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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

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Paul A. Hirschberg and J. Michael Fritsch

Abstract

A five-layer analytic model of quasigeostrophic flow is developed. The model provides exact analytic solutions to the nonlinear quasigeostrophic omega and vorticity equations for various atmospheric temperature and geopotential structures. These solutions yield instantaneous three-dimensional fields of vertical motion and geopotential tendency given some finite-amplitude flow. Hence, unlike traditional eigenvalue analyses that provide time-dependent solutions for simple linearized flows, the five-layer model yields nonlinear diagnostic solutions to initial-value problems.

It is demonstrated that the five-layer model can reproduce many of the disturbance characteristics that are deduced from more traditional analyses of baroclinic instability. It is also shown that, because of its flexible vertical temperature structure specification, it can simulate complex temperature and geopotential structures in the atmosphere. The flexible specification of the total temperature and geopotential structure makes the five-layer model an attractive means for comparing theory with observations. Additionally, the versatility and simplicity of the five-layer model make it a potentially useful research and pedagogical tool.

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Paul A. Hirschberg and J. Michael Fritsch

Abstract

A new five-layer, quasigeostrophic model of baroclinic development is utilized to examine the initial-value problem sensitivity of extratropical cyclogenesis to the variation of stratospheric thermal and geopotential configurations associated with tropopause undulations. Previous studies have suggested that such undulations or potential vorticity anomalies can influence both the structure and evolution of lower-tropospheric cyclones. A series of experiments with the five-layer model are performed to evaluate the sensitivity of the model height, vertical motion, and height tendency patterns to various stratospheric temperature and geopotential distributions. It is found that idealized tropospheric baroclinic systems do not show typically observed characteristics unless certain stratospheric temperature, geopotential, and wind anomaly configurations associated with tropopause undulations are present. Furthermore, for given tropospheric patterns, there are particular lower-stratospheric configurations that optimize the development of model lower-tropospheric cyclones. These stratospheric configurations are functions of 1) the value of the lower-stratospheric temperature anomaly, 2) the amplitude of the tropopause undulation, and 3) the horizontal location of the undulation relative to the tropospheric temperature anomalies. Finally, both the rate of cyclogenesis and the amplification of the tropopause undulation increase if tropospheric static stability is reduced.

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Paul A. Hirschberg and J. Michael Fritsch

Abstract

Top-down height tendency reasoning is explained and examined. This approach uses the assumption of a stratospheric level of insignificant dynamics (LID)—where height and pressure tendencies are considered negligible—to simplify the understanding of cyclone-scale hydrostatic height (pressure) tendency in the troposphere. Quasigeostrophic analytic model results confirm the existence of such a LID for scales less than approximately 5000 km. An examination of a height tendency equation with the LID assumption shows that there must be net integrated local warming (cooling) between the LID and any level below the LID where heights are falling (rising). The local temperature tendency, which from the thermodynamic equation results from advection, diabatic heating, and the product of vertical motion and static stability, reflects the combined actions of all thermodynamic and dynamic processes that together promote hydrostatic height change in isobaric coordinates. In particular, the important dynamic effects of mass-diverging secondary circulations are implicitly contained in the local temperature tendency.

New observational evidence and analytic model simulations supporting the top-down approach for understanding height tendency are also provided. The analytic model simulations show that isolated layers of equivalent diabatic heating and temperature advection do not produce equivalent dynamic responses in the vertical-motion field and height tendency fields. This result is used to explain observations that temperature advections in the upper troposphere /lower stratosphere are associated with larger lower-tropospheric height tendencies than equivalent temperature advections in the lower troposphere.

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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 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, 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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Paul A. Hirschberg, Matthew C. Parke, Carlyle H. Wash, Mark Mickelinc, Roy W. Spencer, and Eric Thaler

Abstract

A statistical analysis is performed on a 6-month global dataset consisting of satellite-derived channel 3 Microwave Sounding Unit (MSU3) brightness temperature and various conventionally derived fields to quantify the potential usefulness of MSU3 analyses in the nowcasting and forecasting of baroclinic waves. High positive spatial and temporal correlations are obtained between the MSU3 brightness temperature and 400–100-mb thickness fields over all wavelengths in the data. Slightly lesser positive correlations are found between the MSU3 and the 200-mb temperature. The MSU3–500-mb and MSU3–50-mb height correlation results indicate a scale dependence in the hydrostatic spreading of thickness anomalies in the vertical. Most significantly, relatively high negative MSU3–500-mb height correlations for the short (≤ synoptic scale) wavelength portion of the data suggest that upper-level thermal anomalies are reflected downward and that MSU3 analyses can be used to track midlevel synoptic-scale baroclinic waves. This conclusion is also supported by relatively high negative MSU3–500-mb vorticity and MSU3–dynamic tropopause correlations along the climatological storm tracks.

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Paul A. Hirschberg, Richard J. Lind, Steven J. Bolduc, and Russell L. Elsberry

Mesoscale weather systems that develop in the central United States are often forced by environmental features that have formed far upstream over the conventional data-sparse Pacific Ocean. Although remotely sensed observations, such as satellite retrievals, are becoming more numerous and accurate, they still may not have the resolution necessary to enhance global model-based analyses and forecasts over this region. These global model products are the primary source of lateral boundary conditions that have been found to have large impacts on the downstream forecast skill of regional mesoscale models over the United States. In addition, the temporal and spatial resolution of the current rawinsonde network along the West Coast may not be sufficient to detect and measure mesoscale flow features as they move inland. During the STORM-FEST experiment in February–March 1992, a “Picket Fence” of seven special rawinsonde stations were interspersed among the seven regular rawinsonde sites from Port Hardy, British Columbia, to San Diego, California. All sites obtained observations every 3 h rather than the normal 12 h. The objective of the Picket Fence was to examine the feasibility of using extra observations in time and space to improve upstream boundary conditions for forecasts of mesoscale weather events in the central United States. As a first step in examining the potential boundary condition impact of the Picket Fence, fluxes of mass, heat, momentum, potential energy, kinetic energy, and moisture across the West Coast resolved with various spatial and temporal combinations of Picket Fence data are compared with the 12-h regular upper-air sites as the standard. When a wave system crossed the middle of the Picket Fence, significantly different fluxes were calculated with the full spatial and 3-h Picket Fence observations. For other systems that crossed near the margins of the Picket Fence, only small changes were detected by the additional observations.

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