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This paper provides an overview of the scientific background and the research objectives of the Research Unit "VolImpact" (Revisiting the volcanic impact on atmosphere and climate - preparations for the next big volcanic eruption, FOR 2820). VolImpact was recently funded by the Deutsche Forschungsgemeinschaft (DFG) and started in spring 2019. The main goal of the research unit is to improve our understanding of how the climate system responds to volcanic eruptions. Such an ambitious program is well beyond the capabilities of a single research group, as it requires expertise from complementary disciplines including aerosol microphysical modelling, cloud physics, climate modelling, global observations of trace gas species, clouds and stratospheric aerosols. The research goals will be achieved by building on important recent advances in modelling and measurement capabilities. Examples of the advances in the observations include the now daily near-global observations of multi-spectral aerosol extinction from the limb-scatter instruments OSIRIS, SCIAMACHY and OMPS-LP. In addition, the recently launched SAGE III/ISS and upcoming satellite missions EarthCARE and ALTIUS will provide high resolution observations of aerosols and clouds. Recent improvements in modeling capabilities within the framework of the ICON model family now enable simulations at spatial resolutions fine enough to investigate details of the evolution and dynamics of the volcanic eruptive plume using the large-eddy resolving version, up to volcanic impacts on larger-scale circulation systems in the general circulation model version. When combined with state-of-the-art aerosol and cloud microphysical models, these approaches offer the opportunity to link eruptions directly to their climate forcing. These advances will be exploited in VolImpact to study the effects of volcanic eruptions consistently over the full range of spatial and temporal scales involved, addressing the initial development of explosive eruption plumes (project VolPlume), the variation of stratospheric aerosol particle size and radiative forcing caused by volcanic eruptions (VolARC), the response of clouds (VolCloud), the effects of volcanic eruptions on atmospheric dynamics (VolDyn), as well as their climate impact (VolClim). [ABSTRACT FROM AUTHOR]

An extension of the dynamic Smgarorinsky model (DSM) is presented. In the so-called dynamic mixed model (DMM), the Leonard stress (i.e., the resolved part of the Reynolds stress) is no longer parameterized in the test filter range by the Smagorinsky model. Instead, it is modeled by the so-called similarity model, which assumes close self-similarity of the resolved and unresolved flow. This implementation is derived first in general and then specifically for spectral models. In the latter case, spectral cut-off filters allow a simple reformulation of the basic tensor equation of the DSM, requiring only the modification of a particular term to obtain the DMM version of the turbulence model. The DMM is then examined in the Kühlungsborn Mechanistic general Circulation Model (KMCM), a spectral CGM, where, starting from the same initial condition, the flow fields are compared in a model run with the DMM and in a control run with the DSM on day zero and day eight. Although most of the results are comparable, there is, already at day zero, a significant difference at the poles: with the DSM, there are certain locations with large values of the Smagorinsky parameter that are not present in the DMM run. This can be explained mathematically by the fact that the convergence of the associated Legendre polynomials in the spectral representation is worse at the poles. Therefore, numerical errors can occur in the spectral transformation, which cancel out in the DMM due to its formulation, allowing the increase of the time steps. [ABSTRACT FROM AUTHOR]

Tidally generated internal waves over Rosemary Bank Seamount, North Atlantic, were investigated using the Massachusetts Institute of Technology general circulation model. The model results were validated against in-situ data collected during the 136th cruise of the RRS 'James Cook' in June 2016. The current study focuses on the sensitivity of the model output to the parameter settings. The estimates of the available potential energy integrated over the model domain were taken as a proxy for evaluating the sensitivity of the model results to the grid steps, horizontal and vertical viscosity, diffusivity, and mixing schemes. It was found that coarse grid models overestimate available potential energy converted to internal tides over seamounts. In fact, the energy conversion rate from barotropic to baroclinic tidal components is sensitive to the grid resolution. The reasons for this tendency are discussed in the paper. • Generation of internal waves over an isolated seamount is considered. • Sensitivity runs were conducted in a wide range of the numerical grid resolution. • The coarse grid models overestimate internal waves potential energy. [ABSTRACT FROM AUTHOR]

A review of the paper "Ocean Surface Temperature Variability: Large Model-Data Differences at Decadal and Longer Periods" by T. Laepple and P. Huybers, published in the 2014 issue of "Proceedings of the National Academy of Sciences" is presented. Topics discussed in the paper include the implications of sea surface temperature (SST) variations for the society and the agreement between observational and General Circulation Model (GCM) estimates of SST variability.

In the present study, the quantity, duration and intensity of heat stress events in Germany as well as their future change and relation with weather types were investigated. A small ensemble of regional climate simulations with the regional climate model (RCM) COSMO-CLM driven by four general circulation models (GCMs) was used to calculate the Universal Thermal Climate Index (UTCI); the UTCI is a well-accepted thermal comfort index which we use here to quantify thermal stress. The variables entering the UTCI were bias corrected with a method that preserves their interdependencies. The projected climate changes cause a significant increase of both the mean UTCI and the number, duration and intensity of heat stress events between the control period (1981-2000) and the projection period (2031-2050). The projected future hourly frequency distribution of the UTCI at a location can be described by a shift to higher UTCI values with an almost constant shape of distribution. The investigations of the projected changes in weather types show no significant changes between the periods covered, with a few exceptions. An exception concerning heat stress events is the increase of summer anticyclonic weather types. Although more anticyclonic weather types in summer lead to an increase in heat stress events, they are not the primary cause of the projected increases. Rather, it turns out that the characteristics of the air masses associated with the weather types change towards warmer and more humid conditions. [ABSTRACT FROM AUTHOR]