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The dichotomous structure of the warm conveyor belt
Author(s) -
MartínezAlvarado O.,
Joos H.,
Chag J.,
Boettcher M.,
Gray S. L.,
Plant R. S.,
Methven J.,
Wernli H.
Publication year - 2014
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.2276
Subject(s) - extratropical cyclone , environmental science , climatology , diabatic , atmospheric sciences , potential vorticity , inflow , lapse rate , convection , parametrization (atmospheric modeling) , troposphere , cyclone (programming language) , geology , meteorology , vorticity , vortex , adiabatic process , physics , radiative transfer , quantum mechanics , field programmable gate array , computer science , computer hardware , thermodynamics
The warm conveyor belt (WCB) of an extratropical cyclone generally splits into two branches. One branch (WCB1) turns anticyclonically into the downstream upper‐level tropospheric ridge, while the second branch (WCB2) wraps cyclonically around the cyclone centre. Here, the WCB split in a typical North Atlantic cold‐season cyclone is analysed using two numerical models: the Met Office Unified Model and the COSMO model. The WCB flow is defined using off‐line trajectory analysis. The two models represent the WCB split consistently. The split occurs early in the evolution of the WCB with WCB1 experiencing maximum ascent at lower latitudes and with higher moisture content than WCB2. WCB1 ascends abruptly along the cold front where the resolved ascent rates are greatest and there is also line convection. In contrast, WCB2 remains at lower levels for longer before undergoing saturated large‐scale ascent over the system's warm front. The greater moisture in WCB1 inflow results in greater net potential temperature change from latent heat release, which determines the final isentropic level of each branch. WCB1 also exhibits lower outflow potential vorticity values than WCB2. Complementary diagnostics in the two models are utilised to study the influence of individual diabatic processes on the WCB. Total diabatic heating rates along the WCB branches are comparable in the two models, with microphysical processes in the large‐scale cloud schemes being the major contributor to this heating. However, the different convective parametrization schemes used by the models cause significantly different contributions to the total heating. These results have implications for studies on the influence of the WCB outflow in Rossby wave evolution and breaking. Key aspects are the net potential temperature change and the isentropic level of the outflow, which together will influence the relative mass going into each WCB branch and the associated negative PV anomalies at the tropopause‐level flow.