EmergenceVelocity.md

Retrieve Emergence Velocity

General Information

• Solver Fortran File: Emergence.F90

• Solver Name: GetEmergenceVelocity

• Required Output Variable(s):

  • (1) EmergenceVelocity

• Required Input Variable(s):

  • (1) Normal Vector

  • (2) Flow Solution

• Optional Output Variable(s): None

• Optional Input Variable(s): None

General Description

This is a pseudo solver (i.e., it only composes a new variable from given, not solving a matrix) to retrieve the emergence velocity from a given surface normal vector and a velocity field. It uses the fact that the scalar-product between surface velocity and surface normal gives (apart from a factor that is very close to unity) the emergence velocity

v_{em} = - u {{\partial h}/{\partial x}} - v {{\partial h}/{\partial y}} + w,

as given in the kinematic boundary condition of the free surface.

{{\partial h}/{\partial t}} - v_{em} = a ||grad F_h||,

where h is the z-coordinate of the free surface, (u,v,w) the components of the ice velocity vector \vec{u}, a the net normal accumulation/ablation. The gradient of the implicit free surface function F_h = z - h

||grad F_h|| = \sqrt{({{\partial h}/{\partial x}})^2 + ({{\partial h}/{\partial y}})^2 + 1},

usually is approximated by unity, as the derivatives of the free surface equation are of the order of the aspect ratio (usually small). Consequently, the surface normal is given by

\vec{n} = {{grad F_h}/{||grad F_h||}} \approx{ {grad F_h}}

and hence \vec{n} =( -{{\partial h}/{\partial x}}, -{{\partial h}/{\partial y}}, 1 ),

and the emergence velocity can be approximated by

v_{em} = \vec{u} . \vec{n}

SIF Contents

The following SIF excerpt additionally contains solvers needed for the surface Normal Vector (Solver 2) and the Flow Solution (not shown here).

!==============================================================================
! /// Compute Normals ///
!==============================================================================
Solver 2
   Exec Solver = "Before Simulation"
   !Exec Solver = Never
   Equation = "NormalVector"
   Procedure = "ElmerIceSolvers" "ComputeNormalSolver"
   Variable = String "Normal Vector"
   Variable DOFs = 3
   Optimize Bandwidth = Logical False
   ComputeAll = Logical False
End
!==============================================================================
! /// Computing emergence velocity ///
!==============================================================================
Solver 3
  Equation = "SMB"
  Procedure = "ElmerIceSolvers" "GetEmergenceVelocity"
  Variable = -dofs 1 EmergenceVelocity
End

GetEmergenceVelocity (Solver 4) is - in contrary to the other Solvers - executed only on the body declared at the free surface boundary, where ComputeNormal = Logical True has to be set. The corresponding Equation 2 has to contain a keyword Convection = “Computed” as well as the name of the variable contining the velocities (usually Flow Solution)

Equation 2
  Name = "Surface Equations"
  Active Solvers(1) = 4 
  Convection = String "Computed"
  Flow Solution Name = String "Flow Solution"
End

Example

An example solving the emergence velocity for a surface velocity and shape distribution computed on a Bueler-profile can be found in [ELMER_TRUNK]/elmerice/Tests/Emergence.

References

Välisuo, I., T. Zwinger, and J. Kohler, 2017. Inverse solution of surface mass balance of Midtre Lovénbreen, Svalbard, Journal of Glaciology, 1-10, doi:10.1017/jog.2017.26