Pressure Transients within MCS Mesohighs and Wake Lows

By animating enhanced coarse surface pressure observations of 12 1985 Preliminary Regional Experiment for Storm-Scale Operational Research Meteorology (PRE-STORM) mesoscale convective systems (MCSs) the authors exposed 92 transitory highs and lows living within virtually all of the systems’ mesohighs and wake lows. A quasi-Lagrangian (feature following, not material following) analysis of the pressure fields produced five primary results. First, these transients, with magnitudes of a few millibars, horizontal dimensions of order 100 km, and average lifetimes of about 2 h, collectively composed spatial and temporal envelopes that contributed at least part of the total pressure field within mesohighs and wake lows. Transients did not apparently favor formation or dissipation in any location of the envelopes. Second, as the MCSs matured, the difference between each complex’s transitory highs’ mean pressure and transitory lows’ mean pressure increased in 78% of the conclusive cases. Apparently, one frequent role of MCSs is locally to magnify storm-scale pressure gradients. Third, transient paths reflect the frequent symmetric-to-asymmetric metamorphoses of the MCSs. Fourth, the temporal fluctuations of the numbers and apparent sizes of transients within a composite MCS partially support theories of MCS upscale evolution. Finally, the composite’s transient numbers and apparent sizes varied almost identically with time in a pattern that closely resembles the fluctuation of stratiform and convective volumetric rain rates of MCSs.