Lars Wiegand

Bio: Lars Wiegand is an academic researcher from University of Leeds. The author has contributed to research in topics: Potential vorticity & Rossby wave. The author has an hindex of 3, co-authored 3 publications receiving 196 citations. Previous affiliations of Lars Wiegand include Deutscher Wetterdienst.

The key role of diabatic processes in modifying the upper-tropospheric wave guide: a North Atlantic case-study

Abstract: This study highlights the importance of diabatic processes for the complex interaction of weather systems in the North Atlantic–European sector during the week of 7–14 September 2008. A chain of events occurred including the extratropical transition (ET) of hurricane Hanna, a subsequently developing extratropical cyclone, the formation of an upper-level potential vorticity (PV) streamer that protruded towards Europe and triggered intense rainfall, and the genesis of a Mediterranean cyclone. A PV perspective is adopted along with trajectory calculations to elucidate the diabatic modification of the midlatitude flow. Important diabatic PV modifications occurred at upper levels, associated with the cross-isentropic transport of low-PV air within warm conveyor belts (WCBs). These were diagnosed during the ET of Hanna and the development of the extratropical cyclone near Newfoundland. The WCBs contributed to the amplification of ridges downstream of each cyclone and to the subsequent elongation of Hanna's upstream trough into a PV streamer. This streamer eventually triggered the Mediterranean cyclogenesis. The second major effect of the diabatic processes occurred on smaller scales, in the low and middle troposphere. The remnants of Hanna's tropical PV core advected moist air towards the baroclinic zone leading to condensational PV production in the lower troposphere. In contrast, in the case of the extratropical cyclone, diabatic PV production occurred within its WCB at mid levels. These diagnostic analyses corroborate the potential of diabatic processes associated with extratropical flow systems for the modification of both the low-level vortices and the upper-level Rossby wave guide. Copyright © 2011 Royal Meteorological Society

Heavy Precipitation at the Alpine South Side and Saharan Dust over Central Europe: A Predictability Study Using TIGGE

Abstract: Around 26 May 2008 a pronounced potential vorticity (PV) streamer penetrated from the North Atlantic into the western Mediterranean Sea followed by widespread dust mobilization over the Maghreb region of northwest Africa and a subsequent northward transport into central Europe. At the same time, strong southerly flow over the Mediterranean Sea caused heavy precipitation and flooding at the windward side of the European Alps. Using continuous and feature-based error measures, as well as ensemble correlation techniques, this study investigates the forecast quality and predictability of synoptic and mesoscale aspects of this high-impact event in operational ensemble predictions from nine meteorological centers participating in The Observing System Research and Predictability Experiment (THORPEX) Interactive Grand Global Ensemble (TIGGE) project. TIGGE is a recently established program providing ensemble forecasts in a standardized format, which allows for an exciting new multimodel approach to investigating the predictability of, for example, high-impact weather and its dynamics. The main conclusions from this study are that 1) the quality of the PV streamer forecasts degrades with lead time showing a general tendency toward too weak Rossby wave; 2) when focusing on the region around the streamer, most models show root-meansquare errors of the same magnitude or larger than the ensemble spread (underdispersive behavior); 3) errors are reduced by about 50% if the comparison is made to each center’s own analysis instead of the ECMWF analysis; 4) peak wind speeds over the Sahara tend to be underpredicted, with differences in model formulation dominating over differences in the representation of the PV streamer; and 5) ensemble-mean multimodel forecasts of 4-day accumulated precipitation appear accurate enough for a successful severe-weather warning.

Equatorward breaking Rossby waves over the North Atlantic and Mediterranean region in the ECMWF operational Ensemble Prediction System

Abstract: The equatorward breaking of Rossby waves is a frequent feature of the synoptic-scale circulation over the North Atlantic. It often creates upper-level disturbances at low latitudes which can cause heavy precipitation in the Mediterranean region and Saharan dust outbreaks. The present study is the first to systematically explore the enormous dynamical information content of 12 years of data from the operational Ensemble Prediction System (EPS) of the European Centre for Medium-Range Weather Forecasts for this particular atmospheric feature. Dynamical precursors, forecast quality and predictability are investigated using a range of verification and analysis tools based on potential vorticity (PV). The main conclusions from this work are: (i) the EPS shows an underdispersive behaviour in the subtropics during PV streamer events; (ii) there is a tendency for too weak Rossby wave breaking and therefore a northward shift of the streamers in the forecast; (iii) strong PV streamers in the medium-range forecasts are preceded by an active wave train in the Subtropics, strongly positive PV anomalies in the Extratropics, and latent heating upstream of the PV streamer. There are no clear indications that blocking downstream is an important factor, in contrast to other studies. Analysis tools developed specifically for EPS data in this study such as ensemble correlation techniques could be applied to other atmospheric phenomena in the future.

