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Kathryn A. Payne
,
Russell L. Elsberry
, and
Mark A. Boothe

Abstract

Because the Joint Typhoon Warning Center (JTWC) has only four dynamical models for guidance in making 96- and 120-h track forecasts, an opportunity exists for improving the consensus forecast by the proper removal of a likely erroneous forecast to form a selective consensus (SCON). Forecast fields from all four models [the U.S. Navy Operational Global Atmospheric Prediction System (NOGAPS), the U.S. Navy version of the Geophysical Fluid Dynamics Laboratory model (GFDN), the Met Office (UKMO) model, and the Global Forecast System (GFS)] were available during the 2005 western North Pacific season to evaluate for the first time the error mechanisms leading to large track errors. As shown previously for the NOGAPS and GFDN models during the 2004 season, error sources related to the midlatitude circulations accounted for about 90% of all large 120-h track errors by all four models during the 2005 season. This dominance of midlatitude-related error source is a major shift from the 72-h errors, which include more errors related to tropical circulations. In the GFS model, 95% of the large errors occurred because of an incorrect depiction of the vertical structure of the tropical cyclone. A systematic error in the GFDN model was identified in which a false anticyclogenesis was predicted downstream of the Tibetan Plateau, which accounted for over 50% of the large GFDN track errors. The consensus spread versus consensus error relationship is examined to isolate those 20%–25% of cases with large spreads and large errors that are candidates for forming an SCON. If the model tracks that contributed to the large errors are eliminated, the average improvement of the SCON forecasts relative to the nonselective consensus is 222 (239) n mi during 2005 (2004), and the corresponding average improvement relative to the JTWC forecasts is 382 (203) n mi. This application of SCON is considered the potential “forecastability” in that it represents the optimum use of the present numerical guidance for consensus forecasting.

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Lester E. Carr III
,
Mark A. Boothe
, and
Russell L. Elsberry

Abstract

An observational study of western North Pacific tropical cyclones (TC) revealed many cases of two TCs whose tracks were altered by processes that were quite different from the mutual advection (Fujiwhara-type) processes. Thus, four conceptual models are proposed to describe these track alterations. A conceptual model called direct interaction is proposed that is a modification of one by Lander and Holland and has three modes: 1) a one-way influence in which the track of a smaller TC that is embedded in the circulation of a larger TC has a cyclonic orbiting motion, but no significant track alteration of the larger TC is apparent; 2) a similar case in which a mutual advection occurs with the tracks of both the smaller and larger TCs being altered; and 3) a subset of 2) in which the mutual advection includes an attraction component such that the two similarly sized TC circulations eventually merge into a larger circulation with a single center. During the 7-yr period (1989–95), the one-way influence, mutual interaction, and merger modes were detected seven, three, and two times.

A semidirect interaction conceptual model is proposed in which the two TCs have a relative cyclonic rotation as in the Lander and Holland model, but the TCs are separated by 10°–20° longitude so that a direct (advective-type) interaction is excluded. Rather, the track alteration is attributed to an environmental flow established by the juxtaposition of a TC on one side and a subtropical anticyclone cell on the opposite side. In an east–west orientation of the two TCs and a subtropical anticyclone cell to the east (west), the height gradient between the western (eastern) TC and the eastern (western) subtropical anticyclone establishes a poleward (equatorward) environmental steering flow across the eastern (western) TC. In the 1989–95 sample, a semidirect interaction that altered the tracks of the eastern or the western TC occurred 18 and 14 times, respectively.

An indirect interaction conceptual model is proposed in which the distinguishing feature is the Rossby wave dispersion-induced anticyclone to the east and equatorward of the western TC. This anticyclone imposes an equatorward (poleward) steering flow across the eastern (western) TC. Several variations of the indirect interaction are possible depending on the separation distance, sizes of each TC, and their relative orientation. During the 7-yr period, an indirect interaction affecting the western TC or the eastern TC occurred 36 and 22 times, respectively.

