Programme Topic 1: Dynamics and cloud processes in the troposphere

Regional change of water availability is one of the most vital consequences of climate change. Therefore, the need for scientifically sound assessments of regional climate and water cycle change scenarios is rapidly growing. Research in Topic 1 focusses on representing the atmospheric components of the water cycle in a realistic way in regional high resolution climate models and in weather forecast models, and on validating the results by advanced statistical analysis.

The Institute of Meteorology and Climate Research contributes to Topic 1 with its divisions IMK-AAF, IMK-TRO, and IMK-ASF. The expertise of IMK-AAF is in aerosols dynamics and effects on climate gained with the unique aerosol chamber (AIDA) measurements. IMK-TRO is experienced in tropospheric dynamics, convection and mesoscale cloud processes.

Long-term experience in remote sensing using operational satellites and surface-based remote sensing in IMK-ASF complements the research portfolio. The research strategy comprises the development and application of numerical models, forced and validated by data from extensive field and chamber experiments.

Prerequisites of scientific progress include more and better observational data, to be collected from aircraft (e.g., HALO), with ground- and satellite based GPS sensors and additional remote sensing instruments (Radar, Lidar). We will improve our understanding of cloud and precipitation processes especially in strong convection conditions by laboratory experiments and process models. The experiments serve to assess the efficiency of aerosols in initiating the ice phase via heterogeneous nucleation and to elucidate the processes of secondary ice formation, whereas the process models on different scales provide insights into all dynamical and hydrometeorological mechanisms. The investigations deal with basic problems, from which improved parametrizations of sub-gridscale aerosol and cloud processes in models will be obtained. Improving operational forecast systems, quantifying the limits of predictability, and assessing hazard and risk in a changing climate are applications of this work.

The challenge to represent the components of the atmospheric water cycle (evapotranspiration, clouds, precipitation) in a realistic way requires major improvements of regional climate models. In particular, much higher resolution of a few kilometres, adequately adapted process descriptions and parametrizations, innovative atmospheric observations with better resolution, as well as sophisticated validation strategies using data beyond those from routine observation networks are required. High spatial resolution is necessary to adequately account for the effects of orography and land use, to improve the results for quantities which depend explicitly on terrain height (like precipitation and temperature) and to avoid the necessity of subgrid-scale parameterisations (e.g. for convective processes). High resolution simulations covering several decades with emphasis on the components of the atmospheric and unsaturated soil-zone water cycle need to be performed for sensitive regions in Europe and Africa.

An important issue is the assessment of the uncertainty of observations and simulation results. Thus, ensemble simulations must be performed whose results are evaluated statistically. Statistics of extreme events (extreme precipitations, droughts), including their uncertainty and their trends, have to be obtained by analysing the distributions derived from the simulations results.

Such advanced simulations require accurate and appropriate meteorological observations. Therefore, innovative in-situ and remote sensing techniques including GPS and other satellite information are needed to initialize and validate the models. The IPCC 2007 Report states with regard to the water cycle, that the modification of cloud properties by aerosols is not well understood and the magnitudes of associated indirect radiative effects are poorly determined. Difficulties in measuring precipitation, soil moisture and total and low cloud amounts as well as trends of extreme events like hurricanes and convective phenomena also belong to key uncertainties of climate change. Hazard and risk assessments are needed to mitigate the impacts of extreme events.

Improved models therefore require an improved process understanding of cloud and precipitation processes on a wide range of scales. The most challenging topic in the formation and development of clouds and the initiation of precipitation is the analysis of processes and influences leading to and sustaining convective clouds. These processes are governed by the abundance and properties of atmospheric aerosol particles as well as hydro- and thermodynamic conditions and depend on the synoptic and mesoscale environment. Research on various scales is involved to improve regional-scale weather prediction and regional/global-scale climate models: to uncover the relevance of microphysical and nanoscale cloud processes, to unravel the complex interactions related to particles in clouds and precipitation, and to assess the impact of the mesoscale and synoptic environment on clouds. On the one hand, this requires laboratory studies and advanced remote sensing measurements from instruments like radar and lidar. On the other hand, numerical models are needed, which describe the significant processes of hydrometeor evolution and their feedback to dynamics.

Responsible and contact: Prof. Dr. Christoph Kottmeier (IMK-TRO), Prof. Dr. Thomas Leisner (IMK-AAF)