Structure and Dynamics of the Arizona Monsoon Boundary

Abstract

The Arizona Monsoon Boundary is defined as the boundary separating two distinctly different air masses over Mexico and the adjacent Pacific during the summer. The structure and dynamics of this boundary are examined by cross-sectional analysis using three different data sources: 1) a composite cross section through the boundary, constructed from the Fleet Numerical Oceanography Center (FNOC) analysis; 2) a time-height cross section, constructed using radiosonde observations at the time the boundary passed through Tucson in 1984; and 3) a cross section through the boundary on 22 July 1985, using high-resolution fields of temperature, moisture, and geopotential height obtained from the VISSR Atmospheric Sounder (VAS). All draw cross sections showed similar structure. In some respects, the Arizona monsoon boundary resembles a midlatitude front (i.e., there is a distinct and relatively sharp air mass change) forced almost entirely by confluence. A direct ageostrophic circulation is produced by this forcing, giving weak ascent on the warm, moist side of the boundary. The gradients and flow associated with the composite boundary are weaker, by a factor of four, than those associated with strong midlatitude fronts. However, the VAS cross section suggests that, at times, the strength of the boundary approaches that of midlatitude fronts. The wind shear suggested by the composite boundary ought to be unstable to baroclinic or barotropic processes and, hence, disturbances developing along the boundary are a distinct possibility. These disturbances are indeed observed and are the subject of a companion paper by Moore et al.