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    TC Rewa 2300 UTC 2 Jan 1994 (a) 850-hPa wind field with vortex. The TC symbol marks observed location; (b) 850-hPa wind field without vortex; (c) 200-hPa wind field with vortex; (d) 200-hPa wind field without vortex; (e) 200–850-hPa vertical wind shear. The contour interval is 10 m s−1. At 850 hPa, light shading indicates regions with winds greater than 10 m s−1. At 200 hPa, light and dark shading indicates regions with winds greater than 20 and 40 m s−1, respectively.

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    Graph of average pressure change over a 6–36-h time period compared to vertical wind shear partitions, without vortex. Here DP(6) is the average change in pressure over 6 h, etc.

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    Same as in Fig. 3, but for broader vertical wind shear partitions, without vortex.

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    Distribution of the time lag between a rise in vertical wind shear (without vortex) above 10 m s−1 and a rise in central pressure of a TC.

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    Best track of TC Gwenda during the period 1–8 Apr 1999. First and final locations are plotted, and then at 24-h intervals in between. The plot shows central pressure in hPa at times DDHH, where DD is day of month and HH is hours UTC, for the times where the symbol is plotted on the track.

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    GMS5 IR image for TC Gwenda (a) 0930 UTC 6 Apr 1999, Gwenda at peak intensity; (b) 0930 UTC 7 Apr 1999, Gwenda 24 h after peak intensity; (c) 1540 UTC 7 Apr 1999, Gwenda at coastal crossing.

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    The average vertical wind shear (without vortex) compared to the central pressure of TC Gwenda, 2–7 Apr 1999, plotted at 6-h intervals.

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    The 850–200-hPa vertical wind shear without vortex for TC Gwenda at (a) 1100 UTC 2 Apr 1999, CP 1007 hPa; (b) 1100 UTC 3 Apr 1999, CP 1004 hPa; (c) 1100 UTC 4 Apr 1999, CP 998 hPa; (d) 1100 UTC 5 Apr 1999, CP 967 hPa; (e) 1100 UTC 6 Apr 1999, CP 900 hPa; (f) 1100 UTC 7 Apr 1999, CP 960 hPa. The contour interval is 10 m s−1. Light and dark shading indicate regions with wind shear greater than 10 and 40 m s−1, respectively.

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    Same as in Fig. 5, but for TC Rewa during the period 26 Dec 1993–19 Jan 1994. First and final locations are plotted, and then at 48-h intervals in between.

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    Average vertical wind shear (without vortex) compared to the central pressure for TC Rewa 28 Dec 1993–20 Jan 1994, plotted at 6-h intervals.

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    Same as in Fig. 8, but for TC Rewa at (a) 1100 UTC 28 Dec 1993, CP 995 hPa; (b) 1100 UTC 30 Dec 1993, CP 975 hPa; (c) 1100 UTC 1 Jan 1994, CP 955 hPa; (d) 1100 UTC 3 Jan 1994, CP 930 hPa; (e) 1100 UTC 5 Jan 1994, CP 980 hPa.

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    Same as in Fig. 8, but for TC Rewa at (a) 1100 UTC 8 Jan 1994, CP 992 hPa; (b) 1100 UTC 10 Jan 1994, CP 994 hPa.

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    Same as in Fig. 8, but for TC Rewa at (a) 1100 UTC 12 Jan 1994, CP 990 hPa; (b) 1100 UTC 14 Jan 1994, CP 975 hPa; (c) 1100 UTC 16 Jan 1994, CP 920 hPa ; (d) 1100 UTC 18 Jan 1994, CP 965 hPa.

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    TC Rewa 1100 UTC, 15 Jan 1994 (a) 200-hPa wind; (b) 200-hPa potential vorticity, calculated between levels 150 and 250 hPa; (c) 200-hPa divergence. Contour intervals are, respectively, 10 m s−1, 0.1 PV units, and 2.5 × 10−6 s−1. TC symbol marks the position of TC Rewa.

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Influence of Environmental Vertical Wind Shear on the Intensity of Hurricane-Strength Tropical Cyclones in the Australian Region

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  • 1 Bureau of Meteorology, Western Australia Regional Office, Perth, Australia
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Abstract

NCEP–NCAR reanalyses have been used to investigate the impact of environmental wind shear on the intensity change of hurricane-strength tropical cyclones in the Australian region. A method of removing a symmetric vortex from objective analyses is used to isolate the environmental flow. A relationship between wind shear and intensity change is documented. Correlations between wind shear and intensity change to 36 h are of the order of 0.4.

