Removed fixed_mri_dt_ratio

Otherwise ref solution crashes on step 938 (t=234.25). Orig fixed dt ratio was
4; allowing a variable dt ratio results in a constant dt ratio of 6.

Exec/RegTests/DensityCurrent/inputs_refsoln

@@ -18,8 +18,11 @@ zlo.type = "SlipWall"
 zhi.type = "SlipWall"
 
 # TIME STEP CONTROL
-erf.fixed_dt       = 0.25     # fixed time step [s] -- Straka et al 1993
-erf.fixed_fast_dt  = 0.0625   # fixed time step [s] -- Straka et al 1993
+# Straka et al 1993:
+#erf.fixed_dt       = 1.5625e-2 # [s]
+#erf.no_substepping = 1
+
+erf.fixed_dt       = 0.25    # large time step [s] -- results in dt ratio = 6
 
 # DIAGNOSTICS & VERBOSITY
 erf.sum_interval   = 1       # timesteps between computing mass
@@ -31,11 +34,12 @@ amr.max_level       = 0       # maximum level number allowed
 
 # CHECKPOINT FILES
 erf.check_file      = chk        # root name of checkpoint file
-erf.check_int       = 57600      # number of timesteps between checkpoints
+erf.check_int       = -1         # number of timesteps between checkpoints
 
 # PLOTFILES
 erf.plot_file_1     = plt        # prefix of plotfile name
-erf.plot_int_1      = 3840       # number of timesteps between plotfiles
+erf.plot_int_1      = 19200      # number of timesteps between plotfiles
+erf.plot_int_1      = 1200       # number of timesteps between plotfiles
 erf.plot_vars_1     = density x_velocity y_velocity z_velocity pressure theta pres_hse dens_hse pert_pres pert_dens
 
 # SOLVER CHOICE
@@ -47,9 +51,9 @@ erf.les_type         = "None"
 #
 # diffusion coefficient from Straka, K = 75 m^2/s
 #
-erf.molec_diff_type = "ConstantAlpha"
+erf.molec_diff_type = "ConstantAlpha" # where alpha == "K" in Straka et al 1993
 erf.rho0_trans = 1.0 # [kg/m^3], used to convert input diffusivities
-erf.dynamicViscosity = 75.0 # [kg/(m-s)] ==> nu = 75.0 m^2/s
+erf.dynamicViscosity = 75.0 # [kg/(m-s)] ==> alpha = 75.0 m^2/s
 erf.alpha_T = 75.0 # [m^2/s]
 
 erf.c_p = 1004.0

Exec/RegTests/DensityCurrent/runscript_refsoln

@@ -1,10 +1,9 @@
 #!/bin/bash
 #SBATCH --account=erf
-#SBATCH --time=1:30:00
+#SBATCH --time=0:30:00
 #SBATCH --job-name=DensityCurrent_ref_soln
-#SBATCH --ntasks=256
-#SBATCH --mail-user [email protected]
-#SBATCH --mail-type BEGIN,END,FAIL
+#SBATCH --ntasks=288
+# Timing with 288 CPU cores on NREL Eagle HPC: 427.1469369 s
 
 # load environment
 . ~/.bash_profile