  We seek to quantify non-normality of the most amplified resolvent modes and
predict their features based on the characteristics of the base or mean
velocity profile. A 2-by-2 model linear Navier-Stokes (LNS) operator
illustrates how non-normality from mean shear distributes perturbation energy
in different velocity components of the forcing and response modes. The inverse
of their inner product, which is unity for a purely normal mechanism, is
proposed as a measure to quantify non-normality. In flows where there is
downstream spatial dependence of the base/mean, mean flow advection separates
the spatial support of forcing and response modes which impacts the inner
product. Success of mean stability analysis depends on the normality of
amplification. If the amplification is normal, the resolvent operator written
in its dyadic representation reveals that the adjoint and forward stability
modes are proportional to the forcing and response resolvent modes. If the
amplification is non-normal, then resolvent analysis is required to understand
the origin of observed flow structures. Eigenspectra and pseudospectra are used
to characterize these phenomena. Two test cases are studied: low Reynolds
number cylinder flow and turbulent channel flow. The first deals mainly with
normal mechanisms and quantification of non-normality using the inverse inner
product of the leading forcing and response modes agrees well with the product
of the resolvent norm and distance between the imaginary axis and least stable
eigenvalue. In turbulent channel flow, structures result from both normal and
non-normal mechanisms. Mean shear is exploited most efficiently by stationary
disturbances while bounds on the pseudospectra illustrate how non-normality is
responsible for the most amplified disturbances at spatial wavenumbers and
temporal frequencies corresponding to well-known turbulent structures.