Blocking and its Response to Climate Change

Abstract: Purpose of Review Atmospheric blocking events represent some of the most high-impact weather patterns in the mid-latitudes, yet they have often been a cause for concern in future climate projections. There has been low confidence in predicted future changes in blocking, despite relatively good agreement between climate models on a decline in blocking. This is due to the lack of a comprehensive theory of blocking and a pervasive underestimation of blocking occurrence bymodels. This paper reviews the state of knowledge regarding blocking under climate change, with the aim of providing an overview for those working in related fields. Recent Findings Several avenues have been identified by which blocking can be improved in numerical models, though a fully reliable simulation remains elusive (at least, beyond a few days lead time). Models are therefore starting to provide some useful information on how blocking and its impacts may change in the future, although deeper understanding of the processes at play will be needed to increase confidence in model projections. There are still major uncertainties regarding the processes most important to the onset, maintenance and decay of blocking and advances in our understanding of atmospheric dynamics, for example in the role of diabatic processes, continue to inform the modelling and prediction efforts. Summary The term ‘blocking’ covers a diverse array of synoptic patterns, and hence a bewildering range of indices has been developed to identify events. Results are hence not considered fully trustworthy until they have been found using several different methods. Examples of such robust results are the underestimation of blocking by models, and an overall decline in future occurrence, albeit with a complex regional and seasonal variation. In contrast, hemispheric trends in blocking over the recent historical period are not supported by different methods, and natural variability will likely dominate regional variations over the next few decades.

Warm Conveyor Belts in the ERA-Interim Dataset (1979–2010): Part I: Climatology and Potential Vorticity Evolution

Abstract: A global climatology of warm conveyor belts (WCBs) is presented for the years 1979–2010, based on trajectories calculated with Interim ECMWF Re-Analysis (ERA-Interim) data. WCB trajectories are identified as strongly ascending air parcels (600 hPa in 2 days) near extratropical cyclones. Corroborating earlier studies, WCBs are more frequent during winter than summer and they ascend preferentially in the western ocean basins between 25° and 50° latitude. Before ascending, WCB trajectories typically approach from the subtropics in summer and from more midlatitude regions in winter. Considering humidity, cloud water, and potential temperature along WCBs confirms that they experience strong condensation and integrated latent heating during the ascent (typically >20 K). Liquid and ice water contents along WCBs peak at about 700 and 550 hPa, respectively. The mean potential vorticity (PV) evolution shows typical tropospheric values near 900 hPa, followed by an increase to almost 1 potential vorticity uni...

Warm Conveyor Belts in the ERA-Interim Dataset (1979–2010). Part II: Moisture Origin and Relevance for Precipitation

Abstract: The role of moisture for extratropical atmospheric dynamics is particularly pronounced within warm conveyor belts (WCBs), which are characterized by intense latent heat release and precipitation formation. Based on the WCB climatology for the period 1979–2010 presented in Part I, two important aspects of the WCB moisture cycle are investigated: the evaporative moisture sources and the relevance of WCBs for total and extreme precipitation. The most important WCB moisture source regions are the western North Atlantic and North Pacific in boreal winter and the South Pacific and western South Atlantic in boreal summer. The strongest continental moisture source is South America. During winter, source locations are mostly local and over the ocean, and the associated surface evaporation occurs primarily during 5 days prior to the start of the WCB ascent. Long-range transport and continental moisture recycling are much more important in summer, when a substantial fraction of the evaporation occurs more th...

Influence of microphysical processes on the potential vorticity development in a warm conveyor belt: a case-study with the limited-area model COSMO

Abstract: The potential vorticity (PV) in warm conveyor belts (WCBs) is strongly influenced by the latent heating associated with the various microphysical processes occurring during the formation of clouds. The first-order effect is that PV increases below the level of maximum diabatic heating and decreases above. Thus, the WCB reaches the upper troposphere with low PV values and has the potential to influence the large-scale dynamics. In order to quantify the influence of different microphysical processes on the diabatic heating rates (DHRs) and associated PV development during the ascent, a Lagrangian analysis is used and applied to a regional model simulation of a selected WCB event. First, the individual DHRs caused by the various microphysical processes are calculated with the COSMO model. Then, the DHRs and the associated changes in PV are evaluated along the WCB trajectories. The relative role of the different microphysical processes is quantified for the latent heating and the diabatic PV modification, for which the gradient of the latent heating and the absolute vorticity are crucially important. It is shown that condensation of vapour and depositional growth of snow each contribute ∼10 K to the total latent heating. However, the diabatic PV modification due to condensation is stronger since it occurs close to the cold frontal low-level maximum of the z-component of the absolute vorticity, ηz, whereas a similar heating rate gradient caused by depositional growth of snow modifies the PV much less since it occurs in a region with much weaker ηz. This highlights the importance of ηz for determining the PV modification due to a certain diabatic heating rate. Furthermore, cooling processes like the evaporation of rain, also co-occurring with high ηz near the surface cold front, have the potential to strongly modify the PV below the WCB. Copyright © 2011 Royal Meteorological Society

Rossby Wave Packets on the Midlatitude Waveguide—A Review

Abstract: Rossby wave packets (RWPs) are Rossby waves for which the amplitude has a local maximum and decays to smaller values at larger distances. This review focuses on upper-tropospheric transient RWPs along the midlatitude jet stream. Their central characteristic is the propagation in the zonal direction as well as the transfer of wave energy from one individual trough or ridge to its downstream neighbor, a process called “downstream development.” These RWPs sometimes act as long-range precursors to extreme weather and presumably have an influence on the predictability of midlatitude weather systems. The paper reviews research progress in this area with an emphasis on developments during the last 15 years. The current state of knowledge is summarized including a discussion of the RWP life cycle as well as Rossby waveguides. Recent progress in the dynamical understanding of RWPs has been based, in part, on the development of diagnostic methods. These methods include algorithms to identify and track RWPs ...