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.
Highlights
It is well documented that atmospheric processes are characterized by various scales of motion, ranging from large-scale, meso-scale, to small-scale turbulent eddies
Results show that convective-scale eddies within the system effec tively redistribute and transfer kinetic energy (KE) from one layer to another
The dynamical interactions between the convective-scale eddies and their meso-; scale smroundings result in a forma tion of an MLJ and a demise of an level jet (LLJ) at the times of intense convection in agreement with observation
Summary
It is well documented that atmospheric processes are characterized by various scales of motion, ranging from large-scale, meso-scale, to small-scale turbulent eddies. E.g., convective storms and squall lines, are affected by both larger and smaller scale eddies. During the developing and mature stages of these mesoscale disturbances, the large-scale environments are considerably modified as a result of scale interactions or "feedback" mechanisms. These dynamic interactions are very complicated in nature and have not been well understood, yet. One way of investigating these feedback processes is to examine the KE balance of the storm's surroundings, e.g., Fuelberg and Printy (1984), etc. Case studies by Ninomiya (1971), Maddox (1980), and many others have shown that storm-environment interactions result in a mid-tropospheric warming and ascent, strong low level convergence and upper-level divergence, regions of upper-level ridging and anticyclonic flow, and the formation of jet maxima on their poleward sides in the storm's suror undings
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