Changeset 2255 for branches/DEV_r1826_DOC/DOC/TexFiles/Chapters/Chap_SBC.tex

Timestamp:

2010-10-13T15:22:26+02:00 (14 years ago)

Author:

rfurner

Message:

changes to river runoff documentation

File:

• branches/DEV_r1826_DOC/DOC/TexFiles/Chapters/Chap_SBC.tex (modified) (4 diffs)

branches/DEV_r1826_DOC/DOC/TexFiles/Chapters/Chap_SBC.tex

@@ -1 @@
-% ================================================================
+ ================================================================
 % Chapter Ñ Surface Boundary Condition (SBC) 
 % ================================================================
@@ -363 +363 @@
 %-------------------------------------------------------------------------------------------------------------
 
-River runoff generally enters the ocean at a nonzero depth rather than through the surface. 
+%River runoff generally enters the ocean at a nonzero depth rather than through the surface. 
+%Many models, however, have traditionally inserted river runoff to the top model cell.
+%This was the case in \NEMO prior to the version 3.3. The switch toward a input of runoff 
+%throughout a nonzero depth has been motivated by the numerical and physical problems 
+%that arise when the top grid cells are of the order of one meter. This situation is common in 
+%coastal modelling and becomes more and more often open ocean and climate modelling 
+%\footnote{At least a top cells thickness of 1~meter and a 3 hours forcing frequency are
+%required to properly represent the diurnal cycle \citep{Bernie_al_JC05}. see also \S\ref{SBC_dcy}.}.
+
+
+%To do this we need to treat evaporation/precipitation fluxes and river runoff differently in the 
+%\mdl{tra\_sbc} module.  We decided to separate them throughout the code, so that the variable 
+%\textit{emp} represented solely evaporation minus precipitation fluxes, and a new 2d variable 
+%rnf was added which represents the volume flux of river runoff (in kg/m2s to remain consistent with 
+%emp).  This meant many uses of emp and emps needed to be changed, a list of all modules which use 
+%emp or emps and the changes made are below:
+
+
+%Rachel:
+River runoff generally enters the ocean at a nonzero depth rather than through the surface.
 Many models, however, have traditionally inserted river runoff to the top model cell.
-This was the case in \NEMO prior to the version 3.3. The switch toward a input of runoff 
-throughout a nonzero depth has been motivated by the numerical and physical problems 
-that arise when the top grid cells are of the order of one meter. This situation is common in 
-coastal modelling and becomes more and more often open ocean and climate modelling 
+This was the case in \NEMO prior to the version 3.3, and was combined with an option to increase vertical mixing near the river mouth.
+
+However, with this method numerical and physical problems arise when the top grid cells are 
+of the order of one meter. This situation is common in coastal modelling and is becoming 
+more common in open ocean and climate modelling 
 \footnote{At least a top cells thickness of 1~meter and a 3 hours forcing frequency are
 required to properly represent the diurnal cycle \citep{Bernie_al_JC05}. see also \S\ref{SBC_dcy}.}.
 
-
-To do this we need to treat evaporation/precipitation fluxes and river runoff differently in the \mdl{tra\_sbc} module.  We decided to separate them throughout the code, so that the variable \textit{emp} represented solely evaporation minus precipitation fluxes, and a new 2d variable rnf was added which represents the volume flux of river runoff (in kg/m2s to remain consistent with emp).  This meant many uses of emp and emps needed to be changed, a list of all modules which use emp or emps and the changes made are below:
-
-
-Rachel:
-
-It is convenient to introduce the river runoff in the model as a surface 
-fresh water flux. This is the default option within NEMO, and there is then
- the option for the user to increase vertical mixing in the vicinity of the rivermouth.
-
-However, this method is not very appropriate for coastal modelling.  As such its now possible 
-to specify, in a NetCDF input file, the temperature and salinity of the river, along with the 
-depth (in metres) which the river should be added to.  This enables to river to be correctly 
-added through the water column, instead of as a surface flux, and also means the temperature 
-or salinity (for low salinity outflow) of the river impacts the surrounding ocean.
-
-For temperature -999 is taken as missing data and the river temperature is taken to be the 
-surface temperatue at the river point.  For the depth parameter a value of -1 means the 
-river is added to the surface box only, and a value of -999 means the river is added through 
-the entire water column.
+As such from VN3.3 onwards it is possible to add river runoff through a non-zero depth, and for the 
+temperature and salinity of the river to effect the surrounding ocean.
+The user is able to specify, in a NetCDF input file, the temperature and salinity of the river, along with the   
+depth (in metres) which the river should be added to.
 
