THE INTERACTIONS BETWEEN A MIDLATITUDE BLOCKING ANTICYCLONE AND SYNOPTIC-SCALE CYCLONES THAT OCCURRED DURING THE SUMMER SEASON

Using the Goddard Laboratory for Atmospheres Goddard Earth Observing S ystem 5-yr analyses and the Zwack-Okossi equation as the diagnostic to ol, the horizontal distribution of the dynamic and thermodynamic forci ng processes contributing to the maintenance of a Northern Hemisphere midlatitude blocking anticyclone that occurred during the summer seaso n were examined. During the development of this blocking anticyclone, vorticity advection, supported by temperature advection, forced 500-hP a height rises at the block center. Vorticity advection and vorticity lilting were also consistent contributors to height rises during the e ntire life cycle. Boundary layer friction, vertical advection of vorti city, and ageostrophic vorticity tendencies (during decay) consistentl y opposed block development. Additionally, an analysis of this blockin g event also showed that upstream precursor surface cyclones were not only important in block development but in block maintenance as well. In partitioning the basic data fields into their planetary-scale (P) a nd synoptic-scale (S) components, 500-hPa height tendencies forced by processes on each scale, as well as by interactions (I) between each s cale, were also calculated. Over the lifetime of this blocking event, the S and P processes were most prominent in the blocked region. Durin g the formation of this block, the I component was the largest and mos t consistent contributor to height rises at the center point. It was a lso shown that the height-rise regions located on the anticyclonic sid e of the jet maxima associated with block development and intensificat ion were primarily composed of the S and I components. Also, the precu rsor cyclones were associated with S or S and I height rises that cont ributed to the formation of this block. Finally, the results of this p aper show that the forcing associated with summer-season blocking even ts are similar to that of their winter-season counterparts neglecting the natural case-to-case variability. In comparing these results to th e results of other papers in this series, however, it is suggested tha t there may be two models for block development.