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Christopher A. Davis
,
Sarah C. Jones
, and
Michael Riemer

Abstract

Simulations of six Atlantic hurricanes are diagnosed to understand the behavior of realistic vortices in varying environments during the process of extratropical transition (ET). The simulations were performed in real time using the Advanced Research Weather Research and Forecasting (WRF) model (ARW), using a moving, storm-centered nest of either 4- or 1.33-km grid spacing. The six simulations, ranging from 45 to 96 h in length, provide realistic evolution of asymmetric precipitation structures, implying control by the synoptic scale, primarily through the vertical wind shear.

The authors find that, as expected, the magnitude of the vortex tilt increases with increasing shear, but it is not until the shear approaches 20 m s−1 that the total vortex circulation decreases. Furthermore, the total vertical mass flux is proportional to the shear for shears less than about 20–25 m s−1, and therefore maximizes, not in the tropical phase, but rather during ET. This has important implications for predicting hurricane-induced perturbations of the midlatitude jet and its consequences on downstream predictability.

Hurricane vortices in the sample resist shear by either adjusting their vertical structure through precession (Helene 2006), forming an entirely new center (Irene 2005), or rapidly developing into a baroclinic cyclone in the presence of a favorable upper-tropospheric disturbance (Maria 2005). Vortex resiliency is found to have a substantial diabatic contribution whereby vertical tilt is reduced through reduction of the primary vortex asymmetry induced by the shear. If the shear and tilt are so large that upshear subsidence overwhelms the symmetric vertical circulation of the hurricane, latent heating and precipitation will occur to the left of the tilt vector and slow precession. Such was apparent during Wilma (2005).

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Observations of the Eyewall Structure of Typhoon Sinlaku (2008) during the Transformation Stage of Extratropical Transition

Annette M. Foerster
,
Michael M. Bell
,
Patrick A. Harr
, and
Sarah C. Jones

Abstract

A unique dataset observing the life cycle of Typhoon Sinlaku was collected during The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC) in 2008. In this study observations of the transformation stage of the extratropical transition of Sinlaku are analyzed. Research flights with the Naval Research Laboratory P-3 and the U.S. Air Force WC-130 aircraft were conducted in the core region of Sinlaku. Data from the Electra Doppler Radar (ELDORA), dropsondes, aircraft flight level, and satellite atmospheric motion vectors were analyzed with the recently developed Spline Analysis at Mesoscale Utilizing Radar and Aircraft Instrumentation (SAMURAI) software with a 1-km horizontal- and 0.5-km vertical-node spacing. The SAMURAI analysis shows marked asymmetries in the structure of the core region in the radar reflectivity and three-dimensional wind field. The highest radar reflectivities were found in the left of shear semicircle, and maximum ascent was found in the downshear left quadrant. Initial radar echos were found slightly upstream of the downshear direction and downdrafts were primarily located in the upshear semicircle, suggesting that individual cells in Sinlaku’s eyewall formed in the downshear region, matured as they traveled downstream, and decayed in the upshear region. The observed structure is consistent with previous studies of tropical cyclones in vertical wind shear, suggesting that the eyewall convection is primarily shaped by increased vertical wind shear during step 2 of the transformation stage, as was hypothesized by Klein et al. A transition from active convection upwind to stratiform precipitation downwind is similar to that found in the principal rainband of more intense tropical cyclones.

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Julian F. Quinting
,
Michael M. Bell
,
Patrick A. Harr
, and
Sarah C. Jones

Abstract

The structure and the environment of Typhoon Sinlaku (2008) were investigated during its life cycle in The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC). On 20 September 2008, during the transformation stage of Sinlaku’s extratropical transition (ET), research aircraft equipped with dual-Doppler radar and dropsondes documented the structure of the convection surrounding Sinlaku and low-level frontogenetical processes. The observational data obtained were assimilated with the recently developed Spline Analysis at Mesoscale Utilizing Radar and Aircraft Instrumentation (SAMURAI) software tool. The resulting analysis provides detailed insight into the ET system and allows specific features of the system to be identified, including deep convection, a stratiform precipitation region, warm- and cold-frontal structures, and a dry intrusion. The analysis offers valuable information about the interaction of the features identified within the transitioning tropical cyclone. The existence of dry midlatitude air above warm-moist tropical air led to strong potential instability. Quasigeostrophic diagnostics suggest that forced ascent during warm frontogenesis triggered the deep convective development in this potentially unstable environment. The deep convection itself produced a positive potential vorticity anomaly at midlevels that modified the environmental flow. A comparison of the operational ECMWF analysis and the observation-based SAMURAI analysis exhibits important differences. In particular, the ECMWF analysis does not capture the deep convection adequately. The nonexistence of the deep convection has considerable implications on the potential vorticity structure of the remnants of the typhoon at midlevels. An inaccurate representation of the thermodynamic structure of the dry intrusion has considerable implications on the frontogenesis and the quasigeostrophic forcing.

