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Nickola M.,Hartebeesthoek Radio Astronomy Observatory | Botha R.C.,Hartebeesthoek Radio Astronomy Observatory | Esau I.,Gc Rieber Climate Institute | Djolov G.D.,University of Pretoria | Combrinck W.L.,Hartebeesthoek Radio Astronomy Observatory
South African Journal of Geology | Year: 2011

A Lunar Laser Ranging (LLR) system is to form part of geodetic instrumentation to be located at a new fundamental space geodetic observatory for South Africa. For optimal efficiency, LLR requires optical resolution or so-called astronomical seeing conditions of -1 arc-second in order to deliverusable ranging data. Site characterisation should include a description of astronomical seeing for various locations on-site and overall atmospheric conditions. Atmospheric turbulence degrades astronomical seeing. In-situ methods of determining astronomical seeing are difficult, time-consuming and costly. We propose the use of a turbulence-resolving model to determine andpredict astronomical seeing at a site. Large Eddy Simulation NERSC (NansenEnvironmental and Remote Sensing Centre) Improved Code (LESNIC) is a turbulence-resolving simulation code which models atmospheric turbulence. It hasbeen used to compile a database of turbulence-resolving simulations, referred to as DATABASE64. This database consists of a collection of LESNIC runs for a stably stratified planetary boundary layer (SBL) over a homogeneous aerodynamically rough surface. Results from DATABASE64 for the nocturnal boundary layer are employed to render profiles of the vertical distribution of optical turbulence (C 2 N profiles). Seeing parameter values are also obtained by making use of DATABASE64 results. The C 2 N profiles and seeing parameter values obtained from DATABASE64 results are compared with general observational results that have been published in the literature. The values obtained are consistent with results from field campaigns as reported. Turbulence-resolving models, such as LESNIC, show potential for delivering and predicting profiles and parameters to characterise astronomical seeing, which are essential prerequisites forestablishing an LLR system at the most suitable site and most suitable on-site location. A two-pronged approach is envisaged - in addition to modelling, quantitative seeing measurements obtained with an on-site seeing monitor will be used to verify and calibrate results produced by the LESNIC model. © 2011 December Geological Society of South Africa.

Esau I.,Gc Rieber Climate Institute | Esau I.,Center for Climate Dynamics | Esau I.,Nizhny Novgorod State Technical University | Luhunga P.,University of Pretoria | And 5 more authors.
Meteorology and Atmospheric Physics | Year: 2012

Links between spatial and temporal variability of Planetary Boundary Layer meteorological quantities and existing land-use patterns are still poorly understood due to the non-linearity of air-land interaction processes. This study describes the results of a statistical analysis of meteorological observations collected by a network of ten Automatic Weather Stations. The stations were in operation in the highveld priority area of the Republic of South Africa during 2008-2010. The analysis revealed localization, enhancement and homogenization in the inter-station variability of observed meteorological quantities (temperature, relative humidity and wind speed) over diurnal and seasonal cycles. Enhancement of the meteorological spatial variability was found on a broad range of scales from 20 to 50 km during morning hours and in the dry winter season. These spatial scales are comparable to scales of observed land-use heterogeneity, which suggests links between atmospheric variability and land-use patterns through excitation of horizontal meso-scale circulations. Convective motions homogenized and synchronized meteorological variability during afternoon hours in the winter seasons, and during large parts of the day during the moist summer season. The analysis also revealed that turbulent convection overwhelms horizontal meso-scale circulations in the study area during extensive parts of the annual cycle. © 2012 Springer-Verlag Wien.

Zilitinkevich S.S.,Finnish Meteorological Institute | Zilitinkevich S.S.,Gc Rieber Climate Institute | Zilitinkevich S.S.,Helsinki Institute of Physics | Esau I.,Gc Rieber Climate Institute | And 5 more authors.
Boundary-Layer Meteorology | Year: 2010

We give a new derivation of the familiar linear relation for the dimensionless velocity gradient in the stably stratified surface layer and provide physical and empirical grounds for its universal applicability in stationary homogeneous turbulence over the whole range of static stabilities from Ri = 0 to very large Ri. Combining this relation with the budget equation for the turbulent kinetic energy we obtain the "equilibrium formulation" of the turbulent dissipation length scale, and recommend it for use in turbulence closure models. © The Author(s) 2010.

Esau I.,Gc Rieber Climate Institute | Esau I.,Center for Climate Dynamics | Repina I.,Russian Academy of Sciences
Advances in Meteorology | Year: 2012

This paper presents analysis of wind climate of the Kongsfjorden-Kongsvegen valley, Svalbard. The Kongsfjorden-Kongsvegen valley is relatively densely covered with meteorological observations, which facilitate joint statistical analysis of the turbulent surface layer structure and the structure of the higher atmospheric layers. Wind direction diagrams reveal strong wind channeled in the surface layer up to 300 m to 500 m. The probability analysis links strong wind channeling and cold temperature anomalies in the surface layer. To explain these links, previous studies suggested the katabatic wind flow mechanism as the leading driver responsible for the observed wind climatology. In this paper, idealized turbulence-resolving simulations are used to distinct between different wind driving mechanisms. The simulations were performed with the real surface topography at resolution of about 60 m. These simulations resolve the obstacle-induced turbulence and the turbulence in the non-stratified boundary layer core. The simulations suggest the leading roles of the thermal land-sea breeze circulation and the mechanical wind channeling in the modulation of the valley winds. The characteristic signatures of the developed down-slope gravity-accelerated flow, that is, the katabatic wind, were found to be of lesser significance under typical meteorological conditions in the valley. © 2012 Igor Esau and Irina Repina.

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