Revising the cloud scheme



Anthropogenic aerosols have an impact on cloud formation, cloud properties and their life times. Comparing those indirect aerosol effects on clouds, or the clouds themselves, in simulations and observations is hampered by changes in cloudiness due to air traffic. Contrail cirrus clouds form on aircraft emitted aerosols in ice supersaturated areas at temperatures below -40°C and can persist in ice supersaturated areas. Air traffic can thus significantly change upper tropospheric cloudiness due to the formation of a large number of small ice crystals in clear air as well as modify microphysical properties of natural clouds by contrail formation within natural clouds. In work package 2 we develop a parameterization for ICON-LEM to study the direct and indirect effects of air traffic on upper tropospheric cloudiness, water budget at air traffic levels, radiation budget at top of the atmosphere, and at the surface. We have calculated an air traffic inventory for the ICON-LEM model from way point data and have implemented a parameterization for contrail ice crystal nucleation in ICON-LEM. We have calculated the ice nucleation with respect to the atmospheric state when air traffic is present. We have perform two simulations, a control run and a perturbed simulation (with contrail ice crystal nucleation). While the total ice water content remains nearly the same after contrail ice crystals nucleation (since the total water content hardly changes), the total number of ice crystals increases drastically. This can be seen when comparing Fig 1 (control run) and Fig 2 (perturbed).

Revised aerosol representation in ICON_LEM and cloud adjustments to aerosol absorption

A prerequisite for realistically simulating the cloud adjustment to aerosol-radiation interactions is a realistic representation of aerosol radiative properties in the model. The aim of work package 3 are new time-varying 3D distributions of cloud condensation nuclei (CCN) and aerosol radiative properties, which will be used in dedicated simulations with the ICON_GCM (global circulation model) and the high-resolved HD(CP)² model to analyze the cloud adjustment effects.

For sensitivity simulations, the available CCN and ice nuclei (IN) climatologies will be merged to one temporally and horizontally constant profile first. A scenario for 2013 (present day) aerosol emissions, and one for 1985 (peak aerosol over Europe) will be developed as basis for CCN scenarios. Simulation results will be evaluated with different kinds of measurements. 3D radiative properties (optical depth, asymmetry parameter and single scattering albedo) will be developed and evaluated. In collaboration with work package 4, additional IN types will be considered. These may include biological aerosols, biomass-burning aerosol and metallic aerosols.

Response of mixed-phase clouds to aerosol perturbations

Atmospheric aerosol particles influence cloud properties by acting as the seeds for cloud droplet and ice particle formation. Work package 4 uses the ICON_LEM to investigate how changes in the concentration of cloud condensing and ice nucleating particles affect the microphysical characteristics of mixed-phase clouds, which comprise both liquid droplets and ice particles. The possible outcomes from perturbing the aerosol size distribution include changes in the thermodynamic phase of the cloud and precipitation formation which affect the cloud lifetime and radiative properties. In addition, the importance of different ice nucleation mechanisms will be estimated.

Modelling of mixed-phase clouds and cloud ice in general comprises many significant challenges. One of these is related to how the models distinguish between different cloud ice categories, such as ice particles, snow, graupel and hail. This task is difficult for traditional microphysical parameterizations that are currently used in the ICON_LEM. Therefore, in the beginning of this work package, a revised microphysical scheme is implemented into the model. The new scheme aims to alleviate the above mentioned challenges by reducing the number of ice categories, but introducing new prognostic equations, which allow a more flexible representation of the ice particle properties.

Statistical relationships between ice clouds and aerosols for model evaluation

The aerosol and CCN (cloud condensation nuclei) profiles derived from COSMO-MUSCAT as an input for ICON-LEM are evaluated using measurements from HD(CP)² supersites. Multiwavelength lidar is the main instrument for this purpose. Statistical measurement data as shown below is used to evaluate the aerosol-model output for certain sites on a statistical basis as shown below.

Furthermore, a mixed-phase cloud data set derived from the HD(CP)² supersites (Cloudnet stations) is used to evaluate ICON-LEM with respect to mixed-phase clouds on a statistical basis. Therefore, a Cloudnet-conform target categorization is derived from the ICON output for which then the same algorithms for mixed-phase cloud characterization can be applied as to the measurements. Finally, the model and the measurements statistics of for example cloud top height vs. ice mass flux can be compared.

Scale- and regime-dependency of cloud-aerosol relationships

Cloud observations from polar-orbiting satellites are used to investigate the scale- and regime dependency of cloud-aerosol relationships. The observed and modelled spatial and temporal distributions of cloud properties are analyzed by, e.g., the construction of probability density functions, also stratified with respect to meteorological parameters. For this, 15 years of MODIS swath cloud products for two spatial domains were collected on pixel-basis from which in turn aggregated data sets were constructed. The focus until now is on the comparison of cloud distributions between the MODIS observations and the ICON-NWP TA simulations, since they provide cloud simulations on a large domain and for a longer time period. Also, the high resolution ICON-LEM simulations and their corresponding aerosol perturbation runs for the German domain as well as available ICON-GCM output will be used in statistical analyses and representativeness studies.

Modelling and Observations

Work package 7 in particular works on the evaluation of the ICON-LES model, we compare the control run and perturbed simulations with observations. It uses a full-domain region (Germany) and performs comparison between the ICON cloud vertical structure (lowest cloud base and higher cloud top) and the retrievals of ground-based ceilometers (DWD network) which yield the lowest cloud base in combination with satellite cloud tops derived from SEVIRI satellite observations. With the ICON-LEM meteogram output available for 39 supersites, it will be possible to derive other products from the simulated variables and analyze these in more detail.

The image shows observed and modeled cloud base heights normalized distributions on 02/05/2013. In blue are shown the ceilometer observations (DWD ceilometer network) in 141 stations around Germany. In orange are displayed modeled cloud bases heights from ICON-LES simulations done at domain 1 level, and in yellow the perturbed simulations doubling the CCN (cloud condensation nuclei) concentrations.

Detectability in observations


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