A multi-scale model for the intraseasonal impact of the diurnal cycle of tropical convection

Abstract

One of the crucial features of tropical convection is the observed variability on multiple spatiotemporal scales, ranging from cumulus clouds on the daily time scale over a few kilometers to intraseasonal oscillations over planetary scales. The diurnal cycle of tropical convection is a significant process, but its large-scale impact is not well understood. Here, we develop a multi-scale analytic model to assess the intraseasonal impact of planetary-scale inertial oscillations in the diurnal cycle. A self-contained derivation of a multi-scale model governing planetary-scale tropical flows on the daily and intraseasonal time scale is provided below, by following the derivation of systematic multi-scale models for tropical convection. This derivation demonstrates the analytic tractability of the model. The appeal of the multi-scale model developed here is that it provides assessment of eddy flux divergences of momentum and temperature and their intraseasonal impact on the planetary-scale circulation in a transparent fashion. Here, we use it to study the intraseasonal impact of a model for the diurnal cycle heating with two local phase-lagged baroclinic modes with the congestus, deep, stratiform life cycle. The results show that during boreal summer, the eddy flux divergence of temperature dominates in the northern hemisphere, providing significant heating in the middle troposphere of the northern hemisphere with large-scale ascent and cooling with subsidence surrounding this heating center. Due to the analytic tractability of the model, such significant eddy flux divergence of temperature is traced to meridional asymmetry of the diurnal cycle heating. In an ideal zonally symmetric case, the resulting planetary-scale circulation on the intraseasonal time scale during boreal summer is characterized by ascent in the northern hemisphere, southward motion in the upper troposphere, descent around the equator and northward motion in the lower troposphere. The intraseasonal impact of the diurnal cycle on the planetary scale also includes negative potential temperature anomalies in the lower troposphere, which suggests convective triggering in the tropics. Furthermore, a fully coupled model for the intraseasonal impact of the diurnal cycle on the Hadley cell shows that the overturning motion induced by the eddy flux divergences of momentum and temperature from the diurnal cycle can strengthen the upper branch of the winter cell of the Hadley circulation, but weaken the lower branch of the winter cell. The corresponding eddy fluxes from the diurnal cycle are very weak for the equinox case with symmetric meridional profiles, and eddy momentum fluxes are small for all scenarios considered here.