From 540f8e0ecaa63267f7a33b03d114cb7699c72a8e Mon Sep 17 00:00:00 2001
From: mahf708 <naser.mahfouz@pnnl.gov>
Date: Fri, 11 Oct 2024 14:45:38 -0700
Subject: [PATCH] add autoconversion radius to namelist

---
 .../eamxx/cime_config/namelist_defaults_scream.xml     |  1 +
 .../src/physics/p3/eamxx_p3_process_interface.cpp      |  1 +
 .../src/physics/p3/impl/p3_autoconversion_impl.hpp     | 10 +++++++++-
 components/eamxx/src/physics/p3/p3_functions.hpp       |  1 +
 4 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/components/eamxx/cime_config/namelist_defaults_scream.xml b/components/eamxx/cime_config/namelist_defaults_scream.xml
index 5344f04618d..d60646e423a 100644
--- a/components/eamxx/cime_config/namelist_defaults_scream.xml
+++ b/components/eamxx/cime_config/namelist_defaults_scream.xml
@@ -205,6 +205,7 @@ be lost if SCREAM_HACK_XML is not enabled.
       <p3_autoconversion_prefactor type="real" doc="Autoconversion linear prefactor in P3">1350.0</p3_autoconversion_prefactor>
       <p3_autoconversion_qc_exponent type="real" doc="Autoconversion qc exponent in P3">2.47</p3_autoconversion_qc_exponent>
       <p3_autoconversion_nc_exponent type="real" doc="Autoconversion nc exponent in P3">1.79</p3_autoconversion_nc_exponent>
+      <p3_autoconversion_radius type="real" doc="Autoconversion droplet radius (meter) in P3">25.0e-6</p3_autoconversion_radius>
       <p3_accretion_prefactor type="real" doc="Accretion linear prefactor in P3">67.0</p3_accretion_prefactor>
       <p3_accretion_qc_exponent type="real" doc="Accretion qc exponent in P3">1.15</p3_accretion_qc_exponent>
       <p3_accretion_qr_exponent type="real" doc="Accretion qr exponent in P3">1.15</p3_accretion_qr_exponent>
diff --git a/components/eamxx/src/physics/p3/eamxx_p3_process_interface.cpp b/components/eamxx/src/physics/p3/eamxx_p3_process_interface.cpp
index e25922df3a1..49b196286d3 100644
--- a/components/eamxx/src/physics/p3/eamxx_p3_process_interface.cpp
+++ b/components/eamxx/src/physics/p3/eamxx_p3_process_interface.cpp
@@ -224,6 +224,7 @@ void P3Microphysics::initialize_impl (const RunType /* run_type */)
   runtime_options.p3_autoconversion_prefactor = m_params.get<double>("p3_autoconversion_prefactor", 1350.0);
   runtime_options.p3_autoconversion_qc_exponent = m_params.get<double>("p3_autoconversion_qc_exponent", 2.47);
   runtime_options.p3_autoconversion_nc_exponent = m_params.get<double>("p3_autoconversion_nc_exponent", 1.79);
+  runtime_options.p3_autoconversion_radius = m_params.get<double>("p3_autoconversion_radius", 25.0e-6);
   runtime_options.p3_accretion_prefactor = m_params.get<double>("p3_accretion_prefactor", 67.0);
   runtime_options.p3_accretion_qc_exponent = m_params.get<double>("p3_accretion_qc_exponent", 1.15);
   runtime_options.p3_accretion_qr_exponent = m_params.get<double>("p3_accretion_qr_exponent", 1.15);
diff --git a/components/eamxx/src/physics/p3/impl/p3_autoconversion_impl.hpp b/components/eamxx/src/physics/p3/impl/p3_autoconversion_impl.hpp
index 76cfbf4e439..b52913ba7c9 100644
--- a/components/eamxx/src/physics/p3/impl/p3_autoconversion_impl.hpp
+++ b/components/eamxx/src/physics/p3/impl/p3_autoconversion_impl.hpp
@@ -19,11 +19,19 @@ ::cloud_water_autoconversion(
 
   // Khroutdinov and Kogan (2000)
   const auto qc_not_small = qc_incld >= 1e-8 && context;
-  constexpr Scalar CONS3 = C::CONS3;
 
   const Scalar p3_autoconversion_prefactor = runtime_options.p3_autoconversion_prefactor;
   const Scalar p3_autoconversion_qc_exponent = runtime_options.p3_autoconversion_qc_exponent;
   const Scalar p3_autoconversion_nc_exponent = runtime_options.p3_autoconversion_nc_exponent;
+  const Scalar p3_autoconversion_radius = runtime_options.p3_autoconversion_radius;
+
+  // TODO: remove this conditional (and keep CONS3 defined as in else) once BFB reqs are satisfied
+  Scalar CONS3;
+  if(p3_autoconversion_radius == 25.0e-6) {
+    CONS3 = C::CONS3;
+  } else {
+    CONS3 = sp(1.0) / (C::CONS2 * pow(p3_autoconversion_radius, sp(3.0)));
+  }
 
   if(qc_not_small.any()) {
     Spack sgs_var_coef;
diff --git a/components/eamxx/src/physics/p3/p3_functions.hpp b/components/eamxx/src/physics/p3/p3_functions.hpp
index fc362717b17..d15100e3f56 100644
--- a/components/eamxx/src/physics/p3/p3_functions.hpp
+++ b/components/eamxx/src/physics/p3/p3_functions.hpp
@@ -113,6 +113,7 @@ struct Functions
     Scalar p3_autoconversion_prefactor = 1350.0;
     Scalar p3_autoconversion_qc_exponent = 2.47;
     Scalar p3_autoconversion_nc_exponent = 1.79;
+    Scalar p3_autoconversion_radius = 25.0e-6;
     Scalar p3_accretion_prefactor = 67.0;
     Scalar p3_accretion_qc_exponent = 1.15;
     Scalar p3_accretion_qr_exponent = 1.15;