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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

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

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

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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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
,
Elliot Abrams
,
Andrea Bleistein
,
William Bua
,
Luca Delle Monache
,
Thomas W. Dulong
,
John E. Gaynor
,
Bob Glahn
,
Thomas M. Hamill
,
James A. Hansen
,
Douglas C. Hilderbrand
,
Ross N. Hoffman
,
Betty Hearn Morrow
,
Brenda Philips
,
John Sokich
, and
Neil Stuart

The American Meteorological Society (AMS) Weather and Climate Enterprise Strategic Implementation Plan for Generating and Communicating Forecast Uncertainty (the Plan) is summarized. The Plan (available on the AMS website at www.ametsoc.org/boardpges/cwce/docs/BEC/ACUF/2011-02-20-ACUF-Final-Report.pdf) is based on and intended to provide a foundation for implementing recent recommendations regarding forecast uncertainty by the National Research Council (NRC), AMS, and World Meteorological Organization. It defines a vision, strategic goals, roles and respon- sibilities, and an implementation road map to guide the weather and climate enterprise (the Enterprise) toward routinely providing the nation with comprehensive, skillful, reliable, and useful information about the uncertainty of weather, water, and climate (hydrometeorological) forecasts. Examples are provided describing how hydrometeorological forecast uncertainty information can improve decisions and outcomes in various socioeconomic areas. The implementation road map defines objectives and tasks that the four sectors comprising the Enterprise (i.e., government, industry, academia, and nongovernmental organizations) should work on in partnership to meet four key, interrelated strategic goals: 1) understand social and physical science aspects of forecast uncertainty; 2) communicate forecast uncertainty information effectively and collaborate with users to assist them in their decision making; 3) generate forecast uncertainty data, products, services, and information; and 4) enable research, development, and operations with necessary information technology and other infrastructure. The Plan endorses the NRC recommendation that the National Oceanic and Atmospheric Administration and, in particular, the National Weather Service, should take the lead in motivating and organizing Enterprise resources and expertise in order to reach the Plan's vision and goals and shift the nation successfully toward a greater understanding and use of forecast uncertainty in decision making.

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