Recurving Western North Pacific Tropical Cyclones and Mid-Latitude Predictability

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL.???, XXXX, DOI: /, 1 2 Recurving Western North Pacific Tropical Cyclones and Mid-Latitude Predictability A. Aiyyer, 1 Corresponding author: A. Aiyyer, Department of Marine, Earth and Atmospheric Sciences, North Carolia State University, Raleigh, NC USA. (aaiyyer@ncsu.edu) 1 North Carolina State University, Raleigh, NC USA.

2 X - 2 A. AIYYER: TROPICAL CYCLONE RECURVATURE AND PREDICTABILTY Abstract Data from an ensemble prediction system are used to examine the impact of recurving tropical cyclones on downstream midlatitude forecasts. The ensemble spread, normalized by its climatology, increases after recurvature and peaks approximately 4-5 days later. It returns to climatological levels within a week after recurvature. Initially, the spread increases around the position of the tropical storm. Subsequently, it increases after extratropical transition, and it is associated with a developing wavepacket in the midlatitude storm track. The enhanced spread propagates downstream approximately at the group speed of the wavepacket. These results suggest that, relative to the model s baseline, recurvature related increase in loss of forecast skill is spatially and temporally localized. Further, energy dispersion of the developing wavepacket may constrain the rate at which the forecast errors propagate downstream.

3 1. Introduction A. AIYYER: TROPICAL CYCLONE RECURVATURE AND PREDICTABILTY X Tropical cyclones often interact with midlatitude flow and undergo extratropical transition (ET). Recurvature of poleward moving tropical cyclones commonly precedes ET. Archambault et al. [2013] report that approximately 36% of all western North pacific cyclones recurve. Cyclone recurvature and subsequent ET are known to force downstream response in the form of amplifying baroclinic wavepackets in the midlatitude stormtrack [e.g. Jones et al., 2003; Riemer et al., 2008; Harr and Dea, 2009; Archambault et al., 2015]. Torn and Hakim [2015] show that the wavepacket associated with ET develops locally and does not require the amplification of a preexisting disturbance in the stormtrack. When forecast models do not capture the details of ET process adequately, their ability to predict the hemispheric flow pattern is also challenged [e.g. Anwender et al., 2008; Harr et al., 2008]. Standard deviation, or the spread among members, is commonly used as a measure of predictive skill of an ensemble forecasting system. Large spread is often assumed to be an indicator of reduced forecast skill. In a case study of Typhoon Nabi(2005), Harr et al. [2008] show that ensemble spread increases about 18 hours after ET. They conclude that ET, not forecast lead time, is the cause of reduced predictability. Anwender et al. [2008] examine five cases of ET and show that plumes of forecast uncertainty extend downstream from ET location. The availability of long term reforecast data using a fixed numerical ensemble prediction system offers the opportunity to extend the results of Harr et al. [2008] and Anwender et al. [2008] to multiple storms. This motivates the present study. Furthermore, while

4 X - 4 A. AIYYER: TROPICAL CYCLONE RECURVATURE AND PREDICTABILTY ensemble spread is often used as a proxy for forecast skill, the two are not always well correlated [e.g. Barker, 1991]. Whitaker and Loughe [1998] show that spread is a better predictor of skill when, relative to climatological values, it is either very large or very small. The objective of this study is to document the ensemble spread for a large number of recurving tropical cyclones in the western North Pacific. The fixed ensemble prediction system used here allows us to define a long-term baseline(climatological) spread and create recurvature-relative composites with statistically robust signals. As in Archambault et al. [2015], we use TC recurvature as the reference point instead of ET since the downstream wavepacket response begins even before ET is completed. 2. Data and Method Ensemble mean and spread of the 500-hPa geopotential height fields are taken from National Oceanic and Atmospheric Administration s (NOAA s) 2nd generation global ensemble reforecast dataset. These are based on a 11-member ensemble using a fixed model and comprise once-daily, 16-day forecasts. Details of the dataset can be found in Hamill et al. [2013]. 500-hPa height fields from the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) project [Dee et al., 2011] are also used. When required, this data is interpolated to the reforecast grid using a bilinear fit. The ERA-Interim data are used to calculate the anomaly correlation and to create lagged composites. Tropical cyclone tracks are obtained from the International Best Track Archive for Climate Stewardship [IBTrACS v03r05; Knapp et al., 2010]. The study period encompasses August-October, These months are most frequently associated

