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Yoshio Kurihara, Morris A. Bender, and Rebecca J. Ross

Abstract

A scheme is presented to improve the representation of a tropical cyclone in the initial condition of a high-resolution hurricane model. In the proposed method, a crudely resolved tropical cyclone in the large-scale analysis is replaced by a vortex that is properly specified for use in the prediction model.

Appropriate filters are used to remove the vortex from the large-scale analysis so that a smooth environmental field remains. The new specified bogus vortex takes the form of a deviation from this environmental held so that it can be easily merged with the latter field at the correct position. The specified vortex consists of both axisymmetric and asymmetric components. The symmetric component is generated by the time integration of an axisymmetric version of the hurricane prediction model. This ensures dynamical and thermodynamical consistency in the vortex structure, including the moisture field, and also compatibility of the vortex with the resolution and physics of the hurricane model. In the course of the integration of the axisymmetric model, the tangential wind component is gradually forced to a target wind profile determined from observational information and empirical knowledge. This makes the symmetric vortex a good approximation to the corresponding real tropical cyclone. The symmetric flow thus produced is used to generate an asymmetric wind field by the time integration of a simplified barotropic vorticity equation, including the beta effect. The asymmetric wind field, which can make a significant contribution to the vortex motion, is then added to the symmetric flow. After merging the specified vortex with the environmental flow, the mass field is diagnosed from the divergence equation with an appropriately controlled time tendency. The wind field remains unchanged at this step of initialization.

Since the vortex specified by the proposed method is well adapted to the hurricane prediction model, problems of initial adjustment and false spinup of the model vortex, a long-standing difficulty in the dynamical prediction of tropical cyclones, are alleviated. It is anticipated that the improvement of the initial conditions can reduce the error in hurricane track forecasting and extend the feasibility of tropical cyclone forecasting to intensity change.

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Morris A. Bender, Robert E. Tuleya, and Yoshio Kurihara

Abstract

A triply-nested, movable mesh model was used to study the effects of a mountain range on a landfalling tropical cyclone embedded in an easterly flow of ∼10 m s−1. The integration domain consisted of a 37° wide and 45° long channel, with an innermost mesh resolution of 1/6°. An idealized mountain range with maximum height of ∼958 meters was placed parallel to the shoreline. The mountain range, which spanned 19° in the north–south direction and 5° in the east–west direction, was centered in the middle of the channel. Results obtained were compared with a previous landfall simulation, performed without the effect of the mountain range included. In particular, comparison was made of the total storm rainfall, maximum wind distribution and storm decay rate. It was found that the storm filled much more rapidly in the simulation run with the mountain included. The mountain range affected the decay rate through reduction in the supply of latent and kinetic energy into the storm circulation during, as well as after, passage of the storm over the mountain. It was found that a low-level, warm and dry region was produced where the storm winds descended the mountain slope.

In order to better isolate the effect of the mountain on the basic easterly flow, a supplemental integration was performed for the flow without the storm. It revealed that the mountain range caused a significant change in the basic flow over the mountain as well as up to several hundred kilometers downstream and extending considerably above the mountain top. A low-level southerly jet was observed to the west of the mountain base.

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Morris A. Bender, Robert E. Tuleya, and Yoshio Kurihara

Abstract

A triply nested, movable mesh model was used to study the behavior of tropical cyclones encountering island mountain ranges. The integration domain consisted of a 37° wide and 45° long channel, with an innermost mesh resolution of 1/6°. The storms used for this study were embedded in easterly flows of ∼5 and ∼10 m s−1 initially. Realistic distributions of island topography at 1/6° resolution were inserted into the model domain for the region of the Caribbean, including the islands of Cuba, Hispaniola, and Puerto Rico; the island of Taiwan; and the region of Luzon in the northern Philippines.

It was found that the islands affected the basic flow as well as the wind field directly associated with the storm system. The combination of these effects caused changes in the track and translational speed of the storm. In particular, in the case of the 5 m s−1 easterly flow, the storm accelerated and veered to the north well before reaching Taiwan. For the other island distributions, the northward deflection of the track and the increase of translational speed occurred near and over the islands. After landfall, the surface pressure underwent rapid filling. As the tropical cyclone passed over Hispaniola, the surface low continued to move along with the upper level vortex as it transversed the mountain range, while over Luzon it became obscure before reforming on the lee side slope of the mountain. In case of Taiwan and the 10 m s−1 easterly zonal flow, secondary surface lows developed behind the mountain range. The upper level vortex in this case became detached from the original surface low and eventually coupled with a secondary one.

