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Eva Regnier and Patrick A. Harr

Abstract

The decision to prepare for an oncoming hurricane is typically framed as a static cost:loss problem, based on a strike-probability forecast. The value of waiting for updated forecasts is therefore neglected. In this paper, the problem is reframed as a sequence of interrelated decisions that more accurately represents the situation faced by a decision maker monitoring an evolving tropical cyclone. A key feature of the decision model is that the decision maker explicitly anticipates and plans for future forecasts whose accuracy improves as lead time declines. A discrete Markov model of hurricane travel is derived from historical tropical cyclone tracks and combined with the dynamic decision model to estimate the additional value that can be extracted from existing forecasts by anticipating updated forecasts, rather than incurring an irreversible preparation cost based on the instantaneous strike probability. The value of anticipating forecasts depends on the specific alternatives and cost profile of each decision maker, but conceptual examples for targets at Norfolk, Virginia, and Galveston, Texas, yield expected savings ranging up to 8% relative to repeated static decisions. In real-time decision making, forecasts of improving information quality could be used in combination with strike-probability forecasts to evaluate the trade-off between lead time and forecast accuracy, estimate the value of waiting for improving forecasts, and thereby reduce the frequency of false alarms.

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William M. Clune, Patrick A. Harr, and L. Robin Brody

Abstract

The Quality Control (QC) Division of the U.S. Navy's Fleet Numerical Oceanography Center (FNOC) is responsible for the quality control of meteorological and oceanographic analyses and forecasts issued to operational users, and for the verification of FNOC numerical model products.

The FNOC QC Division ensures the quality and consistency of data to be included in the meteorological and oceanographic analyses, adding artificial data (“bogus technique”) when needed in sparse areas or in cases of significant discrepancies. Bogus data from various sources have a direct effect on the optimum interpolation analyses for the global forecast model and are used to modify the marine wind field, the spectral wave model, the upper-level winds for the Optimum Path Aircraft Routing System, and tropical cyclone warnings. Bogus sea surface temperature data are used to enhance the FNOC ocean thermal structure analysis.

The FNOC QC performs model verifications on a daily, monthly, and seasonal basis, providing a statistical summary of the performance of the meteorological and oceanographic models and identifying their strengths and weakness.

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Peter M. Klein, Patrick A. Harr, and Russell L. Elsberry

Abstract

Extratropical transition (ET) in the western North Pacific is defined here in terms of two stages: transformation, in which the tropical cyclone evolves into a baroclinic storm; and reintensification, where the transformed storm then deepens as an extratropical cyclone. In this study, 30 ET cases occurring during 1 June–31 October 1994–98 are reviewed using Navy Operational Global Atmospheric Prediction System analyses; hourly geostationary visible, infrared, and water vapor imagery; and microwave imagery. A brief climatology based on these cases is presented for the transformation stage and the subsequent cyclone characteristics of the reintensification stage.

A three-dimensional conceptual model of the transformation stage of ET in the western North Pacific Ocean is proposed that describes how virtually all 30 cases evolved into an incipient, baroclinic low. The three-step evolution of the transformation of Typhoon (TY) David (September 1997) is described as a prototypical example. Four important physical processes examined in each of the three steps include (i) environmental inflow of colder, drier (warm, moist) air in the western (eastern) quadrant of David’s outer circulation that initiates an asymmetric distribution of clouds and precipitation, and a dipole of lower-tropospheric temperature advection; (ii) the interaction between TY David and a preexisting, midlatitude baroclinic zone to produce ascent over tilted isentropic surfaces; (iii) systematic decay and tilt of the warm core aloft in response to vertical shear; and (iv) an evolution of David’s outer circulation into an asymmetric pattern that implies lower-tropospheric frontogenesis.

