AN OBSERVATIONAL STUDY OF BLOCKING WITH REGARD TO THE THEORY OF COHERENT STRUCTURES IN A BAROCLINIC ATMOSPHERE by PAUL JAMES HANCOCK

Considerable theoretical work has been done attempting to model persistent atmospheric flow patterns (blocking) with localized, coherent structures superimposed on a mean westerly wind, in particular by Malguzzi (1984). In Malguzzi's theory, which assumes a baroclinic atmosphere, the existence of the coherent structure solution requires that certain constraints be satisfied by the mean zonal wind field, which is allowed to have both vertical and meridional shear. These constraints imply the existence of a quantum mechanical potential well embedded in the structure of a potential function V(y,z) which Malguzzi 1 4 1 u Uzz + Uz derives. In dimensionless form, V(y,z) = = where y is the meridional gradient of the quasi-geostrophic potential vorticity, u is the mean zonal wind, and the buoyancy frequency (N ) has been assumed constant. The present investigation consists of calculating various profiles of V from real atmospheric data and analyzing their structure, in an attempt to determine the degree of applicability of the coherent structure model to the real atmosphere. Numerous cases are considered. A composite of twelve cases of Atlantic (positive) blocking is looked at, first for a time of five days after onset, and subsequently during an entire time series from 5 days prior to onset until 5 days after the onset of blocking. Also, a composite of twelve cases of Atlantic "negative" blocking, consisting actually of a strong "high index" zonal flow, and the wintertime climatological mean flow are considered, for comparison purposes. Finally, several cases of Atlantic blocking are investigated on an individual basis. For the Atlantic positive blocking composite case at day +5 lag, the results show undeniable evidence supporting the existence of the potential well structure in the V field, although the available data does not extend far enough to the south to capture the entire well. There are some gaps observed in the positive V regions forming the rim of the potential well, however, indicating that the meridional confinement of the waves might not be as complete as assumed in the theory. The negative blocking composite results provide substantially