Confronting ICON-LEM with supersite observations

In work package 1, first steps towards the comparison between model output and observations have been done. Observed liquid cloud fraction profiles over the supersites of JOYCE and RAO Lindenberghave been derived and compared to the liquid cloud fraction profiles obtained from each of the different domains of the ICON-LEM model output. The analysis has been performed so far on 9 simulated days, the profile thus represents a statistical mean over the entire ensemble of available hours.

We found that fog is present at nighttime in the model on all domain resolutions, while it is not present in the observations. Also, the vertical extension of the liquid clouds during daytime is larger in the model output than in the observations, resulting in too high cloud tops.

Further investigations on fog and on boundary layer clouds are currently performed.

Satellite and Ground-Based Synergy and Radiative Impacts for ICON-LEM Evaluation

The aim of work package 2 is to assess the capability of the ICON-LEM to realistically simulate low-level clouds and to accurately represent the resulting cloud radiative effects. Towards this direction, four diagnostic quantities have to be considered: optical thickness, statistical distribution of the liquid water content within the grid, geometrical cloud thickness, and cloud base and top altitudes. For the calculation of the cloud optical thickness, the sub-adiabatic cloud approximation has been employed. Currently, an effort is made to evaluate high-resolution simulations of low-level clouds with observations from passive satellites (METEOSAT SEVIRI, MODIS) over Germany. Research questions addressed:

  • Is ICON-LEM able to realistically simulate low-level clouds (microphysics)?
  • How can we use ICON-LEM to study the influence of the resolution on spatio-temporal scales?
  • What are the implications of the deviations between observations and models?
  • Is the sub-adiabatic cloud assumption a valid concept for model evaluation?

Ultimately, the observed and simulated cloud fields will be used as an input for a radiative transfer model and a radiation closure study will be performed.

Evaluation of the precipitation-evaporation parameterization from supersite observations and simulations

Within work package 3, the evaporation of precipitation will be analyzed in detail. In a first step, cooling rates (CR) are simulated for different assumptions on the initial drop size distribution (DSD) based on a gamma distribution (Fig. I). It is obvious that the DSD strongly influences the cooling rates.

In a second step the assumptions are replaced by observations from collocated micro rain radar (DSD; liquid water content LWC) and Cloudnet (cloud base) observations (Fig. II). Falling rain droplets evaporate resulting in a decreasing drop diameter, a decreasing amount of drops and a decreasing LWC. This evaporation leads to a cooling in the atmosphere (see CR in Fig. II d). In the next step, temperature and humidity profiles from models will be replaced by observations (e.g. from radiosondes).

Soon those CR will be compared to CR gained from microwave radiometer and large eddy simulations (ICON-LEM).

Finally, findings will be implemented in parameterizations of the precipitation evaporation in general circulation models (GCM).

PDF Cloud Schemes for ICON and their interaction with radiation

The overall aim of work package 4 is to improve cloud representation and cloud radiative interactions in the ICON Global Climate Model by implementing and refining a so called probability density function (PDF) parametrization. The core idea of PDF schemes (also sometimes referred to as statistical schemes) is to represent the sub-grid scale variability of one or multiple variables through an analytical PDF for each model cell. Our approach is to use a beta function to describe the total water in each cell. From this beta function we can deduce various parameters such as cloud fraction.

The first main task is to use the many available LES simulations to evaluate and improve the PDF parametrization. The second task is to implement and further develop the beta function PDF scheme in the ICON Global Climate Model. This task is being done in collaboration with work package 5 of S2. The ICON-GCM will then be tested using various simplified and realistic test cases. While the first task will be completed by the end of 2017, the scope and depth of the second task will depend strongly on how smoothly the implementation occurs.

Parameterizing 3D Heating and Cooling Rates

Within work package 5 we aim at systematically characterizing the effect of radiation on boundary layer cloud development on the ICON-LEM resolution, and at studying the relevance for the fast cloud feedback using ICON-GCM.
We use our fast parameterizations for 3D radiative transfer (Jakub and Mayer, 2016; Klinger and Mayer 2016) which are for this purpose adapted to the unstructured ICON LEM grid. Aim of this work package is a systematic characterization of radiation effects on boundary layer clouds and the development of a GCM parameterization, based on these results. Recent results showing the impact of radiation (exact 3D calculation and two common approximations) on cumulus cloud structure and organization (Klinger et al., 2017) is shown in the following figure.

