README.tropical_cyclone
Last updated:  25 March 2011



Description:  This README file provides a short user's guide to the 
idealized tropical cyclone test case.  The initial conditions for this 
case are generated by running ideal.exe.  The initial state is 
motionless (u = v = 0) and horizontally homogeneous, except an analytic 
axisymmetric vortex in hydrostatic and gradient-wind balance is added. 
The initial sea-surface temperature (SST) is constant.  An f-plane is 
assumed.  The lateral boundary conditions are periodic.  All of these 
assumptions can be relaxed/modified should users decide to run something 
more complex. 



Default environment:  the initial horizontally homogeneous environment is 
specified via the "input_sounding" file (as in several other test cases).  
The default file uses the Jordan mean hurricane sounding.  An f-plane 
is assumed by default with f = 5.0e-5  (20 degrees north).   The default 
SST is 28 Celsius.   

Reference:  Jordan, C L., 1958:  Mean soundings for the West Indies area.
J. Meteorology, 15, 91-97. 



Default vortex:  a broad, weak, axisymmetric vortex is specified using an 
analytic equation from Rotunno and Emanuel (1987).  This vortex is placed 
in the middle of the domain.  It is in hydrostatic and gradient-wind 
balance, with the maximum winds at the lowest model level.  Users can 
change the initial vortex by modifying the parameters r0, rmax, vmax, and/or 
zdd in the "module_initialize_tropical_cyclone.F" file, which is located 
in the "dyn_em" directory. 

Reference:  Rotunno, R., and K. A. Emanuel, 1987:  An air-sea interaction 
theory for tropical cyclones.  Part II.  Evolutionary study using a 
nonhydrostatic axisymmetric numerical model.  J. Atmos. Sci., 44, 542-561.



Default physics:  Users should be able to use any physics schemes for this 
test case, although only a few have been tested so far.  The default settings 
are as follows:

  - For simplicity, the Kessler (liquid-only) microphysics scheme is used. 

  - By default, a capped Newtonian relaxation scheme is used on potential 
    temperature (Rotunno and Emanuel 1987) which is a crude approximation 
    for longwave radiation.  This scheme is useful for some idealized studies 
    of maximum tropical cyclone intensity.  (rad_nudge = 1)

       CAUTION:  if you are going to use a real atmospheric radiation scheme, 
       you should turn this option OFF.   (rad_nudge = 0)

  - The YSU PBL scheme and Monin-Obukhov surface-layer scheme are used by 
    default.

  - The alternative Ck/Cd formulations for tropical cyclones are "on" by 
    default (isftcflx=1). 

  - The ocean mixed layer scheme is off by default, and hence SST remains 
    fixed over time. 



Other settings:  the default values of f (Coriolis parameter) and sst (sea-
surface temp.) may be changed in the "module_initialize_tropical_cyclone.F"
file, which is located in the "dyn_em" directory.

The default domain size is 3000 km x 3000 km x 25 km, which is the three-
dimensional equivalent of the Rotunno-Emanuel (1987) setup.  Horizontal 
grid spacing is 15 km, and vertical grid spacing is 1.25 km, also following 
Rotunno and Emanuel (1987).  This resolution is very coarse;  these 
settings were chosen for defaults so the test case can finish quickly, but 
they are probably not appropriate for research-quality work.