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    8   <meta content="Marcus Oliver Letzel" name="AUTHOR">
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    19 <h2 style="line-height: 100%;"><font size="4">2.0 Basic techniques of
     3<meta content="text/html; charset=windows-1252" http-equiv="CONTENT-TYPE"><title>PALM chapter 2.0</title> <meta content="StarOffice 7 (Win32)" name="GENERATOR"> <meta content="Marcus Oliver Letzel" name="AUTHOR"> <meta content="20040719;14534028" name="CREATED"> <meta content="20041117;10385730" name="CHANGED"> <meta content="parallel LES model" name="KEYWORDS"> <style>
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     9<body style="direction: ltr;" lang="en-US"><h2 style="line-height: 100%;"><font size="4">2.0
     10Basic techniques of
    2011the LES model and its parallelization </font>
    21 </h2>
    22 <p style="line-height: 100%;">LES models generally permit the
     12</h2><p style="line-height: 100%;">LES models
     13generally permit the
    2314simulation of turbulent flows, whereby those eddies, that carry the
    2415main energy are resolved by the numerical grid. Only the
    2819simulated directly (they are explicitly resolved) and their effects are
    2920represented by the advection terms. </p>
    30 <p style="font-style: normal; line-height: 100%;">PALM is based on the
     21<p style="font-style: normal; line-height: 100%;">PALM is
     22based on the
    3123non-hydrostatic incompressible Boussinesq equations. It contains a
    3224water cycle with cloud formation and takes into account infrared
    3325radiative cooling in cloudy conditions. The model has six prognostic
    34 quantities in total – u,v,w, liquid water potential temperature
    35 <font face="Thorndale, serif">&#920;</font><sub>l </sub>(BETTS,
     26quantities in total &ndash; u,v,w, liquid water potential
     28<font face="Thorndale, serif">&Theta;</font><sub>l
    36301973), total water content q and subgrid-scale turbulent kinetic energy
    3731e. The
    45391993). The water cycle is closed by using a simplified version of
    4640KESSLERs scheme (KESSLER, 1965; 1969) to parameterize precipitation
    47 processes (MÜLLER and CHLOND, 1996). Incompressibility is
     41processes (M&Uuml;LLER and CHLOND, 1996). Incompressibility is
    4842applied by means of a Poisson equation for pressure, which is solved
    4943with a direct method (SCHUMANN and SWEET, 1988). The Poisson equation
    5852horizontal directions. At the lower surface, either temperature/
    5953humidity or their respective fluxes can be prescribed. </p>
    60 <p style="font-style: normal; line-height: 100%;">The advection terms
     54<p style="font-style: normal; line-height: 100%;">The
     55advection terms
    6156are treated by the scheme proposed by PIACSEK and WILLIAMS (1970),
    6257which conserves the integral of linear and quadratic quantities up to
    6661with the third-order Runge-Kutta scheme. A second-order Runge-Kutta
    6762scheme, a leapfrog scheme and an Euler scheme are also implemented.</p>
    68 <p style="line-height: 100%;">By default, the time step is computed
     63<p style="line-height: 100%;">By default, the time step is
    6965with respect to the different criteria (CFL, diffusion) and adapted
    7066automatically. In case of a non-zero geostrophic
    7167wind the coordinate system can be moved along with the mean wind in
    7268order to maximize the time step (Galilei-Transformation). </p>
    73 <p style="font-style: normal; line-height: 100%;">In principle a model
     69<p style="font-style: normal; line-height: 100%;">In
     70principle a model
    7471run is carried out in the following way: After reading the control
    7572parameters given by the user, all prognostic variables are
    9087corrected with the help of the pressure solver. Following this, all
    9188diagnostic turbulence quantities including possible
    92 Prandtl-layer–quantities are computed. At the end of a time
     89Prandtl-layer&ndash;quantities are computed. At the end of a time
    9390step the data output requested by the user is made
    9491(e.g. statistic of analyses for control purposes or profiles and/or
    9592graphics data). If the given end-time was reached, binary data maybe
    9693be saved for restart. </p>
    97 <p style="font-style: normal; line-height: 100%;">The model is based
     94<p style="font-style: normal; line-height: 100%;">The
     95model is based
    9896on the originally non-parallel LES model which has been operated at the
    9997institute since 1989
    109107Users can choose between a two- and a one-dimensional domain
    110108decomposition. A 1D-decomposition is preferred on machines with a
    111 slow&nbsp; network interconnection. In case of a 1D-decomposition, the
     109slow&nbsp; network interconnection. In case of a 1D-decomposition,
    112111grid points along x direction are
    113112distributed among the individual processors, but in y- and z-direction
    114113all respective grid points belong to the same PE. </p>
    115 <p style="line-height: 100%;">The calculation of central differences or
     114<p style="line-height: 100%;">The calculation of central
     115differences or
    116116non-local arithmetic operations (e.g. global
    117117sums, FFT) demands communication and an appropriate data exchange
    126126y-direction, the data which lie distributed on the individual central
    127127processing elements, have to be collected and/or relocated before.
    128 This happens by means of the routine <tt>MPI_ALLTOALLV</tt>. Certain
     128This happens by means of the routine <tt>MPI_ALLTOALLV</tt>.
    129130global operations like e.g. the search for absolute maxima or minima
    130131within the 3D-arrays likewise require the employment of special MPI
    131132routines (<tt>MPI_ALLREDUCE</tt>). </p>
    132 <p style="line-height: 100%;">Further details of the internal model
     133<p style="line-height: 100%;">Further details of the
     134internal model
    133135structure are described in the <a href="../tec/index.html">technical/numerical
    134136documentation</a>. <br>
    135137&nbsp; </p>
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