The beginning and end of the transformation stage of ET in the western North Pacific is defined based on the interaction of the tropical cyclone circulation with a preexisting, midlatitude baroclinic zone. In particular, cases that complete the transformation stage of ET become embedded in the preexisting, midlatitude baroclinic zone, with the storm center in cold, descending air. Cases that begin transformation but do not become embedded in the baroclinic zone fail to complete transformation and simply dissipate over lower sea surface temperatures and in an environment of vertical wind shear. Use of the conceptual model, together with satellite imagery and high-resolution numerical analyses and forecasts, should assist forecasters in assessing the commencement, progress, and completion of the transformation stage of ET in the western North Pacific, and result in improved forecasts and dissemination of timely, effective advisories and warnings.

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Patrick A. Harr, Russell L. Elsberry, Timothy F. Hogan, and William M. Clune

Abstract

Seventy-two-hour forecasts of sea level cyclones from the Navy Operational Global Atmospheric Prediction System are examined. Cyclones that formed over the North Pacific region of maximum cyclogenesis frequency are included for study. The analysis is oriented to assist the forecaster in evaluating the numerical model guidance by emphasizing verification of operationally oriented factors (i.e., cyclogenesis, explosive deepening).

Initially, systematic errors in forecast intensities and positions are identified. Maximum underforecasting errors (forecast central pressure higher than actual central pressure) occur over the central North Pacific region of climatological maximum cyclone deepening. Maximum overforecasting errors (forecast central pressure lower than the actual central pressure) occur over the region of climatological cyclone dissipation. Maximum position errors also occur over the central North Pacific region of climatological maximum deepening. These systematic error distributions indicate that there are diagnostic relationships between forecast performance, the cyclone track type, and whether the cyclone is deepening or filling at the forecast verification time.

The forecast intensity and position errors are stratified based on the 72-h forecast intensity change, which is one possible measure of forecast accuracy that uses information known at the initial time of the forecast rather than the verifying time. Three classes of intensity change are identified as deepening, filling, and mixed deepening and filling. The systematic intensity errors mainly comprise instances when a 72-h deepening profile was not forecast and a deepening or mixed deepening-filling profile actually occurred. When the category of intensity change is correctly forecast, cyclones forecast to follow a western Pacific track tend to be overforecast, while those forecast to follow a central Pacific track tend to be underforecast. It is hypothesized that one reason for these differences may be due to the relative importance of adiabatic versus diabatic processes involved in the development of cyclones following each track type. Furthermore, central Pacific cyclones become more removed from available initializing data on the Asian continent. Position errors are more sensitive to the forecast track type rather than the forecast central-pressure profile.

Model tendencies based on the forecast intensity change and track type are presented to aid the users of the numerical guidance recognize instances when the forecast performance may be exceptionally high or low.

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Sarah C. Jones, Patrick A. Harr, Jim Abraham, Lance F. Bosart, Peter J. Bowyer, Jenni L. Evans, Deborah E. Hanley, Barry N. Hanstrum, Robert E. Hart, François Lalaurette, Mark R. Sinclair, Roger K. Smith, and Chris Thorncroft

Abstract

A significant number of tropical cyclones move into the midlatitudes and transform into extratropical cyclones. This process is generally referred to as extratropical transition (ET). During ET a cyclone frequently produces intense rainfall and strong winds and has increased forward motion, so that such systems pose a serious threat to land and maritime activities. Changes in the structure of a system as it evolves from a tropical to an extratropical cyclone during ET necessitate changes in forecast strategies. In this paper a brief climatology of ET is given and the challenges associated with forecasting extratropical transition are described in terms of the forecast variables (track, intensity, surface winds, precipitation) and their impacts (flooding, bush fires, ocean response). The problems associated with the numerical prediction of ET are discussed. A comprehensive review of the current understanding of the processes involved in ET is presented. Classifications of extratropical transition are described and potential vorticity thinking is presented as an aid to understanding ET. Further sections discuss the interaction between a tropical cyclone and the midlatitude environment, the role of latent heat release, convection and the underlying surface in ET, the structural changes due to frontogenesis, the mechanisms responsible for precipitation, and the energy budget during ET. Finally, a summary of the future directions for research into ET is given.

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