Untersuchungen zur Entwicklung von Rollenkonvektion in Kaltluftausbrüchen mittels Grobstruktursimulationen

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Parametrization of Turbulent Fluxes over Leads in Sea Ice in a Non-Eddy-Resolving Small-Scale Atmosphere Model

Leads (open-water channels in sea ice) play an important role for surface-atmosphere interactions in the polar regions. Due to large temperature differences between the surface of leads and the near-surface atmosphere, strong turbulent convective plumes are generated with a large impact on the atmospheric boundary layer (ABL). Here, we focus on the effect of lead width on those processes, by means of numerical modeling and turbulence parametrization. We use a microscale atmosphere model in a 2D version resolving the entire convective plume with grid sizes in the range of L/5 where L is the lead width. For the sub-grid scale turbulence, we developed a modified version of an already existing nonlocal parametrization of the lead-generated sensible heat flux including L as parameter. All our simulations represent measured springtime conditions with a neutrally stratified ABL capped by a strong temperature inversion at 300 m height, where the initial temperature difference between the lead surface and the near-surface atmosphere amounts to 20 K. We found that our simulation results obtained with the new approach agree very well with time-averaged results of a large eddy simulation (LES) model for variable lead widths with L ≥ 1 km and different upstream wind speeds. This is a considerable improvement since results obtained with the previous nonlocal approach clearly disagree with the LES results for leads wider than 2 km. In conclusion, considering L as parameter in a nonlocal turbulence parametrization seems to be necessary to study the effect of leads on the polar ABL in non-eddy-resolving small-scale atmosphere models

The Parallelized Large-Eddy Simulation Model (PALM) version 4.0 for atmospheric and oceanic flows: model formulation, recent developments, and future perspectives

In this paper we present the current version of the Parallelized Large-Eddy Simulation Model (PALM) whose core has been developed at the Institute of Meteorology and Climatology at Leibniz Universität Hannover (Germany). PALM is a Fortran 95-based 5 code with some Fortran 2003 extensions and has been applied for the simulation of a variety of atmospheric and oceanic boundary layers for more than 15 years. PALM is optimized for use on massively parallel computer architectures and was recently ported to general-purpose graphics processing units. In the present paper we give a detailed description of the current version of the model and its features, such as an embedded 10 Lagrangian cloud model and the possibility to use Cartesian topography. Moreover, we discuss recent model developments and future perspectives for LES applications.DFG/RA/617/3DFG/RA/617/6DFG/RA/617/16DFG/RA/617/27-

Convection over sea ice leads: large eddy simulation data

The data set consists of time-averaged large eddy simulation (LES) data of simulations of the convection over leads in the polar sea ice regions. The applied model is the Parallelized Large-Eddy Simulation Model (PALM) [Maronga et al. (2015), Raasch & Schröter (2001)]. All of the ten simulations represent a near lead-perpendicular inflow in the atmospheric boundary layer (ABL) over the leads, where the simulations differ in the lead width L, and where partly the flow over two consecutive leads has been simulated. All simulations represent idealized scenarios of a typical springtime ABL in the polar ocean regions. Hence, the data is not georeferenced and the outputs are given with respect to a Cartesian coordinate system. For each simulation, two model output files are available. All files with “xz” refer to vertical cross-sections in lead-perpendicular direction, where the respective model variables are averaged in lead-parallel direction. All files with “xy” refer to values at the surface. All model outputs are averaged in time over a period of 3600 seconds and outputs are available for an output time interval of 900 seconds. The simulations differ in lead width L, in the upstream ABL-averaged wind speed U (or here: ff), and in the surface temperature of sea ice Tice (or here: Surf temp). More details on the respective setup of the scenarios and a description of the applied model version of PALM are given in the corresponding publication (see section 2 and Table 1). More details on the LES variables are given in the PALM-documentation: https://palm.muk.uni-hannover.de/trac/wiki/doc/app/d3pa

Scale analysis of convective clouds

The size distribution of cumulus clouds due to shallow and deep convection is analyzed using satellite pictures, LES model results and data from the German rain radar network. The size distributions found can be described by simple power laws as has also been proposed for other cloud data in the literature. As the observed precipitation at ground stations is finally determined by cloud numbers in an area and individual sizes and rain rates of single clouds, the cloud size distributions might be used for developing empirical precipitation forecasts or for validating results from cloud resolving models being introduced to routine weather forecasts

Parametrizing the effects of lead-generated convective plumes on the polar atmosphere

Even in the central polar ocean regions, the sea ice cover cannot be regarded as a homogeneous and permanently closed surface. Due to divergent sea ice drift, even in winter elongated open-water channels, so-called leads, develop. Leads play an important role for surface-atmosphere interactions since they enable a direct contact between ocean and atmosphere. Especially in winter, when temperature differences amount up to 40 K, strong convective plumes are generated over leads with upward heat fluxes in the order of 10^2 Wm^(-2) . This has considerable effects on the structure of the atmosphere. Those effects mainly depend on the meteorological conditions close to the lead, but also on the lead geometry as, for example, the lead width. To understand the determining processes which lead to the formation and the decay of the plumes, we use numerical model simulations of a lead-perpendicular flow. We use a small-scale atmosphere model, which resolves the entire plume, but not the transport due to sub-grid scale turbulence. Hence, that transport has to be parametrized to close the system of the model equations. Thus, we developed an improved parametrization of the convection over leads, mainly for the turbulent heat flux. A previous parametrization, where one particular lead width (1 km) was considered, acts as a starting point and we derived a more general approach by including the lead width as a parameter. To validate our results, we use time-averaged results of a large eddy simulation (LES) model. We show that also for different lead widths small-scale model results obtained with our modified parametrization agree well with LES. In addition, our approach is still robust against variations of wind speed and surface temperature differences. Thus, our parametrization represents a clear improvement since now variable lead widths can be considered for a detailed investigation of the effects of lead-generated convection on the polar atmosphere

Meteorology and oceanography of the Atlantic sector of the Southern Ocean—a review of German achievements from the last decade

In the early 1980s, Germany started a new era of modern Antarctic research. The Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) was founded and important research platforms such as the German permanent station in Antarctica, today called Neumayer III, and the research icebreaker Polarstern were installed. The research primarily focused on the Atlantic sector of the Southern Ocean. In parallel, the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) started a priority program ‘Antarctic Research’ (since 2003 called SPP-1158) to foster and intensify the cooperation between scientists from different German universities and the AWI as well as other institutes involved in polar research. Here, we review the main findings in meteorology and oceanography of the last decade, funded by the priority program. The paper presents field observations and modelling efforts, extending from the stratosphere to the deep ocean. The research spans a large range of temporal and spatial scales, including the interaction of both climate components. In particular, radiative processes, the interaction of the changing ozone layer with large-scale atmospheric circulations, and changes in the sea ice cover are discussed. Climate and weather forecast models provide an insight into the water cycle and the climate change signals associated with synoptic cyclones. Investigations of the atmospheric boundary layer focus on the interaction between atmosphere, sea ice and ocean in the vicinity of polynyas and leads. The chapters dedicated to polar oceanography review the interaction between the ocean and ice shelves with regard to the freshwater input and discuss the changes in water mass characteristics, ventilation and formation rates, crucial for the deepest limb of the global, climate-relevant meridional overturning circulation. They also highlight the associated storage of anthropogenic carbon as well as the cycling of carbon, nutrients and trace metals in the ocean with special emphasis on the Weddell Sea