Structure and dynamics of the Arizona Monsoon Boundary

The Arizona Monsoon Boundary is defined as the boundary separating two distinctly different air masses over Mexico, the southwestern United States, 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 time-height cross section, constructed using radiosonde observations, at the time the boundary initially passed through Tucson in 1984; (2) a composite cross section through the boundary, constructed from the Fleet Numerical Oceanography Center analysis; and (3) a cross section through the boundary using high-resolution fields of temperature, moisture, and geopotential height obtained from the VISSR Atmospheric Sounder (VAS). All three cross sections showed similar structure. In some respects, the Arizona monsoon boundary resembles a mid-latitude front with a distinct and relatively sharp air mass change across the boundary, 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 mid-latitude fronts. However, the VAS cross section suggests that, at times, the strength of the boundary approaches that of middle-latitude fronts. The wind shear suggested by the composite boundary ought to be unstable to baroclinic or barotropic processes. Disturbances developing along the boundary have been observed. One example of such a disturbance is examined using GOES imagery, lightning strike data, cloud track winds, and VAS data. Satellite images show the disturbance resembling a mid-latitude occluded cyclone, with an apparent low pressure center over northern Baja California and front-like cloud features extending eastward and southward from the low. Lightning strike data show convective activity occurring along the front-like features.