Practical subjective application of the omega equation and Sutcliffe development theory

Abstract

AbstractSutcliffe development theory and the similar omega equation provide an objective framework for the understanding of synoptic‐scale development in the extra‐tropics. Both are widely applied by forecasters in the UK Met. Office, with the omega equation also being used by forecasters working in other meteorological services. Using results from experiments with a quasi‐geostrophic numerical model, this paper examines the pitfalls associated with each, and suggests ways in which these can be avoided. The partitioning between so‐called steering and development terms in the Sutcliffe equation is shown to be misleading, and an alternative form is suggested which is more accurate and circumvents significant cancellation problems associated with the existing form. It is shown that oversimplifiction of the omega equation can lead to it being applied in unsuitable ways. In particular, it is argued that the conversion from omega forcing to vertical velocity is best made at the mid‐tropospheric level, say at 500 hPa, and that therefore diagnostics such as Q‐vectors or their divergence are better applied at this level than, say, at 700 hPa. The so‐called Sutcliffe, or Trenberth, form of the omega equation is recommended as being suitable for subjective application.