Fast response I: Perturbed C02 Simulations

Work package 6.1 employs the ICON model system at a range of resolutions and domain sizes, from local cloud-resolving (ICON-LEM) to global "climate-permitting" (ICON-GCM). High-resolution runs of ICON over different geographical areas and multiple months will be confronted with operational supersite observations as well as advanced satellite products in order to assess for parameterization development and climate sensitivity studies.
A first Opens internal link in current windowsensitivity simulation with the HD(CP)² ICON-LEM with perturbed CO2 concentrations has been prepared and is currently run at the Forschungszentrum Jülich to assess fast cloud responses to CO2 increase.

The figure shows a comparison (from Nam et al., in revision) of the change in effective radiative forcing in response to a quadrupling of CO2 using for two experiments in the ICON-GCM configuration. The similarity between the experiment with a locally imposed CO2 change over Central Europe to the changes over Central Europe from a globally imposed CO2 change suggest that the limited-area HD(CP)² simulation with ICON-LEM will be meaningful and representative. In fact, preliminary results of the perturbed ICON-LEM are imposed onto the plot and it is within the distribution found using the ICON-GCM. The ICON-GCM simulations will help identify the temporal & spatial scales which rapid adjustments statistically significant and guide the study of rapid adjustments with the ICON-LEM.

Fast response II: LEX Proxy Cases

The main aim of work package 6.2 is to determine the effects elevated moisture layers have on boundary layer clouds. Present day cases of observed elevated moisture layers are planned to be used as proxy cases for representing future atmospheric conditions resulting from climate change.  These cases are built using observations recorded during the NARVAL I South Campaign in 2013 [link]. The research flight that is focused on is Research Flight 4 (RF4), which took place on December 14th 2013. The reason this flight was chosen is due to the presence of an elevated moisture layer towards the end of the flight.

Work conducted up to now includes running a number of large eddy simulations for the locations of each of the dropsondes, including a control simulation and a simulation that is nudged towards the observed state. The effect of nudging towards the observation can be seen in the following figures.

Future work will focus on comparing the simulations to additional cloud observations from the campaign. Then the LES will be run for a number of different elevated moisture layers which will vary in amplitude and altitude.

Stochastic closure for nocturnal boundary-layer turbulence under influence of larger scale motions

Work package 7, as a part of the S2 module, investigates the interaction between nocturnal boundary-layer (NBL) turbulence and low-level clouds.

On large scales, this study is focusing on data analysis of long time series from meteorological and remote sensing measurements from several well-established experimental sites in Germany and the Netherlands. By using stochastic clustering methodology, an extended data analysis of the influence of external forcing parameters on the NBL flow regimes has been performed, and regime occupation statistics have been subsequently analyzed. The NBL flow regimes have been represented via the bulk thermal stratification and shear conditions, as influenced by the external larger-scale forcing variables, cloud state and the geostrophic wind speed. The results of this analysis will be consequently used in the development of a stochastic-based turbulence closure for the NBLs that accounts for the large-scale (and often intermittent) effects on the flow. This closure can be used in large-scale models to advance the mixing, and thus cloud and precipitation predictions.

On small scales, when treating the NBL-dynamics explicitly, the initial cloud (state) formation as driven by large-scale motions in NBL is examined. For that purpose, a realistic case study over the Jülich experimental site in Germany has been developed. By comparing high-quality remote sensing observational data of velocity and cloud state, it has been shown that the low-level jet (LLJ) is responsible for the developed turbulence in the NBL and subsequent cloud formation. To further explain and characterize the mechanism of NBL turbulence-cloud formation, a high resolution output (starting from 624 m horizontal resolution and nesting up to 78 m) from the ICOsahedral Nonhydrostatic atmospheric large-eddy model (ICON-LEM) was produced for a domain of 110 km (diameter) over the Jülich experimental site. Comparison between observed and modeled NBL turbulence and low-level clouds as driven by the LLJ have been subject of discussion in this case study.

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