# Higher order velocity schemes

## Higher-Order model basics

The main distinction between so-called "higher-order" models and "0-order" (or "shallow ice") models is that higher-order models attempt a closer approximation to solving the non-linear, Stokes equations. In general, this usually means incorporating some approximation of horizontal-stress gradients - along-flow stretching or compression and across-flow shearing - in addition to the vertical stress gradients that are accounted for in shallow ice models. This is important for several reasons:

- For parts of the ice sheet that we are the most interested in - e.g. ice streams, ice shelves, and other regions of fast flow - horizontal-stress gradients are as or more important than vertical stress gradients. To model the flow in these regions accurately, we need higher-order models.

- Even in some regions of very slow flow horizontal-stress gradients may be important or dominant. For example, ice cores are often recovered at ice divides. Flow modeling is important for interpreting ice core records and using information (such as layer thickness) to infer something about the past flow history in the region. In order to model that flow correctly, one must include horizontal stresses (at the ice divide the surface slope is ~0, in which case vertical stress gradients are also ~0 and horizontal strectching dominates).

- In shallow ice models, we can solve for the velocity field point by point (in map view) because the velocity solution only depends on local variables (slope, thickness) at that point. In general, higher-order models require the simultaneous solution of all velocities in the model domain because the velocity at any one point is coupled (horizontally as well as vertically) to the velocities elsewhere in the domain (the equations are elliptical). This makes higher-order models computationally much more expensive than shallow ice models, requiring more efficient (and complicated) solution techniques.

The term "higher-order" comes from scaling analyses of the Stokes equations for which a scaling parameter λ=H/L - the ratio of the thickness to the horizontal length scale of interest - is used to assign importance to the various terms. Shallow ice models retain only terms of order 0 while "higher-order" models also retain terms of order 1 (and possibly greater).

## Higher-Order model basics

- Available HO schemes (e.g. Full Stokes, Blatter-Pattyn, Bueler & Pollard Hybrids)
- Architecture (transport, and non-linear iteration on diffusivity) of Glimmer an how it differs from a HO model architecture
- Introduce HO CISM (Blatter-Pattyn 1st-order scheme)
- Discretization and solution (elliptic solve, operator splitting approach, etc)
- Boundary conditions + masking, one sided differences, basal BC (no-slip, viscous, and plastic behavior all through Beta^2 implementation)
- Run verification suites; ISMIP-HOM, Ross IS
- Apply HO velocity fields to 1st-order upwind transport (dH/dt) scheme (see Adding a module to Glimmer I)