Mechanics of a Strong Subsynoptic Gravity Wave Deduced from Satellite and Surface Observations

Abstract

The three-dimensional structure and implied dynamics of a strong tropospheric gravity wave event am studied. It is shown that satellite and continuous surface observations reveal the subsynoptic nature of this ?wave of depression? to an extent impossible with conventional data. The observations and theory suggest that the gravity wave originated in the upper troposphere near a jet streak, was quasi-hydrostatic and hence relatively nondispersive and long-lived. The behavior of the wave at upper-tropospheric levels is revealed by sequences of visible and infrared goesynchronous satellite imagery. Quantitative estimates of cloud top temperatures and winds suggest strong subsidence new 300 mb with an isentropic depression as large as 900 m. The upper-level depression and the surface disturbance propagate coherently with a speed of 32 m s?1 indicating that they are part of the same internal gravity wave. The vertical tilt with height is opposite to the propagation direction and thus is consistent with an upper-tropospheric energy source. The negative surface pressure deviation reaches 7 mb and is qualitatively consistent with the field of surface wind divergence. Theory is applied to estimate and explain gravity wave properties throughout the troposphere: vertical tilt (decreasing upward) as large as 1:9 in the lower troposphere; maximum wave energy at upper levels, with maximum wind deviation ? 15 m s?1, horizontal divergence ? 4?10?4 s?1, vertical parcel displacement ? 1 km, local potential temperature deviations of several degrees, pressure perturbations ? 7 mb, and the time to completely establish the wave throughout the troposphere ? 4 h. Further improvement in the description may demand development of ?solitary? wave theory for deep depression waves in shear flow.