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

Abstract The 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 first, leading to the formation of new WCBs.

Download Full-text

The Impact of Initial Condition and Warm Conveyor Belt Forecast Uncertainty on Variability in the Downstream Waveguide in an ECWMF Case Study

Monthly Weather Review ◽

10.1175/mwr-d-18-0333.1 ◽

2019 ◽

Vol 147 (11) ◽

pp. 4071-4089 ◽

Cited By ~ 3

Author(s):

Jeremy D. Berman ◽

Ryan D. Torn

Keyword(s):

Conveyor Belt ◽

Forecast Uncertainty ◽

Ensemble Forecasts ◽

Rossby Wave Breaking ◽

Weather Forecasts ◽

The North ◽

Weather Events ◽

High Impact Weather ◽

Sensitivity Method ◽

The Impact

Abstract Perturbations to the potential vorticity (PV) waveguide, which can result from latent heat release within the warm conveyor belt (WCB) of midlatitude cyclones, can lead to the downstream radiation of Rossby waves, and in turn high-impact weather events. Previous studies have hypothesized that forecast uncertainty associated with diabatic heating in WCBs can result in large downstream forecast variability; however, these studies have not established a direct connection between the two. This study evaluates the potential impact of latent heating variability in the WCB on subsequent downstream forecasts by applying the ensemble-based sensitivity method to European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble forecasts of a cyclogenesis event over the North Atlantic. For this case, ensemble members with a more amplified ridge are associated with greater negative PV advection by the irrotational wind, which is associated with stronger lower-tropospheric southerly moisture transport east of the upstream cyclone in the WCB. This transport is sensitive to the pressure trough to the south of the cyclone along the cold front, which in turn is modulated by earlier differences in the motion of the air masses on either side of the front. The position of the cold air behind the front is modulated by upstream tropopause-based PV anomalies, such that a deeper pressure trough is associated with a more progressive flow pattern, originating from Rossby wave breaking over the North Pacific. Overall, these results suggest that more accurate forecasts of upstream PV anomalies and WCBs may reduce forecast uncertainty in the downstream waveguide.

Download Full-text

Three-dimensional visualization of ensemble weather forecasts – Part 2: Forecasting warm conveyor belt situations for aircraft-based field campaigns

Geoscientific Model Development ◽

10.5194/gmd-8-2355-2015 ◽

2015 ◽

Vol 8 (7) ◽

pp. 2355-2377 ◽

Cited By ~ 6

Author(s):

M. Rautenhaus ◽

C. M. Grams ◽

A. Schäfler ◽

R. Westermann

Keyword(s):

System Integration ◽

Visual Analysis ◽

Weather Forecasting ◽

Three Dimensional ◽

Conveyor Belt ◽

Ensemble Prediction ◽

Wind Fields ◽

Uncertainty Visualization ◽

Ensemble Prediction System ◽

Weather Forecasts

Abstract. We present the application of interactive three-dimensional (3-D) visualization of ensemble weather predictions to forecasting warm conveyor belt situations during aircraft-based atmospheric research campaigns. Motivated by forecast requirements of the T-NAWDEX-Falcon 2012 (THORPEX – North Atlantic Waveguide and Downstream Impact Experiment) campaign, a method to predict 3-D probabilities of the spatial occurrence of warm conveyor belts (WCBs) has been developed. Probabilities are derived from Lagrangian particle trajectories computed on the forecast wind fields of the European Centre for Medium Range Weather Forecasts (ECMWF) ensemble prediction system. Integration of the method into the 3-D ensemble visualization tool Met.3D, introduced in the first part of this study, facilitates interactive visualization of WCB features and derived probabilities in the context of the ECMWF ensemble forecast. We investigate the sensitivity of the method with respect to trajectory seeding and grid spacing of the forecast wind field. Furthermore, we propose a visual analysis method to quantitatively analyse the contribution of ensemble members to a probability region and, thus, to assist the forecaster in interpreting the obtained probabilities. A case study, revisiting a forecast case from T-NAWDEX-Falcon, illustrates the practical application of Met.3D and demonstrates the use of 3-D and uncertainty visualization for weather forecasting and for planning flight routes in the medium forecast range (3 to 7 days before take-off).

