Victor Stepanenko

LakeMIP Kivu: evaluating the representation of a large, deep tropical lake by a set of one-dimensional lake models

The African great lakes are of utmost importance for the local economy (fishing), as well as being essential to the survival of the local people. During the past decades, these lakes experienced fast changes in ecosystem structure and functioning, and their future evolution is a major concern. In this study, for the first time a set of one-dimensional lake models are evaluated for Lake Kivu (2.28°S; 28.98°E), East Africa. The unique limnology of this meromictic lake, with the importance of salinity and subsurface springs in a tropical high-altitude climate, presents a worthy challenge to the seven models involved in the Lake Model Intercomparison Project (LakeMIP). Meteorological observations from two automatic weather stations are used to drive the models, whereas a unique dataset, containing over 150 temperature profiles recorded since 2002, is used to assess the models performance. Simulations are performed over the freshwater layer only (60 m) and over the average lake depth (240 m), since salinity increases with depth below 60 m in Lake Kivu and some lake models do not account for the influence of salinity upon lake stratification. All models are able to reproduce the mixing seasonality in Lake Kivu, as well as the magnitude and seasonal cycle of the lake enthalpy change. Differences between the models can be ascribed to variations in the treatment of the radiative forcing and the computation of the turbulent heat fluxes. Fluctuations in wind velocity and solar radiation explain inter-annual variability of observed water column temperatures. The good agreement between the deep simulations and the observed meromictic stratification also shows that a subset of models is able to account for the salinity- and geothermal-induced effects upon deep-water stratification. Finally, based on the strengths and weaknesses discerned in this study, an informed choice of a one-dimensional lake model for a given research purpose becomes possible.

Numerical modeling of methane emissions from lakes in the permafrost zone

A brief review of published observations of methane fluxes to the atmosphere from bogs and lakes in the permafrost zone is presented. Approaches to modeling the emission of methane from bogs are considered, and their advantages and shortcomings, in particular, from the point of view of their coupling to climate models, are outlined. A one-dimensional model developed by the authors for methane generation, transport, and sink in the ground-water body system and coupled to a hydrothermodynamic model of a water body is described. The approaches used in analogous models for bogs as well as new parametrizations describing lake-specific processes are applied. A parametrization of methane generation in vicinity the lower boundary of the thawed ground zone underneath a water body (talik) is suggested. The results of calibrating this model against available observations of methane emission from the thermokarst Shuchi Lake in northeastern Siberia are discussed.

Simulation of surface energy fluxes and stratification of a small boreal lake by a set of one-dimensional models

Five one-dimensional (1D) lake models were run for the open water season in 2006 for Lake Valkea-Kotinen (Finland) using on-lake measured meteorological forcing. The model results were validated using measurements of water temperature and of eddy covariance (EC) fluxes. The surface temperature is satisfactorily simulated by all models showing slight overestimation (by 0.1–1.1°C). Both sensible and latent heat fluxes are positively biased in respect to EC data, consistent with earlier studies. However, correlation coefficients between EC-fluxes and those simulated are relatively high ranging from 0.55 to 0.74. The skill to simulate vertical temperature profiles by different models is assessed as well. It is found that the lake models underestimate the EC-derived surface drag coefficient, however providing realistic temperature profiles. It is argued that the real momentum flux from the atmosphere is larger than simulated, however it is split up between the wave development and the acceleration of lake currents. Adopting the simple parameterisation for momentum flux partitioning in one of the models showed that this mechanism can be significant. Finally, the effect of including the lake bathymetry data in k-ɛ models was the drastic overheating of water below the thermocline. This is likely to be caused by omitting the heat flux at the lake margins. Thus, the parameterisation of heat flux at the lakes margins should be included in the models; otherwise it is recommended to neglect bathymetry effects for such small water bodies as the Lake Valkea-Kotinen.

Effects of water clarity on lake stratification and lake‐atmosphere heat exchange

Recent progress of including lake subroutines in numerical weather prediction (NWP) models has led to more accurate forecasts. In lake models, one essential parameter is water clarity, parameterized via the light extinction coefficient, Kd, for which a global constant value is usually used. We used direct eddy covariance fluxes and basic meteorological measurements coupled with lake water temperature and clarity measurements from a boreal lake to estimate the performance of two lake models, LAKE and FLake. These models represent two 1D modeling frameworks broadly used in NWP. The results show that the lake models are very sensitive to changes in Kd when it is lower than 0.5 m−1. The progress of thermal stratification depended strongly on Kd. In dark water simulations the mixed layer was shallower, longwave and turbulent heat losses higher and therefore the average water column temperatures lower than in clear water simulations. Thus, changes in water clarity can also affect the onset of ice cover. The more complex LAKE modeled the seasonal thermocline deepening whereas it remained virtually constant during summer in the FLake model. Both models overestimated the surface water temperatures by about 1°C and latent heat flux by >30%, but the variation in heat storage and sensible heat flux were adequately simulated. Our results suggest that, at least for humic lakes, a lake-specific, but not time-depending, constant value for Kd can be used and that a global mapping of Kd would be most beneficial in regions with relatively clear lakes, e.g. in lakes at high altitudes.