5 A. AIYYER: TROPICAL CYCLONE RECURVATURE AND PREDICTABILTY X with recurving tropical cyclones in the western North Pacific [e.g. Archambault et al., 2013]. Here climatology for any variable is defined as the long-term daily mean followed by a 7-day running mean. The resulting daily data is a robust estimate of the synoptic climatology for each grid point. Statistical significance is tested using the two-tailed Welch s t-test with a critical significance level of The null hypothesis is that recurvature based composite mean and the climatological mean are from the same population Recurvature TC recurvature is determined using an objective algorithm that tracks the storm s direction of motion. Recurvature points are also visually inspected to ensure ensure efficacy of the algorithm. Only storms that recurved within the latitude bounds of 20 o 35 o N and within the longitudinal bounds of 100 o 200 o are considered. Since the reforecast data are initialized daily at 00 UTC, and TC locations are available in 6-hourly intervals, recurvature times between 1800 UTC and 0600 UTC are assigned to the nearest 0000 UTC reforecast initialization time. Storms that recurved at 1200 UTC are neglected. This allows us to consider forecasts initialized at one day lags relative to the recurvature time. A total of 146 storms are found to match the above criteria. The time of completion of ET is obtained from the IBTrACS dataset and deemed to be the first instance when storm is classified as extratropical. Of all the storms considered here, the majority (120) underwent ET. The median ET completion time is found to be 2.25 days after recurvature. It should be noted that given the subjectivity in ET

6 X - 6 A. AIYYER: TROPICAL CYCLONE RECURVATURE AND PREDICTABILTY classification, the median ET time may be off by 6-12 hours and individual storms may have larger or smaller lags between recurvature and ET Normalized Spread The measure of forecast skill used here is the normalized spread. It is defined as the ratio of the ensemble spread for each forecast hour and the climatological spread for the same hour. The normalized spread is an indicator of potentially enhanced or reduced forecast skill loss relative to the baseline value for each forecast hour. A ratio greater than unity represents potentially reduced forecast skill. In calculations shown here, we use storm relative composites by shifting the grids such that all storms have a common recurvature point at 27.5 o N and 150 o E. 3. Results Figure 1 shows the Hovmöller diagram for composite anomalies of recurvature-relative 500-hPa heights averaged over o N using the ERA-Interim data. While a weak upstream wave is present, the main features are the ridge around the position of the TC and a developing wavepacket downstream of it. The thin horizontal line in Fig. 1 marks the median time of ET. It can be seen that wavepacket amplification precedes ET and continues after it. The zonal extent of the wavepacket exceeds 150 o of longitude over 6 days. The wavelength of the anomalies within the wavepacket is approximately 60 o. The dashed red line that connects the maxima within the wavepacket depicts the group propagation. The slope of this line yields an estimated group speed of approximately 39 ms 1. This is close to the value found by Archambault et al. [2015]. The dashed blue lines show the phase propagation. The phase speed prior to TC recurvature is approximately 9 ms 1.

7 A. AIYYER: TROPICAL CYCLONE RECURVATURE AND PREDICTABILTY X On the other hand, the phase speed of the ridge associated with the TC is nearly zero. The near-stationarity of the ridge likely stems from the fact that around the days of the recurvature, the composite TC is nearly stationary in the zonal direction. The composite TC during these days is effectively only moving poleward, and the ridge development is tied to the TC forcing. The phase speed of the downstream wavepacket is also small at approximately 3.5 m 1 s. Archambault et al. [2015] argued that TC recurvature amplifies a preexisting Rossby wave train. While a weak upstream wave can be seen Fig. 1, the height anomaly is not statistically significant. We may also draw a distinction between the propagating wavepacket prior to recurvature and the forced wavepacket post recurvature. The latter is nearly stationary as deduced from its small eastward phase speed and has a large group speed indicating rapid energy dispersion and downstream development. The results in Fig. 1 show that, when examined from a climatological perspective, the downstream wavepacket develops locally. This suggests that TC recurvature acts as an impulsive forcing on the midlatitude stormtrack. This is consistent with the conclusion of Torn and Hakim [2015] who used ET as their reference point. This raises a pertinent question: Is TC recurvature also an impulsive forcing on forecast uncertainty? Figure 2 shows the climatological anomaly correlation (AC) and ensemble spread for the 500 hpa height forecasts calculated for the Northern Hemisphere mid-latitude band (30 o 60 o N). The AC drops to a 0.6, a generally accepted value for useful forecasts [e.g. Kalnay, 2003] after about 8 days. Using this threshold as a rough guide, we consider forecasts that are initialized up to 10 days prior to recurvature.

