Multi‐sensor observations of a wave beneath an impacting rear‐inflow jet in an elevated mesoscale convective system

Observations of a wave generated below, and propagating with, an elevated mesoscale convective system (MCS) are presented. The MCS and the wave tracked directly across the 90 km extent of the observational network deployed for the Convective Storm Initiation Project (CSIP) and maintained a coherent structure and velocity during this 1.5 hour period. The rear‐inflow jet of the MCS was observed to impact upon, but not penetrate, the stable undercurrent that was located beneath the MCS, apparently constricting the depth of the undercurrent, which did not participate in the main convective updraught. The wave crest was located approximately 15 km ahead of this constriction. The overall wave led to between 200 and 1000 m of ascent and approximately 1500 m of descent. However, undulations were superimposed on the overall wave, with a wavelength of approximately 7 km and a crest‐to‐trough amplitude of approximately 1000 m. These undulations were associated with double maxima in surface pressure and the elevation of the observed low‐level wind anomaly, where low‐level winds were backed by more than 90°. These winds were never at rest relative to the system and the wave was therefore a ‘gravity wave without stagnation’. The hydrostatic effect expected from the observed lifting in the wave was broadly consistent with the observed surface pressure perturbation of 2 hPa. No cold‐pool outflow was ever observed; instead the observed surface cooling of approximately 4°C was shown to be consistent with reduced solar insolation. The generation of the observed wave structure and its implications for the maintenance of the MCS and numerical modelling of the system are discussed. Copyright © 2010 Royal Meteorological Society