Wave disturbances associated with the red river valley severe weather outbreak of 10–11 April 1979

Summary An analysis of mesoscale gravity wave events during the severe weather outbreak in the Red River Valley on 10–11 April 1979 is presented utilizing surface pressure data and the 3 h rawinsonde data from the AVE-SESAMEI special network. The unique data set provided by the SESAME field experiment makes it possible to relate the wavelike characteristics observed at the surface to the variability of the temperature, humidity, and wind fields over a deep tropospheric layer that act to initiate and sustain the waves over long distances and time periods.Three different wave events (A, B, and C) were identified via spectral analysis and cross-correlation techniques. They all have similar periods, approximately 3 h, but different phase velocities. All three wave events are generated and propagate in the exit region or anticyclonic side of upperlevel jet streaks. Convection and wind shear are shown to be unlikely contributors to the generation of event A, which is probably related to the development of a strong divergent field in association with an upper-tropospheric jet streak and to the ensuing mass adjustment process. Events B and C also appear in a region of strong ageostrophic motion associated with an upper-level jet streak. However, the low values of the Richardson number (Ri) at the critical levels of these two waves suggest vertical wind shear as a likely contributor to their generation and/or maintenance. A linear stability analysis confirms, with unprecedent spatial and temporal resolution, that a modal structure is present in the atmosphere whose characteristics are consistent with those of waves B and C.Three-hourly rawinsonde data show strong temporal and spatial variability throughout the troposphere in the wind, temperature, and humidity fields when the waves are present. Convective systems, as detected by radar, are closely linked to the waves, although not in a consistent manner: cells intensify or develop at the passage of a wave trough in event A, at the passage of a wave ridge in event C, and at the passage of a wave trough or ridge in event B, depending on the geographic location of the cells. For all three events, maximum rainfall recorded at the surface is associated with a wave ridge with a time lag of approximately 1 h.With 20 Figures