8 X - 8 A. AIYYER: TROPICAL CYCLONE RECURVATURE AND PREDICTABILTY Figure 3 shows the area average composite mean (solid line) and median (dashed line) normalized spread as a function of forecast hour. The domain used for averaging covers 30 o 60 o N latitudes and 30 o 120 o longitudes east of the recurvature location, and in effect, encompasses the area impacted by the downstream wavepacket. The individual panels show results for forecasts initialized on different days prior to recurvature: from day-0 to day-10 in two day intervals. Statistically significant values of the composite mean are shown by the thicker solid line, and the 25th and 75th percentile values are bounded within the shaded region. The time of peak normalized spread is marked by the thin vertical line. The time of recurvature is marked by the hurricane symbol, and the median time of ET is noted by the open circle. Several observations can be made from Fig. 3. The time of recurvature marks an increase in domain averaged normalized spread. This is particularly true for forecasts initialized within 6 days of recurvature (Fig. 3 a-d). For forecasts initialized on day-8 and day-10, a small increase can still be seen, albeit not as sharp (Fig. 3 e-f). This implies that irrespective of the forecast initialization time, recurvature is followed by a distinct degradation of forecast skill. This suggests that TC recurvature can be an impulsive forcing on forecast uncertainty. For shorter initialization lead times, the domain averaged normalized spread peaks about 4-5 days after recurvature or about 2-3 days after ET (Fig. 3 a-d). This maximum is statistically significant. On the other hand, for the longer initialization lead time of 10 days, there is no distinct peak after ET and the ensemble spread is indistinguishable from

9 A. AIYYER: TROPICAL CYCLONE RECURVATURE AND PREDICTABILTY X climatology. This is expected as the lead time is well past the threshold of predictability for this case. Furthermore, the normalized spread decreases after it peaks, indicating that the impact of recurvature does not extend to all forecast hours. From Fig 3 a-c, it can be seen that the enhanced spread is no longer significant approximately 5-6 days after recurvature. Thus, from a predictability perspective, the loss of skill due to the presence of TC is indistinguishable from the inherent loss of skill in the model within a week after recurvature. Finally, there is a tendency for reduction in normalized spread just prior to TC recurvature for shorter initialization lead times (Fig. 3 b-c). The reduction, however, does not meet the criterion for statistical significance. Nevertheless, this potential increase in forecast skill prior to recurvature is an intriguing feature of these results. Figure4showstheHovmöllerdiagramfornormalizedspreadaveragedbetween30 60 o N. The panels correspond to the same initialization times as in Fig. 3. For reference, the time of recurvature is marked by the hurricane symbol and ET is shown by the thin horizontal line. Two distinct plumes of enhanced spread are evident for forecasts initialized up to 6 days prior to recurvature. The first plume emanates from the TC and is localized around the longitude of recurvature. The second plume begins after ET and extends downstream. This is similar to the results from case studies by Harr et al. [2008] and Anwender et al. [2008] who found two plumes of enhanced ensemble spread; one associated with ET and the other downstream of it. The downstream extent of the second plume of enhanced spread is roughly the same as that of the wavepacket seen in Fig. 1. The propagation rate of the enhanced spread, as

