Ensemble Forecast of a Typhoon Flood Event

AUGUST 2001 MACKEY AND KRISHNAMURTI 399 Ensemble Forecast of a Typhoon Flood Event BRIAN P. M ACKEY AND T. N . K RISHNAMURTI Department of Meteorology, The Florida State University, Tallahassee, Florida (Manuscript received 9 September 1999, in ®nal form 2 February 2001) ABSTRACT A high-resolution nested regional spectral model and an ensemble prediction system are combined to forecast the track, intensity, and ¯ooding precipitation arising from Typhoon Winnie of August 1997, which eventually reached supertyphoon status. The prediction of ¯oods is operationally challenging since rainfall distributions can have a high degree of spatial and temporal variability. Rare event probabilities, however, can be estimated more readily via ensemble forecasting. This technique is used to evaluate a typhoon ¯ood event in which rainfall amounts greater than 200 mm led to landslides and major ¯ooding of crops. Seven-member ensembles were generated using an EOF-based technique. An experiment was conducted with a regional model resolution of 0.58 latitude. A Mercator transform grid with a grid mesh size of approximately 55 km in the east±west and 48 km in the north±south was employed. The results indicated very accurate track and intensity forecasts for both the control and ensemble mean. Track position errors remained below 150 km through 72 h, while intensity errors were approximately5ms21 at landfall. Qualitatively, the overall 5-day precipitation patterns appeared realistic and compared favorably with the observed data, while, quantitatively, the correlation coef®cient was near 0.6. For stations near and north of where Winnie made landfall, ensemble-based predictions performed well. While the ensemble mean often underestimated the heaviest rainfall totals by approximately 25%±50%, the maximum values within the ensemble spread either exceeded or came within 10%±15% of the station totals. Finally, in a related experiment the horizontal resolution was increased to 0.258 latitude. Even though more precipitation was produced, especially in northeastern China, the ensemble mean was similar to the 0.58 latitude simulation. 1. Introduction spiral bands, and even along inverted troughs to the Tropical cyclones are among the world's most serious north of the cyclone. These troughs are most pronounced threats to life and property with the primary dangers in the western North Paci®c basin and near the coast of being storm surges, strong winds, and ¯ooding due to China. In the case of Typhoon Winnie, heavy rains oc- heavy rainfall. According to a World Meteorological curred in each of these areas. Moreover, coastal bound- Organization (WMO) study (De and Joshi 1998), trop- aries and mountainous terrain provide another important ical cyclones rank second among all natural disasters in means for producing torrential rainfall and landslides. loss of life behind drought. As a result, nearly 900 000 Over northern Taiwan, for instance, the hilly, steep ter- deaths occurred around the world from 1967 to 1991. rain played a role in creating mudslides as Winnie Typhoons in the western North Paci®c Ocean basin an- passed by to the north. Finally, increased convergence nually cause more than $4 billion in total losses based along a coastline to the right of a storm (in the Northern on estimates by the Asian and Paci®c Typhoon Com- Hemisphere) can occur because of blocking and fric- mittee of the WMO (Chen 1995). tional effects of a landmass (Chen 1995). In this case, In this study, the focus will be on the ¯ooding rains the large landmass of China likely served to increase resulting from the passage of Typhoon Winnie of August the convergence and heavy precipitation to the north of 1997, which seriously affected portions of Taiwan and the typhoon's center. eastern China. Intense precipitation in tropical cyclones In order to investigate this event, the Florida State can be traced back to an enhanced hydrological cycle University Nested Regional Spectral Model (FSUNRSM) due to warm sea surface temperatures, sustained high (Cocke 1998) is employed, along with an empirical or- winds, and sea spray (Raj 1998). As a storm approaches land, ¯ooding rains can occur within the core region, thogonal function (EOF) perturbation method following Zhang (1997), to generate ensemble forecasts of the track, intensity, and precipitation. The regional model Corresponding author address: Brian P. Mackey, Department of used here is designed to be compatible with the Florida Meteorology, The Florida State University, 404 Love Building, Tal- lahassee, FL 32306-4520. State University Global Spectral Model (FSUGSM). E-mail: [email protected] Details about the models are available in the appendices. q 2001 American Meteorological Society Unauthenticated | Downloaded 09/27/21 11:17 AM UTC 400 WEATHER AND FORECASTING VOLUME 16 2. Ensemble forecast of ¯oods are scaled to approximate the 3-h forecast error, which is 3 m s21 for the wind ®eld and 0.68 K for the tem- Ensemble forecasting takes into account the inherent perature ®eld. uncertainty in the initial data ®elds