Add option to prescribe an invariant solar constant

components/eamxx/cime_config/namelist_defaults_scream.xml

@@ -491,6 +491,11 @@ be lost if SCREAM_HACK_XML is not enabled.
       <orbital_obliquity COMPSET=".*SCREAM%AQUA.*">0.0</orbital_obliquity>
       <orbital_mvelp type="real">-9999.0</orbital_mvelp>
       <orbital_mvelp COMPSET=".*SCREAM%AQUA.*">0.0</orbital_mvelp>
+
+      <!-- Option for prescribing an invariant solar constant -->
+      <fixed_total_solar_irradiance>-9999.0</fixed_total_solar_irradiance>
+      <fixed_total_solar_irradiance COMPSET=".*SCREAM%RCE.*">551.58</fixed_total_solar_irradiance>
+
       <rad_frequency hgrid="ne4np4">1</rad_frequency>
       <rad_frequency hgrid="ne30np4">2</rad_frequency>
       <rad_frequency hgrid="ne120np4">4</rad_frequency>

components/eamxx/src/physics/rrtmgp/eamxx_rrtmgp_process_interface.cpp

@@ -620,6 +620,10 @@ void RRTMGPRadiation::initialize_impl(const RunType /* run_type */) {
   m_orbital_obliq = m_params.get<double>("orbital_obliquity"   ,-9999);
   m_orbital_mvelp = m_params.get<double>("orbital_mvelp"       ,-9999);
 
+  // Value for prescribing an invariant solar constant (i.e. total solar irradiance at
+  // TOA).  Used for idealized experiments such as RCE. Disabled when value is less than 0.
+  m_fixed_total_solar_irradiance = m_params.get<double>("fixed_total_solar_irradiance", -9999);
+
   // Determine whether or not we are using a fixed solar zenith angle (positive value)
   m_fixed_solar_zenith_angle = m_params.get<double>("Fixed Solar Zenith Angle", -9999);
 
@@ -840,6 +844,12 @@ void RRTMGPRadiation::run_impl (const double dt) {
     shr_orb_decl_c2f(calday, eccen, mvelpp, lambm0,
                      obliqr, &delta, &eccf);
 
+    // Overwrite eccf if using a fixed solar constant.
+    auto fixed_total_solar_irradiance = m_fixed_total_solar_irradiance;
+    if (fixed_total_solar_irradiance >= 0){
+       eccf = fixed_total_solar_irradiance/1360.9;
+    }
+
     // Precompute VMR for all gases, on all cols, before starting the chunks loop
     //
     // h2o is taken from qv

components/eamxx/src/physics/rrtmgp/eamxx_rrtmgp_process_interface.hpp

@@ -95,6 +95,11 @@ class RRTMGPRadiation : public AtmosphereProcess {
   Real m_orbital_obliq;  // Obliquity
   Real m_orbital_mvelp;  // Vernal Equinox Mean Longitude of Perihelion
 
+  // Value for prescribing an invariant solar constant (i.e. total solar irradiance
+  // at TOA).  Used for idealized experiments such as RCE. This is only used when a
+  // positive value is supplied.
+  Real m_fixed_total_solar_irradiance;
+
   // Fixed solar zenith angle to use for shortwave calculations
   // This is only used if a positive value is supplied
   Real m_fixed_solar_zenith_angle;