCAT data record is assessed on a regional scale through comparison with high-quality ground networks in Europe, the United States, India, and Australia. Moreover, an assessment on a global scale is provided by using the triple-collocation (TC) technique allowing us also to compare these data with the latest, fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5), the Early Run version of the Integrated Multi-Satellite Retrievals for Global Precipitation Measurement (IMERG), and the gauge-based Global Precipitation Climatology Centre (GPCC) products. Results show that the SM2RAIN–ASCAT rainfall data record performs relatively well at both a regional and global scale, mainly in terms of root mean square error (RMSE) when compared to other products. Specifically, the SM2RAIN–ASCAT data record provides performance better than IMERG and GPCC in data-scarce regions of the world, such as Africa and South America. In these areas, we expect larger benefits in using SM2RAIN–ASCAT for hydrological and agricultural applications. The limitations of the SM2RAIN–ASCAT data record consist of the underestimation of peak rainfall events and the presence of spurious rainfall events due to high-frequency soil moisture fluctuations that might be corrected in the future with more advanced bias correction techniques. The SM2RAIN–ASCAT data record is freely available at https://doi.org/10.5281/zenodo.3405563 (Brocca et al., 2019) (recently extended to the end of August 2019).

International audienceCLOUDYCOLUMN is one of the 6 ACE-2 projects which took place in June-July 1997, between Portugal and the Canary Islands. It was specifically dedicated to the study of changes of cloud radiative properties resulting from changes in the properties of those aerosols which act as cloud condensation nuclei. This process is also refered to as the aerosol indirect effect on climate. CLOUDYCOLUMN is focused on the contribution of stratocumulus clouds to that process. In addition to the basic aerosol measurements performed at the ground stations of the ACE-2 project, 5 instrumented aircraft carried out in situ characterization of aerosol physical, chemical and nucleation properties and cloud dynamical and microphysical properties. Cloud radiative properties were also measured remotely with radiometers and a lidar. 11 case studies have been documented, from pure marine to significantly polluted air masses. The simultaneity of the measurements with the multi-aircraft approach provides a unique data set for closure experiments on the aerosol indirect effect. In particular CLOUDYCOLUMN provided the 1st experimental evidence of the existence of the indirect effect in boundary layer clouds forming in polluted continental outbreacks. This paper describes the objectives of the project, the instrumental setup and the sampling strategy. Preliminary results published in additional papers are briefly summarized

In general circulation models, clouds are parameterized and radiative transfer calculations are performed using the plane-parallel approximation over the cloudy fraction of each model grid. The albedo bias resulting from the plane-parallel representation of spatially heterogeneous clouds has been extensively studied, but the impact of entrainment-mixing processes on cloud microphysics has been neglected up to now. In this paper, this issue is examined by using large-eddy simulations of stratocumulus clouds and tridimensional calculations of radiative transfer in the visible and near-infrared ranges. Two extreme scenarios of mixing are tested: the homogeneous mixing scheme with constant concentration and reduced droplet sizes, against the inhomogeneous mixing scheme, with reduced concentration and constant droplet sizes. The tests reveal that entrainment-mixing effects at cloud top may substantially bias the simulated albedo. In the worse case, which corresponds to a fragmented and thin stratocumulus cloud, the albedo bias changes from Ϫ3% to Ϫ31% when using both mixing schemes alternatively.