Add readme for LES test case

docs/README_les.md

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+## The ideal em_les test case
+
+This test case produces a large-eddy simulation (LES) of a free
+convective boundary layer (CBL).  The environmental wind (or
+the initial wind profile) is set to zero in this default case.  
+The turbulence of the free CBL is driven/maintained by the surface 
+heat flux, which is specified in the namelist as tke_heat_flux=0.24 
+(in MKS units).
+
+A random perturbation is imposed initially on the mean temperature 
+field at the lowest four grid levels to kick off the
+turbulent motion. Double periodic boundary condition
+is used in x and y.
+
+The default version uses a grid resolution of
+dx=dy=100m and dz=50m, which is considered to be rather coarse 
+for an LES of the CBL.  A typical grid mesh for an LES
+of the CBL is dx=dy=50m and dz=20m.  An LES flow field is more
+accurate when the grid resolution is finer because it resolves
+more turbulent scales.  
+
+This LES version uses the Deardorff's TKE scheme to compute the SGS
+eddy viscosity and eddy diffusivity for turbulent mixing, that is
+diff_opt=2 and km_opt = 2 in the namelist.  The Coriolis parameter
+is set to f = 10^-4/s.
+
+It takes at least 30 minutes of simulation time to spin up the
+turbulent flow field; only after the spin-up, the turbulence inside
+the CBL is considered well established.  A sign of well-established
+turbulence is that the total (i.e., the resolved-scale plus the
+subgrid-scale) heat flux profile should decrease linearly with height
+within the CBL.
+
+To simulate a CBL with a mean wind, change the initial wind
+profile in the input sounding.   
+When pert_coriolis= true is set in the namelist, there is no need to 
+include the geostrophic wind terms in the right-hand sides of
+the u and v equations for LESs with non-zero geostrophic wind. 
+
+Note, parameterization constants, c_s and c_k in this namelist are
+different from the defaults and are the ones recommended to use with LES.
+
+-----
+
+## Shallow convection case (added in V3.7)
+
+- Copy namelist.input_shalconv to namelist.input
+- Copy input_sounding_shalconv to input_sounding
+- Re-run ideal.exe
+
+As LES case but larger domain (100x100), top still at 2 km.
+Differences are sounding 10 K cooler, geostrophic u-wind 10 m/s, isfflx=1 using
+input_sounding's surface theta = 295 K as SST, water surface, monotonic
+moist and scalar advection, mp_physics =6, no other physics except sf_sfclay_physics=1.
+This produces a shallow cloud layer capped by a strong inversion that
+grows in response to warmer SST.