Mean Tracks Research Articles

Track forecast: Operational capability and new techniques - Summary from the Tenth International Workshop on Tropical Cyclones (IWTC-10)

In this paper, we summarize findings from the Tenth International Workshop on Tropical Cyclones (IWTC-10) subgroup on operational track forecasting techniques and capability.The rate of improvement in the accuracy of official forecast tracks (OFTs) appears to be slowing down, at least for shorter lead times, where we may be approaching theoretical limits. Operational agencies continue to use consensus methods to produce the OFT with most continuing to rely on an unweighted consensus of four to nine NWP models. There continues to be limited use of weighted consensus techniques, which is likely a result of the skills and additional maintenance needed to support this approach. Improvements in the accuracy of ensemble mean tracks is leading to increased use of ensemble means in consensus tracks.Operational agencies are increasingly producing situation-dependent depictions of track uncertainty, rather than relying on a static depiction of track forecast certainty based on accuracy statistics from the preceding 5 years. This trend has been facilitated by the greater availability of ensemble NWP guidance, particularly vortex parameter files, and improved spread in ensembles. Despite improving spread-skill relationships, most ensemble NWP systems remain under spread. Hence many operational centers are looking to leverage “super-ensembles” (ensembles of ensembles) to ensure the full spread of location probability is captured. This is an important area of service development for multi-hazard impact-based warnings as it supports better decision making by emergency managers and the community in the face of uncertainty.

Poleward Shift in Tropical Cyclone Tracks in the Northwest Pacific During Warm Periods: Past and Future

AbstractWe used the top age of storm‐relocated coral blocks (SRCBs) from Xisha Islands and Huangyan Island, South China Sea (SCS), in combination with existing paleotempestology records and instrumental climatic data, to reconstruct the spatio‐temporal changes in tropical cyclone (TC) activity over the past millennium, and to project its future trends in the Northwestern Pacific region. We found a positive correlation between the frequencies of TCs, typhoons, and super‐typhoons and the NINO3.4 on the interannual timescale during 1945–2019. TCs latitude has a significant positive correlation with sea surface temperature, with a northward migration rate of 80 ± 11 km per decade during 1951–2019. Coral δ18O can well indicate the month of paleo‐TCs occurrence. The frequency of SRCBs (and by inference, TCs) is relatively low during relatively warm periods (RWP) but high during relatively cold periods (RCP) in the SCS. The spatio‐temporal pattern inferred from paleotempestology records further supports the notion of North‐South migration in TC activity as revealed by the instrumental record in this study. TC activity mainly affected relatively low latitudes (e.g., Philippines, Vietnam, and Hainan, Guangdong, Taiwan) during the RCP (AD 1450–1900), but it migrated from low latitudes towards higher latitudes during the RWP (AD 1000–1450), affecting locations such as Zhejiang, Jiangsu, and Shandong, southwestern Japan, the Korean Peninsula, and even Northeast China. Fujian and northern Guangdong may be in a transition zone of paleo‐TC activity between the North and the South in the past millennium. As global warming continues, the mean tracks and landfall locations of TCs will continue to migrate northward in this region.

Improvements in tropical cyclone forecasting through ensemble prediction system at NCMRWF in India

This paper deals with the comparison of cyclone forecasts from the two versions of the operational global ensemble prediction system (EPS) at the National Centre for Medium Range Weather Forecasting (NEPS). The previous version had a horizontal resolution of 33 km with 44 ensemble members (NEPS) whereas the updated version of this EPS has a resolution of 12 km with 11 members (NEPS-UP). The ensemble mean forecasts from both the models are compared using the direct position (DPE), along (ATE) and cross track (CTE) errors. For the verification of strike probability, Brier Score (BS), Brier Skill Score (BSS), Reliability Diagram, Relative Operating Characteristic (ROC) Curve and Root Mean Square Error (RMSE) in mean Vs Spread in members are used. For verification of intensity, RMSE in maximum wind speed from the ensemble mean forecasts are compared.Comparison of ensemble mean tracks from both models showed lower errors in NEPS-UP for all forecast lead times. The decrease in the DPE, ATE and CTE in NEPS-UP was around 38%, 48% and 15% respectively. NEPS-UP showed lower BS and higher BSS values indicating a better match between observed frequencies and forecast probabilities as well as higher prediction skills. The reliability diagram showed higher accuracy for NEPS-UP as compared to NEPS. The ROC curves showed that for forecasts with higher probabilities the hit rate was high in NEPS-UP. There was a greater consensus between the RMSE and Spread for NEPS-UP at all lead times. It was also seen that the RMSE in mean showed a 41% decrease from NEPS to NEPS-UP. On comparing maximum wind, it was found that for all lead times the RMSE in maximum wind speed for NEPS-UP was lower than NEPS.

