Cloud Resolving Modeling of Tropical Circulations Driven by Large-Scale SST Gradients

Abstract

This paper considers interactions between the moist atmospheric convection and the large-scale flow driven by the large-scale gradient of sea surface temperature. A two-dimensional computational framework is used with the horizontal domain size of 4000 km in which both the convective dynamics and the large-scale flow are resolved. Rotational effects are not considered. Simulations are performed using either a prescribed temperature tendency mimicking the effects of radiative processes or a fully interactive radiation transfer model. The simulations are performed for a period of 60 days with quasi-equilibrium conditions attained after about a month. The time-mean large-scale flow in the simulations features an ascending branch occupied by moist convection over a warm ocean and a cloud-free descending branch over a cold ocean. The time-mean flow for the prescribed radiation case features a complex vertical structure characterized by two somewhat decoupled circulations in the lower and upper troposphere. This is in stark contrast with the predominant first-baroclinic-mode structure typical of the observed large-scale tropical circulations, which is characterized by a single cell. An idealized dry model featuring prescribed convective heat source suggests that the complex vertical structure is directly related to the deviation of the model temperature profile from the climatology. Quasi-two-day oscillations are a major transient feature of the simulations. The oscillations are associated with radiation of gravity waves from the convective branch into the descending branch. Inclusion of the interactive radiation results in a significant modification of the large-scale flow and has a dramatic impact on the strength and horizontal extent of convection. Water vapor and cloud condensate strongly interact with radiative processes to induce these paramount effects on the tropical large-scale circulations.