Download Full-text

Effect of anthropogenic aerosol emissions on precipitation in warm conveyor belts in the western North Pacific in winter – a model study with ECHAM6-HAM

10.5194/acp-2016-722 ◽

2016 ◽

Cited By ~ 1

Author(s):

Hanna Joos ◽

Erica Madonna ◽

Kasja Witlox ◽

Sylvaine Ferrachat ◽

Heini Wernli ◽

...

Keyword(s):

North Pacific ◽

Aerosol Particles ◽

Conveyor Belt ◽

Radiation Balance ◽

Anthropogenic Aerosol ◽

Model Study ◽

Conveyor Belts ◽

Winter Time ◽

Warm Conveyor Belts ◽

Aerosol Emissions

Abstract. While there is a clear impact of aerosol particles on the radiation balance, whether and how aerosol particles influence precipitation is controversial. Here we use the ECHAM6-HAM global cli- mate model coupled to an aerosol module to analyse whether an impact of anthropogenic aerosol particles on the timing and the amount of precipitation from warm conveyor belts in low pressure systems in the winter time North Pacific can be detected. We conclude that while polluted warm con- veyor belt trajectories start with 5–10 times higher black carbon concentrations, the overall amount of precipitation is comparable in pre-industrial and present-day conditions. Precipitation formation is however supressed in the most polluted warm conveyor belt trajectories.

Download Full-text

Sensitivity, Structure, and Dynamics of Singular Vectors Associated with Hurricane Helene (2006)

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-11-048.1 ◽

2012 ◽

Vol 69 (2) ◽

pp. 675-694 ◽

Cited By ~ 15

Author(s):

Simon T. K. Lang ◽

Sarah C. Jones ◽

Martin Leutbecher ◽

Melinda S. Peng ◽

Carolyn A. Reynolds

Keyword(s):

Three Dimensional ◽

Growth Dynamics ◽

Ensemble Forecasts ◽

Weather Forecasts ◽

Singular Vectors ◽

Medium Range ◽

Flow Features ◽

Forecasting System ◽

Diabatic Processes ◽

The Impact

Abstract The sensitivity of singular vectors (SVs) associated with Hurricane Helene (2006) to resolution and diabatic processes is investigated. Furthermore, the dynamics of their growth are analyzed. The SVs are calculated using the tangent linear and adjoint model of the integrated forecasting system (IFS) of the European Centre for Medium-Range Weather Forecasts with a spatial resolution up to TL255 (~80 km) and 48-h optimization time. The TL255 moist (diabatic) SVs possess a three-dimensional spiral structure with significant horizontal and vertical upshear tilt within the tropical cyclone (TC). Also, their amplitude is larger than that of dry and lower-resolution SVs closer to the center of Helene. Both higher resolution and diabatic processes result in stronger growth being associated with the TC compared to other flow features. The growth of the SVs in the vicinity of Helene is associated with baroclinic and barotropic mechanisms. The combined effect of higher resolution and diabatic processes leads to significant differences of the SV structure and growth dynamics within the core and in the vicinity of the TC. If used to initialize ensemble forecasts with the IFS, the higher-resolution moist SVs cause larger spread of the wind speed, track, and intensity of Helene than their lower-resolution or dry counterparts. They affect the outflow of the TC more strongly, resulting in a larger downstream impact during recurvature. Increasing the resolution or including diabatic effects degrades the linearity of the SVs. While the impact of diabatic effects on the linearity is small at low resolution, it becomes large at high resolution.

Download Full-text

Atmospheric processes triggering the Central European floods in June 2013

Natural Hazards and Earth System Sciences Discussions ◽

10.5194/nhessd-2-427-2014 ◽

2014 ◽

Vol 2 (1) ◽

pp. 427-458 ◽

Cited By ~ 6

Author(s):

C. M. Grams ◽

H. Binder ◽

S. Pfahl ◽

N. Piaget ◽

H. Wernli

Keyword(s):

