Seminar: A Review of Mesoscale Gravity Waves and their relation to Spontaneous Balance Adjustment and Deep Convection

Abstract:

Gravity waves are ubiquitous entities in the atmosphere, being generated by topography, wind shear, latent heating, and flow imbalance associated with jets and fronts. Many of these waves display short wavelengths and have little bearing on the weather; however, mesoscale gravity waves (MGW), which have been found to be closely related to the existence of dynamical imbalance, can create coherent precipitation bands composed of ice pellets with lightning, blizzard conditions, or lines of severe convective storms. The MGW sub-class of highly nonlinear waves can create damaging surface winds, disrupt aviation, and drive rapid changes of water level in both coastal and inland water bodies. These large-amplitude waves wreak havoc on local weather forecasts, being largely unanticipated and misunderstood.

Following a short review on the basics of gravity wave dynamics and terminology, this talk will emphasize the nature and diagnosis of unbalanced components of the mass and momentum adjustments leading to MGW generation and the importance this has on high-impact weather. It will be shown that MGW genesis is favored as a jet streak propagates away from the upper-level trough axis and toward the inflection axis between the trough and downstream ridge, especially when the horizontal wavelength between this trough and ridge shortens with time, and the jet streak separates from its geostrophic component at the base of the trough and propagates toward the ridge. This evolution is favored when a synoptic cyclone and associated jet streak develop rapidly, especially when accompanied by latent heating. Increasing separation between the geostrophic wind maximum and the actual jet streak is related to intensifying upper-level flow imbalance commonly associated with strong tropopause folding events, and leads to the repeated development of spontaneous, continuous emission of MGWs. Convective latent heat release not only increases the flow imbalance, but it helps to maintain ducted MGWs, producing even greater imbalance and consequential increase of MGW emission – essentially, a positive feedback process.