modified: checkout.sh to check out modle tag GFS.v16.0.4. changes inc…

…lude:

       1.  Remove constraints on mixing length and background diffusitivity over inversion layers on land
       2.  Enhance mass flux for deep convection, hence to increase subsidzing warming to reduce
           cold bias in the lower tropospehre
       3. Fix a RRTMg solar radiaiton bug which has impact in SW abosrption in the UV region in
          the upper atmospehre.

sorc/checkout.sh

@@ -9,7 +9,7 @@ if [[ ! -d fv3gfs.fd ]] ; then
     rm -f ${topdir}/checkout-fv3gfs.log
     git clone https://github.com/ufs-community/ufs-weather-model fv3gfs.fd >> ${topdir}/checkout-fv3gfs.log 2>&1
     cd fv3gfs.fd
-    git checkout  GFS.v16.0.3
+    git checkout  GFS.v16.0.4
     git submodule update --init --recursive
     cd ${topdir}
 else