Central Europe ◽

Large Scale ◽

Heavy Precipitation ◽

Economic Loss ◽

Precipitation Event ◽

Rossby Wave Breaking ◽

Atmospheric Processes ◽

The North ◽

Upper Level ◽

Warm Conveyor Belts

Abstract. In June 2013 Central Europe was hit by a century flood affecting the Danube and Elbe catchments after a 4 day period of heavy precipitation and causing severe human and economic loss. In this study model analysis and observational data are investigated to reveal the key atmospheric processes that caused the heavy precipitation event. The period preceeding the flood was characterised by a weather regime associated with cool and unusual wet conditions resulting from repeated Rossby wave breaking (RWB). During the event a single RWB established a reversed baroclinicity in the low to mid troposphere in Central Europe with cool air trapped over the Alps and warmer air to the North. The upper-level cut-off resulting from the RWB instigated three consecutive cyclones in eastern Europe that unusually tracked westward during the days of heavy precipitation. Continuous large-scale slantwise ascent in so-called "upside down" warm conveyor belts (WCBs) associated with these cyclones is found as the key process that caused the 4 day heavy precipitation period. Fed by moisture sources from continental evapotranspiration, these WCBs unusually ascended equatorward along the southward sloping moist isentropes. Although "upside down" WCBs are climatologically rare events, they have great potential for causing high impact weather.

Download Full-text

The role of cloud diabatic processes in the life cycle of Atlantic-European weather regimes

10.5194/egusphere-egu21-14238 ◽

2021 ◽

Author(s):

Christian M. Grams

Keyword(s):

Life Cycle ◽

Large Scale ◽

Multiple Scales ◽

Weather Prediction ◽

Time Lag ◽

Conveyor Belt ◽

Life Cycles ◽

Weather Regimes ◽

Diabatic Processes

Weather regimes are quasi-stationary, persistent, and recurrent states of the large-scale extratropical circulation. In the Atlantic-European region these explain most of the atmospheric variability on sub-seasonal time scales. However, current numerical weather prediction (NWP) systems struggle in correctly predicting weather regime life cycles. Latent heat release in ascending air streams injects air into the upper troposphere, which might ultimately result in blocking. Such diabatic outflow is often linked to warm conveyor belt (WCB) activity and has been shown to be involved in upscale error growth up to the regime scale. This study systematically investigates the role of diabatic outflow in the life cycle of Atlantic-European weather regimes.An extended definition of 7 year-round Atlantic-European weather regimes from 37 years of ERA-Interim reanalysis is used. This is based on an EOF analysis and k-means clustering of normalized low-pass-filtered 500hPa geopotential height anomalies. Furthermore an objective regime life cycle is derived. The role of cloud-diabatic processes in European weather regimes is assessed based on time lag analysis of WCB activity at specific life cycle stages.Results indicate that the period prior to regime onset is characterized by important changes in location and frequency of WCB occurrence. Most importantly, prior to the onset of regimes characterized by blocking, WCB activity increases significantly upstream of the incipient blocking even before blocking is detectable and persists over the blocked region later. This suggests that diabatic WCB outflow helps to establish and maintain blocked regimes. Thus it is important to correctly represent cloud-diabatic processes in NWP models across multiple scales in order to predict the large-scale circulation accurately. Ongoing work now systematically investigates the representation of WCB activity in current NWP systems and how this relates to the forecast skill for weather regimes.

Download Full-text

An Examination of Tropical Cyclone Position, Intensity, and Intensity Life Cycle within Atmospheric Reanalysis Datasets

Journal of Climate ◽

10.1175/2011jcli4208.1 ◽

2012 ◽

Vol 25 (10) ◽

pp. 3453-3475 ◽

Cited By ~ 83

Author(s):

Benjamin A. Schenkel ◽

Robert E. Hart

Keyword(s):

