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Christian M. Grams and Heather M. Archambault

), where vertically deep ascent occurs in the lower half of the troposphere that becomes more slantwise along the (moist) isentropes at upper levels. This ascent is supported by upper-level forcing (cf. Fig. 7b ), which is characteristic of PRE convection. Latent heat release (not shown) results in diabatically produced positive PV at lower levels just ahead of the baroclinic zone and marks the center of the PRE (2200 km; Fig. 8a ). Above the maximum of latent heating, PV is diabatically reduced, and

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Maxi Boettcher and Heini Wernli

models were cloud-diabatic heating in a baroclinic background atmosphere producing a positive potential vorticity (PV) anomaly at low-tropospheric levels. The following basic conditions for DRW existence and propagation emerged from these simulations. The vortex of the positive low-level PV anomaly (that is accompanied by a weak SLP minimum) induces a poleward low-level jet of warm moist air at its downstream side. This stream ascends along the poleward-sloping isentropes until condensation occurs

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Julia H. Keller, Christian M. Grams, Michael Riemer, Heather M. Archambault, Lance Bosart, James D. Doyle, Jenni L. Evans, Thomas J. Galarneau Jr., Kyle Griffin, Patrick A. Harr, Naoko Kitabatake, Ron McTaggart-Cowan, Florian Pantillon, Julian F. Quinting, Carolyn A. Reynolds, Elizabeth A. Ritchie, Ryan D. Torn, and Fuqing Zhang

contributing to baroclinic conversion. The upper-tropospheric divergent outflow contributes to the ageostrophic geopotential flux. In an isentropic PV framework, (generalized) vertical motion is represented by diabatic heating, and hence diabatic PV modification is directly diagnosed. The upper-tropospheric divergent outflow is diagnosed as a separate process. More details on the differences between the two frameworks are provided by Teubler and Riemer (2016) and Wirth et al. (2018) . Interpreting PV

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Maxi Boettcher and Heini Wernli

-level vortex leads to upward motion, condensation, and diabatic heating. In this way, the localized low-level PV vortex is steadily renewed in the direction of the thermal wind by diabatic PV production. Recently, some real case studies have been performed that associated the theoretically identified DRW phenomenon with the concept of low-level vortices acting as precursors for explosive cyclone intensification ( Mallet et al. 1999 ; Wernli et al. 2002 ; Moore et al. 2008 ). The winter storm “Lothar

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Franziska Teubler and Michael Riemer

vorticity on an isentropic surface, f being the Coriolis parameter, and σ = − g −1 (∂ p /∂ θ ) (here, g is the acceleration of gravity and p is pressure), v is the wind vector, and is the heating rate constituting a generalized vertical velocity. The diabatic terms are subsumed in the function ; see Davies and Didone (2013) for details. We consider this equation on isentropic surfaces intersecting the midlatitude tropopause (see section 3e below). a. Data Before introducing the

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Olivia Martius and Heini Wernli

-dimensional pathways of the air parcels, the trajectory algorithm traced the specific humidity q , the horizontal wind components u and υ , the potential temperature θ , the equivalent potential temperature θ e , diabatic heating due to condensation (DHR), the angular momentum per unit mass M and the derivative of pressure in the longitudinal direction [first term on the right-hand side of Eq. (1) ]. The DHR has been calculated diagnostically in an approximate way as outlined briefly in Rossa et al

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Jana Čampa and Heini Wernli

these cyclones. However, these explosive cyclones reach their maximum intensity earlier at the surface than at 500 hPa, which highlights the importance of surface fluxes, diabatically produced positive PV anomalies, and reduced static stability for the rapid development. Deveson et al. (2002) classified 16 cyclones from the Fronts and Atlantic Storm Track Experiment (FASTEX) according to the contributions of the forcing from upper and lower levels to the vertical motion ( U / L ratio) and their

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Michael Riemer, Marlene Baumgart, and Sven Eiermann

. Apparently, the decoupling of the cyclone from the upstream trough and the fast movement and final demise of the cyclone are all associated with subsidence attributable to the jet streak. The jet streak therefore has a significant but detrimental impact on the overall evolution of the downstream cyclone. 4) Diabatic contribution to development In this subsection, we exploit the heating terms available from the YOTC data to examine the diabatic vertical motion, ω diab . We use ω diab to briefly comment

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

discussion of the problem of optimal observation network design]. The sensitivity of extratropical SVs to physical processes has been investigated by Coutinho et al. (2004) . They found that so-called moist (diabatic) SVs show enhanced growth due to reduced stability if moist effects are taken into account, and that higher resolution is more appropriate for moist SVs. Ancell and Mass (2006 , 2008) tested the impact of spatial resolution and the representation of physical processes on adjoint

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Marlene Baumgart, Michael Riemer, Volkmar Wirth, Franziska Teubler, and Simon T. K. Lang

error tendency equation. Our tendency equation is based on the local tendency equation for PV in isentropic coordinates, which is given by (e.g., Davies and Didone 2013 ) where describes the nonconservative PV tendency due to diabatic heating and nonconservative momentum change as measured by the parameterization schemes. The heating rate comprises heating due to the cloud, convection, radiation, and turbulence parameterization, while the horizontal wind tendency components of ( and

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