Cyclogenesis Downstream of Extratropical Transition Analyzed by Q-Vector Partitioning Based on Flow Geometry

Riemer, Michael; Baumgart, Marlene; Eiermann, Sven

doi:10.1175/jas-d-14-0023.1

Cited by 15 publications

(42 citation statements)

References 35 publications

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“…; Riemer et al . ; Quinting and Jones, ) and is usually expected to be of larger importance to Rossby‐wave dynamics than direct diabatic PV modification (e.g., Davis et al . ; Riemer and Jones, ; Teubler and Riemer, ).…”

Section: Data and Methods To Quantify The Amplification Of Forecast Umentioning

confidence: 99%

“…Upper-tropospheric divergence (index div) can be associated with dry balanced dynamics and diabatic processes (see, for example, chapter 6.4 in Holton and Hakim, 2013) and is of particular importance during ridge building (Grams et al, 2011;Teubler and Riemer, 2016). Pronounced upper-tropospheric divergence is often associated with latent heat release below (e.g., Davis et al, 1993;Riemer et al, 2014;Quinting and Jones, 2016) and is usually expected to be of larger importance to Rossby-wave dynamics than direct diabatic PV modification (e.g., Davis et al, 1993;Riemer and Jones, 2010;Teubler and Riemer, 2016).…”

Section: Quantitative Pv Framework For the Amplification Of Forecast mentioning

confidence: 99%

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Processes governing the amplification of ensemble spread in a medium‐range forecast with large forecast uncertainty

Baumgart

Riemer

2019

Quart J Royal Meteoro Soc

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This study provides a process‐based perspective on the amplification of forecast uncertainty and forecast errors in ensemble forecasts. A case from the North Atlantic Waveguide and Downstream Impact Experiment that exhibits large forecast uncertainty is analysed. Two aspects of the ensemble behaviour are considered: (a) the mean divergence of the ensemble members, indicating the general amplification of forecast uncertainty, and (b) the divergence of the best and worst members, indicating extremes in possible error‐growth scenarios. To analyse the amplification of forecast uncertainty, a tendency equation for the ensemble variance of potential vorticity (PV) is derived and partitioned into the contributions from individual processes. The amplification of PV variance is, on average for the midlatitudes of the Northern Hemisphere, dominated by near‐tropopause dynamics. Locally, however, other processes can dominate the variance amplification, for example, in the region where tropical storm Karl interacts with the Rossby‐wave pattern during extratropical transition. In this region, the variance amplification is dominated by upper‐tropospheric divergence and tropospheric–deep interaction and is thereby mostly related to (moist baroclinic) cyclone development. The differences between the error growth in the best and worst ensemble members can, to a large part, be attributed to differences in the representation of cut‐off evolution around 3 days, which subsequently amplifies substantially in the highly nonlinear region of the Rossby‐wave pattern until 5 days. In terms of the processes, the differences in error growth are dominated by differences in the error growth by near‐tropopause dynamics. The approach presented provides flow‐dependent insight into the dynamics of forecast uncertainty and forecast errors and helps to understand better the different contributions of specific weather systems to the medium‐range amplification of ensemble spread.

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Section: Data and Methods To Quantify The Amplification Of Forecast Umentioning

confidence: 99%

Section: Quantitative Pv Framework For the Amplification Of Forecast mentioning

confidence: 99%

Processes governing the amplification of ensemble spread in a medium‐range forecast with large forecast uncertainty

Baumgart

Riemer

2019

Quart J Royal Meteoro Soc

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“…This approach diagnoses vertical motion from the non‐divergent wind instead of the geostrophic wind and can thus be considered more accurate, in particular in the vicinity of upper‐level troughs where the flow exhibits strong curvature and the Rossby number is relatively large. The alternative‐balance omega equation reads (Riemer et al , ):