 Namelist options, \np{ln\_rnf\_depth}, \np{ln\_rnf\_sal} and \np{ln\_rnf\_temp} control whether 
@@ -398 +402 @@
 taken as surface temperature respectively.
 
-It is also possible for runnoff to be specified as a negative value for modelling flow through 
-straits, i.e. modelling the baltic flow in and out of the North Sea.  When the flow is out of the 
-domain there is no change in temperature and salinity, regardless of the namelist options used.
-
-The runoff value and attributes are read in in sbcrnf.  The mass/volume addition is added to the 
-divergence term in \rou{sbc\_rnf\_div}.  The dilution effect of the river is automatically applied through 
-the vertical tracer advection, and the direct flux of tracers into the domain is done in trasbc.
-
-
-\colorbox{yellow}{Nevertheless, Pb of vertical resolution and 3D input : increase vertical mixing near river mouths to mimic a 3D river 
-
-All river runoff and emp fluxes are assumed to be fresh water (zero salinity) and at the same temperature as the sea surface.}
-
-\colorbox{yellow}{river mouths{\ldots}}
+The runoff value and attributes are read in in sbcrnf.  
+For temperature -999 is taken as missing data and the river temperature is taken to be the 
+surface temperatue at the river point.
+For the depth parameter a value of -1 means the river is added to the surface box only, 
+and a value of -999 means the river is added through the entire water column. 
+After being read in the temperature and salinity variables are multiplied by the amount of runoff (converted into m/s) 
+to give the heat and salt content of the river runoff.
+After the user specified depth is read ini, the number of grid boxes this corresponds to is 
+calculated and stored in the variable \np{nz\_rnf}.
+The variable \np{h\_dep} is then calculated to be the depth (in metres) of the bottom of the 
+lowest box the river water is being added to (i.e. the total depth that river water is being added to in the model).
+
+The mass/volume addition due to the river runoff is, at each relevant depth level, added to the horizontal divergence (\np{hdivn}) 
+in the subroutine \np{sbc\_rnf\_div} (called from \np{divcur}).
+This increases the diffusion term in the vicinity of the river, thereby simulating a momentum flux.
+The sea surface height is calculated using the sum of the horizontal divergence terms, and so the 
+river runoff indirectly forces an increase in sea surface height. 
+
+The \np{hdivn} terms are used in the tracer advection modules to force vertical velocities.
+This causes a mass of water, equal to the amount of runoff, to be moved into the box above. 
+The heat and salt content of the river runoff is not included in this step, and so the tracer 
+concentrations are diluted as water of ocean temperature and salinity is moved upward out of the box 
+and replaced by the same volume of river water with no corresponding heat and salt addition.
+
+For the linear free surface case, at the surface box the tracer advection causes a flux of water 
+(of equal volume to the runoff) through the sea surface out of the domain, which causes a salt and heat flux out of the model.
+As such the volume of water does not change, but the water is diluted.
+
+For the non-linear free surface case (vvl), no flux is allowed through the surface.
+Instead in the surface box (as well as water moving up from the boxes below) a volume of runoff water 
+is added with no corresponding heat and salt addition and so as happens in the lower boxes there is a dilution effect.
+(The runoff addition to the top box along with the water being moved up through boxes below means the surface box has a large 
+increase in volume, whilst all other boxes remain the same size)
+
+In trasbc the addition of heat and salt due to the river runoff is added.
+This is done in the same way for both vvl and non-vvl.
+The temperature and salinity are increased through the specified depth according to the heat and salt content of the river. 
+
+In the non-linear free surface case (vvl), near the end of the time step the change in sea surface height is redistrubuted 
+through the grid boxes, so that the original ratios of grid box heights are restored.
+In doing this water is moved into boxes below, throughout the water column, so the large volume addition to the surface box is spread between all the grid boxes.
+
+It is also possible for runnoff to be specified as a negative value for modelling flow through straits, i.e. modelling the Baltic flow in and out of the North Sea.
+When the flow is out of the domain there is no change in temperature and salinity, regardless of the namelist options used, as the ocean water leaving the domain removes heat and salt (at the same concentration) with it. 
+
+
+%\colorbox{yellow}{Nevertheless, Pb of vertical resolution and 3D input : increase vertical mixing near river mouths to mimic a 3D river 
+
+%All river runoff and emp fluxes are assumed to be fresh water (zero salinity) and at the same temperature as the sea surface.}
+
+%\colorbox{yellow}{river mouths{\ldots}}
 
