Warm conveyor belts as amplifiers of forecast uncertainty

Pickl, Moritz; Quinting, Julian; Grams, Christian M.

doi:10.1002/qj.4546

Cited by 6 publications

(3 citation statements)

References 62 publications

(102 reference statements)

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“…Further work needs to be done to attribute the systematic errors in Rossby wave structure shown here to model limitations in the representation of diabatic processes". An important study in this direction by Pickl et al (2023) found that the forecast skill of the ECMWF ensemble is generally reduced when the WCB activity is high, and that WCB activity is particularly increased when the error growth is largest.…”

Section: Diabatic Outflows and Forecast Errorsmentioning

confidence: 99%

The importance of diabatic processes for the dynamics of synoptic-scale extratropical weather systems—a review

Wernli,

Gray

2023

Preprint

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Abstract. Many fundamental concepts of synoptic-scale extratropical dynamics are based on the quasi-geostrophic equations of a dry atmosphere. This “dry dynamics” provides the essential understanding of, e.g., the formation of extratropical cyclones and the propagation of Rossby waves, and makes potential vorticity (PV) a materially conserved quantity. Classically, for extratropical weather systems, the importance of so-called “diabatic effects”, e.g. surface fluxes, phase changes of water in clouds, and radiation, has been regarded as secondary compared to the dry dynamical processes. As outlined in this review article, research during the last decades has modified this view of the role of diabatic processes. The combination of complementary research approaches revealed that the non-linear dynamics of extratropical cyclones and upper-tropospheric Rossby waves is— in some cases strongly—affected by diabatic processes. Despite the violation of material PV conservation in the presence of diabatic processes, the concept of PV has been of utmost importance to identify and quantify the role of diabatic processes, and to integrate their effects into the classical understanding based on dry dynamics. This review first summarizes the theoretical concepts of diabatic PV modification and moist PV and slantwise moist convection, and provides a concise overview on early research on diabatic effects until the late 1970s. Two poorly predicted high-impact cyclones affecting eastern North America then triggered an impressive diversity of efforts to investigate the role of diabatic processes for rapid cyclone intensification in the last two decades of the 20th century. These research activities, including the development of sophisticated diagnostics, growing applications of the Lagrangian perspective, real case and idealised numerical experiments, and dedicated field experiments, are reviewed in detail. This historical perspective provides insight about how societal relevance, international collaboration, technical development, and creative science contributed to establishing this important theme of dynamical meteorology. The second part of the review then more selectively outlines important achievements in the last two decades of how diabatic effects, in particular those related to cloud microphysics, affect the structure, dynamics, and predictability of different types of extratropical cyclones and their mesoscale substructures, upper-tropospheric blocks, Rossby waves and their interactions. A novel aspect is the relevance of research on diabatic processes for climate change research. The review closes by highlighting important implications of investigating diabatic processes in extratropical weather systems for the broader field of weather and climate dynamics, its fundamentals and representation in numerical models.

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Section: Diabatic Outflows and Forecast Errorsmentioning

confidence: 99%

The importance of diabatic processes for the dynamics of synoptic-scale extratropical weather systems—a review

Wernli,

Gray

2023

Preprint

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“…The WCB communicated the forecast error from small scales to the upper troposphere and downstream, which led to the missed onset of the block. Other studies also point to the amplification of errors in the WCBs (Pickl et al., 2023) or highlight the generation of errors in potential temperature and potential vorticity in the WCBs, which can lead to downstream errors in the Rossby wave pattern (Berman & Torn, 2019; Martínez‐Alvarado et al., 2016). Moreover, teleconnections from the Caribbean and North Pacific region affect the occurrence of large‐scale weather regimes, including blocking, in the European region (Michel & Rivière, 2011; Michel et al., 2012) and the teleconnection itself is again modulated by WCB activity (Quinting et al., 2024).…”

Section: Introductionmentioning

confidence: 99%

Why Moist Dynamic Processes Matter for the Sub‐Seasonal Prediction of Atmospheric Blocking Over Europe