\nabla_{p} \cdot ((), σ \nabla_{p} ω) + f_{0}^{2} ({}, \frac{\partial^{2} ω}{\partial p^{2}} - \frac{1}{p} \frac{c_{v}}{c_{p}} ((), \frac{\partial ω}{\partial p} + \frac{ω}{p})) = prefix- 2 \nabla_{p} \cdot Q_{AB},

with

Q_{AB} = prefix- \nabla_{p} v_{ψ} \cdot \nabla_{p} α,

where ∇ p is the horizontal gradient on pressure levels, ω the vertical velocity (balanced omega), v ψ the non‐divergent wind, f 0 the constant Coriolis parameter, and c v and c p the specific heats of dry air at constant volume and pressure, respectively. Furthermore, σ = −α 0 θ 0 −1 ∂θ 0 /∂p and α = θ Rp −1 (p/p 0 ) κ where R is the gas constant of dry air, κ = R/c p , and α 0 and θ 0 denote the reference vertical profiles of specific volume and potential temperature, respectively.…”

Section: Methodsmentioning

confidence: 99%

Dynamics of tropical–extratropical interactions and extreme precipitation events in Saudi Arabia in autumn, winter and spring

Vries

Feldstein

Riemer

et al. 2016

Quart J Royal Meteoro Soc

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Extreme precipitation in the arid Middle East can cause flash floods with dramatic societal impacts. This study investigates the synoptic-scale dynamics of three extreme precipitation events that occurred in Saudi Arabia in autumn, winter and spring. Using ERA-Interim reanalysis, soundings and observational precipitation data, we study precipitation characteristics, the synoptic circulations, moisture transport pathways and forcing mechanisms for upward motion. All three cases involved strong tropical-extratropical interactions whereby midlatitude forcing instigated an incursion of tropical moisture over the Arabian Peninsula that fuelled the heavy rainfall. In each case, a midlatitude upper-level trough, associated with anticyclonic Rossby wave breaking, intruded into the subtropics. The phase relationship between this trough and the tropical low-level circulation was consistent with wave amplification through baroclinic growth. Eulerian and Lagrangian analyses reveal moisture transport from nearby and remote tropical regions, leading to above-normal tropospheric moisture content over Saudi Arabia. The autumn case (November 2009) showed a transient midlatitude upper-level trough that interacted with the climatological Red Sea Trough near the surface, being an Active Red Sea Trough' event. The winter case (January 2005) resembled tropical plume-like characteristics and demonstrated the coupling of a midlatitude cyclone and the equatorial low-pressure zone over Africa, an intensified subtropical jet stream, and pronounced moisture fluxes at middle and upper levels. The spring case (April-May 2013) involved a quasi-stationary cut-off low and persistent advection of low-level moist air masses, partly from the south Indian Ocean through cross-equatorial flow. The forcing of ascent was associated with low-level moisture convergence and decreased static stability (autumn case), dynamical lifting (winter case), strong surface sensible heating (spring case), and orographic lifting (all cases), favouring the build-up and release of potential instability. We discuss the three cases from a seasonal perspective and present a synthesis of their common key synoptic features

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“…The divergent flow at upper levels can be predominantly associated with latent heat release below (cf. also Riemer et al 2014) and thus can be interpreted as having an indirect diabatic impact on the ridge. A comprehensive comparison of the PV tendency framework and the more commonly used K e framework can be found in Teubler and Riemer (2016).…”

Section: ) Pv Tendencymentioning

confidence: 99%

Enhanced Tropospheric Wave Forcing of Two Anticyclones in the Prephase of the January 2009 Major Stratospheric Sudden Warming Event

Schneidereit

Peters

Grams

et al. 2017

Monthly Weather Review

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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.

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Cyclogenesis Downstream of Extratropical Transition Analyzed by Q-Vector Partitioning Based on Flow Geometry

Cited by 15 publications

References 35 publications

Processes governing the amplification of ensemble spread in a medium‐range forecast with large forecast uncertainty

Processes governing the amplification of ensemble spread in a medium‐range forecast with large forecast uncertainty

Dynamics of tropical–extratropical interactions and extreme precipitation events in Saudi Arabia in autumn, winter and spring

Enhanced Tropospheric Wave Forcing of Two Anticyclones in the Prephase of the January 2009 Major Stratospheric Sudden Warming Event

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