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1. Introduction An important feature of midlatitude atmospheric dynamics is the existence of upper-tropospheric Rossby waves with synoptic- to planetary-scale wavenumbers. Often a Rossby wave is not strictly circumglobal; rather, its amplitude is spatially inhomogeneous with a relative maximum at a specific location decaying to smaller values at larger distances. This gives rise to so-called Rossby wave packets [RWPs; for a recent review see Wirth et al. (2018) ]. A key feature of RWPs is the
1. Introduction An important feature of midlatitude atmospheric dynamics is the existence of upper-tropospheric Rossby waves with synoptic- to planetary-scale wavenumbers. Often a Rossby wave is not strictly circumglobal; rather, its amplitude is spatially inhomogeneous with a relative maximum at a specific location decaying to smaller values at larger distances. This gives rise to so-called Rossby wave packets [RWPs; for a recent review see Wirth et al. (2018) ]. A key feature of RWPs is the
phase. Beyond this synoptic-scale phase saturation at long lead times (beyond 2 weeks), the results of Buizza and Leutbecher (2015) indicate that there is still forecast skill for large-scale fields. To investigate error growth up to the planetary scale, we employ a complementary diagnostic that filters out phase information and identifies the envelope of the upper-level Rossby waves. This diagnostic is based on finite-amplitude local wave activity (LWA) in the primitive-equation, isentropic
phase. Beyond this synoptic-scale phase saturation at long lead times (beyond 2 weeks), the results of Buizza and Leutbecher (2015) indicate that there is still forecast skill for large-scale fields. To investigate error growth up to the planetary scale, we employ a complementary diagnostic that filters out phase information and identifies the envelope of the upper-level Rossby waves. This diagnostic is based on finite-amplitude local wave activity (LWA) in the primitive-equation, isentropic
. As in Fig. 3 , but for the N = 49 recurving TCs in the DECEL subset. Fig . 15. As in Fig. 3 , but for the N = 49 recurving TCs in the ACCEL subset. The planetary wave pattern strongly influences where the genesis of atmospheric blocking occurs. It still needs to be assessed whether significant differences in planetary-scale flow, which could be related to the different observed blocking patterns, exist between ACCEL and DECEL. For instance, the presence of significantly positive over
. As in Fig. 3 , but for the N = 49 recurving TCs in the DECEL subset. Fig . 15. As in Fig. 3 , but for the N = 49 recurving TCs in the ACCEL subset. The planetary wave pattern strongly influences where the genesis of atmospheric blocking occurs. It still needs to be assessed whether significant differences in planetary-scale flow, which could be related to the different observed blocking patterns, exist between ACCEL and DECEL. For instance, the presence of significantly positive over
for a deceleration of the flow by unresolved orography, either by modifying the roughness length or by including an orographic drag term. Here, we aim to account for the mechanical lifting caused by SSO and its effect on convective initiation with a newly developed stochastic perturbation scheme, called SSOSP. The new scheme closely follows the formulation of the PSP scheme: wind tendencies are randomly perturbed with an amplitude that scales with theoretical gravity waves excited by SSO
for a deceleration of the flow by unresolved orography, either by modifying the roughness length or by including an orographic drag term. Here, we aim to account for the mechanical lifting caused by SSO and its effect on convective initiation with a newly developed stochastic perturbation scheme, called SSOSP. The new scheme closely follows the formulation of the PSP scheme: wind tendencies are randomly perturbed with an amplitude that scales with theoretical gravity waves excited by SSO
upper-level PV structure and can strongly impact downstream development. For WCBs this has, for example, been shown by Grams et al. (2011) and Madonna et al. (2015) . Since the PV anomaly in the outflow usually acts to strengthen the already existing upper-level ridge, a systematic underestimation of low-PV transport by WCBs could contribute to the decrease in Rossby wave amplitude with increasing forecast time found by Gray et al. (2014) . In the continental warm season, only a small fraction