Life Cycle ◽

North Pacific ◽

Strongly Correlated ◽

Sea Level Pressure ◽

Climate Forecast System Reanalysis ◽

Weather Forecasts ◽

The North ◽

Medium Range ◽

The Mean ◽

Modern Era

Abstract The following study examines the position and intensity differences of tropical cyclones (TCs) among the Best-Track and five atmospheric reanalysis datasets to evaluate the degree to which reanalyses are appropriate for studying TCs. While significant differences are found in both reanalysis TC intensity and position, the representation of TC intensity within reanalyses is found to be most problematic owing to its underestimation beyond what can be attributed solely to the coarse grid resolution. Moreover, the mean life cycle of normalized TC intensity within reanalyses reveals an underestimation of both prepeak intensification rates as well as a delay in peak intensity relative to the Best-Track. These discrepancies between Best-Track and reanalysis TC intensity and position can further be described through correlations with such parameters as Best-Track TC age, Best-Track TC intensity, Best-Track TC location, and the extended Best-Track TC size. Specifically, TC position differences within the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40), ECMWF Interim Re-Analysis (ERA-I), and Modern Era Retrospective-Analysis for Research and Applications (MERRA) exhibit statistically significant correlations (0.27 ≤ R ≤ 0.38) with the proximity of TCs to observation dense areas in the North Atlantic (NATL) and western North Pacific (WPAC). Reanalysis TC intensity is found to be most strongly correlated with Best-Track TC size (0.53 ≤ R ≤ 0.70 for maximum 10-m wind speed; −0.71 ≤ R ≤ −0.53 for minimum mean sea level pressure) while exhibiting smaller, yet significant, correlations with Best-Track TC age, Best-Track TC intensity, and Best-Track TC latitude. Of the three basins examined, the eastern North Pacific (EPAC) has the largest reanalysis TC position differences and weakest intensities possibly due to a relative dearth of observations, the strong nearby terrain gradient, and the movement of TCs away from the most observation dense portion of the basin over time. The smaller mean Best-Track size and shorter mean lifespan of Best-Track EPAC TCs may also yield weaker reanalysis TC intensities. Of the five reanalyses, the smaller position differences and stronger intensities found in the Climate Forecast System Reanalysis (CFSR) and Japanese 25-year Reanalysis (JRA-25) are attributed to the use of vortex relocation and TC wind profile retrievals, respectively. The discrepancies in TC position between the Best-Track and reanalyses combined with the muted magnitude of TC intensity and its partially nonphysical life cycle within reanalyses suggests that caution should be exercised when utilizing these datasets for studies that rely either on TC intensity (raw or normalized) or track. Finally, several cases of nonphysical TC structure also argue that further work is needed to improve TC representation while implying that studies focusing solely on TC intensity and track do not necessarily extend to other aspects of TC representation.

Download Full-text

A portrayal of an orographic Warm Conveyor Belt using observations from aircraft, lidar and radar

10.5194/egusphere-egu2020-18794 ◽

2020 ◽

Author(s):

Maxi Boettcher ◽

Andreas Schäfler ◽

Harald Sodemann ◽

Michael Sprenger ◽

Stefan Kaufmann ◽

...

Keyword(s):

Western Europe ◽

Field Experiments ◽

Marine Boundary Layer ◽

Conveyor Belt ◽

Western Mediterranean ◽

Air Mass ◽

Conveyor Belts ◽

The Alps ◽

The Baltic ◽

Warm Conveyor Belts

Warm conveyor belts (WCBs) are important airstreams in extratropical cyclones, leading to the formation of intense precipitation and the transport of substantial amounts of water vapour upward and poleward. This study presents a scenario of a WCB that ascended from western Europe towards the Baltic Sea using aircraft, lidar and radar observations from the field experiments HyMeX and T-NAWDEX-Falcon in October 2012. Trajectories based on the ensemble data assimilation system of the ECMWF are used to quantify probabilistically the occurrence of the WCB and Lagrangian matches between different observations. Despite severe limitations for research flights over Europe, the DLR Falcon successfully sampled WCB air masses during different phases of the ascent. The overall picture of the WCB trajectories revealed measurements in several WCB branches: trajectories that ascended from the East Atlantic over northern France while others had their inflow in the western Mediterranean region and passed across the Alps. For the latter ones, Lagrangian matches coincidentally occurred between lidar water vapour measurements in the inflow of the WCB south, radar measurements during the ascent at and its outflow north of the Alps during a mid-tropospheric flight leg over Germany. The comparison of observations and ensemble analyses reveals a moist bias of the analyses in parts of the WCB inflow and an underestimation of cloud water species in the WCB during ascent. In between, the radar instrument measured strongly precipitating WCB air mass with embedded linking trajectories directly above the melting layer while orographically ascending at the southern slops of the Alps. An inert tracer air mass could confirm the long pathway of WCB air from the inflow in the marine boundary layer until the outflow in the upper troposhpere near the Baltic sea several hours later. This case study illustrates the complexity of the interaction of WCBs with the Alpine topography, which leads to (i) various pathways over and around the Alpine crest and (ii) locally steep WCB ascent with increased cloud content that might result in enhancement of precipitation where the WCB flows over the Alps. The combination of observational data and detailed ensemble-based trajectory calculations reveals important aspects of orographically-modified WCBs.