The intensity changes of the storm near and over the islands were strongly related to the latent energy supply and the vertical coherence of the storm system. Advection of dry air from near or above the mountain tops into the storm area caused significant weakening of all the storms moving with the weaker easterly flow. Storms leaving Hispaniola and moving over open sea quickly reintensified as their vertical structure remained coherent. On the other hand, storms leaving Luzon were disorganized and did not reintensify until several hours later when the vertical coherence of the systems was reestablished.

Although these experiments were performed for an idealized experimental design and basic flow, many observed storms have exhibited similar behavior in track deviation and decay. This implies that the effect of detailed topography should be considered if an accurate forecast of the storm direction and behavior is to be made.

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Morris A. Bender, Timothy P. Marchok, Charles R. Sampson, John A. Knaff, and Matthew J. Morin

Abstract

The impact of storm size on the forecast of tropical cyclone storm track and intensity is investigated using the 2016 version of the operational GFDL hurricane model. Evaluation was made for 1529 forecasts in the Atlantic, eastern Pacific, and western North Pacific basins, during the 2014 and 2015 seasons. The track and intensity errors were computed from forecasts in which the 34-kt (where 1 kt = 0.514 m s−1) wind radii obtained from the operational TC vitals that are used to initialize TCs in the GFDL model were replaced with wind radii estimates derived using an equally weighted average of six objective estimates. It was found that modifying the radius of 34-kt winds had a significant positive impact on the intensity forecasts in the 1–2 day lead times. For example, at 48 h, the intensity error was reduced 10%, 5%, and 4% in the Atlantic, eastern Pacific, and western North Pacific, respectively. The largest improvements in intensity forecasts were for those tropical cyclones undergoing rapid intensification, with a maximum error reduction in the 1–2 day forecast lead time of 14% and 17% in the eastern and western North Pacific, respectively. The large negative intensity biases in the eastern and western North Pacific were also reduced 25% and 75% in the 12–72-h forecast lead times. Although the overall impact on the average track error was neutral, forecasts of recurving storms were improved and tracks of nonrecurving storms degraded. Results also suggest that objective specification of storm size may impact intensity forecasts in other high-resolution numerical models, particularly for tropical cyclones entering a rapid intensification phase.

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Morris A. Bender, Rebecca J. Ross, Robert E. Tuleya, and Yoshio Kurihara

Abstract

The initialization scheme designed at GFDL to specify a more realistic initial storm structure of tropical cyclones was tested on four real data cases using the GFDL high-resolution multiply nested movable mesh hurricane model. Three of the test cases involved Hurricane Gloria (1985) in the Atlantic basin; the fourth involved Hurricane Gilbert (1988) in the Gulf of Mexico. The initialization scheme produced an initial vortex that was well adapted to the forecast model and was much more realistic in size and intensity than the storm structure obtained from the NMC T80 global analysis. As a result, the erratic storm motion seen in previous integrations of the GFDL model has been nearly eliminated with dramatic improvements in track forecasts during the first 48 h of the prediction. Using the new scheme, the average 24-h and 48-h forecast error for the four test cases was 58 and 94 km, respectively, compared with 143 and 191 km for the noninitalized forecasts starting from the global analysis. The average National Hurricane Center operational forecast error at 24 and 48 h was 118 and 212 km for the same four cases. After 48 h the difference in the average track error became small between the integrations starting from the global analysis and the forecasts starting from the fields obtained by the initialization scheme.

With accurate specification of the initial vortex structure, changes in the storm intensity were also well predicted in them cases. The model correctly forecasted the rapid intensification of Hurricane Gloria just after the system was first upgraded to a hurricane. The model storm intensification also ceased at approximately the same time as observed, with gradual weakening as the storm moved north and approached the east coast of the United States. In the forecast of Hurricane Gilbert, the model storm initially weakened as it moved over the Yucatan Peninsula and underwent only moderate reintensification after moving over the Gulf of Mexico, in good agreement with observations.

