Nonlinear Perturbation Growth at Mesoscale Related to Upscaling Processes from a Mesoscale Convective System

Abstract

In this study, the nonlinearity in a weather forecast was examined in an environment containing a mesoscale convective system. The nonlinearity was quantified by the relative nonlinearity as the extent to which the initial opposite-sign perturbed state vector does not keep the same magnitude and opposite direction in a forecast time. A pair of 18-h forecast experiments with initial perturbations of different signs was conducted for a heavy rainfall event in western Japan on 13 August 2021.

Despite the initially different signs, the perturbations had random structures at convective scales over 2 h, taking the relative nonlinearity value 1.72 as previous studies have shown. However, the perturbations had the same sign on the meso-α scale at 11 h, taking the relative nonlinearity value greater than 1.72. This result suggested that this nonlinear signal was found not only on the convective scale but also on the meso-α scale. The nonlinear signal upscaled from convective to mesoscale, indicating a transition to a nonlinear regime at the mesoscale. Additional experiments showed that this meso-α scale nonlinear signal originated from the front with high convective activities in the initial field through the emission of gravity waves via the moist physics.