Dissipation - Revision history

Emillour at 14:47, 21 August 2025

2025-08-21T14:47:57Z

Emillour at 13:57, 21 August 2025

2025-08-21T13:57:05Z

Emillour at 11:14, 21 August 2025

2025-08-21T11:14:24Z

Emillour at 06:52, 1 June 2023

2023-06-01T06:52:11Z

Emillour at 07:31, 31 May 2023

2023-05-31T07:31:20Z

Martin.turbet at 10:11, 9 March 2023

2023-03-09T10:11:39Z

LTeinturier: /* How to change it in the model */

2022-05-11T09:08:34Z

‎How to change it in the model

LTeinturier: /* Description */

2022-05-11T09:04:16Z

‎Description

LTeinturier at 08:44, 11 May 2022

2022-05-11T08:44:44Z

LTeinturier: Created page with "In the LMD grid point model, nonlinear interactions between explicitly resolved scales and subgrid-scale processes are parameterized by applying a scale-selective horizontal d..."

2022-05-11T08:37:19Z

Created page with "In the LMD grid point model, nonlinear interactions between explicitly resolved scales and subgrid-scale processes are parameterized by applying a scale-selective horizontal d..."

New page

In the LMD grid point model, nonlinear interactions between explicitly resolved scales
and subgrid-scale processes are parameterized by applying a scale-selective horizontal dis-
sipation operator based on an n time iterated Laplacian $$\Delta^n$$. For the grid point model, for
instance, this can be written:

\begin{align}
\label{def:Wns}
\frac{\partial q}{\partial t} = \frac{(-1)^n}{\tau_{diss}}(\delta x)^{2n}\Delta^nq
\end{align}

where $$\delta x$$ is the smallest horizontal distance represented in the model and $$\tau_{diss}$$ is the dissipation timescale for a structure of scale $$\delta x$$. These operators are necessary to ensure the grid point model numerical stability. In practice, the operator is separately applied to three components :
* the divergence of the flow,
* the vorticity of the flow,
* potential temperature.

We classically use n = 2, n = 1,and n = 2.

In practise, the values of $$n$$ and $$\tau_{diss}$$ are prescribed in the ''run.def'' with the keys:
*nitergdiv
*nitergrot
*niterh
for the values of $$n$$ on each field, and the associated $$\tau$$:
*tetagdiv
*tetagrot
*tetatemp

@@ Line 35: / Line 35: @@
 In [[The_run.def_Input_File|run.def]], there is also a key ''idissip'' (depreciated) or ''dissip_period'' which is the rate (in dynamical steps) at which to apply the dissipation (and thus impacts on the overall time step of the dissipation).
-In addition there are multiplicative factors on the dissipation that are applied going to the upper layers of the atmosphere (as it has been noted that usually more dissipation is required in the upper atmosphere than the lower atmosphere). These coefficients are:
+In addition there are multiplicative factors on the dissipation that are applied going to the upper layers of the atmosphere (usually more dissipation is required in the upper atmosphere than the lower atmosphere since perturbations traveling upwards will grow in amplitude). These coefficients are:
 * dissip_fac_mid : dissipation multiplicative factor in the middle atmosphere
 * dissip_fac_up : dissipation multiplicative factor in the upper atmosphere
 == Good to know rules and rules of thumb ==

@@ Line 1: / Line 1: @@
 Note that there are some slides from the LMDZ Earth Climate Model tutorial sessions:
 https://lmdz.lmd.jussieu.fr/utilisateurs/formation/winter_2024
-in the "Dynamics: grid/temporal discretization/stability/dissipation" course which can be helpful to learn about dissipation in the PCM.
+in the "Dynamics: grid/temporal discretization/stability/dissipation" course which can be helpful to learn about dissipation in the PCM,
 == Description==
-In the LMD grid point model, nonlinear interactions between explicitly resolved scales
+In the LMDZ grid point model, nonlinear interactions between explicitly resolved scales
 and subgrid-scale processes are parameterized by applying a scale-selective horizontal dissipation operator based on an $$n$$ time iterated Laplacian $$\Delta^n$$. For the grid point model, for
 instance, this can be written:
@@ Line 14: / Line 15: @@
 \end{align}
-where $$q$$ is a field component on which disspation is applied, $$\delta x$$ is the smallest horizontal distance represented in the model and $$\tau_{diss}$$ is the dissipation timescale for a structure of scale $$\delta x$$. These operators are necessary to ensure the grid point model numerical stability. In practice, the operator is separately applied to three components :
+where $$q$$ is a field component on which dissipation is applied, $$\delta x$$ is the smallest horizontal distance represented in the model and $$\tau_{diss}$$ is the dissipation timescale for a structure of scale $$\delta x$$. These operators are necessary to ensure the grid point model numerical stability. In practice, the operator is separately applied to three components :
 * the divergence of the flow,
 * the vorticity of the flow,
 * potential temperature.
-We classically use n = 1 for the divergence of the flow and n = 2 for flow vorticity and potential temperature.
+We classically use n = 1 for the divergence of the flow (<code>nitergdiv=1</code> in [[The_run.def_Input_File|run.def]]) and n = 2 for flow vorticity and potential temperature (<code>nitergrot=2</code> and <code>niterh=2</code> in [[The_run.def_Input_File|run.def]]).
 == How to change it in the model ==
@@ Line 27: / Line 28: @@
 *nitergrot
 *niterh
-for the values of $$n$$ on each field, and the associated time scales $$\tau$$:
+for the values of $$n$$ on each field, and the associated time scales $$\tau$$ (in s):
 *tetagdiv
 *tetagrot
@@ Line 34: / Line 35: @@
 In [[The_run.def_Input_File|run.def]], there is also a key ''idissip'' (depreciated) or ''dissip_period'' which is the rate (in dynamical steps) at which to apply the dissipation (and thus impacts on the overall time step of the dissipation).
-In addition there are multiplicative factors on the dissipation that are applied going to the upper layers of the atmosphere (as it has been noted that usually more dissipation is required in the upper atmospher than the lower atmosphere). These coefficients are:
+In addition there are multiplicative factors on the dissipation that are applied going to the upper layers of the atmosphere (as it has been noted that usually more dissipation is required in the upper atmosphere than the lower atmosphere). These coefficients are:
 * dissip_fac_mid : dissipation multiplicative factor in the middle atmosphere
 * dissip_fac_up : dissipation multiplicative factor in the upper atmosphere
 == Good to know rules and rules of thumb ==
-* If your simulation shows numerical instabilities, a good idea is to increase dissipation. This means decreasing parameters $$\tau$$.
+* If your simulation shows numerical instabilities, a good idea is to increase dissipation. This means decreasing parameters $$\tau$$ and/or increasing dissip_fac_up (and to a lesser extent dissip_fac_mid).
 * Optimal values for the dissipation timescales depends on the resolution of the horizontal grid. The higher the resolution, the more dissipation we need.
 * Because dissipation is applied using an Explicit Euler time marching scheme, it is liable to be unstable if the time step is too large. This is tested at run-time by the model which will generate an error message of the likes of:

