POTENTIAL VORTICITY ON ISENTROPIC SURFACES - CLIMATOLOGY AND DIAGNOSTICS

A 25-year dataset of potential vorticity on the 315-K isentropic surfa ce is built from the National Meteorological Center (NMC) final analys is archive. potential vorticity is calculated from the nonlinear gradi ent wind balance using temperature and geopotential fields, since the wind field is not available in the early part of the archive. The vali dity of this calculation is assessed by comparing the results with pot ential vorticity obtained directly from European Centre for Medium-Ran ge Weather Forecasts (ECMWF) analyzed winds. The error due to the nonl inear balance approximation turns out to be smaller than the differenc e between ECMWF and NMC analysis. The possibility of studying diabatic forcing as a residual of the equation of potential vorticity conserva tion is examined. The average potential vorticity forcing found in thi s way is consistent with the authors knowledge of the mean diabatic he ating. The amplitude of the residual decreases through the analysis pe riod, reflecting the improvement in the observational network and in t he analysis schemes. Next the authors demonstrate that this dataset ca n be used for studies of transient-mean flow interactions. The authors present diagnostics of the transient feedback by separating the contr ibution of vortical and thermal terms on the isentropic surface. Also, the contribution of high-frequency (periods less than 10 days) and lo w-frequency (periods greater than 10 days) transients is examined. On the 315-K surface, transients act mostly in reducing the potential vor ticity gradient through thermal terms and accelerate the zonal flow th rough vortical forcing. Finally, these diagnostics are also applied to a long ensemble of blocking events, and the authors study the anomaly of transient feedback during these events. It is found that transient s have primarily an advective effect, forcing the dipole structure to retrograde westward. Thermal and vortical terms have a very distinct a ction on the blocking anomaly. Vortical forcing is constructive and ad vective, whereas thermal forcing is dissipative and makes the dipole r otate clockwise.