Mesoscale air transport at a midlatitude squall line in Europe – a numerical analysis

Abstract

This study examines a fast-propagating squall line over Western and Central Europe. A high-resolution regional weather prediction model is used to analyze the threedimensional wind field and the corresponding air mass transport induced by the mesoscale convective system. The squall line can be divided into a bow-shaped strong part with a continuous line of heavy convective precipitation, and a weaker part exhibiting several isolated multi-cell storms. In the strong part, a deep and intense rear inflow jet occurs with relative wind speeds of 12-18 m s −1 . This part is also characterized by moderate to strong wind shear, in contrast to the weaker part of the squall line where a shallow cold pool surges under the potentially unstable presquall air.To investigate the air mass transport along the squall line, in the model simulations passive fluid tracers are initialized in several vertical layers with different thermodynamic conditions. At the strong part of the squall line a very efficient vertical tracer transport occurs. Potentially warm low-level air is lifted from the lowermost 2 km to the upper troposphere where an organized horizontal outflow is observed. The potentially coldest presquall air descends just in front of the updraught region and partly penetrates the convective precipitation area. In the weaker part of the squall line, the potential instability is not significantly reduced because the vertical air mass transport is less effective there. Additionally, passive tracers are used to analyze the composition of the cold pool which turns out to be a very heterogeneous mixture of descending post-frontal cold air and presquall air originating at different heights.

Mesoscale simulations of atmospheric CO2 variations using a high‐resolution model system with process‐based CO2 fluxes

“…For high‐resolution simulations (1.1 km), deep cumulus convection is assumed to be explicitly resolved. For the nesting simulations, the hybrid mass‐flux convection scheme HYMACS (Kuell and Bott, 2008; Kuell and Bott, 2011) is used for shallow, mid‐level and deep convection because at a grid spacing of 2.8 km deep convection is only partly resolved (Uebel and Bott, 2015). The transport of CO 2 mixing ratios was included as a passive fluid tracer and feedbacks on radiative transfer is neglected.…”

The local wavelet‐based organization index – Quantification, localization and classification of convective organization from radar and satellite data

“…The supercell developed within a high-energy air mass and a heavily sheared environment ahead of the 2014 Pentecostal storm over western Germany (Barthlott et al, 2017;Mathias et al, 2017). Details of the squall line and its development can be found in Uebel and Bott (2015). The supercell on 09 June 2014 is characterized by a sharp and intense hook echo with rain rates above 150 mm⋅hr −1 .…”

Meteorological aspects of heavy precipitation in relation to floods – An overview

“…Examples of lifting mechanisms that tend to occur in bands are sea breezes, (coastal) fronts, drylines and large outflow boundaries (low-level convergence lines) (Curtis, 2017;Iwai et al, 2018;Meng et al, 2013;Smith et al, 2000). Squall lines also are considered as (quasi-)linearly organised mesoscale convective systems with a precipitation band and usually a cold pool in the convective region adjoined by a large region of stratiform precipitation, the stratiform region (Meng et al, 2013;Uebel and Bott, 2015). The classic squall line will develop ahead or along a cold front or dry line boundary due to the presence of significant moisture and strong upper level divergence.…”

“…A strong squall line with strong low/mid-level wind shear consists of a continuous line of deep convection with a deep and strong rear-inflow jet related to organised horizontal convective outflow, while a weak one exhibit several isolated multi-cell thunderstorms with a shallow and weak rear-inflow jet (Uebel and Bott, 2015). A descending convectively generated rear inflow jet (density current) occurring under the stratiform part of the squall line, can function as a barrier (cold pool) to increase the convection of incoming moist and warm air at the other side of the squall line.…”