← Older revision		Revision as of 11:14, 21 August 2025
Line 1:		Line 1:
		+	Note that there are some slides from the LMDZ Earth Climate Model tutorial sessions:
		+	https://lmdz.lmd.jussieu.fr/utilisateurs/formation/winter_2024
		+	in the "Dynamics: grid/temporal discretization/stability/dissipation" course which can be helpful to learn about dissipation in the PCM.

	== Description==		== Description==

@@ Line 29: / Line 29: @@
 *tetatemp
-In [The_run.def_Input_File|run.def]], there is also a key ''idissip'' or ''dissip_period'' which is the rate (in dynamical steps) at which to apply the dissipation (and thus impacts on the overall time step of the dissipation).
+In [[The_run.def_Input_File|run.def]], there is also a key ''idissip'' (depreciated) or ''dissip_period'' which is the rate (in dynamical steps) at which to apply the dissipation (and thus impacts on the overall time step of the dissipation).
 In addition there are multiplicative factors on the dissipation that are applied going to the upper layers of the atmosphere (as it has been noted that usually more dissipation is required in the upper atmospher than the lower atmosphere). These coefficients are:

@@ Line 16: / Line 16: @@
 * potential temperature.
-We classically use n = 2, n = 1,and n = 2.
+We classically use n = 1 for the divergence of the flow and n = 2 for flow vorticity and potential temperature.
 == How to change it in the model ==
-In practise, the values of $$n$$ and $$\tau_{diss}$$ are prescribed in the [https://lmdz-forge.lmd.jussieu.fr/mediawiki/Planets/index.php/The_run.def_Input_File run.def] with the keys:
+In practice, the values of $$n$$ and $$\tau_{diss}$$ are prescribed in the [[The_run.def_Input_File|run.def]] with the keys:
 *nitergdiv
 *nitergrot
 *niterh
-for the values of $$n$$ on each field, and the associated $$\tau$$:
+for the values of $$n$$ on each field, and the associated time scales $$\tau$$:
 *tetagdiv
 *tetagrot
 *tetatemp
-In [https://lmdz-forge.lmd.jussieu.fr/mediawiki/Planets/index.php/The_run.def_Input_File run.def], there is also a key ''idissip'' which is the frequency (in dynamical steps) at which to apply the dissipation.
+In [The_run.def_Input_File|run.def]], there is also a key ''idissip'' or ''dissip_period'' which is the rate (in dynamical steps) at which to apply the dissipation (and thus impacts on the overall time step of the dissipation).
 == Good to know rules and rules of thumb ==
-*If your simulation shows numerical instabilities, a good idea is to increase dissipation. This means decreasing parameters $$\tau$$.
+* If your simulation shows numerical instabilities, a good idea is to increase dissipation. This means decreasing parameters $$\tau$$.
 * Optimal values for the dissipation timescales depends on the resolution of the horizontal grid. The higher the resolution, the more dissipation we need.
 [[Category:Generic-Model]]

← Older revision		Revision as of 10:11, 9 March 2023
Line 34:		Line 34:
	*If your simulation shows numerical instabilities, a good idea is to increase dissipation. This means decreasing parameters $$\tau$$.		*If your simulation shows numerical instabilities, a good idea is to increase dissipation. This means decreasing parameters $$\tau$$.
	* Optimal values for the dissipation timescales depends on the resolution of the horizontal grid. The higher the resolution, the more dissipation we need.		* Optimal values for the dissipation timescales depends on the resolution of the horizontal grid. The higher the resolution, the more dissipation we need.
		+
		+	[[Category:Generic-Model]]