upper-level PV structure and can strongly impact downstream development. For WCBs this has, for example, been shown by Grams et al. (2011) and Madonna et al. (2015) . Since the PV anomaly in the outflow usually acts to strengthen the already existing upper-level ridge, a systematic underestimation of low-PV transport by WCBs could contribute to the decrease in Rossby wave amplitude with increasing forecast time found by Gray et al. (2014) . In the continental warm season, only a small fraction
1. Introduction Tropical transition (TT) describes the phenomenon when a tropical cyclone (TC) emerges from an extratropical cyclone ( Davis and Bosart 2003 , 2004 ). During TT, the extratropical cyclone transforms from a cold- to a warm-core system. A cascade of events commonly precedes the TT: anticyclonic wave breaking (e.g., Thorncroft et al. 1993 ; Postel and Hitchman 1999 ) causes an upper-level precursor potential vorticity (PV) trough to penetrate into the (sub)tropics ( Galarneau et
1. Introduction Tropical transition (TT) describes the phenomenon when a tropical cyclone (TC) emerges from an extratropical cyclone ( Davis and Bosart 2003 , 2004 ). During TT, the extratropical cyclone transforms from a cold- to a warm-core system. A cascade of events commonly precedes the TT: anticyclonic wave breaking (e.g., Thorncroft et al. 1993 ; Postel and Hitchman 1999 ) causes an upper-level precursor potential vorticity (PV) trough to penetrate into the (sub)tropics ( Galarneau et
) and http://www.cosmo-model.org for more details on the computational methods]. The model is set up with a horizontal resolution of 0.025° (about 2.5 km at 35°N) and 57 vertical levels up to 30-km height, with an enhanced vertical resolution in the planetary boundary layer. Shallow convection is parameterized using the mass-flux scheme of Tiedtke (1989) , while middle and high convection are explicitly computed. For all parameterized processes, the default setup of COSMO is used ( Doms et al
) and http://www.cosmo-model.org for more details on the computational methods]. The model is set up with a horizontal resolution of 0.025° (about 2.5 km at 35°N) and 57 vertical levels up to 30-km height, with an enhanced vertical resolution in the planetary boundary layer. Shallow convection is parameterized using the mass-flux scheme of Tiedtke (1989) , while middle and high convection are explicitly computed. For all parameterized processes, the default setup of COSMO is used ( Doms et al
structural evolution In this section the storm structure and environmental conditions in the simulation will briefly be described at four selected stages during Karl’s transition. This description provides the context for the Lagrangian analysis presented in the subsequent sections. a. Synoptic overview A synoptic overview of Karl is given by Pasch and Zelinsky (2016) : Karl originated from a low pressure system close to the Cabo Verde Islands that developed from an easterly wave crossing the west coast
structural evolution In this section the storm structure and environmental conditions in the simulation will briefly be described at four selected stages during Karl’s transition. This description provides the context for the Lagrangian analysis presented in the subsequent sections. a. Synoptic overview A synoptic overview of Karl is given by Pasch and Zelinsky (2016) : Karl originated from a low pressure system close to the Cabo Verde Islands that developed from an easterly wave crossing the west coast
of the stationwise (left) mean bias of the ensemble median and (right) mean ensemble range of the EPS as functions of the lead time. The black line indicates the average over all samples. The temporal development of the bias and ensemble range shown in Fig. 4 indicates another meteorological effect, the evening transition of the planetary boundary layer. When the sun sets, the surface and low-level air that have been heated over the course of the day cool down and thermally driven
of the stationwise (left) mean bias of the ensemble median and (right) mean ensemble range of the EPS as functions of the lead time. The black line indicates the average over all samples. The temporal development of the bias and ensemble range shown in Fig. 4 indicates another meteorological effect, the evening transition of the planetary boundary layer. When the sun sets, the surface and low-level air that have been heated over the course of the day cool down and thermally driven
