Performance Test of the Pasquill Stability Scheme

Hillary Chapman
University of Wisconsin-Milwaukee
MS Student

“In 1961, Frank Pasquill proposed a method for classifying atmospheric stability based on routinely available surface observations – namely wind speed, cloud cover, and the strength of incoming solar radiation. Stability is classified into six categories: extremely unstable (A); moderately unstable (B); slightly unstable (C); neutral (D); slightly stable (E); and moderately stable (F). These categories are ultimately meant to be used to determine the rate of diffusion of windblown pollutants, but since their inception, the classes have often seen use outside of their originally intended purpose. In this thesis, the performance of the Pasquill scheme is tested in order to determine whether it is appropriate to use it in a non-diffusion related setting. Stability derived using the Pasquill stability scheme is compared to stability derived from temperature lapse rates, using surface and upper air data obtained from the NOAA NCEI for six sites across the Eurasian continent for the years 2000-2010. The Pasquill scheme is found to be biased towards neutral stability, with 57% of all cases determined to be class D – but the actual surface-100m temperature lapse rates were found to be biased towards stable conditions, with 70% of all cases falling into the stable stability range. The Pasquill scheme did perform best under stable conditions, with over 90% the E and F classes occurring when stable conditions were actually present. However, the scheme performed poorly during unstable conditions, correctly predicting an unstable class in only 57% of all unstable cases. The Pasquill method perfomed the worst under neutral conditions, correctly with neutral conditions present for only 5% of the cases when class D was predicted”.

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