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David E. Kofron
,
Elizabeth A. Ritchie
, and
J. Scott Tyo

Abstract

As a tropical cyclone moves poleward and interacts with the midlatitude circulation, the question of whether it will undergo extratropical transition (ET) and, if it does, whether it will reintensify or dissipate, is a complex problem. Uncertainties include the tropical cyclone, the midlatitude circulation, the subtropical anticyclone, and the nonlinear interactions among these systems. A large part of the uncertainty is due to a lack of an understanding of when extratropical transition begins and how it progresses. In this study, absolute potential vorticity and isentropic, or Ertel’s, potential vorticity is examined for its ability to more consistently determine significant times (i.e., beginning or end) of the ET life cycle using the Navy Operational Global Assimilation and Prediction System gridded analyses.

It is found that isentropic potential vorticity on the 330-K potential temperature isentropic level is a good discriminator for examining the extratropical transition of tropical cyclones. At this level, a consistent “ET time” is defined as when the TC-centered circular average of isentropic potential vorticity reaches a minimum value. All 82 tropical cyclones moving into the midlatitudes meet this criterion. The completion of extratropical transition for the reintensifying cases is defined as when the storm exceeds an isentropic potential vorticity threshold value of 1.6 PVU at the 330-K potential temperature isentropic level. The success rate of this threshold value for the completion of extratropical transition for the reintensification cases is found to be 94.3% with a 27.6% false-alarm rate.

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Liang Hu
,
Elizabeth A. Ritchie
, and
J. Scott Tyo

Abstract

The deviation angle variance (DAV) is a parameter that characterizes the level of organization of a cloud cluster compared with a perfectly axisymmetric tropical cyclone (TC) using satellite infrared (IR) imagery, and can be used to estimate the intensity of the TC. In this study, the DAV technique is further used to analyze the relationship between satellite imagery and TC future intensity over the North Atlantic basin. The results show that the DAV of the TC changes ahead of the TC intensity change, and this can be used to predict short-term TC intensity. The DAV-IR 24-h forecast is close to the National Hurricane Center (NHC) 24-h forecast, and the bias is lower than NHC and other methods during weakening periods. Furthermore, an improved TC intensity forecast is obtained by incorporating all four satellite bands. Using SST and TC latitude as the other two predictors in a linear regression model, the RMSE and MAE of the DAV 24-h forecast are 13.7 and 10.9 kt (1 kt ≈ 0.51 m s−1), respectively, and the skill space of the DAV is about 5.5% relative to the Statistical Hurricane Intensity Forecast model with inland decay (Decay-SHIFOR) during TC weakening periods. Considering the DAV is an independent intensity technique, it could potentially add value as a member of the suite of operational intensity forecast techniques, especially during TC weakening periods.

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Klaus Dolling
,
Elizabeth A. Ritchie
, and
J. Scott Tyo

Abstract

This study extends past research based on the deviation angle variance (DAV) technique that utilizes digital brightness temperatures from longwave infrared satellite images to objectively measure the symmetry of a tropical cyclone (TC). In previous work, the single-pixel DAV values were used as an objective estimator of storm intensity while maps of the DAV values indicated areas where tropical cyclogenesis was occurring. In this study the spatial information in the DAV maps is utilized along with information from the Cooperative Institute for Research in the Atmosphere’s extended best-track archive and the Statistical Hurricane Intensity Prediction Scheme model to create multiple linear regression models of wind radii parameters for TCs in the North Atlantic basin. These models are used to estimate both symmetric, and by quadrant, 34-, 50-, and 64-kt wind radii (where 1 kt = 0.51 m s−1 1) on a half-hourly time scale. The symmetric model assumes azimuthal symmetry and has mean absolute errors of 38.5, 23.2, and 13.5 km (20.8, 12.5, and 7.3 n mi) for the 34-, 50-, and 64-kt wind radii, respectively, which are lower than results for most other techniques except for those based on AMSU. The asymmetric model independently estimates radii in each quadrant and produces mean absolute errors for the wind radii that are generally highest in the northwest quadrant and lowest in the southwest quadrant similar to other techniques. However, as a percentage of the average wind radii from aircraft reconnaissance, all quadrants have similar errors.

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Miguel F. Piñeros
,
Elizabeth A. Ritchie
, and
J. Scott Tyo

Abstract

This paper describes results from a near-real-time objective technique for estimating the intensity of tropical cyclones from satellite infrared imagery in the North Atlantic Ocean basin. The technique quantifies the level of organization or axisymmetry of the infrared cloud signature of a tropical cyclone as an indirect measurement of its maximum wind speed. The final maximum wind speed calculated by the technique is an independent estimate of tropical cyclone intensity. Seventy-eight tropical cyclones from the 2004–09 seasons are used both to train and to test independently the intensity estimation technique. Two independent tests are performed to test the ability of the technique to estimate tropical cyclone intensity accurately. The best results from these tests have a root-mean-square intensity error of between 13 and 15 kt (where 1 kt ≈ 0.5 m s−1) for the two test sets.

