Warm Conveyor Belts in Idealized Moist Baroclinic Wave Simulations

• Published in: Journal of the atmospheric sciences Vol. 70; no. 2; pp. 627 - 652
• Main Authors: SCHEMM, Sebastian, WERNLI, Heini, PAPRITZ, Lukas
• Format: Journal Article
• Language: English
• Published: Boston, MA American Meteorological Society 01.02.2013
• Subjects: Air currents; Algorithms; Anomalies; Ascent; Baroclinic flow; Baroclinic waves; Belt conveyors; Belts; Cold; Computer simulation; Cyclogenesis; Cyclones; Drying; Earth, ocean, space; Eddy kinetic energy; Energy budget; Evolution; Exact sciences and technology; Experiments; External geophysics; General circulation. Atmospheric waves; Jet stream; Jet streams (meteorology); Kinetic energy; Lower troposphere; Meteorology; Outflow; Perturbation; Phase transitions; Physics of the high neutral atmosphere; Potential vorticity; Saturation; Simulation; Studies; Tracking; Troposphere; Upper level potential vorticity; Upper troposphere; Vorticity; Warm air; Winds; Zonal winds; Northern Hemisphere; Warm conveyor belt; f plane approximation; Turbulence energy; energy balance; atmospheric circulation; Modeling; Cyclogenesis; Baroclinic waves
• ISSN: 0022-4928; 1520-0469
• DOI: 10.1175/jas-d-12-0147.1

This idealized modeling study of moist baroclinic waves addresses the formation of moist ascending airstreams, so-called warm conveyor belts (WCBs), their characteristics, and their significance for the downstream flow evolution. Baroclinic wave simulations are performed on the f plane, growing from a finite-amplitude upper-level potential vorticity (PV) perturbation on a zonally uniform jet stream. This nonmodal approach allows for dispersive upstream and downstream development and for studying WCBs in the primary cyclone and the downstream cyclone. A saturation adjustment scheme is used as the only difference between the dry and moist simulations, which are systematically compared using a cyclone-tracking algorithm, with an eddy kinetic energy budget analysis, and from a PV perspective. Using trajectories and a selection criterion of maximum ascent, forward- and rearward-sloping WCBs in the moist simulation are identified. No WCB is identified in the dry simulation. Forward-sloping WCBs originate in the warm sector, move into the frontal fracture region, and ascend over the bent-back front, where maximum latent heating occurs in this simulation. The outflow of these WCBs is located at altitudes with prevailing zonal winds; they hence flow anticyclonically ("forward") into the downstream ridge. In case of a slightly weaker ascent, WCBs curve cyclonically ("rearward") above the cyclone center. A detailed analysis of the PV evolution along the WCBs reveals PV production in the lower troposphere and destruction in the upper troposphere. Consequently, WCBs transport low-PV air into their outflow region, which contributes to the formation of distinct negative PV anomalies. They, in turn, affect the downstream flow and enhance downstream cyclogenesis.