Finite-Amplitude Convection in a Conditionally Unstable Stratification
Journal of the Meteorological Society of Japan
In order to obtain a basis for better understanding of the properties of atmospheric moist convection, numerical experiments are performed using a simplified model in which the pseudoadiabatic assumption is adopted. Therefore, the model does not include evaporation and drag force of liquid water and cloud microphysical processes. Furthermore, it is assumed that motions are always moist adiabatic in ascending area. The present study is made as an extension of a previous study (Yamasaki, 1972) in
... (Yamasaki, 1972) in which Kuo (1961Kuo ( , 1965)'s perturbation theory for moist convection was re-examined with a complete set of boundary conditions. The objective of this paper is to study how the results obtained from the perturbation theory are modified by finite-amplitude effects. In particular, it is attempted to clarify the effects of finite-amplitude on the horizontal scale of ascending motion and on the interaction among convective cells. The present study is restricted to such situations that neither ambient wind nor large-scale convergence exist. It is shown that finite-amplitude effects act to make the size of ascending area larger than that expected from the perturbation theory, particularly in the lower part of convection. However, it is rather emphasized that the characteristic size of ascending area is not significantly modified by finite-amplitude effects. This means that the characteristic size in finite-amplitude convection can be approximately predicted from the perturbation theory. When the initial size of ascending area is small or large compared to the size expected from the perturbation theory, the characteristic size tends to approach the theoretical value during the growing stage. Therefore, the size of ascending area at the mature stage does not depend much on the initial size. Although these results are not necessarily true for actual atmospheric convection because of some assumptions used, it may be expected that the results obtained in this paper will provide us with a useful basis for further studies of moist convection. The results as to the size of ascending area are also discussed in comparison with those obtained by .