Typically a critical wind shear value of ∼10 m s−1 represents a change from intensification to dissipation. Wind shear values of less than ∼10 m s−1 favor intensification, with values between ∼2 and 4 m s−1 favoring rapid intensification. Shear values greater than ∼10 m s−1 are associated with weakening, with values greater than 12 m s−1 favoring rapid weakening. There appears to be a time lag between the onset of increased vertical wind shear and the onset of weakening, typically between 12 and 36 h.

A review of synoptic patterns during intensification-weakening cycles revealed the juxtaposition of a low-level anticyclone on the poleward side of the storm and an approaching 200-hPa trough to the west. In most cases, intensification commences under weak shear with the approach of the trough, but just prior to the onset of high shear. Further, based on described cases when wind shear was weak but no intensification occurred, it is suggested that weak shear is a necessary but not a sufficient condition for intensification. It is illustrated here that the remote dynamical influence of upper-level potential vorticity anomalies may offset the negative effects of environmental shear.

* Current affiliation: Bureau of Meteorology, New South Wales Regional Office, Sydney, Australia

+ Current affiliation: Bureau of Meteorology Research Centre, Melbourne, Australia

# Current affiliation: Meteorological Institute, University of Munich, Munich, Germany

Corresponding author address: Linda A. Paterson, WARO, P.O. Box 1370, West Perth, 6872 Western Australia, Australia. Email: l.paterson@bom.gov.au

Abstract

NCEP–NCAR reanalyses have been used to investigate the impact of environmental wind shear on the intensity change of hurricane-strength tropical cyclones in the Australian region. A method of removing a symmetric vortex from objective analyses is used to isolate the environmental flow. A relationship between wind shear and intensity change is documented. Correlations between wind shear and intensity change to 36 h are of the order of 0.4.

Typically a critical wind shear value of ∼10 m s−1 represents a change from intensification to dissipation. Wind shear values of less than ∼10 m s−1 favor intensification, with values between ∼2 and 4 m s−1 favoring rapid intensification. Shear values greater than ∼10 m s−1 are associated with weakening, with values greater than 12 m s−1 favoring rapid weakening. There appears to be a time lag between the onset of increased vertical wind shear and the onset of weakening, typically between 12 and 36 h.

A review of synoptic patterns during intensification-weakening cycles revealed the juxtaposition of a low-level anticyclone on the poleward side of the storm and an approaching 200-hPa trough to the west. In most cases, intensification commences under weak shear with the approach of the trough, but just prior to the onset of high shear. Further, based on described cases when wind shear was weak but no intensification occurred, it is suggested that weak shear is a necessary but not a sufficient condition for intensification. It is illustrated here that the remote dynamical influence of upper-level potential vorticity anomalies may offset the negative effects of environmental shear.

* Current affiliation: Bureau of Meteorology, New South Wales Regional Office, Sydney, Australia

+ Current affiliation: Bureau of Meteorology Research Centre, Melbourne, Australia

# Current affiliation: Meteorological Institute, University of Munich, Munich, Germany

Corresponding author address: Linda A. Paterson, WARO, P.O. Box 1370, West Perth, 6872 Western Australia, Australia. Email: l.paterson@bom.gov.au

1. Introduction

The behavior of tropical cyclones (TCs) as they intensify or dissipate over the ocean, spin down over the land, or transition into extratropical cyclones is a significant forecast problem in the Australian region (see, e.g., Dare and Davidson 2004). Understanding and prediction of this behavior is important for research and forecast operations. While there have been steady improvements in tropical cyclone track forecasting, improvements in intensity forecasting have not kept pace and large forecast errors in intensity out to 72 h can occur. The current study was motivated by the need for improved understanding and prediction of TC intensity change.

It is widely accepted that vertical wind shear is a major factor in the intensity change of tropical cyclones (e.g., DeMaria and Kaplan 1999; Fitzpatrick 1997; Hanley et al. 2001). This is consistent with operational experience in the Australian region. Aspects of the dynamical impact of wind shear on vortex intensity change are discussed in Jones (2000), DeMaria (1996), Wang and Holland (1996), Frank and Ritchie (2001), Corbosiero and Molinari (2002, 2003), Zehr (2003), Rogers et al. (2003), and the references therein. To summarize, it is suggested that wind shear results in the downshear tilt of the vortex, changes in conditional instability, and the upshear descent and downshear ascent with associated asymmetries in convection. The resiliency of TC vortices in shear has most recently been addressed by Reasor et al. (2004). They argue that the secondary circulation contributes indirectly to the resiliency, but that a vortex Rossby wave damping mechanism suppresses departures from the upright state. It is thus of interest to know (i) the magnitude (and variability) of environmental wind shear that TCs can tolerate before undergoing decay, and (ii) how the changes in wind shear generally occur.

