On the scale interaction between African easterly waves and convention

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1998.Includes bibliographical references (p. 78-85).In this study a -plane quasi-geostrophic channel model is used to study the dynamics of African waves. In part I the structure and energetics of the African easterly waves are well replicated compared to observations. Highly simplified physical processes, including Ekman damping and relaxation, are included to study the dynamic equilibrium problem. Similar to the results of observation, the barotropic process is important during the linear stage of the life cycle. The structure during the nonlinear stage is similar to observations, especially the upper-level meridional wind field. During the nonlinear stage, baroclinic processes dominate. Afterwards, the system reaches saturation and stays in an oscillatory state due to the nonlinear wave-mean flow interaction. Simple bulk aerodynamic formulae are used to parameterize the surface heat fluxes. Ekman damping works to spin down the circulation. Relaxation is used to restore the zonal mean field back to its initial condition and works a simple way to mimic the radiative processes which maintains the strength of the environment. The experiment with Ekman damping and relaxation shows that the system reaches a state of dynamic equilibrium. The relaxation restores the strength of the jet and the kinetic energy is dissipated due to Ekman damping. The results of different sensitivity experiments are compared to the control run to see the effects of parameterized processes. In part II we deal with the quasi-equilibrium problem. Wind-induced heat and water vapor fluxes from the ocean surface are also calculated using the bulk aerodynamic formulae. By adopting the Emanuel convection scheme, we put together different diabatic processes including radiative cooling, Ekman damping and convection. Although physical processes are idealized, the results demonstrate strong interaction between the circulation of African disturbances and convection. The scenario is that the African waves works as the synoptic-scale forcing to modify the entropy of the subcloud layer. Convection, in response to the large-scale forcing, redistributes the heat and water vapor upward. The interaction of convection and African easterly waves ultimately reaches a state of quasi-equilibrium. Two-dimensional perturbations are introduced at the lower boundary as the system reaches a two-dimensional equilibrium state. The structure and evolution of the African easterly waves are well replicated. The results show that wave disturbances grow in the expense of zonal mean available potential energy and kinetic energy trough barotropic and baroclinic energy conversions. The trough has a cold core below and warm core above structure. The geographic distribution of precipitation rate shows that the convection is enhanced by the dynamic forcing of the approaching mid-level trough of the African waves. The structure and evolution of the African disturbances is modified by the convective processes. The results show much resemblance to the observations. Sensitivity experiments show that higher SST with fixed temperature gradient results in higher and weaker jet and weaker vertical wind shear below the jet. According to Miller and Lindzen's results, the effect of higher SST on jet properties is a possible cause that leads to later organization of convections over the Atlantic. Larger temperature gradient with fixed SST gives rise to a stronger jet. The results are insensitive to the initial background humidity but sensitive to the surface water vapor flux over the land.by Bruce, Ting-Hsin Kuo.S.M