Changeset 1010 for trunk/DOC

Timestamp:

Jul 24, 2013, 1:36:44 PM (11 years ago)

Author:

emillour

Message:

Common dynamics: Improved sponge layer scheme (top_bound):

• Sponge tendencies are now computed analytically, instead of using a Forward Euler approximation.
• Sponge tendencies are now added within top_bound.

EM

Location:

trunk/DOC

Files:

• chantiers/commit_importants.log (modified) (1 diff)
• documentation/top_bound.pdf (modified) (previous)
• documentation/top_bound.tex (modified) (4 diffs)

trunk/DOC/chantiers/commit_importants.log

@@ -1225 +1225 @@
 Ehouarn: Some cleanup in the dynamics: ismax.F,ismin.F and cray.F (scopy/ssum)
 moved to 'bibio' (rather than being in dyn3d and dyn3dpar).
+
+**********************
+**** commit_v1010 ****
+**********************
+Ehouarn: Improved sponge layer scheme (top_bound):
+- Sponge tendencies are now computed analytically, instead than using
+  a Forward Euler approximation.
+- Sponge tendencies are now added within top_bound.

trunk/DOC/documentation/top_bound.tex

@@ -37 +37 @@
 
 \vspace{1cm}
-S\'ebastien Lebonnois
+S\'ebastien Lebonnois , Ehouarn Millour
 
 \vspace{1cm}
@@ -43 +43 @@
 \end{center}
 
-%\section{Theoretical aspects}
+\section{Theoretical aspects}
+Because of the inevitable numerical boundary at the top of the model,
+upward travelling waves are found to non-physically reflect down into the
+atmosphere.
+A common remedy to this unwanted behaviour is to apply a sponge layer near
+the top of the model in order to quench these waves and avoid significant
+reflection thereof.
+In practice such quenching is done by adding a dissipative term which forces
+a relaxation of potential temperature and/or winds of the form:
+\[
+ A(t)=A_m+A_0 \exp(-\lambda t )
+\]
+Where $A_m$ is the value towards which $A$ is to asymptotically relax, and
+$\lambda$ is the inverse of the characteristic relaxation time scale.
+As there is no obvious value of $A_m$ towards which to relax, in practice
+it is often chosen to be either the zonal average of $A$ (evaluated at time $t$,
+i.e. conveniently ignoring that $A_m$ then is in fact not time-independent),
+or zero (at least for winds, since this would have little physical meaning for
+potential temperature).
 
 \section{Pratical aspects in the code}
@@ -50 +68 @@
 flag is set to {\em True} in \textsf{gcm.def} 
 (this parameter also controls the application of a second step in the 
-horizontal dissipation).
+determination of vertical variation of coefficients for
+the horizontal dissipation, see \textsf{inidissip.F} and
+\textsf{disspi\_horiz.pdf} document).
 
 The tendencies for the upper boundary sponge layer are computed separately in
-the \textsf{top\_bound.F} routine, called from \textsf{leapfrog.F}. 
-These tendencies are \textsf{dutop}, \textsf{dvtop} and \textsf{dhtop}, in
-unit/s. 
+the \textsf{top\_bound.F} routine (called from \textsf{leapfrog.F}) and
+added in place. 
+The resulting sponge tendency \textsf{dutop}, in m/s, is also given as an output for
+diagnostics.
 
 Three parameters may be adjusted in the \textsf{gcm.def} file: 
@@ -74 +95 @@
   averaged value.
   \end{itemize}
-\item \textsf{tau\_top\_bound}: damping rate (in /s) in the top layer.
+\item \textsf{tau\_top\_bound}: damping rate ($\lambda$ in equation above,
+expressed in Hz) in the topmost layer.
 \end{itemize}