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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.
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.
Abstract
Synoptic-scale error growth near the tropopause is investigated from a process-based perspective. Following previous work, a potential vorticity (PV) error tendency equation is derived and partitioned into individual contributions to yield insight into the processes governing error growth near the tropopause. Importantly, we focus here on the further amplification of preexisting errors and not on the origin of errors. The individual contributions to error growth are quantified in a case study of a 6-day forecast. In this case, localized mesoscale error maxima have formed by forecast day 2. These maxima organize into a wavelike pattern and reach the Rossby wave scale around forecast day 6. Error growth occurs most prominently within the Atlantic and Pacific Rossby wave patterns. In our PV framework, the error growth is dominated by the contribution of upper-level, near-tropopause PV anomalies (near-tropopause dynamics). Significant contributions from upper-tropospheric divergent flow (prominently associated with latent heat release below) and lower-tropospheric anomalies [tropospheric-deep (i.e., baroclinic) interaction] are associated with a misrepresentation of the surface cyclone development in the forecast. These contributions are, in general, of smaller importance to error growth than near-tropopause dynamics. This result indicates that the mesoscale errors generated near the tropopause do not primarily project on differences in the subsequent baroclinic growth, but instead directly project on the tropopause evolution and amplify because of differences in the nonlinear Rossby wave dynamics.
Abstract
Synoptic-scale error growth near the tropopause is investigated from a process-based perspective. Following previous work, a potential vorticity (PV) error tendency equation is derived and partitioned into individual contributions to yield insight into the processes governing error growth near the tropopause. Importantly, we focus here on the further amplification of preexisting errors and not on the origin of errors. The individual contributions to error growth are quantified in a case study of a 6-day forecast. In this case, localized mesoscale error maxima have formed by forecast day 2. These maxima organize into a wavelike pattern and reach the Rossby wave scale around forecast day 6. Error growth occurs most prominently within the Atlantic and Pacific Rossby wave patterns. In our PV framework, the error growth is dominated by the contribution of upper-level, near-tropopause PV anomalies (near-tropopause dynamics). Significant contributions from upper-tropospheric divergent flow (prominently associated with latent heat release below) and lower-tropospheric anomalies [tropospheric-deep (i.e., baroclinic) interaction] are associated with a misrepresentation of the surface cyclone development in the forecast. These contributions are, in general, of smaller importance to error growth than near-tropopause dynamics. This result indicates that the mesoscale errors generated near the tropopause do not primarily project on differences in the subsequent baroclinic growth, but instead directly project on the tropopause evolution and amplify because of differences in the nonlinear Rossby wave dynamics.
Abstract
It has been suggested that upper-tropospheric Rossby wave packets propagating along the midlatitude waveguide may play a role for triggering severe weather. This motivates the search for robust methods to detect and track Rossby wave packets and to diagnose their properties. In the framework of several observed cases, this paper compares different methods that have been proposed for these tasks, with an emphasis on horizontal propagation and on a particular formulation of a wave activity flux previously suggested by Takaya and Nakamura. The utility of this flux is compromised by the semigeostrophic nature of upper-tropospheric Rossby waves, but this problem can partly be overcome by a semigeostrophic coordinate transformation. The wave activity flux allows one to obtain information from a single snapshot about the meridional propagation, in particular propagation from or into polar and subtropical latitudes, as well as about the onset of wave breaking. This helps to clarify the dynamics of individual wave packets in cases where other, more conventional methods provide ambiguous or even misleading information. In some cases, the “true dynamics” of the Rossby wave packet turns out to be more complex than apparent from the more conventional diagnostics, and this may have important implications for the predictability of the wave packet.
Abstract
It has been suggested that upper-tropospheric Rossby wave packets propagating along the midlatitude waveguide may play a role for triggering severe weather. This motivates the search for robust methods to detect and track Rossby wave packets and to diagnose their properties. In the framework of several observed cases, this paper compares different methods that have been proposed for these tasks, with an emphasis on horizontal propagation and on a particular formulation of a wave activity flux previously suggested by Takaya and Nakamura. The utility of this flux is compromised by the semigeostrophic nature of upper-tropospheric Rossby waves, but this problem can partly be overcome by a semigeostrophic coordinate transformation. The wave activity flux allows one to obtain information from a single snapshot about the meridional propagation, in particular propagation from or into polar and subtropical latitudes, as well as about the onset of wave breaking. This helps to clarify the dynamics of individual wave packets in cases where other, more conventional methods provide ambiguous or even misleading information. In some cases, the “true dynamics” of the Rossby wave packet turns out to be more complex than apparent from the more conventional diagnostics, and this may have important implications for the predictability of the wave packet.