10 X - 10 A. AIYYER: TROPICAL CYCLONE RECURVATURE AND PREDICTABILTY estimated by the slope of the dashed blue line in each panel, is approximately 36 ms 1. This is nearly the same value as the group speed of the developing wavepacket estimated from Fig. 1. This suggests that the rate of downstream development of the wavepacket yields an estimate for the propagation rate of forecast uncertainty associated with the TC recurvature and ET. Furthermore, as noted from forecasts initialized prior to recurvature (Figs. 4 b-f), there is no evidence of significant upstream spread enhancement prior to recurvature. This, along with Fig. 3, suggests that TC recurvature is a localized forcing on forecast uncertainty. This complements the findings of Torn and Hakim [2015] that developing wavepackets associated with ET, on average, do not have significant upstream precursors and are likely locally forced. A few additional observations can be made from Fig. 4. There are two significant maxima in the downstream spread plume. One is at 60 o and the other 120 o from recurvature location (Fig. 4a-d). Comparing this with the structure of the wavepacket in Fig 1, we note that these maxima are nearly one wavelength apart. In the forecast initialized 6 days prior to recurvature, the enhanced spread is not statistically significant but the general pattern is similar to the shorter range forecasts. Consistent with Fig. 3f, there are no significant differences from climatology for the forecast initialized 10 day prior to recurvature. Figure 5 shows the spatial distribution of the composite height anomalies from ERA- Interim(contours; negative values dashed) and normalized spread(only statistically significant values shown in shading) for selected forecast times. The composites are calculated

11 A. AIYYER: TROPICAL CYCLONE RECURVATURE AND PREDICTABILTY X in recurvature-relative frame. Hour 0 marks the recurvature time at which the TC is located at 0 o longitude and 27.5 o N latitude. Forecasts from three different initialization times relative to the date of recurvature are shown in the columns from left to right: Day-0; Day-2 and Day-4 respectively. The ERA height anomalies are lagged composites corresponding to the forecast verification time and are common to each row in Fig. 5. From Day-2 and Day-4 initializations, it can be noted that the downstream ridge has already formed prior to recurvature time (see panels for T=-48 and T=-24). A weak upstream disturbance in the midlatitude stormtrack can also be seen in these panels but the primary wavepacket at subsequent times(t=0h and later in all three columns) clearly originates from the TC location. This again supports the conclusion that TC forcing results in a new wavepacket formation. Further, it also indicates that the midlatitude response begins even earlier than the time of recurvature. Two other aspects of this wavepacket noted earlier are clearly evident. The cyclonic and anticyclonic anomalies appear to be nearly stationary even as the peak amplitude in the wavepacket shifts eastward over time. These are consistent with near-zero phase and large group speeds reported earlier from Fig. 1. A few additional observations can be made by examining the normalized spread that is shaded in the panels of Fig. 5. The general pattern of spread is similar at each forecast hour relative to recurvature, irrespective of the initialization time. The TC location is consistently associated with increased spread indicating poor predictability of the TC structure and/or track in this ensemble system. Interestingly, the ridge downstream from the TC is associated with decreased spread. This accounts for the reduction in the do-

12 X - 12 A. AIYYER: TROPICAL CYCLONE RECURVATURE AND PREDICTABILTY main averaged spread prior to TC recurvature seen in Fig. 3 b,c. Generally, even after recurvature, ridges in the wavepacket appear to have lower spread while the troughs have enhanced spread. Taken with the near stationarity of the wavepacket, this also explains the local maxima in spread at locations 60 o and 120 o from recurvature point noted earlier fromfig. 4. Thesetwopointscorrespondtothelocationofthetroughs. Figure5confirms that even after accounting for the climatological increase with forecast lead time, ensemble spread continues to be elevated for several days post recurvature and shifts downstream over time. 4. Conclusions The impact of tropical cyclones on the downstream response is investigated using NOAA s 2nd generation global ensemble reforecast dataset. We consider 146 western North Pacific storms occurring during Aug-Oct, The ensemble spread normalized by its climatology is used as the measure of forecast skill. The large sample size allows us to extend the results of previous case studies on midlatitude predictability. Recurvature of TC and its subsequent ET forces a downstream response in the form of a developing wavepacket in the midlatitude stormtrack. This is consistent with several previous studies [e.g. Jones et al., 2003; Riemer et al., 2008; Harr and Dea, 2009; Archambault et al., 2015]. The downstream response is already underway at the time of TC recurvature, and it continues to amplify after ET which typically occurs 2 days after the former. The forced wavepacket is nearly stationary and is associated with energy dispersion and downstream development.

