Guidelines for Idealized Simulations Using MM5

Two choices:

1. Create a pressure-level file (DATAGRID or RAWINS output type) for input to INTERP
2. Create a sigma-level MM5 input file

1. Pressure-level file

Advantages: Do not have to worry about terrain-following coordinate. Only need to specify variables on constant p surfaces (e.g constant, simple gradients). INTERP will take care of vertical interpolation to model levels, and initialize p' in hydrostatic balance and w. Also creates boundary file.

Disadvantages: Difficult to use reference state other than standard log p profile unless also modifying INTERP and MM5.

2. Sigma-level file

Advantages: More control over reference state (have to change MM5 slightly if not using a log p profile). Able to set initial state exactly equal to reference state (removes pressure-gradient force errors because initial p'=0).

Disadvantages: Harder to set initial fields in terrain-following coordinate. Need to calculate heights of grid-points to set initial values. If reference profile is not log p, need to set reference state T0(i,j,k) and PS0(i,j) with PS0 being a surface pressure consistent with T0 profile. If initial state virtual temp not equal to reference state T0 need to calculate p' for hydrostatic balance [see method in INTERP which uses virtual temp profile in each column and surface actual (not reference) pressure to integrate p' upwards.] Surface actual pressure needs to be consistent with virtual temp profile in terrain regions. Need to estimate initial w (may be OK to use zero everywhere). Have to create a boundary file consistent with initial state.

Notes on running model with idealized soundings

1. In no-wind situations with terrain, motion may develop if initial state is not identical to reference state due to pressure-gradient force errors.

2. Even if ref state equals initial state motion may develop due to horizontal diffusion on sigma-surfaces (Using ITPDIF=1 helps).

3. Both the above effects become less important, and even negligible, when a flow is imposed.

4. When there is an initial wind that is not in geostrophic or thermal wind balance with the temperature and pressure gradient, Coriolis effects will lead to rotation. This can be avoided by subtracting off the initial wind in the model's Coriolis calculation, assuming it is geostrophic and there is no pressure gradient in the model input. With a frictional layer, this geostrophic wind may not necessarily equal the initial wind, or if it does the initial model wind will change with time due to friction.

5. There are no open radiative or periodic lateral boundary options in the standard model. Values have to be specified there. Frictional turning or radiative cooling or PBL processes may lead to development of gradients at the boundary. Also waves may be partially reflected there. Options are to move the boundaries far away, or nest, or develop your own boundary conditions (mods for periodic conditions exist, and possibly radiative conditions in the future, but these are not supported pieces of code).

6. If using periodic conditions, you should set the map-scale factors to 1, Coriolis parameter to constant, and latitude and longitude to constant in the model input.