A Kinetic Energy Budget of a Subtropical Prefrontal Rainband Based on Dual-Doppler Measurements

Abstract

Dual-Doppler data collected during the Taiwan Area Mesoscale Experiment (TAMEX) were used to study the kinetic energy balance of a subtropical prefrontal rainband over the Taiwan Strait. Fields of the system-relative wind and reflectivity were derived in a horizontal domain of 36 km by 40 km using the objective analysis scheme with 1-km grid spacing in all three directions, except in the lowest two levels where the height increment was chosen to be 0.5 km. There were ten analysis levels in the vertical ranging from 0.4 to 8.8 km. Vertical velocities were computed from the an elastic continuity equation by integrating downward with variational adjustment. Subsequently, fields of perturbation pressure and temperature were retrieved from a detailed wind field using the three momentum equations. The Doppler-derived winds and retrieved thermodynamic variables are then used to compute the magnitude of each term in the kinetic energy budget equation. Results show that the vertical total of the horizontal generation term acts as the main source of kinetic energy, while vertical totals of dissipation and the horizontal flux convergence/divergence of kinetic energy provide the main sinks. The horizontal flux convergence/divergence of kinetic energy is nearly balanced by the vertical flux convergence/divergence at most levels. In a similar manner, the vertical generation of kinetic energy term is almost in balance with the total buoyancy term. The computed tendencies show the decrease of mean kinetic energy at low levels and the increase at middle levels, which are attributable to the generation and redistribution of kinetic energy and latent heat releases by organized convection associated with the rainband. These findings are consistent with the weakening of a low-level jet and the formation of a middle-level jet at the times of convection as revealed by upper air observations. The budget study further demonstrates that the storm's meso-γ-scale environment is modified by areas of convection through scale interaction or ”feedback” mechanisms.