Changes between Version 9 and Version 10 of 2016WP/NocWad

Timestamp:

2016-06-09T10:32:04+02:00 (8 years ago)

Author:

mike.bell

Comment:

--

2016WP/NocWad

@@ -321 +321 @@
 1. Main points
 
-
+A. History and Objectives: Jason and Hedong have previous experience of WAD in POLCOMS and FVCOM respectively. They have been working on WAD in NEMO since 2012. WAD is a key capability for NOC and it would be valuable for NOC to have it in NEMO. The Met Office needs WAD to work in NEMO for an upgrade to CMEMS north-west European shelf system expected to be required within 1-2 years. The aim is to implement  a non intrusive, accurate, stable and efficient formulation of WAD within NEMO. In order to
+be non intrusive a scheme using a minimum depth of water (@ 5-10 cm) at all potentially wet point is being implemented (similar to POM and ROMS implementation). 
+We should publish results of our implementation. To do this we will need to compare results with other schemes (e.g. POM and ROMS implementations) for 
+simple test cases.      
+
+B. Review of WAD code: The main changes to an ocean model and issues associated with an implementation of WAD are: 
+
+- a time-varying land-sea mask (wdmask) is required; the initial set up of the grid needs to allow negative depths (domgr)
+
+- the depth of water at ocean points cannot be allowed to go negative; some care with flux limiters is needed to ensure this. The splitting of the barotropic and baroclinic timesteps is complicated by cells wetting and drying within the baroclinic time-step
+
+- the thin film of water at dry cells results in an artificial horizontal pressure gradient (hpg) along a sloping bottom
+
+- some implementations (e.g. POM) have adjusted the bottom friction 
+
+- the 3D equations can be unstable in a very thin layer of water (with many levels); e.g. problems with CFL for vertical advection or diffusion 
+ 
+C. The WAD implementation by ROMS is particularly simple. The flux limiter simply specifies that there is no flux out of a dry cell. This allows
+wetting of dry cells and (in the no wind case) ensures that the artificial hpgs have no impact on the model 
+
+D. The time-step splitting could give stability problems. In order to separate issues it would be valuable to run without time-step splitting. 
+We can do this by removing the updates calculated from the barotropic time-steps that are made on the baroclinic time-step.
+
+E. Andrew presented results using Hedong's code for a tidally driven sloping channel. It seems that when the tide is going out, strong horizontal
+sea surface height gradients develop (before the layers dry out). Either the pressure gradients are being mis-calculated or the outward
+fluxes of water are being mis-calculated. 
 
 2. Actions