Diagnosis and Dynamics of Forecast Error Growth

Abstract

Consideration is given to the diagnosis and dynamics of synoptic and subsynoptic forecast error from a potential vorticity (PV) perspective. A depiction of the extratropical ''forecast minus analysis'' PV pattern on a cross-tropopause isentropic surface serves to illustrate characteristic features of the PV-error field, and these features relate both to the instigation, development, and breaking of Rossby waves at the tropopause, and to surface cyclones and anticyclones. An outline is provided of a three-component diagnostic approach for studying PV forecast error. The approach exploits the quintessential PV concepts of quasi conservation, inversion, and attribution, and its essence is illustrated qualitatively by reference to one particular synoptic sequence over the North Atlantic. It also provides a framework for assessing the dynamics of possible mechanisms for generating realized PV-error features. The approach offers a conceptually attractive and diagnostically useful method of analyzing, assessing, and understanding the dynamics of forecast error growth.

A Composite Perspective of the Extratropical Flow Response to Recurving Western North Pacific Tropical Cyclones

“…Many studies suggest that large numerical model forecast errors may result from a failure of the numerical model to adequately capture diabatically driven ridge amplification (e.g., Davies and Didone 2013;Gray et al 2014), whether associated with recurving TCs (e.g., Henderson et al 1999;Torn 2010), mesoscale convective systems (e.g., Dickinson et al 1997;Rodwell et al 2013), or warm conveyor belts of explosively deepening extratropical cyclones (e.g., Doyle et al 2014). A question to be addressed is whether forecast error and uncertainty associated with the negative PV advection by the irrotational wind are systematic sources of forecast error and uncertainty in the midlatitudes.…”

“…The irrotational and nondivergent wind fields are computed for each individual case prior to compositing. All derived variables, including PV advection, Q vectors, and Q-vector divergence (described subsequently in section 2d), and PV frontogenesis 1 [i.e., the rate of change of the magnitude of the horizontal PV gradient; e.g., Davies and Rossa (1998), Cordeira (2011, p. 105)], are computed on isobaric levels from the composite fields.…”

The Key Role of Diabatic Outflow in Amplifying the Midlatitude Flow: A Representative Case Study of Weather Systems Surrounding Western North Pacific Extratropical Transition

“…Because their general evolution follows dry dynamics that can be represented at grid scale in numerical weather prediction (NWP) models, Rossby waves may be expected to feature a high degree of predictability (Grazzini and Vitart 2015). However, major forecast uncertainty and error in the midlatitudes in current NWP models have recently been shown to be linked to a misrepresentation of the Rossby wave pattern and various authors pointed to the potential role of moist processes in large-scale flow modification (e.g., Grams et al 2011;Davies and Didone 2013;Rodwell et al 2013;Gray et al 2014;Teubler and Riemer 2016). Enhanced forecast uncertainty for the large-scale flow evolution has also been documented downstream of extratropical transition (ET; Jones et al 2003;Anwender et al 2008;Aiyyer 2015;Grams et al Supplemental information related to this paper is available at the Journals Online website: http://dx.doi.org/10.1175/ MWR-D-15-0419.s1.…”

Modulation of surface meteorological parameters by extratropical planetary-scale Rossby waves

“…Moreover, knowledge of the evolution of coherent Rossby waves is crucial for weather forecasting since they set the stage for weather systems to evolve. Furthermore, analyses of forecast errors of numerical weather prediction models have revealed a close link between the propagation of the error patterns and the propagation of upper-level Rossby waves (Davies and Didone, 2013;Grazzini, 2015). Therefore, the study of the characteristics of extratropical Rossby waves, such as propagation, breaking, and their organization, is imperative for understanding and forecasting regional and local weather.…”