7B.6
An explicit numerical study of the intertropical convergence zone in an aqua-planet with uniform sea surface temperature
Changhai Liu, NCAR, Boulder, CO; and M. W. Moncrieff
Explicit two-dimensional numerical modeling of convective features in an equatorial beta plane was conducted with a nonhydrostatic cloud-resolving model. The model was forced by constant sea surface temperature on an aqua planet and by horizontally homogeneous radiative cooling. Two distinct patterns of the spatial distribution of convective activities in the tropics were identified. The first pattern corresponded to enhanced off-equator convection, namely double intertropical convergence zones (ITCZs) that straddled the equator during the early period of the integration. The second pattern was characterized by enhanced equatorial convection, namely a single ITCZ on the equator during the later quasi-equilibrium period.
Three physical mechanisms behind the modeled convective behavior were suggested and explored through an additional experiment in which time-and space-independent surface fluxes were applied and through a set of dry simulations involving the nonlinear atmospheric response to the ITCZ-like heat sources located at various latitudes. The first was the wind-induced surface flux variability which played a vital role in the formation of the single ITCZ on the equator in the control simulation. The second was the enhanced low-level convergence by the planetary rotation regarding the response to convective heating, which favored an ITCZ displaced from the equator. The third concerned the trapping of convection-generated subsidence warming and drying by the Coriolis force, which preferred an equatorial ITCZ. The combination of the last two conflicting dynamical processes resulted in a double ITCZs positioned at a finite distance (approximately 800~1500 km) away from the equator in the control simulation and also in the sensitivity experiment with uniform surface fluxes.
Session 7B, Large-Scale Dynamics and Convection II (Parallel with Sessions 7C and 7D)
Tuesday, 30 April 2002, 4:00 PM-5:30 PM
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