A study of quasi-millennial extratropical winter cyclone activity over the Southern Hemisphere

The winter extratropical cyclone activity in the Southern Hemisphere during the last one thousand years within a global climate simulation was analyzed by tracking cyclones, and then clustering them into ten clusters consecutively for each hundred years. There is very strong year-to-year variability for Southern Hemispheric winter extratropical cyclone numbers and larger variations on centennial time scale, more so than for its Northern Hemispherical counterparts. However, no obvious trend can be found. The mean tracks of clusters over the Southern Indian Ocean and near New Zealand shift poleward from the eleventh to the twentieth century while the clusters in the central Southern Pacific shift equatorward. Storm track clusters with largest deepening rates are found over the Southwestern Indian Ocean. In the twentieth century, rapidly deepening cyclones appear more often while long lifespan cyclones appear less frequently. The winter storm activity in the Southern Hemisphere is closely related to the Antarctic Oscillation. The cyclone frequency over the Indian Ocean and South Pacific Ocean can be associated with the Indian Ocean Dipole and El Nino-Southern Oscillation respectively.

ENSO Effect on East Asian Tropical Cyclone Landfall via Changes in Tracks and Genesis in a Statistical Model

AbstractImprovements on a statistical tropical cyclone (TC) track model in the western North Pacific Ocean are described. The goal of the model is to study the effect of El Niño–Southern Oscillation (ENSO) on East Asian TC landfall. The model is based on the International Best-Track Archive for Climate Stewardship (IBTrACS) database of TC observations for 1945–2007 and employs local regression of TC formation rates and track increments on the Niño-3.4 index and seasonally varying climate parameters. The main improvements are the inclusion of ENSO dependence in the track propagation and accounting for seasonality in both genesis and tracks. A comparison of simulations of the 1945–2007 period with observations concludes that the model updates improve the skill of this model in simulating TCs. Changes in TC genesis and tracks are analyzed separately and cumulatively in simulations of stationary extreme ENSO states. ENSO effects on regional (100-km scale) landfall are attributed to changes in genesis and tracks. The effect of ENSO on genesis is predominantly a shift in genesis location from the southeast in El Niño years to the northwest in La Niña years, resulting in higher landfall rates for the East Asian coast during La Niña. The effect of ENSO on track propagation varies seasonally and spatially. In the peak activity season (July–October), there are significant changes in mean tracks with ENSO. Landfall-rate changes from genesis– and track–ENSO effects in the Philippines cancel out, while coastal segments of Vietnam, China, the Korean Peninsula, and Japan show enhanced La Niña–year increases.

Climate change effects on North Atlantic cyclones

We investigated the possible climate change impacts on midlatitude northwest Atlantic storms using a mesoscale atmospheric model driven by the second‐generation Coupled Global Climate Model (CGCM2) environmental fields, for control (1975–1994) and high‐CO2 (2040–2059) years of the Intergovernmental Panel on Climate Change (IPCC) IS92a scenarios. Using ensemble experiments, we show that for a given storm initialization location, the high‐CO2 climate change scenario results in slightly higher mean peak storm intensity than that of the control climate. However, the impact of averaged environmental scenario fields on storm intensity and track is dependent on the storm initialization location within these averaged scenario fields. Under the climate change scenario, storms that are initially located south of 35°N and west of 65°W exhibit the largest differences in storm intensities and tracks, compared to the present climate. Storms with trajectories that move close to the upper level steering jet exhibit the largest differences in mean tracks, moving closer to the North American coast.

Ensemble forecasting of tropical cyclone motion: comparisonbetween regional bred modes and random perturbations

Random perturbations (RPs) and a modified version for breeding of growing modes are used with a regional baroclinic mesoscale model to perform ensemble forecasting of tropical cyclone motion. Based on a sample of six cases, similar conclusions are found as in previous barotropic modeling studies. Even after introducing a larger spatial correlation into the RPs using a multi-quadric analysis scheme, the skill of this ensemble mean track prediction is almost always lower than that of the control forecast in the cases considered. The track prediction performance of the ensemble using regional bred modes (RBMs) as perturbations has a higher average skill. At nearly all forecast intervals except less than 24 h when the initial position error still dominates, the ensemble mean tracks in all six cases are improved over the control forecast. In the 6 h–24 h range, the success rate (ratio of the cases with a forecast improvement to the total number of cases) has a value of 10/24. In the 30 h–48 h range, the success rate increases to 20/24, but drops to 18/24 in the 54 h–72 h range. A relative skill score (RSS) is used to compare the skills of the two perturbation methodologies. It is found that the average RSSs of using RBMs are significantly higher than the corresponding ones of RPs at the 99% confidence level in all three 24-h periods. Note that the above conclusion is only based on ensemble mean forecasts. All of the possibilities from an ensemble-based probabilistic track distribution are not explored in this paper. The ensemble spreads in these RBM ensembles are large enough to include the verifying tracks in all the cases considered. It is also found that the ensemble spread is well correlated with the average error in an ensemble when using RBMs, but not with the ensemble mean forecast error in both methodologies.

