Downstream Development of Baroclinic Waves As Inferred from Regression Analysis

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Abstract
The structure and evolution of transient disturbances in the Northern Hemisphere winter season are examined using one-point regression maps and longitude-height sections derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses for seven winter seasons. With the use of unfiltered time series of normalized 300-mb meridional wind perturbations at a grid point in the Pacific storm track as the reference time series, regression statistics for perturbations in the horizontal wind, geopotential height, temperature, and vertical velocity are derived. The resulting perturbation fields exhibit characteristics of midlatitude baroclinic waves, such as a westward tilt with height in the velocity and height fields and eastward tilt in the temperature field, with typical wavelengths of 4000 km and periods of around 4 days. The main difference between the results of this work and previous similar analyses is in the propagation characteristics of the baroclinic wave trains. The wave trains found here exhibit characteristics of downstream development, with successive perturbations developing toward the downstream side of existing perturbations. An analysis of the eddy kinetic energy budget of the wave train indicates that downstream radiation of ageostrophic geopotential fluxes by existing perturbations triggers the development of new eddies downwind, with baroclinic conversion becoming important only during the later part of the life cycle of a downstream developed wave. The Appendix suggests that the difference between these results and those of previous works is mainly due to time filtering obscuring the delicate signal of downstream development.

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