Downstream development of baroclinic waves
Downstream development of baroclinic waves is studied using models and observations. Localized perturbations in a baroclinic basic state spread out towards the downstream (and upstream) direction by radiation of energy via ageostrophic geopotential fluxes. The energy budget of these perturbations is examined, and their growth is triggered by the convergence of ageostrophic geopotential fluxes radiated from upstream (downstream) disturbances, while their decay stage is dominated by downstream (upstream) radiation of energy, with baroclinic and barotropic conversion playing only secondary roles. A variable Coriolis parameter and surface friction act to favor downstream over upstream development. Downstream development of baroclinic waves in zonally inhomogeneous environments are studied to understand the energetics and maintenance of storm tracks. In experiments with zonally varying baroclinicity introduced by zonal variations in heating, eddies grow due to baroclinic conversion over regions with high baroclinicity and then form coherent wave groups through downstream development. These wave groups propagate into regions with weak baroclinicity and extend the storm track deep into such regions. An eddy kinetic energy equation is derived using simple parameterizations of the various sources and sinks to diagnose the zonal distribution of eddy kinetic energy in a storm track. The effects of mountains on eddy distribution is examined briefly. Apart from generating stationary waves which modulate the baroclinicity of the atmosphere, mountains may directly affect eddies by reducing their vertical phase tilt, making them less efficient in extracting energy from the mean flow. Atmospheric observations analyzed by the European Centre for Medium Range Weather Forecasting are examined. Synoptic scale waves in the Northern Hemisphere storm track regions display clear signal of downstream development, and their energy budgets are very similar to those found in modelling studies.
- Research Organization:
- Princeton Univ., NJ (United States)
- OSTI ID:
- 7043465
- Country of Publication:
- United States
- Language:
- English
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