A Lagrangian‐based analysis of extratropical cyclones. II: A detailed case‐study

A detailed case-study analysis is undertaken of the occurrence of significant coherent ensembles of trajectories (CETs) that accompany Atlantic cyclogenesis. The approach is based upon the Lagrangian-based analysis scheme introduced in Part I. The analysis reveals the existence of several distinct moist ascending (and dry descending) CETs. Two types of moist ascending CETs associated with strong condensational processes ascend from the boundary layer of the warm sector, either in the close vicinity of the cyclone centre, or in the trailing cold-frontal region. The latter is the main contributor to the precipitation along the cold front during the early phase of the development. Another type of moist CETs—distinguished by maximum water vapour flux—travels at low levels through the warm sector and ascends to mid-tropospheric levels in the warm-frontal region, where it produces strong precipitation around the time of maximum storm intensity. CETs with an upper-level origin are linked to the classical dry intrusion. Different CETs correspond to: strong descent of upper-level tropospheric air that penetrated to the lower troposphere, the movement of stratospheric air toward the centre of the maturing cyclone, and an event of stratosphere-to-troposphere exchange upstream of the upper-level trough. Two different visualizations of time sequences of the moist ascending CETs indicate that they represent well-defined ‘flexible tubes’ or airstreams that exist over a time period comparable with the duration of the main cyclone intensification. They provide a translating, flexible but coherent ‘river bed’ for the quasi-continuous ascent of moist boundary-layer air to the upper troposphere. These ‘tubes’ are akin to the concept of warm conveyor belts, and the present analysis method objectively diagnoses their geometrical and physical characteristics and potential-vorticity pattern. Some consideration is also given to the relationship of CETs and ‘tropospheric rivers’, and to possible shortcomings of the approach.

[1]  P. Persson.  Simulations of the potential vorticity structure and budget of fronts 87 IOP8 , 1995 .

[2]  Yongti Zhu,et al.  Tropospheric rivers? – A pilot study , 1992 .

[3]  M. Shapiro.  Turbulent Mixing within Tropopause Folds as a Mechanism for the Exchange of Chemical Constituents between the Stratosphere and Troposphere , 1980 .

[4]  F. Ludlam,et al.  Isentropic relative-flow analysis and the parcel theory , 1966 .

[5]  Louis W. Uccellini,et al.  A model-based diagnostic study of the rapid development phase of the Presidents' Day cyclone , 1988 .

[6]  Images in weather forecasting , 1997 .

[7]  K. Browning,et al.  Structure of a frontal cyclone , 1994 .

[8]  Toby N. Carlson,et al.  Airflow Through Midlatitude Cyclones and the Comma Cloud Pattern , 1980 .

[9]  K. Browning,et al.  Diagnostic study of a narrow cold‐frontal rainband and severe winds associated with a stratospheric intrusion , 1994 .

[10]  R. J. Reed,et al.  A Model-aided Study of the Origin and Evolution of the Anomalously High Potential vorticity in the Inner Region of a Rapidly Deepening Marine Cyclone , 1992 .

[11]  K. Browning,et al.  Variation of frontal and precipitation structure along a cold front , 1996 .

[12]  M. Stoelinga.  A Potential Vorticity-Based Study of the Role of Diabatic Heating and Friction in a Numerically Simulated Baroclinic Cyclone , 1996 .

[13]  David M. Schultz,et al.  The Structure and Evolution of a Simulated Midlatitude Cyclone over Land , 1993 .

[14]  K. Browning,et al.  Structure of low‐level jet streams ahead of mid‐latitude cold fronts , 1973 .

[15]  C. Appenzeller,et al.  PV morphology of a frontal-wave development , 1996 .

[16]  B. Hoskins,et al.  A POTENTIAL VORTICITY PERSPECTIVE OF THE STORM OF 15–16 OCTOBER 1987 , 1988 .

[17]  Yongti Zhu,et al.  Tropospheric rivers: A one‐year record and a possible application to ice core data , 1994 .

[18]  B. Hoskins,et al.  A new look at the ?-equation , 1978 .

[19]  Heini Wernli,et al.  A Lagrangian‐based analysis of extratropical cyclones. I: The method and some applications , 1997 .

[20]  Heini Wernli,et al.  Structure and evolution of an isolated semi‐geostrophic cyclone , 1993 .

[21]  E. Danielsen.  Stratospheric source for unexpectedly large values of ozone measured over the Pacific Ocean during Gametag, August 1977 , 1980 .

[22]  K. Browning,et al.  Air motion and precipitation growth in frontal systems , 1980 .

[23]  Ying-Hwa Kuo,et al.  Thermal Structure and Airflow in a Model Simulation of an Occluded Marine Cyclone , 1992 .

[24]  K. Emanuel,et al.  Potential Vorticity Diagnostics of Cyclogenesis , 1991 .

[25]  T. W. Harrold.  Mechanisms influencing the distribution of precipitation within baroclinic disturbances , 1973 .

[26]  J. Namias.  The Use of Isentropic Analysis in Short Term Forecasting , 1939 .

[27]  Toby N. Carlson,et al.  Mid-Latitude Weather Systems , 1991 .

[28]  Yongti Zhu,et al.  Atmospheric rivers and bombs , 1994 .

[29]  R. Stewart,et al.  Winter storm structure and melting‐induced circulations , 1989 .