Elevated Heat Source and the Eastward Propagation of the Low-level Response

The responses to prescribed shallow heating in the tropics are investigated with a dry primitive equation model of the atmosphere in this study. Special emphasis is placed on the adjustment process in the first few days when heating is turned on. The results show that: 1) the elevated heat source induces a complicated evolution of surface horizontal convergence with the convergence center propagating eastward, which can explain the propagation of the Madden–Julian Oscillation; 2) the surface convergence is largely confined in the heating area and shows little outward propagation when the heat base is at the surface.

Detailed analysis indicates that the heating causes wind-pressure imbalance which leads to the adjustment process until a balanced state is reached. The surface responses to elevated heating take time and are relatively weak. The convergence center in the elevated heating case migrates to the east after a couple of days when the Kevin wave structure in the responses shaped up. In contrast, the surface-based heating results in immediate mass deficit at the surface and thus the strong convergence at heat source.

The boundary layer friction does not necessarily cause the eastward migration of the convergence in our simulations. The apparent phase lag between convective heating and surface convergence seen in previous studies is the result of implicitly lifting the heat base by inserting a boundary layer with a prescribed depth. The strong friction in the boundary layer does not change the induced surface convergence structure but reduces the amplitude of the convergence.