Russell L. Elsberry Applications oi Department of Meteorology Naval Postgraduate School Tropical Cyclone Models Monterey, Calif. 93940 Abstract and Eastern Pacific hurricanes (Sanders et al., 1977). A The extensive research in tropical cyclone modeling during the balanced barotropic model for typhoon movement pre- 1960s and 1970s has resulted in a number of applications for diction has been run by the Japan Meteorological real-time track prediction. A review of the characteristics of Agency. Similar barotropic models are also being de- these 3-dimensional dynamical models is given, including a veloped in India (Singh and Saha, 1976), Taiwan discussion of procedures for initializing and tracking the (Tsay, 1977), and the Phillipines. In some cases the model storm. Some limited verifications of track forecasts are described. An outlook for the future is presented; both in initial fields are based on hand analyses. Until objective terms of numerical model improvements, and for large-scale analysis schemes arc developed and tested with the and inner-scale data required to implement the improved models on an operational basis, it is unlikely that these models. models will have a practical application. 1. Introduction 2. General description of models At the Sixth Technical Exchange Conference in 1970, S. Rosenthal of the National Hurricane Research Laboratory reported results from the first 3-dimensional A few of the physical characteristics of some selected numerical model with the physical processes and hori- baroclinic models are shown in Table 1. The National zontal resolution necessary to resolve tropical cyclones. Meteorological Center (NMC) Moveable Fine Mesh Since that time a number of 3-dimensional models have (MFM) has been used on an operational basis for several been developed—both for research purposes and for years (Hovermale and Livezey, 1977). A Tropical operational prediction of tropical cyclone tracks. The Cyclone Model (TCM) has been operationally tested characteristics of these operational models, and those for Pacific Ocean storms at the Fleet Numerical being tested with operational data, will be reviewed. Weather Center (FNWC), as described by Hinsman Some of this material must be considered as interim (1977) and Mihok and Hinsman (1977). The model since several of the models to be described are in various (Madala and Hodur, 1977) being developed jointly by stages of development and testing. After describing the Naval Research Laboratory (NRL) and the Naval generally the characteristics of the models, I shall Environmental Prediction Research Facility (NEPRF) discuss the initial fields that are required and the is intended to replace the FNWC-TCM after further methods of tracking the model tropical cyclone. An testing with operational data. Following this, a nested evaluation of the track forecasts will then be presented. grid version is to be developed and tested by NRL and Some of the results or conclusions based on rather NEPRF. Some limited testing of the Pennsylvania limited sample sizes may have to be reexamined in the State University (PSU) mesoscale model for several light of additional cases. The last section of this paper western Pacific typhoon cases is being done at the Naval will be concerned with the outlook for the future and Post-graduate School (NPS). The primary purpose of will focus on some of the data problems that are being these tests (Hacunda, 1978) is to compare the predic- encountered in the operational use of fine-scale tropical tions with the FNWC-TCM and NRL/NEPRF models cyclone models. using the same input data. Anthes (1978) has shown that the PSU mesoscale model (with 60 km resolution) This survey is limited to 3-dimensional models, and improved the forecasts in 32 cases over Europe and the thus excludes a number of barotropic models that have United States. These further tests over data-sparse proved very useful for tropical cyclone motion fore- oceans, together with the different physics represented casts. Experience with these models has helped antici- by typhoon circulations, will provide additional in- pate the types of problems that will occur during opera- formation regarding the applicability of these fine mesh tional implementation of the more complex baroclinic models. The last model listed in Table 1 is the Japanese models. The best known of the barotropic models is Meteorological Agency (J MA) movable multiply- the SANBAR model, which was developed by Sanders nested grid (MNG) developed by Ookochi (1978). and Burpee (1968) and subsequently modified by Pike This model has been tested on a few typhoons approach- (1972) and by Sanders et al. (1975). Various versions of ing Japan. this model have been extensively tested with Atlantic The entries in Table 1 suggest wide variances in model characteristics, and consequently it is not possible 0003-0007/79/070750-13$07.25 to