Abstract
Tropospheric forcing of planetary wavenumber 2 is examined in the prephase of the major stratospheric sudden warming event in January 2009 (MSSW 2009). Because of a huge increase in Eliassen–Palm fluxes induced mainly by wavenumber 2, easterly angular momentum is transported into the Arctic stratosphere, deposited, and then decelerates the polar night jet. In agreement with earlier studies, the results reveal that the strongest eddy heat fluxes, associated with wavenumber 2, occur at 100 hPa during the prephase of MSSW 2009 in ERA-Interim. In addition, moderate conditions of the cold phase of ENSO (La Niña) contribute to the eddy heat flux anomaly. It is shown that enhanced tropospheric wave forcing over Alaska and Scandinavia is caused by tropical processes in two ways. First, in a climatological sense, La Niña contributes to an enhanced anticyclonic flow over both regions. Second, the Madden–Julian oscillation (MJO) has an indirect influence on the Alaskan ridge by enhancing eddy activity over the North Pacific. This is manifested in an increase in cyclone frequency and associated warm conveyor belt outflow, which contribute to the maintenance and amplification of the Alaskan anticyclone. The Scandinavian ridge is maintained by wave trains emanating from the Alaskan ridge propagating eastward, including an enhanced transport of eddy kinetic energy. The MSSW 2009 is an extraordinary case of how a beneficial phasing of La Niña and MJO conditions together with multiscale interactions enhances tropospheric forcing for wavenumber 2–induced zonal mean eddy heat flux in the lower stratosphere.
Abstract
Tropospheric forcing of planetary wavenumber 2 is examined in the prephase of the major stratospheric sudden warming event in January 2009 (MSSW 2009). Because of a huge increase in Eliassen–Palm fluxes induced mainly by wavenumber 2, easterly angular momentum is transported into the Arctic stratosphere, deposited, and then decelerates the polar night jet. In agreement with earlier studies, the results reveal that the strongest eddy heat fluxes, associated with wavenumber 2, occur at 100 hPa during the prephase of MSSW 2009 in ERA-Interim. In addition, moderate conditions of the cold phase of ENSO (La Niña) contribute to the eddy heat flux anomaly. It is shown that enhanced tropospheric wave forcing over Alaska and Scandinavia is caused by tropical processes in two ways. First, in a climatological sense, La Niña contributes to an enhanced anticyclonic flow over both regions. Second, the Madden–Julian oscillation (MJO) has an indirect influence on the Alaskan ridge by enhancing eddy activity over the North Pacific. This is manifested in an increase in cyclone frequency and associated warm conveyor belt outflow, which contribute to the maintenance and amplification of the Alaskan anticyclone. The Scandinavian ridge is maintained by wave trains emanating from the Alaskan ridge propagating eastward, including an enhanced transport of eddy kinetic energy. The MSSW 2009 is an extraordinary case of how a beneficial phasing of La Niña and MJO conditions together with multiscale interactions enhances tropospheric forcing for wavenumber 2–induced zonal mean eddy heat flux in the lower stratosphere.
Abstract
A tropical cyclone (TC) undergoing extratropical transition (ET) may support the amplification of a Rossby wave train in the downstream midlatitudes. Within the context of downstream baroclinic development, the TC acts as an additional source of eddy kinetic energy (
The present study uses ensemble sensitivity analysis to examine the sensitivity of downstream Rossby wave train amplification to the 
Abstract
A tropical cyclone (TC) undergoing extratropical transition (ET) may support the amplification of a Rossby wave train in the downstream midlatitudes. Within the context of downstream baroclinic development, the TC acts as an additional source of eddy kinetic energy (
The present study uses ensemble sensitivity analysis to examine the sensitivity of downstream Rossby wave train amplification to the
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.
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.
Abstract
Recurving tropical cyclones (TCs) undergoing extratropical transition (ET) may substantially modify the large-scale midlatitude flow pattern. This study highlights the role of diabatic outflow in midlatitude flow amplification within the context of a review of the physical and dynamical processes involved in ET. Composite fields of 12 western North Pacific ET cases are used as initial and boundary conditions for high-resolution numerical simulations of the North Pacific–North American sector with and without the TC present. It is demonstrated that a three-stage sequence of diabatic outflow associated with different weather systems is involved in triggering a highly amplified midlatitude flow pattern: 1) preconditioning by a predecessor rain event (PRE), 2) TC–extratropical flow interaction, and 3) downstream flow amplification by a downstream warm conveyor belt (WCB). An ensemble of perturbed simulations demonstrates the robustness of these stages. Beyond earlier studies investigating PREs, recurving TCs, and WCBs individually, here the fact that each impacts the midlatitude flow through a similar sequence of processes surrounding ET is highlighted. Latent heat release in rapidly ascending air leads to a net transport of low-PV air into the upper troposphere. Negative PV advection by the diabatically driven outflow initiates ridge building, accelerates and anchors a midlatitude jet streak, and overall amplifies the upper-level Rossby wave pattern. However, the three weather systems markedly differ in terms of the character of diabatic heating and associated outflow height, with the TC outflow reaching highest and the downstream WCB outflow producing the strongest negative PV anomaly.
