SCALE AND PATTERN SEPARATION MODEL

The circulation at any level within the atmosphere shows features of varying scale (size) and pattern. At the surface, there are micro-lows, troughs, ridges, migratory cyclones and anti-cyclones, and the large-scale semi-permanent pressure systems. Aloft, there are troughs, ridges, highs, and lows with varying wave-lengths and amplitudes. These in turn are all part of the still larger scale system, the planetary vortex.

Interaction of features either at the same level or at differing levels is a major problem faced by synoptic analysts and forecasters. The problem stems from distortion in the circulation patterns caused by the interaction of small-scale and large-scale features. The classical example, for instance, occurs in the vertical; the long-wave patterns are distorted by short waves. The distortion makes for subjective positioning of the long-waves, and the positions are usually inaccurate. An inaccurate analysis then leads to inaccurate prognoses. To overcome such subjective determinations, the scale and separation model was developed.

The scale and separation model provides an objective measure of scale while retaining characteristic recognizable patterns. It separates features of various size into separate parts. For example, it takes the 500-millibar field and separates it into a short-wave field (500-millibar SD), a long-wave field (500-millibar SL), a residual field (500-millibar SR), and a planetary vortex field (500-millibar SV). The model uses a smoothing process to separate each field. For example, if the small-scale features (SD) are smoothed out of the total field (Z), a residual field (SR) remains. The residual field contains the large-scale disturbance features (SL) and the planetary vortex (SV). The SR field at 500 millibars is ideal for locating long-wave troughs. A more massive smoothing process continues on the residual (SR) field until only the planetary vortex remains. The large-scale disturbance pattern is obtained by subtracting the planetary vortex field from the residual field (SL = SR SV). The smoothing relationships, in the order given above, are ZSD=SR and SRSV=SL.  

To be totally accurate, the amount of smooth-ing required to remove any one scale should vary depending on the time of year. However, the FLENUMOCEANCEN model employs only the October smoothing value so as not to disrupt component continuity. This results in sL features being somewhat weaker than they should be in summer, and the SD features being somewhat stronger. The reverse is true in winter.

Learning Objective: Recognize the parameter used in FLENUMOCEANCENs atmospheric frontal model and the strengths and weakness of the final product.

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