13 A. AIYYER: TROPICAL CYCLONE RECURVATURE AND PREDICTABILTY X The normalized spread exhibits a few notable characteristics. The time of recurvature marks an increase in spread irrespective of the day the forecasts are initialized prior to TC recurvature. This suggests that TC recurvature is also a localized impulsive forcing on forecast skill and does not merely represent a further amplification of increasing spread prior to recurvature. As in the case studies of Harr et al. [2008] and Anwender et al. [2008], we note two plumes of enhanced spread. The first is associated with the poleward propagating TC and is localized around the point of recurvature. The second plume, that extends downstream starts after ET and is associated with a developing wavepacket in the midlatitude stormtrack. On average, the normalized spread associated with the downstream wavepacket peaks around 4-5 days after recurvature, or equivalently 2-3 days after ET. Thereafter, it decreases to near-climatological levels in 5-6 days. This implies that from a predictability perspective, the loss of skill due to TC recurvature is also localized in time, and becomes indistinguishable from the inherent loss of skill in the model within a week after recurvature. The results also indicate that there is a tendency for the domain averaged spread to decrease slightly prior to recurvature indicating potential increase in predictability. The decrease is seen to be associated with the ridge that develops downstream of the TC. Within the developing wavepacket, in general, ridges are associated with reduced spread while troughs are associated with enhanced spread. Anwender et al.[2008] also report that the troughs in the downstream response were associated with large forecast uncertainty. Since the downstream wavepacket is nearly stationary, the wavelength of the packet pro-

14 X - 14 A. AIYYER: TROPICAL CYCLONE RECURVATURE AND PREDICTABILTY vides an estimate for the recurvature-relative distance where the spread might be low or high, depending on whether a ridge or a trough is present. The rate at which the enhanced spread propagates eastward is nearly the same as the group speed of the developing wavepacket. This suggests that the downstream energy dispersion of the wavepacket constrains the rate at which the degradation of ensemble skill spreads eastward. This also implies that the configuration of the midlatitude waveguide plays a key role in determining the predictability of the downstream response. For example, zonal and meridional inhomogeneity in the deformation and potential vorticity gradients within the waveguide will determine how much the wavepacket will amplify or how far downstream it will propagate. This may also account for the observation by Archambault et al. [2013] that the strength of the downstream response was not correlated with the intensity of the TC. Previous studies have also suggested that the large scale flow characteristics such as the strength of the westerlies or presence of large amplitude trough are important in mediating the downstream response [e.g. Harr and Dea, 2009; Riemer and Jones, 2014]. In summary, the results show that, relative to the model s baseline, recurvature related increase in loss of forecast skill is spatially and temporally localized and is associated with a developing downstream wavepacket. This complements the finding of Torn and Hakim [2015] that extratropical transition is a localized forcing for a developing wavepacket in the midlatitude stormtrack Acknowledgments. This work is supported in part by National Science Foundation grant NOAA s 2nd generation global ensemble reforecast dataset were obtained

15 A. AIYYER: TROPICAL CYCLONE RECURVATURE AND PREDICTABILTY X from Earth System Research Laboratory( The ERA-Interim data were obtained National Center for Atmospheric Research s Data Support Section ( We are grateful to two anonymous reviewers who provided numerous constructive suggestions. References Anwender, D., P. A. Harr, and S. C. Jones (2008), Predictability Associated with the Downstream Impacts of the Extratropical Transition of Tropical Cyclones: Case Studies, Mon. Wea. Rev., 143, Archambault, H. M., D. Keyser, L. F. Bosart, C. A. Davis and J. M. Cordeira (2015), A Composite Perspective of the Extratropical Flow Response to Recurving Western North Pacific Tropical Cyclones, Mon. Wea. Rev., 143, Archambault, H. M., L. F. Bosart, D. Keyser, and J. M. Cordeira (2013), A Climatological Analysis of the Extratropical Flow Response to Recurving Western North Pacific Tropical Cyclones, Mon. Wea. Rev., 141, Barker, T. W. (1991), The Relationship between Spread and Forecast Error in Extendedrange Forecasts., J. Clim., 4, Dee, D. P., and co-authors (2011), The ERA-Interim reanalysis: configuration and performance of the data assimilation system, Quart. J. Roy. Meteor. Soc., 137, Hamill, T. M., G. T. Bates, J. S. Whitaker, D. R. Murray, M. Fiorino, T. J. Galarneau, Jr., Y. Zhu, and W. Lapenta(2013), NOAA s Second-Generation Global Medium-Range Ensemble Reforecast Dataset, Bull. Amer. Meteor. Soc., 94,