The satellite altimeter data derived mean sea surface GSFC98

The GSFC98 mean sea surface (MSS) was computed on a 2' oceanwide grid for latitudes below 80°. The data used included 3‐years of TOPEX data (Cycles 9 to 119), 1.5‐years of ERS‐1 35‐day repeat cycle (Cycles 1 to 18), 2‐years of Geosat ERM data (Cycles 1 to 42), 2 ERS‐1 168‐day repeat cycles, and 18 months of the Geosat Geodetic Mission data. All non‐TOPEX satellite altimeter data were adjusted to the mean of TOPEX data in 2° × 30° blocks. After the adjustment, the mean sea surface height was gridded into 2' nodes using least squares collocation and an iteration procedure to reduce the ocean variability. To validate the mean sea surfaces, three comparisons were made. The GSFC98 MSS, along with OSU95 [Yi, 1995] and CSR95 [Kim et al., 1995], were compared with 6‐years of TOPEX and 3‐years of ERS‐2 mean tracks which were not used in the MSS computations. Finally, the marine gravity anomalies were computed from the three MSS implied geoid undulations using the inverse Stokes integral. The marine gravity anomalies were compared with ship gravity data in selected areas. The ship gravity comparison is an independent assessment of the quality of the MSSs, especially at intermediate and short wavelengths. Finally, the inter‐comparisons between the mean sea surfaces were made. The root mean square values of the differences were 6.8, 6.8, and 7.2 cm between GSFC98/OSU95, GSFC98/CSR95, and OSU95/CSR95. The differences agree well with the error estimation of GSFC98 MSS.

Storm tracks and zonal mean flow variability: a comparison between observed and simulated data

A statistical technique is used to analyze the relation between monthly mean zonal flow and storm tracks activity in the observations and numerical simulations (ECHAM4 model). The singular value decomposition technique (SVD) has been used to correlate storm tracks and monthly mean wintertime anomaly fields. The analysis has been performed on data from January 1980 to December 1989 (NMC analyses) and on an ensemble of AGCM simulations with prescribed SST for the same period, separately in the Euro-Atlantic and Pacific sectors. We found good correlation between storm tracks activity and zonal flow in both regions. In both data and simulations the dominant SVD modes show that the storm tracks spatial displacement is in conjunction with jet shifts in the same direction. Our analysis suggests that the model is highly sensitive to the equatorial ocean forcing. Although the model produces an excessive response to El Nino and La Nina phases, it shows good capability of simulating the dynamical connection between storm tracks and jet.

Anomalous behavior of the irreversible magnetization and time relaxation inYBa2Cu3O7single crystals with splayed tracks

We have studied the angular dependence of the irreversible magnetization and its time relaxation in YBa$_2$Cu$_3$O$_7$ single crystals with one or two families of columnar defects inclined with respect to the c-axis. At high magnetic fields, the magnetization shows the usual maximum centered at the mean tracks' orientation and an associated minimum in the normalized relaxation rate. In contrast, at low fields we observe an anomalous local minimum in the magnetization and a maximum in the relaxation rate. We present a model to explain this anomaly based on the slowing down of the creep processes arising from the increase of the vortex-vortex interactions as the applied field is tilted away from the mean tracks' direction.

Temporal changes in eddy energy of the oceans

Insight into the sources of eddy energy in the ocean can be obtained by studying the degree and nature of its temporal changes. As a preliminary to a dynamical and modeling discussion, we provide a description of the changes in variability on the annual and interannual time scales as seen in the TOPEX/POSEIDON altimeter data and in current meter records with a focus on the North Atlantic and North Pacific Oceans. The patterns of change and corresponding ones in the wind stress are globally very variable and intricate, with few easy generalizations possible. Over most of the subtropical oceans and along major mean fronts, seasonal variations of the eddy energy are negligible. There are, however, regions that show a pronounced annual cycle in eddy energy, notably the northeastern Pacific, the northern and eastern North Atlantic, as well as the tropical oceans. In those locations a strong correlation of a time-varying altimetric eddy kinetic energy on annual and longer periods with wind stress forcing is found, and trends present in near-surface eddy kinetic energy can be related to drifts in meteorological wind stress fields (mean and storm tracks) over the four-year TOPEX/POSEIDON record. While the seasonal cycles in mooring data are found generally to agree with altimetry, the only statistically significant evidence for interannual trends in current meter data appears in the long duration eastern Atlantic site mooring near 33°N, 22°W.

