FRONTS AND FRONTAL WAVES OVER THE SOUTHERN CONE

Jérôme Patoux & Robert A. Brown

Department of Atmospheric Sciences University of Washington 408 ATG Building Box 351640 Seattle, WA 98195-1640 Ph.: (206) 543-2480 Fax: (206) 543-0308 E-mail: jerome@atmos.washington.edu

ABSTRACT

Numerous cold fronts associated with mature extratropical storms sweep the coast of Chile every year. The large expanses of the Southern Pacific Ocean offer an ideal setting for up to 4000-km long cold fronts to develop as their parent cyclones travel from Australia to South America. These fronts sometimes break and spawn secondary cyclones that deepen significantly in one or two days. Examples of such frontal waves will be shown and discussed. An attribution technique applied to scatterometer wind measurements is used to diagnose the development of those frontal waves.

Necessary conditions for their development are revealed.

INTRODUCTION

Atmospheric fronts have been observed to survive their parent cyclone for several days over the Southern Ocean (Patoux 2003, Patoux et al. 2004). Some of these fronts simply weaken and decay. Others strengthen and sometimes give birth to frontal waves and secondary cyclones. These cyclones are of intermediate size, have very large growth rates and can have damaging effects (Parker 1998). They are well known to forecasters in Western Europe where they often appear on the tail of mature cyclones off the coast of the British Isles. Our understanding of frontal wave development has been improved by such experiments as FASTEX (Joly et al. 1997). However, they have not been studied in the Southern Hemisphere.

Since July 1999, the SeaWinds-on-QuikSCAT (QS) scatterometer has been providing us with dense, high-resolution measurements of the surface wind field over the world ocean. The surface structure of marine atmospheric fronts over the Southern Ocean has been studied and has revealed complex features such as 4000-km long trailing cold fronts, double cold fronts and the so-called "T-bone". With a 1700-km-wide swath and a 25-km grid-spacing, QS offers an unprecedented look at marine surface winds.

Here QS surface winds are used in combination with a kinematic decomposition to analyze the development of frontal waves off the coast of Chile. The methodology and results are briefly described in the following sections. All graphs and figures are available at

//www.atmos.washington.edu/ ~jerome.

MATERIAL AND METHODS

The tools developed by Patoux and Brown (2001, 2002) and Patoux et al. (2003)are used to compute surface pressure fields from QS surface wind measurements and to correct the winds for errors due to rain contamination and to the geometry of the antenna. The divergence and vorticity of the corrected surface winds are then calculated. They are confronted to infrared satellite imagery obtained from the Global Hydrology Resource Center (GHRC). A history of the development of the frontal wave is obtained by building a time series of pressure and divergence maps and satellite images.

The technique described by Bishop (1996) was adapted to QS surface wind measurements to diagnose the impact of the environmental flow on the development of the front. A box is drawn around the front in such a way as to enclose all the vorticity and divergence characterizing the front. A non-divergent and an irrotational wind fields are attributed to these elements of vorticity and divergence. The environmental flow is obtained by subtracting them from the total wind.

RESULTS

The environmental flow typically induces strong deformation along an axis that lies at a slight angle with the front. The front is thus effectively "stretched" and rotated by the environmental flow. A time series of that stretching shows that the frontal wave grows when stretching deformation along the front decreases significantly. Frontogenesis by the environmental flow is also shown to decrease at that time. These results are consistent with observational studies by Rivals et al. (1996) and Renfrew et al. (1997).

CONCLUSION

The advent of scatterometers offers an unprecedented opportunity to study the development of fronts and frontal waves over the Southern Ocean, where few observational studies have been possible in the past. Bishop's (1996) kinematic decomposition can be applied directly to measured surface winds. Our findings are qualitatively consistent with past theoretical and observational results.

ACKNOWLEDGMENTS

This work is sponsored by NASA grant NS033A-01, administered through Oregon State University, and NASA grant NAG8-1424.

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