Abstract
Recurving tropical cyclones (TCs) undergoing extratropical transition (ET) may substantially modify the large-scale midlatitude flow pattern. This study highlights the role of diabatic outflow in midlatitude flow amplification within the context of a review of the physical and dynamical processes involved in ET. Composite fields of 12 western North Pacific ET cases are used as initial and boundary conditions for high-resolution numerical simulations of the North Pacific–North American sector with and without the TC present. It is demonstrated that a three-stage sequence of diabatic outflow associated with different weather systems is involved in triggering a highly amplified midlatitude flow pattern: 1) preconditioning by a predecessor rain event (PRE), 2) TC–extratropical flow interaction, and 3) downstream flow amplification by a downstream warm conveyor belt (WCB). An ensemble of perturbed simulations demonstrates the robustness of these stages. Beyond earlier studies investigating PREs, recurving TCs, and WCBs individually, here the fact that each impacts the midlatitude flow through a similar sequence of processes surrounding ET is highlighted. Latent heat release in rapidly ascending air leads to a net transport of low-PV air into the upper troposphere. Negative PV advection by the diabatically driven outflow initiates ridge building, accelerates and anchors a midlatitude jet streak, and overall amplifies the upper-level Rossby wave pattern. However, the three weather systems markedly differ in terms of the character of diabatic heating and associated outflow height, with the TC outflow reaching highest and the downstream WCB outflow producing the strongest negative PV anomaly.
Abstract
Stochastic perturbations allow for the representation of small-scale variability due to unresolved physical processes. However, the properties of this variability depend on model resolution and weather regime. A physically based method is presented for introducing stochastic perturbations into kilometer-scale atmospheric models that explicitly account for these dependencies. The amplitude of the perturbations is based on information obtained from the model’s subgrid turbulence parameterization, while the spatial and temporal correlations are based on physical length and time scales of the turbulent motions. The stochastic perturbations lead to triggering of additional convective cells and improved precipitation amounts in simulations of two days with weak synoptic forcing of convection but different amounts of precipitation. The perturbations had little impact in a third case study, where precipitation was mainly associated with a cold front. In contrast, an unphysical version of the scheme with constant perturbation amplitude performed poorly since there was no perturbation amplitude that would give improved amounts of precipitation during the day without generating spurious convection at other times.
Abstract
Stochastic perturbations allow for the representation of small-scale variability due to unresolved physical processes. However, the properties of this variability depend on model resolution and weather regime. A physically based method is presented for introducing stochastic perturbations into kilometer-scale atmospheric models that explicitly account for these dependencies. The amplitude of the perturbations is based on information obtained from the model’s subgrid turbulence parameterization, while the spatial and temporal correlations are based on physical length and time scales of the turbulent motions. The stochastic perturbations lead to triggering of additional convective cells and improved precipitation amounts in simulations of two days with weak synoptic forcing of convection but different amounts of precipitation. The perturbations had little impact in a third case study, where precipitation was mainly associated with a cold front. In contrast, an unphysical version of the scheme with constant perturbation amplitude performed poorly since there was no perturbation amplitude that would give improved amounts of precipitation during the day without generating spurious convection at other times.
Abstract
An accurate representation of synoptic-scale Rossby waves in numerical weather forecast models is very important as these waves are closely linked to weather formation at the surface. Enhanced potential vorticity (PV) gradients at the tropopause levels act as waveguides for synoptic-scale Rossby waves, so spatial errors in the waveguides imply errors in the amplification and propagation of Rossby waves. This paper focuses on evaluating the forecast representation of these waveguides and presents an object-based forecast verification tool. In both forecast and the verification data, Rossby waveguide objects are defined based on enhanced PV gradient fields on isentropic surfaces. The tool automatically pairs the complex objects, compares their properties, and assesses the number of objects without a matching partner in either the forecast or the reanalysis. In the last step, error measures are calculated for the area and the location of the objects. As proof-of-concept application of the method for the year 2008, five lead times of the Integrated Forecast System (IFS) from the ECMWF are compared with the ECMWF reanalysis dataset. The majority of the waveguide objects are found to be in the correct position, and there are no systematic positional errors; however, the forecast objects and hence the areas of enhanced PV gradients are smaller.
