Circulation conservation in the outflow of warm conveyor belts and consequences for Rossby wave evolution

Abstract

Rossby waves on the jet stream are associated with meridional motions, displacing air and the strong potential vorticity (PV) gradient on isentropic surfaces. Poleward motion along sloping isentropic surfaces typically results in ascent and a ridge of air with low PV values. Latent heating in the ascending warm conveyor belt (WCB) enables air to cross isentropic surfaces so that the WCB outflow into a ridge occurs in a higher isentropic layer than the inflow. However, the PV impermeability theorem states that there can be no PV flux across isentropic surfaces, so how can heating alter the PV pattern of a Rossby wave? Here, the ways in which heating in WCBs can influence Rossby wave evolution at tropopause level are explained in the context of the PV impermeability theorem. First, a WCB outflow volume is defined by the upper tropospheric air in a ridge that has experienced net heating over the last few days, using a tracer within short global model forecasts. Second, the boundary of this outflow volume is tracked backwards using isentropic trajectories allowing quantification of the degree to which circulation is conserved, as predicted by theory, even though the WCB transports mass into the volume from lower isentropic layers. This diabatic flux of mass into the outflow volume results in an increase in density and expansion in the outflow area, the partition being determined approximately by PV inversion. The area expansion, combined with conservation of circulation, implies stronger anticyclonic vorticity. The relative vorticity change from divergent outflow can be as large as the decrease relative to the background planetary vorticity associated with poleward displacement of the circuit. The additional anticyclonic relative motion results in enhanced anticyclonic overturning of PV contours on the eastern flank of the ridge, altering qualitatively the nonlinear evolution of the Rossby wave.

Integration-based extraction and visualization of jet stream cores

“…Jet streams are fast and coherent air streams in the upper troposphere and lower stratosphere, which are hundreds of kilometers wide and only a few kilometers thick. The center of the jet stream, which is called the "jet core", often exceeds 50 m s −1 and can occasionally reach wind magnitudes of 100 m s −1 (Koch et al, 2006;Schiemann et al, 2009;Ahrens and Henson, 2018). Jet streams and their variation are of high interest, (cf.…”

“…An additional constraint is the assumption that the flow is oriented eastwards, cf. Schiemann et al (2009). Alternatively, Archer and Caldeira (2008) locally considered mass and mass-fluxweighted averages to detect the jets.…”

“…This is because the atmospheric circulation contains and is governed by numerous three-dimensional flow features and processes that significantly impact the weather (Nielsen-Gammon, 2001;Harnik et al, 2016). For example, near the tropopause, the boundary between the troposphere and the stratosphere, we can find fast coherent air streams called jet streams (Koch et al, 2006;Schiemann et al, 2009). The wind speed of jet streams often exceeds 50 m s −1 at their core, as they meander from west to east around the planet (Ahrens and Henson, 2018).…”

Jet stream dynamics from a potential vorticity gradient perspective: The method and its application to a kilometre‐scale simulation

“…Its application in the context of upper level wind dates back to at least Kleinschmidt (1955). Owing its success to its conservation under adiabatic flow and its invertibility property, PV has since helped illuminate, among other things, the amplification of extratropical cyclone growth through latent heat release (Gyakum, 1983a; Gyakum, 1983b; Boyle and Bosart, 1986; Stoelinga, 1996; Wernli et al ., 2002; Ahmadi‐Givi et al ., 2004; Binder et al ., 2016; Martínez‐Alvarado et al ., 2016) by generating or sustaining lower tropospheric positive PV anomalies that mutually interact with upper level PV anomalies (Hoskins et al ., 1985; Hoskins and Berrisford, 1988; Whitaker et al ., 1988; Davis and Emanuel, 1991; Kuo et al ., 1991; Reed et al ., 1992; Rossa et al ., 2000; Čampa and Wernli, 2012; Schemm and Wernli, 2014; Attinger et al ., 2021), the evolution and diabatic modification of downstream development via attenuated upper level Rossby wave amplitudes (Grams and Wernli, 2011; Davies and Didone, 2013; Schemm et al ., 2013; Oertel et al ., 2019b), blocking formation (Pfahl et al ., 2015; Steinfeld et al ., 2020; Saffin et al ., 2021), and surface frontogenesis, including frontal‐wave developments (Thorpe and Emanuel, 1985; Joly and Thorpe, 1990; Davies et al ., 1991; Bishop and Thorpe, 1994; Appenzeller and Davies, 1996; Fehlmann and Davies, 1999; Dacre and Gray, 2006; Schemm and Sprenger, 2015; Attinger et al ., 2021).…”

Interaction of microphysics and dynamics in a warm conveyor belt simulated with the ICON model

“…On larger scales, where vertical diabatic heating gradients dominate, diabatic heating during slantwise WCB ascent increases low-level PV values, which can lead to an intensification of cyclones (Rossa et al, 2000;Joos and Wernli, 2012;Binder et al, 2016). In the upper troposphere, PV values are decreased, which can modify the upper-level flow evolution (e.g., Wernli and Davies, 1997;Pomroy and Thorpe, 2000;Grams et al, 2011;Madonna et al, 2014;Methven, 2015;Saffin et al, 2021). Hence, as ascending air parcels pass through the quasi-vertical PV dipole related to mid-tropospheric diabatic heating from cloud formation, their PV values on average increase before they decrease to values close to their initial PV value (e.g., Madonna et al, 2014;Methven, 2015).…”