P1 – Cloud Modelling

The aim of P1 is to provide a model-based and most realistic relation between large- and smallscale cloud physical properties and their resulting radiative properties.

Regional and local simulations will be carried out with the ICON model for different cloud scenes. These high-resolution simulations will be performed in close consultation with the observational work of the partner projects in order to capture existing and future high-resolution observations from space and ground. Based on long-term observations, cloud regimes will be identified and further analysed with respect to model-observation differences both in cloud macrophysical and radiative properties. For the latter, very high-resolution model-runs on the hectometer scale with microphysical parameterization of different degrees of complexity will provide the input for 3D Monte-Carlo radiative transfer simulations, which in turn provide the radiation closure for the assessment of 3D biases in cloud radiative effects and passive remote sensing from space. Furthermore, the high-resolution regional modelling will put the column-based observations at the ground-based super sites as well as the pixel-based observations from space into a larger context of weather and seasonal regimes.

Principal Investigators:

• Vera Schemann, University of Cologne
• Fabian Senf, Leibniz Institute for Tropospheric Research

Work Programme

In order to address these research questions, the following work programme ist anticipated.

Setup of simulations

We will define two dedicated setups for the application and evaluation of hectometer simulations. One setup will cover the region around Lindenberg for the planned C3SAR campaign, but also the FESSTVAL campaign, performed 2021 in Lindenberg. Another setup will target the HOPE campaign at the Jülich Observatory for Cloud Evolution (JOYCE).

Especially limited area simulations are strongly influenced by the applied large-scale forcing. This includes domain sizes and resolution as well as surface structure and synoptic boundary forcing. Therefore, we will perform several sensitivity studies to define a suitable model-setup for the campaign. The sensitivity studies will help to identify specific sensitivities or uncertainties in the virtual model world, which should then be constrained by additional observations at the C3SAR campaign.

Reconstruction and Synthesis

While observations are usually limited in dimension, models can provide a consistent four dimensional context of specific atmospheric situations and observations. In combination with the works performed in Project 2 and Project 3, we will create a synthesis of observations and simulations to reconstruct the full 4-dimensional context.

Detailed cloud microphysics

Within this project, we will perform detailed cloud microphysics simulations. Thereby, we will focus on meteorological situations that are characterized by weak synoptic-scale forcing and in which shallow and mainly liquid-containing cumuliform clouds have formed. The emphasis on clouds dominated primarily by liquid-phase processes creates important simplifications for the initial application of the detailed cloud microphysics model SPECS. Online nested simulations using ICON-LEM will be set up and run, targeting a horizontal mesh size of 100 m or less for the innermost nest. Reference simulations will be performed using standard 2-moment bulk microphysics. As an extension towards more detail, ICON coupled to SPECS is rerun in the innermost nest. These detailed and computationally expensive simulations are driven with the respective next coarser nest with bulk microphysics.

The factors influencing the cloud radiative effect are analyzed, based on the high-resolution ICON simulations. Therefore, the 3D radiative transfer model MYSTIC is run, using both bulk cloud microphysics data and detailed spectral cloud microphysics data. Furthermore, MYSTIC calculation with bulk microphysics will be compared with the results of 1D online ecRAD calculations. Thus, the differences between the 3D effects to those without microphysical details can be quantified.

Bias identification

It is expected that the deviations between 1D and 3D cloud radiative effects will strongly depend on the spatial cloud structure and the heterogeneity of cloud properties, so that, for example, opaque stratiform clouds will show much less biases than broken clouds. And the different structural properties of cloud will be associated with different weather situations. Based on the classification of cloud regimes, done within this Research Unit, we will use this information and apply it to the simulation of selected periods as well as on selected case studies. By this we can investigate the influence of different weather situations and cloud regimes on the bias between 1D and 3D simulated radiative effects.

Furthermore, the scale dependence of the biases will be investigated. The effects are part of a cascade in which coarser-scale cloud structures and their mesoscale organization are associated with the respective weather regimes, while finer-scale effects arise from turbulent mixing processes in the cloud interior and boundary regions as well as from microphysical variabilities. The scale dependence of cloud radiative effect biases is thus related with the governing processes. This raises the exciting question of at which scales (in terms of modeling, this means at which mesh sizes) what level of process detail is needed to represent the most important parts of the radiation-cloud interaction and its spatiotemporal variability.