Convectively generated gravity waves during TWP-ICE

Description

All convective clouds emit gravity waves. As they propagate, the gravity waves emitted by clouds displace air parcels vertically, possibly producing further clouds. These waves also transport momentum and energy large distances from the site of their generation, exerting a stress on the atmosphere wherever they dissipate. The larger project examines the part played by convectively generated gravity waves in the formation of cirrus and in the subgrid-scale momentum transport and associated large-scale stresses imposed on the troposphere and stratosphere. The work reported here focuses specifically on the characteristics of the convectively generated gravity waves analysed from the radiosonde soundings taken during the Tropical Warm Pool-International Cloud Experiment (TWP-ICE). Fluctuations in the ascent rate of the radiosonde are used to characterize the high-frequency waves, whereas the fluctuations in the horizontal winds are used to determine the properties of the inertia-gravity waves. A basic assumption underpinning both approaches is that the perturbations in the radiosonde sounding represent gravity waves. Following May et al. (2008), TWP-ICE is divided into three periods. As the character of the convection changes during the experiment, so does the character of the gravity wave spectrum. Regime 1 is characterized by active monsoon convection. In this regime, the wave field, like the convection, shows little diurnal variation. Regime 3 is characterized by diurnally forced break convection embedded in deep easterlies. Accordingly, the gravity wave field is of higher frequency and exhibits a strong diurnal variation. Regime 2 is characterized by suppressed convection with strong westerlies associated with deep tropical lows to the south of Darwin. In this regime, the high frequency wave activity is modulated on a time scale of 3–4 days. Interestingly, the meridional wind just above the top of the tropopause fluctuates with a similar period. It appears that the convectively generated high-frequency gravity wave activity is modulated by the large amplitude low-frequency inertia-gravity wave.