Finally, in the case where the track of Hurricane Gloria was well forecast, the distribution of the maximum low-level wind was accurately predicted as the storm moved up the east coast of the United States. During this period the model successfully reproduced many observed features such as large asymmetries in the wind field, with strongest winds occurring well east of the storm center, and a sharp decrease of the wind speed at the coast. Although the asymmetry in the wind distribution was reproduced to a first order in the forecast starting with the global analysis, the agreement with observations was much better with the specified vortex, primarily due to a more realistic radius of maximum wind and storm intensity.

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Yoshio Kurihara, Morris A. Bender, Robert E. Tuleya, and Rebecca J. Ross

Abstract

The prediction capability of the GFDL triply nested, movable mesh model, with finest grid resolution of ⅙ degree, was investigated using several case studies of Hurricane Gloria ( 1985) during the period that the storm approached and moved up the east coast of the United States. The initial conditions for these experiments were interpolated from an NMC T80 global analysis at 0000 UTC 25 September and 1200 UTC 22 September. The integrations starting from 0000 UTC 25 September were run 72 h, while those starting on 1200 UTC 22 September were run 132 h. The lateral boundary conditions were obtained from either an integration of the NMC T80 forecast model or the T80 global analysis, or were fixed to the initial value.

The model's predicted track of Gloria for each integration was compared against the best track determined by the National Hurricane Center (NHC). For the case starting from 0000 UTC 25 September using a forecasted boundary condition, the model successfully forecasted significant acceleration of the storm's movement after 48 h. The 72 h forecast error was about 191 km, compared to 480 km for the official track forecast made by the NHC.

To examine the model's skill in simulating the storm structure, distributions of the low level maximum wind and total storm rainfall during passage of the model storm are shown and compared with observed values. The model successfully reproduced many observed features such as the occurrence of strong winds well east of the storm center, with an abrupt decrease of the wind field along the coastline. When the storm track was accurately forecasted, the total storm rainfall amounts agreed well with the observed values. In both the model integration and observations, a significant structural change took place as the storm accelerated toward the north with little significant precipitation occurring south of the storm center and heavy precipitation spreading well north of the storm. It appears that the gross features of the structure of the storm's outer region resulted from the interaction of the vortex with its environment.

Sensitivity of the model forecast to the lateral boundary condition and the horizontal resolution was also investigated. The storm's track error was greatly affected after the boundary error propagated by advection to the storm region. The impact of the horizontal resolution on the forecast was such that the model with one degree resolution produced a fairly good track forecast up to 48 h, but failed to simulate some of the main structural features.

In the experiments starting from the 0000 UTC September 25 initial field, the interior storm structure did not develop, and the storm exhibited too large a radius of maximum wind throughout the integration. However, the integrations starting from 1200 UTC September 22 developed a more intense storm, with a more realistic radius of maximum wind. These differences were due to the spinup time necessary for the storm to develop in the model when starting from a coarse resolution global analysis which did not adequately resolve the fine structure of the storm interior. This indicates the importance of proper specification of the storm in the initial field.

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Morris A. Bender, Isaac Ginis, Robert Tuleya, Biju Thomas, and Timothy Marchok

Abstract

The past decade has been marked by significant advancements in numerical weather prediction of hurricanes, which have greatly contributed to the steady decline in forecast track error. Since its operational implementation by the U.S. National Weather Service (NWS) in 1995, the best-track model performer has been NOAA’s regional hurricane model developed at the Geophysical Fluid Dynamics Laboratory (GFDL). The purpose of this paper is to summarize the major upgrades to the GFDL hurricane forecast system since 1998. These include coupling the atmospheric component with the Princeton Ocean Model, which became operational in 2001, major physics upgrades implemented in 2003 and 2006, and increases in both the vertical resolution in 2003 and the horizontal resolution in 2002 and 2005. The paper will also report on the GFDL model performance for both track and intensity, focusing particularly on the 2003 through 2006 hurricane seasons. During this period, the GFDL track errors were the lowest of all the dynamical model guidance available to the NWS Tropical Prediction Center in both the Atlantic and eastern Pacific basins. It will also be shown that the GFDL model has exhibited a steady reduction in its intensity errors during the past 5 yr, and can now provide skillful intensity forecasts. Tests of 153 forecasts from the 2004 and 2005 Atlantic hurricane seasons and 75 forecasts from the 2005 eastern Pacific season have demonstrated a positive impact on both track and intensity prediction in the 2006 GFDL model upgrade, through introduction of a cloud microphysics package and an improved air–sea momentum flux parameterization. In addition, the large positive intensity bias in sheared environments observed in previous versions of the model is significantly reduced. This led to the significant improvement in the model’s reliability and skill for forecasting intensity that occurred in 2006.