A fourth conceptual model of track alterations involving two TCs is proposed in conjunction with a reverse-oriented monsoon trough formation. The distinguishing feature of this conceptual model is the combination of the peripheral anticyclones of both TCs as the eastern TC moves into an east–west orientation and has a separation of 10°–20° longitude. In the 1989–95 sample, a reverse-oriented trough formation involving two TCs occurred seven times.

The frequency of track alterations whenever two TCs are present emphasizes that forecasters must give special attention to such situations. The four conceptual models proposed here emphasize that the physical mechanisms are complex and in the vast majority of cases cannot be attributed to the mutual advection (Fujiwhara-type) process implied in the Lander and Holland model.

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Russell L. Elsberry
,
James R. Hughes
, and
Mark A. Boothe

Abstract

Two approaches are developed and tested to improve the unweighted position consensus for 96-, 108-, and 120-h tropical cyclone track guidance in the western North Pacific. A weighted position guidance technique uses a weighting factor for each model that is inversely proportional to how far the 60-, 66-, and 72-h positions of that model are from the corresponding positions of the 11-member position consensus. The weighted position consensus of 96-, 108-, and 120-h track errors for a sample of 24 storms during the 2006 season are consistently smaller than for the unweighted position consensus.

In the second approach, a weighted motion vector consensus is developed that uses the same weighting factors as in the weighted position consensus, except that the weights are applied to 12-h motion vectors between 84 and 120 h. This weighted motion vector consensus has substantially smaller errors than the unweighted position consensus, and results in smoother tracks when one or more of the model tracks drops out of the consensus. It is proposed that the weighted motion vector consensus would provide improved guidance for the 96-, 108-, and 120-h tropical cyclone track forecasts.

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Russell L. Elsberry
,
Tara D. B. Lambert
, and
Mark A. Boothe

Abstract

Five statistical and dynamical tropical cyclone intensity guidance techniques available at the National Hurricane Center (NHC) during the 2003 and 2004 Atlantic and eastern North Pacific seasons were evaluated within three intensity phases: (I) formation; (II) early intensification, with a subcategory (IIa) of a decay and reintensification cycle; and (III) decay. In phase I in the Atlantic, the various techniques tended to predict that a tropical storm would form from six tropical depressions that did not develop further, and thus the tendency was for false alarms in these cases. For the other 24 depressions that did become tropical storms, the statistical–dynamical techniques, statistical hurricane prediction scheme (SHIPS) and decay SHIPS (DSHIPS), have some skill relative to the 5-day statistical hurricane intensity forecast climatology and persistence technique, but they also tend to intensify all depressions and thus are prone to false alarms. In phase II, the statistical–dynamical models SHIPS and DSHIPS do not predict the rapid intensification cases (≥30 kt in 24 h) 48 h in advance. Although the dynamical Geophysical Fluid Dynamics Interpolated model does predict rapid intensification, many of these cases are at the incorrect times with many false alarms. The best performances in forecasting at least 24 h in advance the 21 decay and reintensification cycles in the Atlantic were the three forecasts by the dynamical Geophysical Fluid Dynamics Model-Navy (interpolated) model. Whereas DSHIPS was the best technique in the Atlantic during the decay phase III, none of the techniques excelled in the eastern North Pacific. All techniques tend to decay the tropical cyclones in both basins too slowly, except that DSHIPS performed well (12 of 18) during rapid decay events in the Atlantic. This evaluation indicates where NHC forecasters have deficient guidance and thus where research is necessary for improving intensity forecasts.