A recent observational study by Gallina and Velden (2002) showed that a critical shear value (between 200 and 850 hPa), above which TCs on average fill is 7–8 m s−1 in the Atlantic and 9–10 m s−1 in the western Pacific. Palmer and Barnes (2002) also found that a single maximum shear value of 35 to 40 kt (∼17–20 m s−1) within 7° of the TC center can trigger decay within 12–24 h. For TC formation, Zehr (1992) observed that tropical cyclones did not develop when the shear exceeded 12.5–15 m s−1. Hanley et al. (2001) found a clear relationship between wind shear and intensity change, although their dataset contained examples of filling (deepening) for weak (strong) shear. Similar quantitative evaluation of how vertical wind shear affects TCs in the Australian basin has not been undertaken. The study presented here (i) uses a unique method of vortex extraction from numerical analyses to study the impact of environmental vertical wind shear on the intensity change of hurricane-strength tropical cyclones, and (ii) documents synoptic patterns often associated with wind shear and intensity change in the Australian region. With regard to (ii), we foreshadow the finding that intensification often commences as the wind shear begins to increase. This increase is often associated with the approach of an upper trough, which while seemingly at first providing a favorable environment for intensification, eventually evolves into a high shear situation favoring dissipation. The trough interaction problem is ongoing and has been addressed in numerous studies (e.g., Molinari et al. 1998).

The study will mostly focus on the history of storms of at least hurricane strength that eventually underwent oceanic decay. This stratification necessarily means that numerous intensifiers are included in the sample. These are a valuable set that can be contrasted with the weakening systems. Indeed we will suggest that many storms undergo an intensification then weakening cycle, which may be related to the same evolving environment that is initially conducive to intensification but later is associated with increasing shear and hence weakening. We note that the set of intensifiers used here is a subset of all Australian region intensifiers.

The paper is structured in the following way. Section 2 details the datasets and methodology used. Sections 3 and 4 provide a statistical evaluation of the relationship between wind shear and intensity change. Section 5 documents large-scale structures associated with intensification and decay. Section 6 is the summary and conclusions.

2. Data and methodology

a. Data

The tropical cyclone data were extracted from the Australian national tropical cyclone database for the period November 1984–April 1999. Although data are available in the database from 1910, 10-min mean wind speeds, which were used to calculate intensity change, were not recorded until November 1984. Further, this period is part of the satellite era when we might expect more consistency in intensity estimates.

For the purpose of this study the Australian region is defined to be the area between 10°–30°S and 105°–160°E. The southern latitude constraint was applied to exclude systems that may have been experiencing extratropical transition. Some TC cases had best tracks that started or concluded just outside of the previously defined Australian region, these were still included. Cases were also limited to the official tropical cyclone season in the Australian region, the months November–April. From this dataset we then selected TCs that had attained hurricane strength (10-min mean wind speed greater than 34 m s−1) at some stage during their life cycle and that had subsequently weakened by 30% or more in a 24-h period. This criteria defines the TC dataset used in this study. The TCs that made landfall in the 24-h weakening period were excluded as it was assumed that this was the main reason for weakening rather than the effects of vertical wind shear. From a total of 141 systems, 29 met the criteria and resulted in the availability of approximately 1050 wind shear and central pressure values for statistical processing.

Despite applying some constraints to the areas and months there remain some limitations in the dataset. The “best-track” data in the national database are produced by postanalysis of all the available data for each particular system. Historically this has consisted mainly of applying the Dvorak technique to infrared (IR) and visible (VIS) satellite imagery, a subjective process with little verifying data available in the Australian region. In more recent years this technique has been enhanced by the availability of passive microwave data and scatterometer wind data. With regard to absolute intensity estimates, errors are probably of the order of 10%–20% for wind speed, which we assume is mostly a bias, rather than random error. Since we are here dealing with intensity change, provided that the estimates are internally consistent, bias errors are mostly eliminated and net errors reduced. Operational experience and constraints within the application of the Dvorak method suggest that in cases of rapid weakening (developing) our intensity estimates are too high (low). But until more research is done, the estimates from the best-track data are still useful for the application here.

Some quality control has been applied to the dataset to minimize the influence of other factors that might affect intensity change. Tropical Cyclones Frank, Joy, and Daphne were close to land for long periods and TC Justin weakened due to cooler SSTs, perhaps associated with very slow movement or poleward motion. Two systems (TC Nicholas and TC Jacob) were poorly analyzed by the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis scheme and were also excluded from the dataset.

