2.5
Surface potential temperature as an analysis and forecasting tool
Eric G Hoffman, Plymouth State University, Plymouth, NH
Surface analysis has long been a key component to weather analysis and forecasting. Current surface analysis techniques are based upon the Norwegian conceptual model for cyclone development first espoused in a series of papers published around the early 1920's (e.g., Bjerknes 1919; Bjerknes and Solberg 1922). Criticism of the Norwegian Cyclone Model (NCM) and of surface analysis practices resulted in a number of papers suggesting revisions of surface analysis methodology and practice (e.g., Mass 1991; Sanders and Doswell 1995) as well as a workshop for operational and research meteorologists at what is now the National Centers for Environmental Prediction (NCEP, formerly the National Meteorological Center, NMC) as reported by Uccellini (1992). Dr. Frederick Sanders was involved in several of these critical reviews over the last 10 years and has been committed to changing the way surface analysis and fronts are viewed.
Dr. Fred Sanders first proposed an alternate method of surface analysis in an invited talk at the 10th Cyclone Workshop (Val Morin, PQ, September 1997), which has subsequently been published in Monthly Weather Review (Sanders, 1999). This method centers on the routine analysis of surface potential temperatures. The term "front" is reserved by Dr. Sanders for those surface features that have: either a moderate or strong surface potential temperature gradient (defined by Sanders to be 8 K 220 km-1 and 8 K 110 km-1, respectively) AND an associated pressure trough and wind shift. Surface features that have significant wind shifts or pressure troughs without a moderate or strong potential temperature gradients are labeled baroclinic troughs. All areas of strong potential temperature gradient not associated with significant pressure troughs are referred to as non-frontal baroclinic zones.
Sanders and Hoffman (2002) used routine analyses of surface potential temperature to construct a climatology of surface baroclinic zones. Sanders and Hoffman (2002) present frequencies of strong and moderate baroclinic zones that are summarized by state. The results of that study are extended in this work. This one-year (July 1999- July 2000) climatology is reconsidered using gridded data instead of counts by state. The Barnes objective analysis scheme in GEMPAK is used to produce 0.5° x 0.5° gridded analyses every three hours over the continental United States, southern Canada, northern Mexico, and adjacent coastal waters. Composite maps of surface potential temperature gradient for the entire year as well as for each season are constructed from these gridded analyses. These maps will show additional sub-synoptic scale detail of the composite surface potential temperature field that is not readily seen in Sanders and Hoffman (2002). In addition, grid point frequencies of strong and moderate surface potential temperature analyses are constructed. Again, these analyses confirm the basic results of Sanders and Hoffman (2002) but give additional geographic and sub-synoptic detail.
Lastly, this paper examines two case studies to highlight the usefulness of surface potential temperature as an analysis and forecasting tool. The first case study from November of 2001 will look at the evolution of the fronts and surface potential temperature boundaries associated with a strong extra-tropical cyclone in the Midwestern United States. The results show that moderate and strong potential temperature gradients are often associated with warm and stationary fronts but that cold frontal features evolve and change character from a baroclinic trough to a front and back to a baroclinic trough. The second case examines the evolution of a non-frontal baroclinic zone and demonstrates the usefulness of potential temperature in identifying diabatic frontogenetical and frontolytical processes.
Recorded presentationSession 2, Mesoscale Weather Systems (Room 617)
Monday, 12 January 2004, 10:45 AM-12:00 PM, Room 617
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