Merge branch 'v8_add_1.6_ln_p_diag_interp' into develop (PR #1042)

This merge updates the isobaric height diagnostic to vertically interpolate
linearly in ln(p) rather than linearly in p.

The isobaric height diagnostic interpolated pressure as a function of height to
the w points to later interpolate height to pressure levels. This introduces a
low bias in the diagnosed height fields. Interpolating ln(p) to the w levels
alleviates this bias.

src/core_atmosphere/diagnostics/mpas_isobaric_diagnostics.F

@@ -608,7 +608,10 @@ subroutine interp_diagnostics(mesh, state, time_lev, diag)
         do iCell = 1, nCells
            w1 = (height(k,iCell)-height(k-1,iCell)) / (height(k+1,iCell)-height(k-1,iCell))
            w2 = (height(k+1,iCell)-height(k,iCell)) / (height(k+1,iCell)-height(k-1,iCell))
-           pressure2(k,iCell) = w1*pressure(k,iCell) + w2*pressure(k-1,iCell)
+           ! pressure2(k,iCell) = w1*pressure(k,iCell) + w2*pressure(k-1,iCell)
+           !
+           ! switch to use ln(pressure) for more accurate vertical interpolation, WCS 20230407
+           pressure2(k,iCell) = exp(w1*log(pressure(k,iCell))+w2*log(pressure(k-1,iCell)))
         enddo
         enddo
         k = 1
@@ -618,7 +621,10 @@ subroutine interp_diagnostics(mesh, state, time_lev, diag)
            z2 = 0.5*(height(k+1,iCell)+height(k+2,iCell))
            w1 = (z0-z2)/(z1-z2)
            w2 = 1.-w1
-           pressure2(k,iCell) = w1*pressure(k,iCell)+w2*pressure(k+1,iCell)
+           ! pressure2(k,iCell) = w1*pressure(k,iCell)+w2*pressure(k+1,iCell)
+           !
+           ! switch to use ln(pressure) for more accurate vertical interpolation, WCS 20230407
+           pressure2(k,iCell) = exp(w1*log(pressure(k,iCell))+w2*log(pressure(k+1,iCell)))
         enddo
 
        !calculation of total pressure at cell vertices (at mass points):