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branches/2016/dev_INGV_UKMO_2016/DOC/TexFiles/Chapters/Chap_STP.tex
r4147 r7351 1 \documentclass[NEMO_book]{subfiles} 2 \begin{document} 1 3 2 4 % ================================================================ 3 % Chapter 2 �Time Domain (step.F90)5 % Chapter 2 ——— Time Domain (step.F90) 4 6 % ================================================================ 5 7 \chapter{Time Domain (STP) } … … 21 23 22 24 Having defined the continuous equations in Chap.~\ref{PE}, we need now to choose 23 a time discretization. In the present chapter, we provide a general description of the \NEMO 25 a time discretization, a key feature of an ocean model as it exerts a strong influence 26 on the structure of the computer code ($i.e.$ on its flowchart). 27 In the present chapter, we provide a general description of the \NEMO 24 28 time stepping strategy and the consequences for the order in which the equations are 25 29 solved. … … 158 162 \end{equation} 159 163 164 %%gm 165 %%gm UPDATE the next paragraphs with time varying thickness ... 166 %%gm 167 160 168 This scheme is rather time consuming since it requires a matrix inversion, 161 169 but it becomes attractive since a value of 3 or more is needed for N in … … 188 196 189 197 % ------------------------------------------------------------------------------------------------------------- 190 % Hydrostatic Pressure gradient 191 % ------------------------------------------------------------------------------------------------------------- 192 \section{Hydrostatic Pressure Gradient --- semi-implicit scheme} 193 \label{STP_hpg_imp} 198 % Surface Pressure gradient 199 % ------------------------------------------------------------------------------------------------------------- 200 \section{Surface Pressure Gradient} 201 \label{STP_spg_ts} 202 203 ===>>>> TO BE written.... :-) 194 204 195 205 %\gmcomment{ 196 206 %>>>>>>>>>>>>>>>>>>>>>>>>>>>> 197 207 \begin{figure}[!t] \begin{center} 198 \includegraphics[width=0.7\textwidth]{ ./TexFiles/Figures/Fig_TimeStepping_flowchart.pdf}208 \includegraphics[width=0.7\textwidth]{Fig_TimeStepping_flowchart} 199 209 \caption{ \label{Fig_TimeStep_flowchart} 200 210 Sketch of the leapfrog time stepping sequence in \NEMO from \citet{Leclair_Madec_OM09}. … … 209 219 %>>>>>>>>>>>>>>>>>>>>>>>>>>>> 210 220 %} 211 212 The range of stability of the Leap-Frog scheme can be extended by a factor of two213 by introducing a semi-implicit computation of the hydrostatic pressure gradient term214 \citep{Brown_Campana_MWR78}. Instead of evaluating the pressure at $t$, a linear215 combination of values at $t-\rdt$, $t$ and $t+\rdt$ is used (see \S~\ref{DYN_hpg_imp}).216 This technique, controlled by the \np{nn\_dynhpg\_rst} namelist parameter, does not217 introduce a significant additional computational cost when tracers and thus density218 is time stepped before the dynamics. This time step ordering is used in \NEMO219 (Fig.\ref{Fig_TimeStep_flowchart}).220 221 222 This technique, used in several GCMs (\NEMO, POP or MOM for instance),223 makes the Leap-Frog scheme as efficient224 \footnote{The efficiency is defined as the maximum allowed Courant number of the time225 stepping scheme divided by the number of computations of the right-hand side per time step.}226 as the Forward-Backward scheme used in MOM \citep{Griffies_al_OS05} and more227 efficient than the LF-AM3 scheme (leapfrog time stepping combined with a third order228 Adams-Moulthon interpolation for the predictor phase) used in ROMS229 \citep{Shchepetkin_McWilliams_OM05}.230 231 In fact, this technique is efficient when the physical phenomenon that232 limits the time-step is internal gravity waves (IGWs). Indeed, it is233 equivalent to applying a time filter to the pressure gradient to eliminate high234 frequency IGWs. Obviously, the doubling of the time-step is achievable only235 if no other factors control the time-step, such as the stability limits associated236 with advection, diffusion or Coriolis terms. For example, it is inefficient in low resolution237 global ocean configurations, since inertial oscillations in the vicinity of the North Pole238 are the limiting factor for the time step. It is also often inefficient in very high239 resolution configurations where strong currents and small grid cells exert240 the strongest constraint on the time step.241 221 242 222 % ------------------------------------------------------------------------------------------------------------- … … 288 268 %>>>>>>>>>>>>>>>>>>>>>>>>>>>> 289 269 \begin{figure}[!t] \begin{center} 290 \includegraphics[width=0.90\textwidth]{ ./TexFiles/Figures/Fig_MLF_forcing.pdf}270 \includegraphics[width=0.90\textwidth]{Fig_MLF_forcing} 291 271 \caption{ \label{Fig_MLF_forcing} 292 272 Illustration of forcing integration methods. … … 424 404 } 425 405 %% 406 \end{document}
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