Use btr time step for RK4

Author: cbegeman

polaris/tasks/ocean/barotropic_gyre/forward.py

@@ -1,5 +1,5 @@
 import time
-from math import ceil, floor, pi
+from math import ceil, pi
 
 import numpy as np
 
@@ -190,9 +190,24 @@ def dynamic_model_config(self, at_setup):
                 f'maximum value is {nu_max}'
             )
 
-        dt = floor(dt_max / 5.0)
-        dt_str = get_time_interval_string(seconds=dt)
-        dt_btr_str = get_time_interval_string(seconds=dt / 20.0)
+        time_integrator = config.get('barotropic_gyre', 'time_integrator')
+        time_integrator_map = dict([('RK4', 'RungeKutta4')])
+        if model == 'omega':
+            if time_integrator in time_integrator_map.keys():
+                time_integrator = time_integrator_map[time_integrator]
+            else:
+                print(
+                    'Warning: mapping from time integrator '
+                    f'{time_integrator} to omega not found, '
+                    'retaining name given in config'
+                )
+        if time_integrator == 'RK4':
+            dt = dt_max / 20.0
+            dt_str = get_time_interval_string(seconds=dt)
+        else:
+            dt = dt_max / 5.0
+            dt_str = get_time_interval_string(seconds=dt)
+            dt_btr_str = get_time_interval_string(seconds=dt_max / 20.0)
 
         nu_max = stability_parameter_max * (resolution * 1.0e3) ** 2.0 / dt
         if nu > nu_max:
@@ -234,17 +249,6 @@ def dynamic_model_config(self, at_setup):
                 )
 
         slip_factor_dict = {'no-slip': 0.0, 'free-slip': 1.0}
-        time_integrator = config.get('barotropic_gyre', 'time_integrator')
-        time_integrator_map = dict([('RK4', 'RungeKutta4')])
-        if model == 'omega':
-            if time_integrator in time_integrator_map.keys():
-                time_integrator = time_integrator_map[time_integrator]
-            else:
-                print(
-                    'Warning: mapping from time integrator '
-                    f'{time_integrator} to omega not found, '
-                    'retaining name given in config'
-                )
 
         replacements = dict(
             dt=dt_str,