 %IF( ln_rnf ) THEN                                     ! increase diffusivity at rivers mouths
@@ -417 +458 @@
 %ENDIF
 
-\gmcomment{  word doc of runoffs:
-
-In the current \NEMO setup river runoff is added to emp fluxes, these are then applied at just the sea surface as a volume change (in the variable volume case this is a literal volume change, and in the linear free surface case the free surface is moved) and a salt flux due to the concentration/dilution effect.  There is also an option to increase vertical mixing near river mouths; this gives the effect of having a 3d river.  All river runoff and emp fluxes are assumed to be fresh water (zero salinity) and at the same temperature as the sea surface.
-Our aim was to code the option to specify the temperature and salinity of river runoff, (as well as the amount), along with the depth that the river water will affect.  This would make it possible to model low salinity outflow, such as the Baltic, and would allow the ocean temperature to be affected by river runoff.  
-
-The depth option makes it possible to have the river water affecting just the surface layer, throughout depth, or some specified point in between.
-
-To do this we need to treat evaporation/precipitation fluxes and river runoff differently in the tra_sbc module.  We decided to separate them throughout the code, so that the variable emp represented solely evaporation minus precipitation fluxes, and a new 2d variable rnf was added which represents the volume flux of river runoff (in kg/m2s to remain consistent with emp).  This meant many uses of emp and emps needed to be changed, a list of all modules which use emp or emps and the changes made are below:
+%\gmcomment{  word doc of runoffs:
+%
+%In the current \NEMO setup river runoff is added to emp fluxes, these are then applied at just the sea surface as a volume change (in the variable volume case this is a literal volume change, and in the linear free surface case the free surface is moved) and a salt flux due to the concentration/dilution effect.  There is also an option to increase vertical mixing near river mouths; this gives the effect of having a 3d river.  All river runoff and emp fluxes are assumed to be fresh water (zero salinity) and at the same temperature as the sea surface.
+%Our aim was to code the option to specify the temperature and salinity of river runoff, (as well as the amount), along with the depth that the river water will affect.  This would make it possible to model low salinity outflow, such as the Baltic, and would allow the ocean temperature to be affected by river runoff.  
+
+%The depth option makes it possible to have the river water affecting just the surface layer, throughout depth, or some specified point in between.
+
+%To do this we need to treat evaporation/precipitation fluxes and river runoff differently in the tra_sbc module.  We decided to separate them throughout the code, so that the variable emp represented solely evaporation minus precipitation fluxes, and a new 2d variable rnf was added which represents the volume flux of river runoff (in kg/m2s to remain consistent with emp).  This meant many uses of emp and emps needed to be changed, a list of all modules which use emp or emps and the changes made are below:
 
 }