Frontal depressions over the Mediterranean sea and central southern Europe. Les perturbations frontales au-dessus de la mer Méditerranée et de l'Europe centrale méridionale

An analysis of the frequency of occurrence of frontal depressions over central-southern Europe and the Mediterranean Sea was carried out based on daily synoptic charts for a nine year period from 1971 to 1979. The frontal depressions are classified into five types (A, B, C, D, E) based on the regions in which they originally formed. Tables and charts are presented showing the annual and seasonal frontal depression frequencies over the area. Also, the variation of average frequency of frontal depressions with latitude and longitude is demonstrated. The distribution of the frequency of frontal depressions fits in with generally accepted models of hemispheric circulation. The mean tracks of the frontal depression centres were determined. The ten individual tracks are shown together in a map and they emphasize the way in which so many frontal depression centres migrate towards Corsica and nothern Italy from various points in the west of the study area.

Northeastward reverse migration of birds over Nova Scotia, Canada, in autumn

During 4 seasons of study, small numbers of birds flew overland to the NNE-E, counter to the main SW migration, intermittently throughout the ‘autumn’ (31 July – 16 November). Radars also detected overwater reverse migration (RM) from New England to Nova Scotia and from Nova Scotia toward Newfoundland. RM occurred at all hours of the day and night, especially when few birds were migrating SW. Most but not all cases of RM occurred with following SW winds. RM was more common with cloud and/or poor visibility than in fair weather, but was not restricted to cloudy occasions. Mean tracks were correlated with wind direction, but were not consistently downwind. Tracks tended to be closer to downwind in early than in late autumn, and on clear than on overcast nights. Dispersion in tracks was not discernibly related to weather variables, time of day or night, or magnetic disturbance. Eleven hypotheses concerning the reasons for reverse migration in autumn are evaluated; cases of RM recorded in this study are attributable to at least three of these hypotheses (late summer dispersal, hurricanes, dawn reorientation toward coast) and possibly to several others.

Southeastward shorebird migration over Nova Scotia and New Brunswick in autumn: a radar study

Shorebirds departed over the Atlantic intermittently by night and (in lesser numbers) day in late summer and early autumn. Migrations were broad-front and usually began inland (including Bay of Fundy), often 1–2 h before sunset. Tracks were straight, with daily means of 110–170° (overall mean 134°). About 69% were flying on tracks directed too far east to reach South America without a course change; the roles of wind drift and heading adjustments in effecting this course change are uncertain. Ground speeds were 74 ± SD 17 km/h; altitudes were often high (mean 2.0 km, 5% of flocks above 4.4 km, highest 6.65 km). Many departed with W–NW winds and fair weather behind cold fronts or E or N of high pressure areas, but some took off ahead of or caught up with and penetrated cold fronts. Mean tracks were correlated with wind direction, but were rarely downwind. Their deviation from downwind was unrelated to visibility, cloud, or magnetic disturbance (K index). Dispersion of tracks did not change consistently during nights or days and was unrelated to wind or K index. Dispersion at night was slightly greater in inclement than fair weather.

A digital orthogonal model for nonlinear processes with two-level inputs

This paper describes a method by which the class of nonlinear processes with switched two-level inputs and finite settling times can be identified and an adaptive model of the process constructed. The adaptive model uses only the process input-output records. After a suitable identification time (approximately 14-70 times the settling time of the process) the model approximates the plant performance using a mean square error criteria and tracks any changes in the plant parameters. The past of the two-level input is stored in a digital shift register tapped at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</tex> points, thus forming a function space comprised of 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sup> nonoverlapping or orthogonal cells. By averaging the output wave-form during the time that a cell is occupied a coefficient is obtained which characterizes the output for that input condition. A basic assumption about the input waveform statistics reduces the number of characterizing coefficients from 1024 to approximately 50. The model that is evolved is a small synchronous digital computer. The model is quite versatile as it is independent of the type of process nonlinearity and can adapt to systems with different settling times. The model is ideal for use in predictive adaptive control systems where a fast time predictive model is utilized.