# Dry pressure, Pd
# Dry surface pressure = Pds
# Model top pressure = Pt
# Mass in column, Pc = Pds - Pt
# Total dry pressure
# Pd = BF ( Pds - Pt ) + ( eta - BF ) ( P0 - Pt ) + Pt
# Hybrid coordinate: mu is still d(Pd)/d(eta)
# new MUT = d Pd / d eta = d BF / d eta * ( Pcb + Pc ) + ( 1 - d BF / d eta ) * ( P0 - Pt )
# Define two columnar constants, function only of eta and other constants, specifically for the "mu" replacements.
# C1 = d BF / d eta
# C2 = ( 1 - d BF / d eta ) * ( P0 - Pt )
# Total field
# new MUT(i,k,j) = C1(k) * ( Pcb(i,j) + Pc(i,j) ) + C2(k)
# Base-state, background field
# new MUB(i,k,j) = C1(k) * Pcb(i,j) + C2(k)
# Perturbation field
# new MU(i,k,j) = C1(k) * Pc(i,j)
# Define two columnar constants, function only of eta and other constants, specifically for the "pressure" replacements.
# C3 = BF
# C4 = ( eta - BF ) * ( P0 - Pt )
# new dry pressure Pd(i,k,j) = C3(k) * ( Pds(i,j) - Pt ) + C4(k) + Pt
#