em_grav2d_x test case:

The default version of this test case reproduces the
100 m resolution 2D gravity current.  This test case
was the focus of a workshop.  The results of this workshop
are published in

Straka et al, NUMERICAL-SOLUTIONS OF A NONLINEAR DENSITY-CURRENT - A
BENCHMARK SOLUTION AND COMPARISONS, 
INT J NUMER METH FL 17 (1): 1-22 JUL 15 1993
 
This test is 2d (x,z) and uses a fixed physical viscosity, hence the
solution converges.  It also uses a finite length domain with periodic
conditions, so tests with non-zero horizontal translation are also
appropriate.  The reference shows tests are several different
resolutions and these are quite illuminating
 
There are a number of interesting features in this very non-linear
solution (eddy structure, mins and maxes of temperature and velocity,
propagation of the gravity current, etc.)

Solution results from the WRF-ARW core can be found on the web at
http://www.mmm.ucar.edu/individual/skamarock/test_cases/test_cases.html

A number of versions test cases can be run using namelist.input files
and input_sounding files provided in the test directory.  

Two input_sounding files are provided

(1) input_sounding.um=0
(2) input_sounding.um=20

The first has no mean wind, the second has a constant horizontal
environmental wind of 20 m/s.  The no-mean wind case produces a 
symmetric gravity current, while the 20 m/s wind translates the
current (see the web page for further details and examples).
Copy the input_sounding.um=? into input_sounding to run the
desired case.  The input_sounding file is an ascii file, so
it can be edited to change the mean wind speed to other values
if desired.

Three namelist.input files are in this directory and they provide
for three different resolutions (dx, dz = 100, 200 and 400 m).  
Again, see the web page listed above for example solutions at
the different resolutions.  As with the sounding file, copy the
namelist.input.??? into namelist.input to run the desired case.

The default case is 100 m resolution and um=0.