Abstract
Abstract. Jet streams and potential vorticity (PV) gradients along upper-level ridges, troughs and zonal flows form a waveguide that governs midlatitude dynamics. Warm conveyor belt (WCB) outflows often inject low-PV air into ridges, and the representation of WCBs is seen as a source of uncertainty for downstream forecasts. Recent studies have highlighted the presence of mesoscale structures with negative PV in WCBs, the impact of which, on large-scale dynamics, is still debated. Here, fine-scale observations of cloud and wind structures acquired with airborne Doppler radar and dropsondes provide rare information on the WCB outflow of the Stalactite cyclone and the associated upper-level ridge on 2 October 2016 during the North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX). The observations show a complex tropopause structure associated with two jet stream cores along the northwestern edge of the ridge. A reference convection-permitting simulation with full physics reproduces the observed structures and reveals the presence of elongated negative PV bands along the eastern jet stream core. In contrast, a sensitivity experiment with heat exchanges due to cloud processes being cut off shows lower cloud tops, weaker jet stream cores, a ridge less extended westward and the absence of negative PV bands. A Lagrangian analysis based on online trajectories shows that the anticyclonic branch of the WCB outflow enters the eastern jet stream core in the reference simulation, while it is absent in the sensitivity experiment. The anticyclonic ascents and negative PV bands originate from the same region near the cyclone's bent-back front. The most rapid ascents coincide with mid-level convective cells identified by clustering analysis, which are located in a region of conditional instability below the jet stream core and above a low-level jet. Horizontal PV dipoles are found around these cells, with the negative poles reaching absolute negative values, and the convective cells thus appear as the source of the negative PV bands. The results show that mid-level convection within WCBs accelerates the jet stream and may influence the downstream large-scale circulation.
Highlights
Jet streams and potential vorticity (PV) gradients along upper-level ridges, troughs and zonal flows form a waveguide that governs the propagation of Rossby waves (Hoskins and Ambrizzi, 1993)
Blanchard et al (2020) showed that, among three types of organized convection they found in a Warm conveyor belt (WCB) region, only mid-level convection is associated with coherent negative PV bands
This paper focuses on the WCB outflow associated with the Stalactite cyclone located close to the Icelandic coast on 2 October 2016
Summary
Jet streams and potential vorticity (PV) gradients along upper-level ridges, troughs and zonal flows form a waveguide that governs the propagation of Rossby waves (Hoskins and Ambrizzi, 1993). It has been shown that analyses and short-term forecasts tend to underestimate the peak jet stream wind, the vertical wind shear and the abruptness of the change in wind shear across the tropopause (Schäfler et al, 2020) This calls for a better understanding of processes controlling PV gradients. The low-PV air resulting from the negative anomalies is transported into the upper-level ridge by the WCB outflow, where it is advected toward high-PV air by the associated divergent wind, and this impacts both the jet stream and the PV gradient at the tropopause (Grams et al, 2011). Blanchard et al (2020) showed that, among three types of organized convection they found in a WCB region, only mid-level convection is associated with coherent negative PV bands These studies further suggest that the mesoscale negative PV structures may accelerate the jet stream locally and potentially influence the downstream circulation.
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