as well as the model An ensemble forecast can easily provide much more formulation of the atmosphere. This introduces a prob- information than a single, deterministic model run since abilistic aspect to the model forecasts as well as a sig- each individual forecast within the ensemble suite can ni®cant challengeÐto ensure that an ensemble includes be assigned a probability of occurrence. Ensemble ¯ood the full range of possible initial states so that all potential forecasts are especially bene®cial as rainfall distribu- forecasts can be considered. This is practically impos- tions may exhibit large variability on all time and space sible to accomplish. Nevertheless, by suf®ciently per- scales. Model-based precipitation errors can quickly be- turbing the initial conditions, a wide range of plausible come signi®cant even over small areas. Ensembles are forecasts is attainable. It can be shown that the best also useful tools in evaluating extreme or unusual perturbations are those that (a) adequately sample the weather events, such as ¯oods. If an ensemble ade- error probability distribution, and (b) give distinct, yet quately samples all possible forecasts, then such an reasonable, outcomes. For example, if it is known a event is likely to be well predicted by the ®rst moment, priori that two forecast results are quite similar, then it and an understanding of the uncertainty is possible from is unnecessary to run both cases. the second moment. Hence, it is obvious that probabil- The perturbation method employed in this study is ities of an extreme event can be estimated more readily based upon EOFs. An ensemble is generated speci®cally via ensemble forecasting (Hamill and Colucci 1998). for tropical weather prediction (Zhang 1997; Zhang and Very few studies have addressed the use of ensemble Krishnamurti 1997, 1999). Other methods currently in NWP methods to forecast tropical cyclone±related use around the world are more suited for midlatitude ¯oods. First, a successful prediction of a typhoon's prediction and therefore may not be adequate for trop- heavy rain areas is based upon an accurate forecast of ical studies due to the differences in dynamics, physics, the typhoon's path, strength, and structure. In addition, and causes and rates of perturbation growth. For in- air±sea interactions and moisture, temperature, and wind stance, singular vectors (Buizza and Palmer 1995; Mol- ®elds surrounding the typhoon must be accurately rep- teni et al. 1996) are predicated on linear dynamical in- resented as well as boundary parameters, such as sea stability and leave out physical processes. In addition, surface temperatures and topography. By using a proven the breeding method (Toth and Kalnay 1997) determines high-resolution spectral model and an ensemble predic- perturbations for the entire globe, and it largely depends tion system, a detailed case study of Typhoon Winnie upon baroclinic instabilities over the midlatitudes. is performed where the observed track and intensity is The EOF-based ensemble technique examines only compared to that from a 5-day ensemble forecast. This two of the most important tropical variablesÐtemper- study will focus on the rainfall and subsequent ¯ooding ature and windÐpartly in order to keep computations caused by Winnie. By comparing station and satellite- at a minimum. A moisture parameter may also be uti- derived precipitation data to the model ensemble, a qual- lized, however. As a ®rst step, random temperature and itative assessment of the forecast skill is given. A ®nal wind perturbations with amplitudes comparable to fore- goal of this study is to examine what effect, if any, a cast errors are added to the control analysis. Then, both higher-resolution regional forecast would have on the the unperturbed (control) and the perturbed analyses are precipitation patterns and amounts. integrated for an optimal 36 h using the complete ver- In this research, the ensemble maps will attempt to sion of the FSUGSM with full physics. During this time, convey useful probabilistic information to the user. An it is presupposed that only linear growth has occurred. ensemble mean chart of the total precipitation is one From both forecasts, time series of difference ®elds of such product. It is best to treat this type of map with temperature and wind are constructed every 3 h begin- caution, however, as some smoothing of the maximum ning with hour 6 of the forecast. This ensures that ran- rainfall is probable. The ensemble mean will tend to dom effects do not degrade the process. Next, scalar enhance characteristics that are similar to all members EOF analysis is performed on the temperature time se- while giving little signi®cance to their differences (Siv- ries while a complex EOF technique is used for the wind illo and Ahlquist 1997). Marked smoothing is especially series. Ultimately, the ®rst three eigenvectors (modes) noticeable if the rainfall probability distribution is bi- are added to and subtracted from the control analysis modal, or doubly peaked in two separate areas.

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