Wandel,

Büeler,

Knippertz

et al. 2024

JGR Atmospheres

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In recent years, there has been growing evidence that latent heat release in midlatitude weather systems such as warm conveyor belts (WCBs) contributes significantly to the onset and maintenance of blocking anticyclones (blocked weather regimes). Still, numerical weather prediction (NWP) and climate models struggle to correctly predict and represent atmospheric blocking in particular over Europe. Here, we elucidate the representation of WCB activity in 20 years of extended winter (1997–2017) of European Centre for Medium‐Range Weather Forecast's IFS reforecasts around the onset of blocking over Europe (EuBL) employing different perspectives. First, we show that the model struggles to predict EuBL onsets already at 10–14 days lead time in line with a misrepresentation of WCB activity in the ensemble mean. However, we also find cases with accurate EuBL forecasts even in pentad 4 (15–19 days). This subset of successful forecasts at extended‐range lead times goes in line with accurate WCB forecasts over the North Atlantic several days prior to the blocking onset. Second, investigating the time‐lagged relationship of blocking onset and WCB activity, we find that WCB activity over the North Atlantic emerges well prior to the onset of the block and that different pathways into EuBL exist in the reforecasts compared to reanalysis. Finally, we find indication of predictability associated with a Rossby wave train emerging from the North Pacific. Although our study can not disentangle the roles of intrinsic predictability limits and model deficiencies, we show that correct predictions of EuBL go along with distinct patterns of WCB activity.

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“…As WCBs and their impact on the large-scale flow are sensitive to the conditions provided by the background flow (e.g. low-level moisture supply (Schäfler and Harnisch, 2015) or baroclinicity (Grams et al, 2018)), WCBs act as a dynamical hinge between the lower and upper troposphere and between the synoptic and large scale, which makes WCBs very relevant for the growth and propagation of forecast errors (Martínez-Alvarado et al, 2016;Berman and Torn, 2019;Maddison et al, 2019;Pickl et al, 2023).…”

Section: Introductionmentioning

confidence: 99%

Towards a process-oriented understanding of the impact of stochastic perturbations on the model climate

Deinhard,

Grams

2023

Preprint

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Abstract. Stochastic parametrisation techniques have been used by operational weather centres for decades to produce ensemble forecasts and to represent uncertainties of the forecast model. Their use has been demonstrated to be highly beneficial, as it increases the reliability of the forecasting system and reduces systematic biases. Despite the random nature of the perturbation techniques, the response of the model can be nonlinear and the mean state of the model can change. In this study, we attempt to provide a process-based understanding how stochastic model perturbations affect the model climate. Previous work has revealed sensitivities of the occurrence of diabatically driven, rapidly ascending air streams to the stochastically perturbed parametrisation tendencies (SPPT) scheme. Such strongly ascending air streams are linked to different weather phenomena, such as precipitation and upper-tropospheric ridge building in the midlatitudes, which raises the question whether these processes are also influenced by stochastic perturbations. First, we analyse if rapidly ascending air streams also show sensitivities to a different perturbation technique - the stochastically perturbed parametrisations (SPP) scheme, which directly represents parameter uncertainty in parametrisations and has recently been developed at the European Centre for Medium-Range Weather Forecasts (ECMWF). By running a set of sensitivity experiments with the Integrated Forecasting System (IFS) and by employing a Lagrangian detection of ascending air streams, we show that SPP results in a systematic increase of the occurrence of ascending air parcel trajectories compared to unperturbed simulations. This behaviour is very similar to that of SPPT, albeit some regional differences are apparent. We further show that the one-sided response to the stochastic forcing cannot be attributed to a single process (e.g. convection parametrisation), but rather that perturbations to different parametrisations have similar effects. Thereafter, we link the frequency changes of ascending air streams to closely related weather phenomena. Whereas the signal of increased ascending motion is directly transmitted to global precipitation sums for all analysed schemes, changes to the amplitude of the upper-level Rossby wave pattern are more subtle. In agreement with the trajectory analysis, both SPPT and SPP increase the waviness of the upper-level flow and thereby reduce a systematic bias of the model, even though the order of magnitude is small. Our study presents a coherent process chain that enables to understand how stochastic perturbations systematically affect the model climate. We argue that weather systems which are characterised by threshold behaviour on the one hand and that serve as a dynamical hinge between spatial scales on the other hand can convert zero-mean perturbations into an asymmetric response and project it onto larger scales.

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Warm conveyor belts as amplifiers of forecast uncertainty

Cited by 6 publications

References 62 publications

The importance of diabatic processes for the dynamics of synoptic-scale extratropical weather systems—a review

The importance of diabatic processes for the dynamics of synoptic-scale extratropical weather systems—a review

Why Moist Dynamic Processes Matter for the Sub‐Seasonal Prediction of Atmospheric Blocking Over Europe

Towards a process-oriented understanding of the impact of stochastic perturbations on the model climate

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