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Lisa-Ann Quandt
,
Julia H. Keller
,
Olivia Martius
, and
Sarah C. Jones

Abstract

The Euro–Russian atmospheric blocking pattern in the summer of 2010 was related to high-impact weather, including a mega–heat wave in Russia. A set of scenarios for the synoptic evolution during the onset, mature stage, and decay of the block are extracted from the THORPEX Interactive Grand Global Ensemble multimodel ensemble forecast. These scenarios represent the key features of the forecast variability of the block and of the resulting surface impacts. Two heat indices and a fire index are computed to highlight the forecast variability in societal impacts. The study is a proof of concept, showing how information about surface impacts can be derived from available operational ensemble forecasts in an effective manner, and pointing to possible difficulties in this approach. Comparing the forecast for the heat wave’s impact on large spatial domains, and on a near-gridpoint scale, identifies challenges forecasters may face when predicting the development of a heat wave.

Although the block’s onset was highly predictable, the increase in temperature and the extension of the heat-affected area differed between the scenarios. During the mature stage of the block, the variability of its western flank had a considerable influence on the precipitation and heat distribution. Since the blocking was maintained after the analyzed decay in two of three scenarios, the predictability of the decay was low in this forecast. The heat wave ended independently from the block’s decay, as the surface temperature and the impact indices decreased in all scenarios.

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Simon T. K. Lang
,
Sarah C. Jones
,
Martin Leutbecher
,
Melinda S. Peng
, and
Carolyn A. Reynolds

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.

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Hilke S. Lentink
,
Christian M. Grams
,
Michael Riemer
, and
Sarah C. Jones

Abstract

Extratropical transition (ET) can cause high-impact weather in midlatitude regions and therefore constitutes an ongoing threat at the end of a tropical cyclone’s (TC) life cycle. Most of the ET events occur over the ocean, but some TCs recurve and undergo ET along coastal regions; however, the latter category is less investigated. Typhoon Sinlaku (2008), for example, underwent ET along the southern coast of Japan. It was one of the typhoons that occurred during the T-PARC field campaign, providing unprecedented high-resolution observational data. Sinlaku is therefore an excellent case to investigate the impact of a coastal region, and in particular orography, on the evolution of ET. Here, observations from T-PARC are employed to verify high-resolution simulations of Sinlaku. In addition, a sensitivity simulation is performed with the orography of Japan removed. The presence of orography causes blocking of low-level, cool midlatitude air north of Japan. Without this inflow of cool air, ET is delayed. Only once Sinlaku moves away from the orographic barrier does the cool, dry environmental air penetrate equatorward, and ET continues. On a local scale, evaporatively cooled air from below Sinlaku’s asymmetric precipitation field could be advected toward the cyclone center when orography was favorable for it. Changes in the vortex structure, as known from mature TCs interacting with orography, were only minor due to the high translation speed during ET. This study corroborates that orography can impact ET by modulating both the synoptic-scale environmental conditions and the mesoscale cyclone structure during ET.

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Lisa-Ann Quandt
,
Julia H. Keller
,
Olivia Martius
,
Joaquim G. Pinto
, and
Sarah C. Jones

Abstract

In summer 2010, the weather conditions in the Euro-Russian sector were affected by a long-lasting atmospheric block that led to a heat wave in Russia and floods in Pakistan. Following previous studies describing the block’s predictability, the present study aims to investigate uncertainties in the upper-level wave pattern and diabatic processes that were responsible for the block’s forecast variability during its onset, mature, and decay phases. With this aim, an ensemble sensitivity analysis (ESA) is performed for three medium-range THORPEX Interactive Grand Global Ensemble multimodel ensemble forecasts, one associated with each phase of the block’s life cycle. The ESA revealed that the block’s predictability was influenced by forecast uncertainties in the general wave pattern and in the vertically integrated water vapor transport (IVT), used here as a proxy for diabatic processes. These uncertainties are associated with spatial shifts and intensity changes of synoptic waves and IVT during the whole life cycle of the block. During the onset phase, specific features include an Atlantic precursor block and the occurrence of several cyclones. During the mature stage, the blocking ridge itself was highly predictable, while forecast uncertainties in the wave pattern and in IVT primarily were associated with uncertainties in the block’s western flank. During the decay phase, the ESA signals were less intense, but the forecast variability significantly depended on the transformation of the block into a high-over-low pattern. It can be concluded that ESA is suitable to investigate the block’s forecast variability in multimodel ensembles.

