Lin Lin identified a bug in computing rain drop effective diameter as well as a bug in multiplying the immersion freezing number rate by cld_frac_l rather than the ooverlapped fraction between liquid and ice. Both bugs are fixed here

Author: hassanbeydoun

components/eam/src/physics/p3/scream/micro_p3.F90

@@ -1030,7 +1030,7 @@ subroutine p3_main_part3(kts, kte, kbot, ktop, kdir, &
          ze_rain(k) = nr(k)*(mu_r(k)+6._rtype)*(mu_r(k)+5._rtype)*(mu_r(k)+4._rtype)*           &
               (mu_r(k)+3._rtype)*(mu_r(k)+2._rtype)*(mu_r(k)+1._rtype)/bfb_pow(lamr(k), 6._rtype)
          ze_rain(k) = max(ze_rain(k),1.e-22_rtype)
-         diag_eff_radius_qr(k) = 1.5_rtype/lamr(k)
+         diag_eff_radius_qr(k) = 0.5_rtype*(mu_r(k)+3._rtype)/lamr(k)
       else
          qv(k) = qv(k)+qr(k)
          th_atm(k) = th_atm(k)-inv_exner(k)*qr(k)*latent_heat_vapor(k)*inv_cp
@@ -2839,7 +2839,7 @@ subroutine back_to_cell_average(cld_frac_l,cld_frac_r,cld_frac_i,
    ni2nr_melt_tend   = ni2nr_melt_tend*cld_frac_i       ! Change in number due to melting
    nc_collect_tend   = nc_collect_tend*il_cldm     ! Cloud # change due to collection of cld water by ice
    ncshdc  = ncshdc*il_cldm    ! Number change due to shedding, occurs when ice and liquid interact
-   nc2ni_immers_freeze_tend  = nc2ni_immers_freeze_tend*cld_frac_l      ! Number change associated with freexzing of cld drops
+   nc2ni_immers_freeze_tend  = nc2ni_immers_freeze_tend*il_cldm      ! Number change associated with freexzing of cld drops
    nr_collect_tend   = nr_collect_tend*ir_cldm     ! Rain number change due to collection from ice
    ni_selfcollect_tend   = ni_selfcollect_tend*cld_frac_i       ! Ice self collection
    qidep   = qidep*cld_frac_i       ! Vapor deposition to ice phase

components/eamxx/src/physics/p3/impl/p3_back_to_cell_average_impl.hpp

@@ -62,7 +62,7 @@ ::back_to_cell_average(
   ni2nr_melt_tend.set(context, ni2nr_melt_tend * cld_frac_i);    // Change in number due to melting
   nc_collect_tend.set(context, nc_collect_tend * il_cldm);  // Cloud # change due to collection of cld water by ice
   ncshdc.set(context, ncshdc * il_cldm); // Number change due to shedding, occurs when ice and liquid interact
-  nc2ni_immers_freeze_tend.set(context, nc2ni_immers_freeze_tend * cld_frac_l);   // Number change associated with freexzing of cld drops
+  nc2ni_immers_freeze_tend.set(context, nc2ni_immers_freeze_tend * il_cldm);   // Number change associated with freexzing of cld drops
   nr_collect_tend.set(context, nr_collect_tend * ir_cldm);  // Rain number change due to collection from ice
   ni_selfcollect_tend.set(context, ni_selfcollect_tend * cld_frac_i);    // Ice self collection
   qv2qi_vapdep_tend.set(context, qv2qi_vapdep_tend * cld_frac_i);    // Vapor deposition to ice phase

components/eamxx/src/physics/p3/impl/p3_main_impl_part3.hpp

@@ -118,7 +118,7 @@ ::p3_main_part3(
         ze_rain(k).set(qr_gt_small, nr(k)*(mu_r(k)+6)*(mu_r(k)+5)*(mu_r(k)+4)*
                        (mu_r(k)+3)*(mu_r(k)+2)*(mu_r(k)+1)/pow(lamr(k), sp(6.0))); // once f90 is gone, 6 can be int
         ze_rain(k).set(qr_gt_small, max(ze_rain(k), sp(1.e-22)));
-        diag_eff_radius_qr(k).set(qr_gt_small, sp(1.5) / lamr(k));
+        diag_eff_radius_qc(k).set(qr_gt_small, sp(0.5) * (mu_r(k) + 3) / lamr(k));
       }
 
       if (qr_small.any()) {