Numerical modeling of the influence of cool skin on the heat balance and thermal regime of a water body

The influence that cool skin has on the energy exchange between the atmosphere and the ocean is investigated in this work. For this purpose, a series of numerical experiments with the use of the one-dimensional LAKE model of a water body were performed. Three types of cold-skin parameterization were used in this model. The data of in situ measurements in the coastal zone of the Black Sea, in the Arctic Ocean, and over Lake Sparkling served as the boundary and initial conditions. It has been established from the results of experiments that the LAKE model with the incorporated parameterization of the cool skin successfully reproduces cold-skin characteristics, namely, the difference between the temperature of the cool skin surface and the water temperature below the skin. The results of numerical experiments are within the variability of the results of in situ measurements. It has been shown that the presence of a cool skin reduces the heat losses of a body of water. The numerical experiments showed that the presence of a cool skin can change the thermal regime of a water body and its stratification by changing the heat balance at the surface. This result can be important for the modeling of many processes inside a body of water and at its surface, for example, gas and heat exchange.

Large-eddy simulation of stratified turbulent flows over heterogeneous landscapes

Large-eddy simulation (LES) runs are performed to calculate flows over heterogeneous surfaces imitating small forest lakes. Regularities in the turbulent exchange of heat and momentum over such objects are examined. A weak sensitivity of turbulence characteristics over a “lake” to thermal stratification is noted. Problems of the representativeness of field eddy covariance measurements of turbulent fluxes over such objects are discussed.

Experimental study of heat and momentum exchange between a forest lake and the atmosphere in winter

The article presents the results of an experimental study of turbulent heat exchange between the surface of a frozen lake surrounded by forest and the atmospheric boundary layer. Heat and momentum fluxes were measured at three levels by an eddy covariance (EC) technique. Additionally, the heat fluxes were estimated by a surface energy balance method using a temperature profile measured in the snow cover and net longwave and shortwave radiation. The results of the measurements show that the eddy covariance fluxes correlate well with those obtained by the surface energy balance method, with a tendency of underestimation. The presence of wind-shear effects at treetop height demonstrated recently in a Large Eddy Simulation (LES) [3] was supported in our measurements by the fact that the momentum flux increased with height from the surface. The negative sensible heat flux increased with height most of the time. We suggest that this phenomenon may partially be caused by the high negative heat fluxes above the surface formed when warm advection occurs at altitudes of ~100 m. During the warm advection events, Monin-Obukhov similarity theory (MOST) fails to reproduce the sharp increase of the negative heat flux at the surface layer. Beyond the warm advection events, the MOST calculations agree well with the EC fluxes, however, with some systematic underestimation bias.

Numerical simulation of the structure and evolution of a polar mesocyclone over the Kara Sea. Part 1. Model validation and estimation of instability mechanisms

Numerical experiments based on the WRF model were conducted to analyze the structure and evolution of the polar mesoscale cyclone developed over the Kara Sea on September 29-30, 2008. It was found that baroclinic instability in the lower troposphere and convective instability (including that due to the wind-induced surface heat exchange) did not play a significant role. Significant contribution was made by the downward advection of potential vorticity from the upper troposphere and by the conditional instability of second kind. It is demonstrated that if water phase transitions are not taken into account, the mesocyclone intensity is reduced by 7-20% and the time of its development increases by 4 hours. The advection of potential vorticity was not the only process causing the intensification of the lower potential vorticity anomaly associated with cyclonic circulation.

Development of lake parametrization in the INMCM climate model

Land surface schemes (LSS or terrestrial models) are a crucial component of both Numerical Weather Prediction (NWP) systems and climate models. An important land- surface type is lakes. This paper presents the mechanism of incorporation of a model LAKE into a coupled general circulation model of the atmosphere and ocean INMCM4, with a space resolution 2° to 1.5° and 21 levels in height, with two-way interaction. A new map for 14 land types distribution was created using a digital map of inland waters for the entire globe. The digital map of water bodies includes the fraction of lake area on the land surface, and the average depth of water bodies, both on the grid of the climate model. This digital map is based on a dataset consisting of 14 000 freshwater lakes. In order to increase the time step in the LAKE model, the k-e parameterization has been replaced by the parameterization of Henderson-Sellers. With the amended INMCM4 model, numerical experiments were carried out to simulate global climate during the second half of the XX century. The effects of the new lake parameterization on the surface temperature and heat fluxes are analyzed.

The implementation of regional atmospheric model numerical algorithms for CBEA-based clusters

Regional atmospheric models are important tools for short-range weather predictions and future climate change assessment. The further enhancement of spatial resolution and development of physical parameterizations in these models need the effective implementation of the program code on multiprocessor systems. However, nowadays typical cluster systems tend to grow into very huge machines with over petaflop performance, while individual computing node design stays almost unchanged, and growth is achieved simply by using more and more nodes, rather than increasing individual node performance and keeping adequate power consuming. This leads to worse scalability of data-intensive applications due to increasing time consumption for data passing via clusters interconnect. Especially some of numerical algorithms (e.g. those solving the Poisson equation) satisfactorily scaling at previous generation cluster systems do not utilize the computational resources of clusters with thousands cores effectively. This prompts to study the performance of numerical schemes of regional atmospheric models on processor architectures significantly different from those used in conventional clusters. Our approach focuses on improving the performance of time explicit numerical schemes for Reynolds-averaged equations of atmospheric hydrodynamics and thermodynamics by parallelization on CellBE processors. The optimization of loops for numerical schemes with local data dependence pattern and with independent iterations is presented. Cell-specific workloading managers are built on top of existing numerical schemes implementations, conserving the original source code layout and bringing high speed-ups over serial version on QS22 blade server. Intercomparison between Cell and other multicore architectures is also provided. Targeting the next generation of MPI-CellBE hybrid cluster architectures, out method aims to provide additional scalability to MPI-based codes of atmospheric models and related applications.