Effects of easterlies at the upper troposphere on the development of tropical mesoscale convective systems

Convectively generated gravity waves and their effects on the development of convective systems over the western Pacific ocean are investigated, using a numerical model. A simulation is initialized with the sounding of R/V Vickers on 11 February 1993 during the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE), with west winds at low levels and easterlies aloft. It was a convectively active day when discrete regeneration of a new convective system to the west of an old convective system was observed by Halverson et al. (1999).

The simulation shows that the discrete regeneration of a new convective system to the west of an old convective system is due to westward propagating gravity waves. The westward propagating gravity waves are preferentially exited by multicellar squall lines propagating eastward. This is due to the westward motion of convective cells within the squall lines relative to winds at low levels, as discussed by Fovell et al. (1992). These westward propagting waves are reflected by an upper layer of weak stability and strong vertical wind shear, caused by the mechanism proposed by Linzen and Tung (1976). Therefore these waves can propagate horizontally and regenerate a new convective system to the west of an old convective system with an averaged speed of 10 m/s relative to the ground. An additional simulation shows that the discrete regeneration of a new convective system to the west of an old convective system can hardly be seen if easterlies aloft are reduced. Thus we conclude that strong easterlies are very important for the discrete regeneration of a new convective system to the west of an old convective system.

This regeneration, which implies westward-moving heating, may be related to the excitation of the 2-day westward-propagating inertio-gravity waves with an averaged speed of 10-20 m/s relative to the ground.