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subroutine da_ref_pres(height, ref_pres),3
!---------------------------------------------------------------------------
! Purpose: To calculate the reference pressure from the height.
!---------------------------------------------------------------------------
implicit none
real, intent(in) :: height
real, intent(out) :: ref_pres
real, parameter :: rovg = gas_constant/gravity
real :: aa, bb, cc, hh, htop, ziso, p0iso
if (trace_use_frequent) call da_trace_entry
("da_ref_pres")
call da_ref_height(ptop, htop)
bb = rovg * t0
aa = rovg * base_lapse * 0.5
hh = min(htop, height)
cc = (-bb + sqrt(bb * bb - 4.0 * aa * hh))/(2.0*aa)
ref_pres = base_pres * exp(cc)
if (base_temp > 0.0) then
p0iso=base_pres*exp((base_temp-t0)/base_lapse)
cc=log(p0iso/base_pres)
ziso=-(aa*cc+bb)*cc
if (height > ziso) then
ref_pres = p0iso/exp((height-ziso)/(rovg*base_temp))
end if
end if
if (trace_use_frequent) call da_trace_exit("da_ref_pres")
end subroutine da_ref_pres