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Ryan M. Kehoe
,
Mark A. Boothe
, and
Russell L. Elsberry

Abstract

The Joint Typhoon Warning Center has been issuing 96- and 120-h track forecasts since May 2003. It uses four dynamical models that provide guidance at these forecast intervals and relies heavily on a consensus of these four models in producing the official forecast. Whereas each of the models has skill, each occasionally has large errors. The objective of this study is to provide a characterization of these errors in the western North Pacific during 2004 for two of the four models: the Navy Operational Global Atmospheric Prediction System (NOGAPS) and the U.S. Navy’s version of the Geophysical Fluid Dynamics Laboratory model (GFDN). All 96- and 120-h track errors greater than 400 and 500 n mi, respectively, are examined following the approach developed recently by Carr and Elsberry. All of these large-error cases can be attributed to the models not properly representing the physical processes known to control tropical cyclone motion, which were classified in a series of conceptual models by Carr and Elsberry for either tropical-related or midlatitude-related mechanisms. For those large-error cases where an error mechanism could be established, midlatitude influences caused 83% (85%) of the NOGAPS (GFDN) errors. The most common tropical influence is an excessive direct cyclone interaction in which the tropical cyclone track is erroneously affected by an adjacent cyclone. The most common midlatitude-related errors in the NOGAPS tracks arise from an erroneous prediction of the environmental flow dominated by a ridge in the midlatitudes. Errors in the GFDN tracks are caused by both ridge-dominated and trough-dominated environmental flows in the midlatitudes. Case studies illustrating the key error mechanisms are provided. An ability to confidently identify these error mechanisms and thereby eliminate likely erroneous tracks from the consensus would improve the accuracy of 96- and 120-h track forecasts.

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Russell L. Elsberry
,
Mark A. Boothe
,
Greg A. Ulses
, and
Patrick A. Harr

Abstract

A statistical postprocessing technique is developed and tested to reduce the U.S. Navy global model (NOGAPS) track forecast errors for a sample of western North Pacific tropical cyclones during 1992–96. The key piece of information is the offset of the initial NOGAPS position relative to an updated (here best-track) position that will be known by 6 h after the synoptic times, which is when the NOGAPS forecast is actually available for use by the forecaster. In addition to the basic storm characteristics, the set of 24 predictors includes various segments in the 0–36-h NOGAPS forecast track as well as a 0–36-h backward extrapolation that is compared with the known recent track positions. As statistically significant regressions are only found for 12–36 h, the original 36-h to 72-h NOGAPS track segment is simply translated to the adjusted 36-h position. For the development sample, the adjusted NOGAPS track errors are reduced by about 51 n mi (95 km) at 12 h, 35 n mi (65 km) at 36 h, and 28 n mi (52 km) at 72 h. Independent tests with a 1997 western North Pacific sample, 1995–97 Atlantic sample, and 1996–97 eastern and central North Pacific sample of NOGAPS forecasts have similar improvements from the postprocessing technique. Thus, the technique appears to have a more general applicability to Northern Hemisphere tropical cyclones.

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Jeng-Ming Chen
,
Russell L. Elsberry
,
Mark A. Boothe
, and
Lester E. Carr III

Abstract

A simple statistical-synoptic technique for tropical cyclone (TC) track forecasting to 72 h in the western North Pacific is derived. This technique applies to the standard (S) pattern/dominant ridge region (S/DR) and poleward/poleward-oriented (P/PO) combinations, which are the two most common and represent about 73% of all situations. Only eight predictors that involve present and past 12-h and 24-h positions, intensity, and date are used. The track predictions are simple to calculate and understand; are available in near–real time each 6 h; apply at all intensities, as compared to the complex global or regional dynamical model predictions that are only available each 12 h at about 3–4 h after synoptic time; are not calculated for weak TCs; and tend to have accurate predictions only for tropical storm stage and above. The statistical-synoptic technique for S/DR cases has an improvement (skill) relative to the operational climatology and persistence (WPCLPR) technique of 12% after 12 h and 24% after 72 h if the TC remains in the S/DR pattern/region for the entire 72 h. The statistical-synoptic technique for P/PO cases have an improvement relative to WPCLPR of 11% after 12 h and about 13% for 72-h forecasts if the TC remains in P/PO for the entire 72 h.

