Erica Madonna

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

AbstractA 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

AbstractThe 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...

Global Relationship between Fronts and Warm Conveyor Belts and the Impact on Extreme Precipitation

AbstractExtratropical cyclones are responsible for many extreme precipitation events in the midlatitudes. Warm conveyor belts (WCBs) and fronts are known to be related to the uplift and hence the precipitation within cyclones. The authors have investigated the link between WCBs and fronts and how such a link impacts the occurrence of extreme precipitation events. WCB trajectories have been calculated from the ERA-Interim dataset, and low-level (below 790 hPa) and midlevel (790–600 hPa) WCBs have been considered. These have been matched with objectively identified fronts (i.e., characterized by an overlap of WCB and front somewhere along the front). About 10% of cold fronts, 8% of warm fronts (identified using a thermal criterion), and 15% of wind fronts (identified using a wind shift method) are matched with WCBs, while up to 70% of WCBs are matched with fronts. Some WCBs, especially in the Southern Hemisphere, are not matched with either type of front (up to 70% east of Australia). The relationship betwe...

On the Co-Occurrence of Warm Conveyor Belt Outflows and PV Streamers*

AbstractThe co-occurrence of warm conveyor belts (WCBs), strongly ascending moist airstreams in extratropical cyclones, and stratospheric potential vorticity (PV) streamers, indicators for breaking Rossby waves on the tropopause, is investigated for a 21-yr period in the Northern Hemisphere using Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data. WCB outflows and PV streamers are respectively identified as two- and three-dimensional objects and tracked during their life cycle. PV streamers are more frequent than WCB outflows and nearly 15% of all PV streamers co-occur with WCBs during their life cycle, whereas about 60% of all WCB outflows co-occur with PV streamers. Co-occurrences are most frequent over the North Atlantic and North Pacific in spring and winter. WCB outflows are often located upstream of the PV streamers and form earlier, indicating the importance of diabatic processes for downstream Rossby wave breaking. Less frequently, PV streamers occur ...

Daily to Decadal Modulation of Jet Variability

AbstractThe variance of a jet’s position in latitude is found to be related to its average speed: when a jet becomes stronger its variability in latitude decreases. This relationship is shown to hold for observed midlatitude jets around the world and also across a hierarchy of numerical models. North Atlantic jet variability is shown to be modulated on decadal timescales, with decades of a strong, steady jet being interspersed with decades of a weak, variable jet. These modulations are also related to variations in the basin-wide occurrence of high-impact blocking events. A picture emerges of complex multidecadal jet variability in which recent decades do not appear unusual. We propose an underlying barotropic mechanism to explain this behaviour, related to the change in refractive properties of a jet as it strengthens, and the subsequent effect on the distribution of Rossby wave breaking.

Global Climatologies of Eulerian and Lagrangian Flow Features based on ERA-Interim

AbstractThis paper introduces a newly compiled set of feature-based climatologies identified from ERA-Interim (1979–2014). Two categories of flow features are considered: (i) Eulerian climatologies of jet streams, tropopause folds, surface fronts, cyclones and anticyclones, blocks, and potential vorticity streamers and cutoffs and (ii) Lagrangian climatologies, based on a large ensemble of air parcel trajectories, of stratosphere–troposphere exchange, warm conveyor belts, and tropical moisture exports. Monthly means of these feature climatologies are openly available at the ETH Zurich web page (http://eraiclim.ethz.ch) and are annually updated. Datasets at higher resolution can be obtained from the authors on request. These feature climatologies allow studying the frequency, variability, and trend of atmospheric phenomena and their interrelationships across temporal scales. To illustrate the potential of this dataset, boreal winter climatologies of selected features are presented and, as a first applicati...