Processes Generating Convection Cells near Sumatra Island in the Monsoon Season

Abstract

   We investigated processes generating convection cells in tropical regions using a high-resolution, 3-dimensional, non-hydrostatic model. Convection cells near Sumatra Island were reproduced by using reanalysis data of the Japan Meteorological Agency Climate Data Assimilation System. Downscaled experiments with a horizontal grid interval of 1 km showed that convection cells were generated by the passage of updrafts of low-level waves over an area of large-scale convergence or over the leading edge of a weak cold pool. The large-scale convergence gradually decreased the temperature and increased the relative humidity of the atmosphere below a height of 2 km. The atmosphere above the leading edge of the weak cold pool was almost saturated. These atmospheric conditions were favorable for the generation of convection. The updrafts of low-level waves also decreased the temperature and increased the relative humidity in the updraft regions. These changes caused by the low-level waves triggered new convection cells. The low-level waves had the structure of gravity waves, as shown by the temporal lag of about 90°, of temperature minimums behind the updraft maximums. The changes in the lower atmosphere caused by the large-scale convergence created conditions that allowed convection cells to be generated easily, but the intensity of the low-level waves determined the timing of the generation.