diff --git a/src/MNH/bl89.f90 b/src/MNH/bl89.f90
index d92be3851353028f4b87c86152e25dd56eaf91d2..e34f0524c01349592c51d663f4e18221a7f61418 100644
--- a/src/MNH/bl89.f90
+++ b/src/MNH/bl89.f90
@@ -264,6 +264,7 @@ zsqrt_tke(:, : ) = Br_pow( ztkem, 0.5 )
 !ZBL89EXP is defined here because (and not in ini_cturb) because XCED is defined in read_exseg (depending on BL89/RM17)
 ZBL89EXP = Br_log( 16. ) / ( 4. * Br_log( XKARMAN )+ Br_log( XCED ) - 3. * Br_log( XCMFS) )
 #endif
+ZUSRBL89 = 1./ZBL89EXP
 !-------------------------------------------------------------------------------
 !
 !*       2.    Virtual potential temperature on the model grid