Download Full-text

3-D visualization of ensemble weather forecasts – Part 2: Forecasting warm conveyor belt situations for aircraft-based field campaigns

Geoscientific Model Development Discussions ◽

10.5194/gmdd-8-2161-2015 ◽

2015 ◽

Vol 8 (2) ◽

pp. 2161-2212 ◽

Cited By ~ 2

Author(s):

M. Rautenhaus ◽

C. M. Grams ◽

A. Schäfler ◽

R. Westermann

Keyword(s):

System Integration ◽

Visual Analysis ◽

Weather Forecasting ◽

Conveyor Belt ◽

Ensemble Prediction ◽

Wind Fields ◽

Uncertainty Visualization ◽

Ensemble Prediction System ◽

Conveyor Belts ◽

Warm Conveyor Belts

Abstract. We present the application of interactive 3-D visualization of ensemble weather predictions to forecasting warm conveyor belt situations during aircraft-based atmospheric research campaigns. Motivated by forecast requirements of the T-NAWDEX-Falcon 2012 campaign, a method to predict 3-D probabilities of the spatial occurrence of warm conveyor belts has been developed. Probabilities are derived from Lagrangian particle trajectories computed on the forecast wind fields of the ECMWF ensemble prediction system. Integration of the method into the 3-D ensemble visualization tool Met.3D, introduced in the first part of this study, facilitates interactive visualization of WCB features and derived probabilities in the context of the ECMWF ensemble forecast. We investigate the sensitivity of the method with respect to trajectory seeding and forecast wind field resolution. Furthermore, we propose a visual analysis method to quantitatively analyse the contribution of ensemble members to a probability region and, thus, to assist the forecaster in interpreting the obtained probabilities. A case study, revisiting a forecast case from T-NAWDEX-Falcon, illustrates the practical application of Met.3D and demonstrates the use of 3-D and uncertainty visualization for weather forecasting and for planning flight routes in the medium forecast range (three to seven days before take-off).

Download Full-text

Warm Conveyor Belts and Their Role for Cloud Radiative Forcing in the Extratropical Storm Tracks

Journal of Climate ◽

10.1175/jcli-d-18-0802.1 ◽

2019 ◽

Vol 32 (16) ◽

pp. 5325-5343 ◽

Cited By ~ 2

Author(s):

Hanna Joos

Keyword(s):

Radiative Forcing ◽

Climate Models ◽

Storm Tracks ◽

Shortwave Radiation ◽

Ice Clouds ◽

Cloud Band ◽

Cloud Radiative Forcing ◽

The North ◽

Conveyor Belts ◽

Warm Conveyor Belts

Abstract The link between cloud radiative forcing (CRF) and warm conveyor belts (WCBs), which are strongly ascending airstreams in extratropical cyclones, is investigated based on ERA-Interim reanalysis from 1979 to 2011. Clouds associated with WCBs can be liquid, mixed phase, or ice clouds. They interact with the longwave and shortwave radiation in different ways and thus strongly influence Earth’s radiative budget in the extratropical storm tracks in a complex way. In this study, WCBs are identified with a Lagrangian method, where WCBs are represented by trajectories that rise at least 600 hPa in 48 h in the vicinity of an extratropical cyclone, and CRF is traced along all WCB trajectories during the considered 30-yr period. The results show that due to the poleward ascent of WCBs, they exhibit negative net cloud forcing (NetCRF) in the southern part of the associated cloud band, whereas in their northern part, NetCRF gets positive due to the lack of sunlight in the winter months. This nonuniform CRF along WCBs from low to high latitudes increases the meridional NetCRF gradient. Furthermore, in their outflow regions in the North Atlantic, where WCBs are mainly associated with ice clouds, WCBs contribute up to 10 W m−2 to the global climatological NetCRF maximum in winter. The results highlight the importance of WCBs in modulating the radiative budget in the extratropics. Furthermore, the results emphasize the need for a correct representation of WCBs in climate models to correctly simulate the cloud–circulation coupling.

Download Full-text