add default default

Author: peverwhee

bld/namelist_files/namelist_defaults_cam.xml

@@ -576,6 +576,7 @@
 <rrtmgp_coefs_sw_file>src/physics/rrtmgp/data/rrtmgp-gas-sw-g112.nc</rrtmgp_coefs_sw_file>
 
 <!-- Pressure top for blending region for radiation heating -->
+<p_top_for_equil_rad>                  0.D0 </p_top_for_equil_rad>
 <p_top_for_equil_rad  rad="rrtmgp">    2.D0 </p_top_for_equil_rad>
 <p_top_for_equil_rad  waccmx="1" >     1.D0 </p_top_for_equil_rad>
 <p_top_for_equil_rad  waccm_phys="1" > 1.D0 </p_top_for_equil_rad>