Abstract
An accurate representation of synoptic-scale Rossby waves in numerical weather forecast models is very important as these waves are closely linked to weather formation at the surface. Enhanced potential vorticity (PV) gradients at the tropopause levels act as waveguides for synoptic-scale Rossby waves, so spatial errors in the waveguides imply errors in the amplification and propagation of Rossby waves. This paper focuses on evaluating the forecast representation of these waveguides and presents an object-based forecast verification tool. In both forecast and the verification data, Rossby waveguide objects are defined based on enhanced PV gradient fields on isentropic surfaces. The tool automatically pairs the complex objects, compares their properties, and assesses the number of objects without a matching partner in either the forecast or the reanalysis. In the last step, error measures are calculated for the area and the location of the objects. As proof-of-concept application of the method for the year 2008, five lead times of the Integrated Forecast System (IFS) from the ECMWF are compared with the ECMWF reanalysis dataset. The majority of the waveguide objects are found to be in the correct position, and there are no systematic positional errors; however, the forecast objects and hence the areas of enhanced PV gradients are smaller.
Abstract
Many studies have highlighted the importance of recurving tropical cyclones (TCs) in triggering Rossby waves. This study investigates the impact of western North Pacific (WNP), south Indian Ocean, and North Atlantic recurving TCs on the amplitude and frequency of synoptic-scale Rossby wave packets (RWPs) over a 30-yr period. The results indicate a significant increase of RWP frequency downstream of WNP and south Indian Ocean TCs. A statistically significant RWP amplitude anomaly downstream of these TCs suggests that RWPs, which are associated with TCs, are stronger than those that generally occur in midlatitudes. North Atlantic TCs do not seem to be associated with a statistically significant increase in RWP frequency and amplitude downstream.
Processes that contribute to Rossby wave amplification are identified by creating composites for WNP TCs with and without downstream development. Potential vorticity, eddy kinetic energy, and quasigeostrophic forcing diagnostics highlight dynamical mechanisms that contribute to the synergistic interaction between the TC and the midlatitude flow. The existence of an upstream Rossby wave favors a downstream development. Diabatically enhanced upper-level divergent flow that can be attributed to the nonlinear interaction between the TC and the midlatitude flow impedes the eastward propagation of the upstream trough, amplifies the downstream ridge, and intensifies the jet. The amplified midlatitude flow provides upper-level forcing, which helps to maintain the predominantly diabatically driven divergent flow.
Forecast uncertainties that are related to these complex TC–midlatitude flow interactions may spread into downstream regions. A climatological analysis of ensemble reforecast data emphasizes the importance of TC–midlatitude flow interactions and Rossby wave amplification on downstream predictability.
Abstract
Many studies have highlighted the importance of recurving tropical cyclones (TCs) in triggering Rossby waves. This study investigates the impact of western North Pacific (WNP), south Indian Ocean, and North Atlantic recurving TCs on the amplitude and frequency of synoptic-scale Rossby wave packets (RWPs) over a 30-yr period. The results indicate a significant increase of RWP frequency downstream of WNP and south Indian Ocean TCs. A statistically significant RWP amplitude anomaly downstream of these TCs suggests that RWPs, which are associated with TCs, are stronger than those that generally occur in midlatitudes. North Atlantic TCs do not seem to be associated with a statistically significant increase in RWP frequency and amplitude downstream.
Processes that contribute to Rossby wave amplification are identified by creating composites for WNP TCs with and without downstream development. Potential vorticity, eddy kinetic energy, and quasigeostrophic forcing diagnostics highlight dynamical mechanisms that contribute to the synergistic interaction between the TC and the midlatitude flow. The existence of an upstream Rossby wave favors a downstream development. Diabatically enhanced upper-level divergent flow that can be attributed to the nonlinear interaction between the TC and the midlatitude flow impedes the eastward propagation of the upstream trough, amplifies the downstream ridge, and intensifies the jet. The amplified midlatitude flow provides upper-level forcing, which helps to maintain the predominantly diabatically driven divergent flow.
Forecast uncertainties that are related to these complex TC–midlatitude flow interactions may spread into downstream regions. A climatological analysis of ensemble reforecast data emphasizes the importance of TC–midlatitude flow interactions and Rossby wave amplification on downstream predictability.