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Liang Hu
,
Elizabeth A. Ritchie
, and
J. Scott Tyo

Abstract

The deviation angle variance (DAV) is a parameter that characterizes the level of organization of a cloud cluster compared with a perfectly axisymmetric tropical cyclone (TC) using satellite infrared (IR) imagery, and can be used to estimate the intensity of the TC. In this study, the DAV technique is further used to analyze the relationship between satellite imagery and TC future intensity over the North Atlantic basin. The results show that the DAV of the TC changes ahead of the TC intensity change, and this can be used to predict short-term TC intensity. The DAV-IR 24-h forecast is close to the National Hurricane Center (NHC) 24-h forecast, and the bias is lower than NHC and other methods during weakening periods. Furthermore, an improved TC intensity forecast is obtained by incorporating all four satellite bands. Using SST and TC latitude as the other two predictors in a linear regression model, the RMSE and MAE of the DAV 24-h forecast are 13.7 and 10.9 kt (1 kt ≈ 0.51 m s−1), respectively, and the skill space of the DAV is about 5.5% relative to the Statistical Hurricane Intensity Forecast model with inland decay (Decay-SHIFOR) during TC weakening periods. Considering the DAV is an independent intensity technique, it could potentially add value as a member of the suite of operational intensity forecast techniques, especially during TC weakening periods.

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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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Elizabeth A. Ritchie
,
Kimberly M. Wood
,
David S. Gutzler
, and
Sarah R. White

Abstract

Forty-three eastern North Pacific tropical cyclone remnants with varying impact on the southwestern United States during the period 1992–2005 are investigated. Of these, 35 remnants (81%) brought precipitation to some part of the southwestern United States and the remaining 8 remnants (19%) had precipitation that was almost entirely restricted to Mexico, although cloud cover did advect over the southwestern United States in some of these cases. Although the tropical cyclone–strength winds rapidly diminish upon making landfall, these systems still carry a large quantity of tropical moisture and, upon interaction with mountainous topography, are found to drop up to 30% of the local annual precipitation.

Based on common rainfall patterns and large-scale circulation features, the tropical cyclones are grouped into five categories. These include a northern recurving pattern that is more likely to bring rainfall to the southwestern United States; a southern recurving pattern that brings rainfall across northern Mexico and the Gulf Coast region; a largely north and/or northwestward movement pattern that brings rainfall to the west coast of the United States; a group that is blocked from the southwest by a ridge, which limits rainfall to Mexico; and a small group of cases that are not clearly any of the previous four types. Composites of the first four groups are shown and forecasting strategies for each are described.

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Steven R. Felker
,
Brian LaCasse
,
J. Scott Tyo
, and
Elizabeth A. Ritchie

Abstract

Intensity changes following the multistage process of extratropical transition have proven to be especially difficult to forecast because of the extremely similar storm evolutions prior to and during the first stages of the transformation from a warm-cored axisymmetric tropical storm to a cold-cored asymmetrical extratropical low pressure system. In this study, differences in surrounding synoptic environments between dissipating and reintensifying extratropical transitioning tropical cyclones are used to develop a predictive technique for extratropical transition intensity change that can be used to enhance the standard numerical guidance. Using a set of all historical transitioning storms between 2000 and 2008 in the western North Pacific, common differences between 850-hPa potential temperature fields surrounding extratropical transition intensifiers and extratropical transition dissipaters, respectively, were identified. These features were then used as inputs into a support vector machine classification system in the hopes of creating a robust prediction system. Once the system was trained on a random subset of the data (80%), performance was tested on the remaining test set (20%). Overall, it was found that the prediction system was able to correctly predict extratropical transition intensity outcome in >75% of the test cases at 72 h prior to extratropical transition. This paper discusses the feature selection and classification system used, as well as the performance results, in detail.

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Elizabeth A. Ritchie
,
Genevieve Valliere-Kelley
,
Miguel F. Piñeros
, and
J. Scott Tyo

Abstract

This paper describes results from an improvement to the objective deviation angle variance technique to estimate the intensity of tropical cyclones from satellite infrared imagery in the North Atlantic basin. The technique quantifies the level of organization of the infrared cloud signature of a tropical cyclone as an indirect measurement of its maximum wind speed. The major change described here is to use the National Hurricane Center’s best-track database to constrain the technique. Results are shown for the 2004–10 North Atlantic hurricane seasons and include an overall root-mean-square intensity error of 12.9 kt (6.6 m s−1, where 1 kt = 0.514 m s−1) and annual root-mean-square intensity errors ranging from 10.3 to 14.1 kt. A direct comparison between the previous version and the one reported here shows root-mean-square intensity error improvements in all years with a best improvement in 2009 from 17.9 to 10.6 kt and an overall improvement from 14.8 to 12.9 kt. In addition, samples from the 7-yr period are binned based on level of intensity and on the strength of environmental vertical wind shear as extracted from Statistical Hurricane Intensity Prediction Scheme (SHIPS) data. Preliminary results suggest that the deviation angle variance technique performs best at the weakest intensity categories of tropical storm through hurricane category 3, representing 90% of the samples, and then degrades in performance for hurricane categories 4 and 5. For environmental vertical wind shear, there is far less spread in the results with the technique performing better with increasing vertical wind shear.

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Greg J. Holland
,
Lance M. Leslie
,
Elizabeth A. Ritchie
,
Gary S. Dietachmayer
,
Peter E. Powers
, and
Mark Klink

Abstract

The design concept and operational trial of a fully interactive analysis and numerical forecast system for tropical-cyclone motion are described. The design concept emphasizes an interactive system in which forecasters can test various scenarios objectively, rather than having to subjectively decide between conflicting forecasts from standardized techniques. The system is designed for use on a personal computer, or workstation, located on the forecast bench. A choice of a Barnes or statistical interpolation scheme is provided to analyze raw or bogus observations at any atmospheric level or layer mean selected by the forecaster. The track forecast is then made by integration of a nondivergent barotropic model.

An operational trial during the 1990 tropical-cyclone field experiments in the western north Pacific Ocean indicated that the system can be used very effectively in real time. A series of case-study examples is presented.

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