An Intermediate-Complexity Model of Tropical Dynamics for Mean and Extreme Precipitation

The broad expectation under global warming is for the mean precipitating zones in the tropics to narrow and intensify, and for the mean non-precipitating zones to expand and weaken. Relatedly, at fast timescales, both tropical precipitation extremes and dry durations are poised to increase. However, the time-mean features scale at very different rates than the fast time-scale features. We present a model of intermediate complexity to investigate the tropical wet and dry zone changes under global warming at both the time-mean and fast timescale ends of the temporal spectrum.

The model generates precipitation in response to an imposed sea surface temperature (SST) pattern. The model contains a fully dynamic boundary layer, which helps set the boundary layer moist static energy (MSE). Convection is parameterized based on the observed precipitation-buoyancy relationship, wherein precipitation is produced in response to changes in both the stability (defined as the MSE difference between the boundary layer and the lower-free troposphere) and the lower-free tropospheric subsaturation. Radiative fluxes are calculated using a linearized version of the Rapid Radiative Transfer Model for GCMs (RRTMG). In the free-troposphere, the weak temperature gradient (WTG) assumption is assumed, which removes equatorial wave variability. Fast timescale variations are instead represented by a stochastic term, permitting an evaluation of extremes in precipitation and dry times.

The mean state of the model is sensitive to the entrainment rate in convection scheme and parameters in the radiative scheme. The mean state precipitation pattern is also sensitive to the degree of downdraft-induced cooling and drying of the boundary layer. Strong downdraft-induced effects produce a double intertropical convergence zone (ITCZ); while a single ITCZ is produced for moderate to weak downdraft-induced changes to the boundary layer. Preliminary work also indicates that the model can reproduce the anticipated changes to precipitation extremes under warming, with contributions from both dynamic and thermodynamic feedbacks. This model is expected to provide process-oriented guidance for the analysis of more complex climate models.