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Paul A. Hirschberg
and
J. Michael Fritsch

Abstract

An analytic quasigeostrophic model is used to examine the sensitivity of type B cyclogenesis to the vertical structure of the troposphere given a particular stratospheric temperature configuration. It is found that there is an optimal tropospheric configuration that produces the largest negative height tendency at the center of the 1000-mb model cyclone. Based on the response of the 1000-mb height tendencies, altering the baroclinicity in the model planetary boundary layer (PBL) does not significantly affect the instantaneous quasigeostrophic dynamics of the deep atmosphere. Rather, the PBL temperature anomalies affect the development of lower-tropospheric model lows by hydrostatically shifting or steering the cyclone centers to locations beneath more (or less) favorable deep atmospheric quasigeostrophic conditions for development.

Diagnostic analyses of three individual stratospheric-tropospheric model configurations are also performed to examine the dynamics that drive the height (pressure) tendency field. Generally, the analytic model findings confirm previous observational and numerical investigations of height tendency mechanisms and support the notion of a stratospheric level of insignificant dynamics. In the optimal development case, the 1000-mb low is located almost directly underneath the region of strongest 200-mb temperature advection associated with a tropopause undulation (potential vorticity anomaly). This strong lower-stratospheric warm advection instantaneously overwhelms adiabatic cooling in the stratosphere and troposphere so that there are height falls over and downstream of the 1000-mb low. When the static stability is lowered in the troposphere and raised in the stratosphere to realistic “warm-sector” values, the vertical motion increases, and the local warming in the stratosphere and cooling in the troposphere decrease. The reduced tropospheric cooling results in larger net local column warming that intensifies the 1000-mb height falls. The intensified vertical circulation also acts to amplify the tropopause undulation. As the amplitude of the undulation increases, characteristics of the occlusion process can be identified.

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Paul A. Hirschberg
and
J. Michael Fritsch

Abstract

The hypothesis that the development of extratropical cyclones is influenced by the evolution of tropopause undulations is described and examined. These undulations exhibit large temperature and potential vorticity anomalies, and are often observed prior to and during surface cyclogenesis. Typically, an undulation has a half wavelength of approximately 2000 km and a vertical amplitude of over 200 mb. Warm and cold temperature anomalies which lie respectively over the low and under the high portions of the undulation, are often embedded within strong upper-level flow, so that large temperature advections are found upstream and over developing cyclones. A case analysis of a cyclone event indicates that the distributions of tropospheric height change and vorticity change can be strongly sensitive to the undulation-related temperature changes in the lower stratosphere, especially near 200 mb.

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Paul A. Hirschberg
and
J. Michael Fritsch

Abstract

A case study of a developing cyclone is used to show that the three-dimensional distribution of height change during development can be strongly sensitive to temperature changes which occur in the lower stratosphere in association with an evolving tropopause undulation. Quantitative analysis of the storm with a geopotential height tendency equation indicates that a synergistic process developed between the stratosphere and troposphere, whereby the vertical motion pattern maintained and intensified the upper-level temperature anomalies while the subsequent upper-level temperature advection led to an enhanced vertical circulation. Using the results of this diagnostic study, a conceptual model is constructed. The conceptual model is based on the hydrostatic and wind-field adjustments that occur as tropopause undulations propagate over favored regions of tropospheric warm advection and less stable air.

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Paul A. Hirschberg
,
Perry C. Shafran
,
Russell L. Elsberry
, and
Elizabeth A. Ritchie

Abstract

Analyses and forecasts from a modern data assimilation and modeling system are used to evaluate the impact of a special rawinsonde dataset of 3-h soundings at seven sites interspersed with the seven regular sites along the West Coast (to form a so-called picket fence to intercept all transiting circulations) plus special 6-h rawinsondes over the National Weather Service Western Region. Whereas four intensive observing periods (IOPs) are available, only two representative IOPs (IOP-3 and IOP-4) are described here. The special observations collected during each 12-h cycle are analyzed with the National Centers for Environmental Prediction (NCEP) Eta Data Assimilation System in a cold start from the NCEP–National Center for Atmospheric Research reanalyses as the initial condition. Forecasts up to 48 h with and without the special picket fence observations are generated by the 32-km horizontal resolution Eta Model with 45 vertical levels.

The picket fence observations had little impact in some cases with smooth environmental flow. In other cases, relatively large initial increments were introduced offshore of the picket fence observations. However, these increments usually damped as they translated downstream. During IOP-3, the increments amplified east of the Rocky Mountains after only 24 h. Even though initially small, the increments in IOP-4 grew rapidly to 500-mb height increments ∼20–25 m with accompanying meridional wind increments of 5–8 m s−1 that contributed to maxima in shear vorticity. Many of the downstream amplifying circulations had associated precipitation increments ∼6 mm (6 h)−1 between the control and experimental forecasts. The equitable threat scores against the cooperative station set for the first 24-h forecasts during IOP-3 had higher values at the 0.50- and 0.75-in-thresholds for the picket fence dataset. However, the overall four-IOP equitable threat scores were similar.

Although the classical synoptic case was not achieved during the picket fence, these model forecasts suggest that such observations around the coast of the United States would impact the downstream forecasts when added in dynamically unstable regions. An ultimate picket fence of continuous remotely observing systems should be studied further.

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