16 X - 16 A. AIYYER: TROPICAL CYCLONE RECURVATURE AND PREDICTABILTY Harr, P. A., and J. M. Dea (2009), Downstream Development Associated with the Extratropical Transition of Tropical Cyclones over the Western North Pacific, Mon. Wea. Rev., 137, Harr, P. A., D. Anwender, and S. C. Jones (2008), Predictability Associated with the Downstream Impacts of the Extratropical Transition of Tropical Cyclones: Methodology and a Case Study of Typhoon Nabi (2005), Mon. Wea. Rev., 136, Jones, S. C., and co-athors (2003), The Extratropical Transition of Tropical Cyclones: Forecast Challenges, Current Understanding, and Future Directions, Wea. Forecasting, 18, Kalnay, E.(2003), Atmospheric modeling, data assimilation and predictability, Cambridge University Press, pp Knapp, K. R., M. C. Kruk, D. H. Levinson, H. J. Diamond, and C. J. Neumann (2010), The International Best Track Archive for Climate Stewardship (IBTrACS), Bull. Amer. Meteor. Soc., 91, Riemer, M., and S. C. Jones (2014), Interaction of a tropical cyclone with a highamplitude, midlatitude wave pattern: Waviness analysis, trough deformation and track bifurcation, Quart. J. Roy. Meteor. Soc., 140, Riemer, M., S. C. Jones, and C. A. Davis (2008), The impact of extratropical transition on the downstream flow: An idealized modelling study with a straight jet, Quart. J. Roy. Meteor. Soc., 134, Torn, R. D., and G. Hakim (2015), Comparison of Wave Packets Associated with Extratropical Transition and Winter Cyclones, Mon. Wea. Rev., 143,

17 A. AIYYER: TROPICAL CYCLONE RECURVATURE AND PREDICTABILTY X Whitaker, J. S., and A. F. Loughe (1998), The Relationship between Ensemble Spread and Ensemble Mean Skill, Mon. Wea. Rev., 126,

18 X - 18 A. AIYYER: TROPICAL CYCLONE RECURVATURE AND PREDICTABILTY Figure 1. Hovmöller diagram of composite ERA-Interim 500 hpa height anomalies (contour interval of 1.0 dam; statistically significant values shaded) averaged over o N latitude band. Hurricane symbol marks the recurvature point. Dashed blue and red lines show respectively the phase and group propagation. The thin black horizontal line marks the median time of ET.

19 A. AIYYER: TROPICAL CYCLONE RECURVATURE AND PREDICTABILTY X - 19 Figure 2. Climatological anomaly correlation (solid line) and ensemble spread (dashed line; units: m) for 500 hpa heights. Period covered: Aug-Oct

20 X - 20 A. AIYYER: TROPICAL CYCLONE RECURVATURE AND PREDICTABILTY Figure 3. Composite normalized spread as a function of forecast hour for forecasts initialized at selected days prior to recurvature. The solid line shows mean, dashed line shows median and the shaded region shows the 25th 75th percentile range. Statistically significant values of the mean are shown by thicker line. Hurricane symbol marks the time of recurvature, circle shows the median time of ET, and the thin vertical line marks the peak spread.

21 A. AIYYER: TROPICAL CYCLONE RECURVATURE AND PREDICTABILTY X - 21 Figure 4. Hovmöllerdiagramsofnormalizedspreadaveragedoverthe30-60 o Nlatitudebandfor forecasts initialized at selected days prior to recurvature. Shading denotes statistically significant values. Contours from.8 to 1.6 with interval.05. The hurricane symbol marks recurvature point and the thin horizontal line marks the median time of ET.

22 X - 22 A. AIYYER: TROPICAL CYCLONE RECURVATURE AND PREDICTABILTY Figure 5. Composite anomalies of ERA Interim 500 hpa heights (contours with negative values dashed) at selected lags relative to recurvature; and composite normalized spread (only statistically significant values shown in shading) at corresponding forecast hours. Left, center and right columns are respectively for forecasts initialized on the day of recurvature; 2 days prior and 4 days prior.