Gravity waves in the mesosphere generated by tropospheric convention

Abstract

The observed cold temperatures in the summer mesosphere are dynamically maintained primarilythrough upwelling induced in response to the action of a zonal drag force caused by thebreaking of upward propagating gravity waves. Tropospheric convective storms are believedto be important sources of gravity waves in the summer mesosphere, but little is known aboutthe characteristics of mesospheric gravity waves generated by convection. As a first attempt tomodel such waves a nonhydrostatic cloud-resolving numerical model is used to simulate a2-dimensional squall line in a domain of width 2048 km and depth 90 km. The simulationproduces a broad spectrum of convectively generated gravity waves. These propagate into themiddle atmosphere, forming a fan-like pattern of waves with amplitudes increasing with height,and eventually reach breaking amplitudes in the mesosphere. The resultant mesospheric wavebreakingproduces strong zonal forcing, which is eastward to the east of the storm center andwestward to the west of the storm center. Breaking of upward propagating waves also generateshigh frequency downward propagating secondary waves of short horizontal wavelength, andlong vertical wavelength. The secondary waves have only a small influence on the net verticaltransfer of momentum, but produce a strong signature in perturbation vertical velocity, featuringalternating positive and negative interference with the primary upward propagating modes.