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Christopher Lucas, Peter T. May, and Robert A. Vincent

Abstract

An algorithm to detect frontal zones in time–height cross sections of horizontal wind from wind profiler measurements is described. The algorithm works by identifying regions with 1) a strong horizontal temperature gradient, estimated by using a quasigeostrophic thermal wind retrieval, 2) a strong temporal increase in the signal-to-noise ratio at a given range gate, and/or 3) a strong temporal shift in the horizontal winds at a given range gate. The type (e.g., cold or warm) of front is determined by examining the advection field and the characteristics of the boundary. Most weight is given to the horizontal temperature gradient component of the algorithm.

A springtime frontal system and an associated baroclinic wave over South Australia are examined using both routine synoptic observations and analyses as well as data from the profiler. Synoptic observations depict a prefrontal trough and two cold fronts at the surface and a deep trough in upper levels. The tropopause is identified at ∼6 km in one sounding. The algorithm successfully identifies the one main cold front and the lowered tropopause in the polar air. There are also hints of a prefrontal trough and a descending tropopause with the onset of the main cold front. After the passage of the upper trough, the ascending tropopause and the so-called jet front or trailing front are also identified by the algorithm. The latter represents the passage of the upper-level baroclinic wave and the reappearance of a strong jet stream.

Other regions are spuriously identified as fronts. These regions could be the reflection of some short-term meteorological phenomena, such as gravity waves; deviations from the assumed quasi-geostrophy; or simply reflections of noise in the analysis. An examination of the effect of random measurement uncertainties on the frontal analysis gives an estimate of error of around 2 K (100 km)−1 in the horizontal temperature gradient calculations for typical wind errors. The errors on the retrieved advection vary, depending on the wind speed, but are around 25 K day−1 for a ∼20 m s−1 wind speed. These values are typical of the noise in those fields, suggesting that the spuriously defined fronts likely reflect uncertainties in the data rather than actual meteorological phenomena.

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Peter T. May, Greg J. Holland, and Warner L. Ecklund

Abstract

Wind profiler and serial sounding observations extending to the upper troposphere are used to analyze Tropical Storm Flo (1990) as it passed within 115 km of the experimental site on Saipan. These data resolve details of the circulation and precipitation structure of the storm and its rainbands. Analysis of principal and secondary rainbands in outer radii indicate that there are considerable similarities with previous studies. Although the bands contained distinct precipitation maxima, there is no evidence of active convection and the mean structure is similar to that observed in the stratiform regions of squall lines. The vertical circulations in the rainbands are weak and complex, but distinct azimuthal wind maxima are observed that have maxima of relative vorticity and inertial stability on the inner edge. The divergence fields for the entire analysis period are strongly coherent and are indicative of vertically propagating gravity waves generated in the near inertially neutral outflow layer. The analysis thus demonstrates the usefulness of wind profilers for tropical cyclone observations.

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Peter T. May, Thomas D. Keenan, Rod Potts, James W. Wilson, Rob Webb, Andrew Treloar, Elly Spark, Sue Lawrence, Elizabeth Ebert, John Bally, and Paul Joe

Abstract

The Sydney 2000 Olympic Games World Weather Research Programme Forecast Demonstration Project (WWRP FDP) aimed to demonstrate the utility and impact of modern nowcast systems. The project focused on the use of radar processing systems and products for nowcasting, including severe weather. The forecast problems facing the Australian Bureau of Meteorology (BoM) on these short timescales during the FDP are briefly described. The observing system is then discussed and enhancements to the network that supported the Olympic Games forecast requirements and the WWRP FDP project are outlined. In particular, issues related to radar calibration and quality control are discussed in some detail. The paper concludes with a brief discussion on the observing system requirements to meet such modern nowcast systems, areas of further development, and impacts that the FDP had on BoM nowcasting systems. The need for end-to-end design of systems from data gathering, to analysis and product generation is emphasized.

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