South Africa - Inkaba.org

Site characterisation:
Astronomical seeing from a turbulence-resolving model
M. Nickola 1,2 , I. Esau 3 , G.D. Djolov 2 , W.L. Combrinck 1 and R.C. Botha 1
1. Hartebeesthoek Radio Astronomy Observatory (HartRAO) Space Geodesy programme, South Africa,
marisa@hartrao.ac.za
2. University of Pretoria (UP), Department of Geography, Geoinformatics and Meteorology, South Africa,
george.djolov@up.ac.za
3. Nansen Environmental and Remote Sensing Center (NERSC), Bergen, Norway
igor.ezau@nersc.no
ABSTRACT
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. LLR requires optical seeing (or astronomical seeing)
conditions at ~ 1 arc second resolution level in order to deliver usable data. Site characterisation should include a
description of astronomical seeing for various locations on-site as well as 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 and predict
astronomical seeing at a site.
Large Eddy Simulation NERSC (Nansen Environmental and Remote Sensing Centre) Improved Code (LESNIC) is
a turbulence-resolving simulation code with which to model atmospheric turbulence. It has been 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
C profiles). Seeing parameter values are also obtained by making use of
2
distribution of optical turbulence ( N
2
DATABASE64 results. The C profiles and seeing parameters values obtained from DATABASE64 results are
N
compared with observational results that have been published in the literature. These values obtained are
consistent with results from field campaigns as reported in the literature.
Turbulence-resolving models, such as LESNIC, show potential for delivering and predicting profiles and
parameters to characterise astronomical seeing, which are essential prerequisites for establishing an LLR system at
the most suitable site and most suitable on-site location.
KEYWORDS: astronomical seeing, atmospheric turbulence, LESNIC, LLR.
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