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Fix incorrect qrn gradient scaling in WRFDA radial velocity adjoint #2356

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Fix incorrect qrn gradient scaling in WRFDA radial velocity adjoint #2356

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5 changes: 3 additions & 2 deletions var/da/da_radar/da_radial_velocity_adj.inc
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Original file line number Diff line number Diff line change
Expand Up @@ -4,6 +4,7 @@ subroutine da_radial_velocity_adj(rv,p,u,v,w,qrn,ps,x,y,z,qrn9,rho)
! Purpose: adjoint of da_radial_velocity_lin
! History:
! 08/2017 - bug fix for Vt (Siou-Ying Jiang, CWB, Taiwan)
! 06/2026 - bug fix for qrn adjoint gradient scaling (Weiyu Yang, NUIST, China)
!-----------------------------------------------------------------------

implicit none
Expand All @@ -20,7 +21,7 @@ subroutine da_radial_velocity_adj(rv,p,u,v,w,qrn,ps,x,y,z,qrn9,rho)
real :: qrrc
real :: qrn_g, qrn9_g

qrn_g = qrn *1000. ! kg/kg -> g/kg
qrn_g = 0.0 ! Initialize the local adjoint accumulator to 0.0 here
qrn9_g= qrn9*1000. ! kg/kg -> g/kg
qrrc = 0.01 ! g/kg

Expand All @@ -37,7 +38,7 @@ subroutine da_radial_velocity_adj(rv,p,u,v,w,qrn,ps,x,y,z,qrn9,rho)

if (qrn9_g > qrrc) then
qrn_g = qrn_g + vt*0.675*alpha*qrn9_g**(-0.875)*rho**0.125
qrn = qrn_g * 0.001 ! g/kg -> kg/kg
qrn = qrn + qrn_g * 1000. ! Proper adjoint accumulation following the transpose logic (* 1000.0)
end if

if (trace_use) call da_trace_exit("da_radial_velocity_adj")
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