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Sarah C. Jones
,
Patrick A. Harr
,
Jim Abraham
,
Lance F. Bosart
,
Peter J. Bowyer
,
Jenni L. Evans
,
Deborah E. Hanley
,
Barry N. Hanstrum
,
Robert E. Hart
,
François Lalaurette
,
Mark R. Sinclair
,
Roger K. Smith
, and
Chris Thorncroft

Abstract

A significant number of tropical cyclones move into the midlatitudes and transform into extratropical cyclones. This process is generally referred to as extratropical transition (ET). During ET a cyclone frequently produces intense rainfall and strong winds and has increased forward motion, so that such systems pose a serious threat to land and maritime activities. Changes in the structure of a system as it evolves from a tropical to an extratropical cyclone during ET necessitate changes in forecast strategies. In this paper a brief climatology of ET is given and the challenges associated with forecasting extratropical transition are described in terms of the forecast variables (track, intensity, surface winds, precipitation) and their impacts (flooding, bush fires, ocean response). The problems associated with the numerical prediction of ET are discussed. A comprehensive review of the current understanding of the processes involved in ET is presented. Classifications of extratropical transition are described and potential vorticity thinking is presented as an aid to understanding ET. Further sections discuss the interaction between a tropical cyclone and the midlatitude environment, the role of latent heat release, convection and the underlying surface in ET, the structural changes due to frontogenesis, the mechanisms responsible for precipitation, and the energy budget during ET. Finally, a summary of the future directions for research into ET is given.

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Françoise Guichard
,
Nicole Asencio
,
Christophe Peugeot
,
Olivier Bock
,
Jean-Luc Redelsperger
,
Xuefeng Cui
,
Matthew Garvert
,
Benjamin Lamptey
,
Emiliano Orlandi
,
Julia Sander
,
Federico Fierli
,
Miguel Angel Gaertner
,
Sarah C. Jones
,
Jean-Philippe Lafore
,
Andrew Morse
,
Mathieu Nuret
,
Aaron Boone
,
Gianpaolo Balsamo
,
Patricia de Rosnay
,
Bertrand Decharme
,
Philip P. Harris
, and
J.-C. Bergès

Abstract

An evaluation of precipitation and evapotranspiration simulated by mesoscale models is carried out within the African Monsoon Multidisciplinary Analysis (AMMA) program. Six models performed simulations of a mesoscale convective system (MCS) observed to cross part of West Africa in August 2005.

Initial and boundary conditions are found to significantly control the locations of rainfall at synoptic scales as simulated with either mesoscale or global models. When initialized and forced at their boundaries by the same analysis, all models forecast a westward-moving rainfall structure, as observed by satellite products. However, rainfall is also forecast at other locations where none was observed, and the nighttime northward propagation of rainfall is not well reproduced. There is a wide spread in the rainfall rates across simulations, but also among satellite products.

The range of simulated meridional fluctuations of evapotranspiration (E) appears reasonable, but E displays an overly strong zonal symmetry. Offline land surface modeling and surface energy budget considerations show that errors in the simulated E are not simply related to errors in the surface evaporative fraction, and involve the significant impact of cloud cover on the incoming surface shortwave flux.

The use of higher horizontal resolution (a few km) enhances the variability of precipitation, evapotranspiration, and precipitable water (PW) at the mesoscale. It also leads to a weakening of the daytime precipitation, less evapotranspiration, and smaller PW amounts. The simulated MCS propagates farther northward and somewhat faster within an overall drier atmosphere. These changes are associated with a strengthening of the links between PW and precipitation.

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Paolo M. Ruti
,
Oksana Tarasova
,
Julia H. Keller
,
Greg Carmichael
,
Øystein Hov
,
Sarah C. Jones
,
Deon Terblanche
,
Cheryl Anderson-Lefale
,
Ana P. Barros
,
Peter Bauer
,
Véronique Bouchet
,
Guy Brasseur
,
Gilbert Brunet
,
Phil DeCola
,
Victor Dike
,
Mariane Diop Kane
,
Christopher Gan
,
Kevin R. Gurney
,
Steven Hamburg
,
Wilco Hazeleger
,
Michel Jean
,
David Johnston
,
Alastair Lewis
,
Peter Li
,
Xudong Liang
,
Valerio Lucarini
,
Amanda Lynch
,
Elena Manaenkova
,
Nam Jae-Cheol
,
Satoru Ohtake
,
Nadia Pinardi
,
Jan Polcher
,
Elizabeth Ritchie
,
Andi Eka Sakya
,
Celeste Saulo
,
Amith Singhee
,
Ardhasena Sopaheluwakan
,
Andrea Steiner
,
Alan Thorpe
, and
Moeka Yamaji

Abstract

Whether on an urban or planetary scale, covering time scales of a few minutes or a few decades, the societal need for more accurate weather, climate, water, and environmental information has led to a more seamless thinking across disciplines and communities. This challenge, at the intersection of scientific research and society’s need, is among the most important scientific and technological challenges of our time. The “Science Summit on Seamless Research for Weather, Climate, Water, and Environment” organized by the World Meteorological Organization (WMO) in 2017, has brought together researchers from a variety of institutions for a cross-disciplinary exchange of knowledge and ideas relating to seamless Earth system science. The outcomes of the Science Summit, and the interactions it sparked, highlight the benefit of a seamless Earth system science approach. Such an approach has the potential to break down artificial barriers that may exist due to different observing systems, models, time and space scales, and compartments of the Earth system. In this context, the main future challenges for research infrastructures have been identified. A value cycle approach has been proposed to guide innovation in seamless Earth system prediction. The engagement of researchers, users, and stakeholders will be crucial for the successful development of a seamless Earth system science that meets the needs of society.

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