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Andrew T. Hazelton, Morris Bender, Matthew Morin, Lucas Harris, and Shian-Jiann Lin

Abstract

The 2017 Atlantic hurricane season had several high-impact tropical cyclones (TCs), including multiple cases of rapid intensification (RI). A high-resolution nested version of the GFDL finite-volume dynamical core (FV3) with GFS physics (fvGFS) model (HifvGFS) was used to conduct hindcasts of all Atlantic TCs between 7 August and 15 October. HifvGFS showed promising track forecast performance, with similar error patterns and skill compared to the operational GFS and HWRF models. Some of the larger track forecast errors were associated with the erratic tracks of TCs Jose and Lee. A case study of Hurricane Maria found that although the track forecasts were generally skillful, a right-of-track bias was noted in some cases associated with initialization and prediction of ridging north of the storm. The intensity forecasts showed large improvement over the GFS and global fvGFS models but were somewhat less skillful than HWRF. The largest negative intensity forecast errors were associated with the RI of TCs Irma, Lee, and Maria, while the largest positive errors were found with recurving cases that were generally weakening. The structure forecasts were also compared with observations, and HifvGFS was found to generally have wind radii larger than the observations. Detailed examination of the forecasts of Hurricanes Harvey and Maria showed that HifvGFS was able to predict the structural evolution leading to RI in some cases but was not as skillful with other RI cases. One case study of Maria suggested that the inclusion of ocean coupling could significantly reduce the positive bias seen during and after recurvature.

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Yoshio Kurihara, Morris A. Bender, Robert E. Tuleya, and Rebecca J. Ross

Abstract

The hurricane model initialization scheme developed at GFDL was modified to improve the representation of the environmental fields in the initial condition. The filter domain defining the extent of the tropical cyclone in the global analysis is determined from the distribution of the low-level disturbance winds. The shape of the domain is generally not circular in order to minimize the removal of important nonhurricane features near the storm region. An optimum interpolation technique is used to determine the environmental fields within the filter domain. Outside of the domain, the environmental fields are identical to the original global analysis. The generation process of the realistic and model-compatible vortex has also undergone some minor modifications so that reasonable vortices are produced for various data conditions. The upgraded hurricane prediction system was tested for a number of cases and compared against the previous version and yielded an overall improvement in the forecasts of storm track. The system was run in an automated semioperational mode during the 1993 hurricane season for 36 cases in the Atlantic and 36 cases in the eastern Pacific basin. It demonstrated satisfactory skill in the storm track forecasts in many cases, including the abrupt recurvature of Hurricane Emily in the Atlantic and the landfall of Hurricane Lidia onto the Pacific coast of Mexico.

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Morris A. Bender, Timothy Marchok, Robert E. Tuleya, Isaac Ginis, Vijay Tallapragada, and Stephen J. Lord

Abstract

The hurricane project at the National Oceanic and Atmospheric Administration (NOAA) Geophysical Fluid Dynamics Laboratory (GFDL) was established in 1970. By the mid-1970s pioneering research had led to the development of a new hurricane model. As the reputation of the model grew, GFDL was approached in 1986 by the director of the National Meteorological Center about establishing a collaboration between the two federal organizations to transition the model into an operational modeling system. After a multiyear effort by GFDL scientists to develop a system that could support rigorous requirements of operations, and multiyear testing had demonstrated its superior performance compared to existing guidance products, operational implementation was made in 1995. Through collaboration between GFDL and the U.S. Navy, the model was also made operational at Fleet Numerical Meteorology and Oceanography Center in 1996. GFDL scientists continued to support and improve the model during the next two decades by collaborating with other scientists at GFDL, the National Centers for Environmental Prediction (NCEP) Environmental Modeling Center (EMC), the National Hurricane Center, the U.S. Navy, the University of Rhode Island (URI), Old Dominion University, and the NOAA Hurricane Research Division. Scientists at GFDL, URI, and EMC collaborated to transfer key components of the GFDL model to the NWS new Hurricane Weather Research and Forecasting Model (HWRF) that became operational in 2007. The purpose of the article is to highlight the critical role of these collaborations. It is hoped that the experiences of the authors will serve as an example of how such collaboration can benefit the nation with improved weather guidance products.

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