evaluate directly the differences in the model pre- © 1979 American Meteorological Society dictions. The FNWC and JMA models have minimal 750 Vol. 60, No. 7, July 1979 Unauthenticated | Downloaded 10/11/21 01:38 AM UTC Bulletin American Meteorological Society 751 TABLE 1. Characteristics of several baroclinic models being applied for prediction of tropical cyclone motion based on operational data (see text for explanation). Lateral Vertical Number Grid size Number Relocatable boundary Agency-Model coordinate of layers (km) of points grid conditions* NMC-MFM <r 10 60 50 X 50 Yes OW FNWC-TCM p 3 205 32 X 24 No OW NRL/NEPRF a- 5 60 51 X 51 No OW PSU/NPS c 5 120 40 X 40 No OW JMA-MNG c 3 291 31 X 31 No OW 145 31 X 31 Yes TW 73 31 X 31 Yes TW 36 31 X 31 Yes TW * OW—One-way interaction; TW—Two-way interaction. vertical resolution with only three layers, which are Each of these models must be provided time-de- able to represent the inflow, outflow, and an inter- pendent values along the outer boundaries from some mediate layer of the mature typhoon circulation. Al- other prediction model. The interface boundary condi- though the NRL/NEPRF and PSU/NPS models have tion is of the one-way (OW) type (Phillips and Shukla, improved resolution with five layers, only the NMC- 1973), in that no feedback of information is provided MFM has adequate vertical resolution. However, it from the tropical cyclone model to the hemispheric should be noted that the tendency for tropical observa- model. An early version (Hinsman, 1977) of the FNWC- tions to be distributed in two layers—near the surface TCM model was in a channel, and thus required no and at predominately jet aircraft levels—makes initiali- information from the larger scale model after the initial zation of a 10-layer model rather difficult. time. Hodur and Burk (1977) have demonstrated that It has been known for some time that finer spatial one-way boundary conditions improve the TCM fore- grids are required to predict tropical circulations than casts during recurvature situations. The JMA nested are commonly used for mid-latitude prediction models. grid model has two-way interaction conditions for the Except for the FNWC-TCM model, each of the models inner grids. This requires a simultaneous integration on in Table 1 attempts to resolve the inner structure of the all grids rather than a sequential integration as in one- tropical cyclone circulation. It is generally assumed way integration. Various interface boundary conditions that the primary interaction between the vortex and have been employed to permit mass, momentum and the steering current can be resolved on the 60 km grid energy to flow between the grids (Elsberry, 1978). Al- used by the NMC and NRL/NEPRF models. How- though these conditions can never be perfect because ever, to attempt intensity forecasts one must resolve of the differences in resolvable waves with different the inner region of the typhoon (Elsberry, 1975 ; Jones, grid lengths, it appears that acceptable numerical solu- 1977a). This will require a nested grid arrangement be- tions have been obtained (see e.g., Anthes, 1976; Jones, cause the entire domain cannot be covered with the 1977a,b; Sobel, 1976). approximately 10 km resolution that will be necessary Note that the requirement in every case for lateral for the innermost grid. The second characteristic that boundary conditions from a hemispheric model places will be required for intensity forecasts will be an ability an important operational constraint on these tropical to move the inner grids with the storm. The JMA model cyclone forecast models. The normal procedure would possesses both of these characteristics, although the be to await the completion of the hemispheric forecast inner grid presently has only 36 km resolution. The model run. Allowing for the collection and analysis of other models in Table 1 have a uniform grid lattice the initial data, this means that the tropical model will that generally covers at least a 3000 km square. Even not be initiated until 6 h or more after map time (/0). this may be inadequate for 72 h forecasts of fast-moving Consequently the model guidance may not actually be storms. All of the models can be positioned at the initial used by the forecaster until to + 12 h. For example, time to minimize the chance that the expected storm Jarvinen (1977) points out that the SANBAR forecast motion will bring the storm center near the boundaries. is not available to the National Hurricane Center until Only the NMC model has the capability of being re- to + 4 h, and the 48 h NMC-MFM forecast appears at located during the forecast. This relocatability feature to + 8 h. Likewise, Mihok and Hinsman (1977) have allows one to keep the storm near the center of the attempted to reduce the greater than to + 9 h lag time domain, away from adverse boundary effects, and con- for the FNWC-TCM.