Assuming a perfect knowledge of the S/DR to P/PO and P/PO to S/DR transitions, a simple blending of a composite post-transition track with the statistical-synoptic technique is tested. For the 72-h forecasts initiated 12 h before the S/DR to P/PO transition, the statistical-synoptic track error is about 290 n mi (537 km) compared to 410 n mi (759 km) for WPCLPR. For corresponding P/PO to S/DR transition, the statistical-synoptic technique 72-h error is 215 n mi (398 km) compared to about 485 n mi (898 km) for WPCLPR.

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Louis L. Lussier III
,
Blake Rutherford
,
Michael T. Montgomery
,
Mark A. Boothe
, and
Timothy J. Dunkerton

Abstract

The tropical cyclogenesis sequence of Hurricane Sandy is examined. It is shown that genesis occurs within a recirculating Kelvin cat’s-eye flow of a westward-propagating tropical wave. The cat’s-eye flow is able to provide a protective environment for the mesoscale vortex to grow and is characterized by gradual column moistening and increased areal coverage of deep cumulus convection. These findings are generally consistent with a recently proposed tropical cyclogenesis sequence referred to as the “marsupial paradigm.” Sandy’s cyclogenesis provides a useful illustration of the marsupial paradigm within a partially open recirculating region, with the opening located south of the pouch center. It is suggested that the opening acts to enhance the genesis process because it is adjacent to an environment characterized by warm, moist air, conditions favorable for tropical cyclogenesis. From a dynamical perspective, accretion of low-level cyclonic vorticity filaments into the developing vortex from several sources (the South American convergence zone, an easterly wave located west of the pre-Sandy wave, and cyclonic vorticity generated along Hispaniola) is documented. Organization and growth of the nascent storm is enhanced by this accretion of cyclonic vorticity. A Lagrangian trajectory analysis is used to assess potential contributions to Sandy’s spinup from a Caribbean gyre and the easterly wave that formed Hurricane Tony. This analysis indicates that these features are outside of the Lagrangian flow boundaries that define the pre-Sandy wave and do not directly contribute to spinup of the vortex. Finally, the effectiveness of forecasts from the U.S. and European operational numerical weather prediction models is discussed for this case.

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Michael T. Montgomery
,
Christopher Davis
,
Timothy Dunkerton
,
Zhuo Wang
,
Christopher Velden
,
Ryan Torn
,
Sharanya J. Majumdar
,
Fuqing Zhang
,
Roger K. Smith
,
Lance Bosart
,
Michael M. Bell
,
Jennifer S. Haase
,
Andrew Heymsfield
,
Jorgen Jensen
,
Teresa Campos
, and
Mark A. Boothe

The principal hypotheses of a new model of tropical cyclogenesis, known as the marsupial paradigm, were tested in the context of Atlantic tropical disturbances during the National Science Foundation (NSF)-sponsored Pre-Depression Investigation of Cloud Systems in the Tropics (PREDICT) experiment in 2010. PREDICT was part of a tri-agency collaboration, along with the National Aeronautics and Space Administration's Genesis and Rapid Intensification Processes (NASA GRIP) experiment and the National Oceanic and Atmospheric Administration's Intensity Forecasting Experiment (NOAA IFEX), intended to examine both developing and nondeveloping tropical disturbances.

During PREDICT, a total of 26 missions were flown with the NSF/NCAR Gulfstream V (GV) aircraft sampling eight tropical disturbances. Among these were four cases (Fiona, ex-Gaston, Karl, and Matthew) for which three or more missions were conducted, many on consecutive days. Because of the scientific focus on the Lagrangian nature of the tropical cyclogenesis process, a wave-relative frame of reference was adopted throughout the experiment in which various model- and satellite-based products were examined to guide aircraft planning and real-time operations. Here, the scientific products and examples of data collected are highlighted for several of the disturbances. The suite of cases observed represents arguably the most comprehensive, self-consistent dataset ever collected on the environment and mesoscale structure of developing and nondeveloping predepression disturbances.

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