To isolate the wind shear effects we have taken only data points where the central pressure is at or below 990 hPa. There are several reasons for doing this. 1) In the early and late stages of a tropical cyclone’s life cycle (i.e., weak storms) it is more difficult to obtain accurate Dvorak intensity estimates due to uncertainties in storm structure and poorly defined cloud features on satellite images. Removing data points where the central pressure is above 990 hPa eliminates poorer-quality estimates. 2) During the early stages of a system’s development, when central pressures were between 1010 and 990 hPa, the wind shear was typically low to moderate but the change in central pressure was often very small. During this early stage, processes other than wind shear (e.g., convection and system-scale intensification) were likely playing a more critical role. 3) At the other end of the spectrum in the final decaying stage when central pressures were above 990 hPa, the wind shear was often high but the change in central pressure was again very small. During this late stage the influence of wind shear may have already occurred. Thus, to isolate the wind shear influence we have chosen to filter these points from the dataset. However, we will show in the next section that this processing only increases the correlations between wind shear and intensity change by about 0.1.

b. Methodology

NCEP–NCAR reanalysis fields (Kalnay et al. 1996), based on a multivariate objective analysis method in which mass observations can influence the wind analysis, were used to calculate vertical wind shear values. Although there is a serious lack of real-time wind data over the oceans of the Australian region, the use of satellite retrievals and cloud motion vectors in a multivariate objective analysis should partially offset the paucity of observations and define the large-scale environment in which we are interested here.

Wind shear was calculated in two ways: (i) as a vector difference between the 200- and 850-hPa wind, averaged over a 3° latitude–longitude square, centered on the best-track location of the storm (to eliminate the symmetric component of the circulation), and (ii) by computing environmental vertical wind shear at grid points from analyses in which the storm circulation had been removed via the vortex specification in the Tropical Cyclone Limited Area Prediction System (TC-LAPS). It will be shown that the results using the conventional method (i) and the no-vortex method (ii) are quite consistent, but that method (ii) has some useful advantages and will be mostly used here. The technique used in (ii) follows exactly the vortex enhancement method described in detail in Davidson and Weber (2000) and is illustrated in Fig. 1, which shows 850- and 200-hPa wind fields, with and without the analyzed circulation for TC Rewa (December 1993–January 1994), together with 200–850-hPa wind shear. After removal of the environmental (large scale) zonal and meridional wind fields, computed by application of a modified Barnes’ scheme in combination with a low-pass filter (Barnes 1964; Weber and Smith 1995), from the corresponding total wind fields (shown in the examples in Figs. 1a,c) at all mandatory pressure levels, the remaining residual wind fields are used to locate the vortex center (defined as the minimum/maximum of relative vorticity on the Southern/Northern Hemisphere). If such a center is found, the zonal and meridional wind fields are transformed to radial and tangential wind fields relative to this center. Otherwise, no further actions are taken at the pressure level in question and all pressure levels above. An azimuthal Fourier analysis relative to the vortex center produces the axisymmetric radial and tangential wind fields, which are then transformed back to zonal and meridional wind fields and subtracted from the corresponding residual fields. In the final step, the fields resulting from the last operation are added back to the environmental wind fields computed earlier and represent the environmental wind fields as shown in the examples in Figs. 1a,d. Although the partitioning into symmetric vortex and environment can never be unique, the figure illustrates the efficiency with which the method removes the symmetric circulation and suggests that the technique is providing a relatively realistic representation of the storm’s environment. However, we wish to emphasize that it is not possible to determine if remnants of the TC circulation remain as part of the environment as defined here. A possible interpretation is that changes in wind shear may be associated with changes in storm intensity. However, we will illustrate that wind shear change can be associated with neighboring synoptic-scale systems, thus suggesting that our diagnosed wind shear is mostly environmental rather than TC related.

Because of deficiencies in both the observational network and short-term model forecasts used to first guess the analysis, the assimilation method cannot always locate the storm accurately. Some systems appeared to be poorly analyzed compared to the best-track positions (TC Nicholas in 1985 and TC Jacob in 1996). It was noted that if the reanalysis fields located the cyclone some way from the observed position, there were understandably large differences between the two shear calculations. The shear calculation with vortex extracted was considered more reliable and will be mostly used here. The method searches around the observed location and removes the analyzed vortex even when it differs markedly from the best-track location. The shear calculation is then made over the observed storm location. The method has the significant advantage of eliminating spurious wind shear associated with incorrect analysis of storm location. Finally we note that the shear values are earth relative. We have not attempted to use storm-relative shear, which may be important for TCs that are accelerating poleward and when the motion is in the same direction as the shear. Our decision to exclude most storms undergoing extratropical transition via a latitude constraint appears to mostly eliminate these cases anyway.

3. Statistical results

Using the above methodology, values of wind shear were computed at nominal time t, and compared with both central pressure change and wind speed change at times t + 6, t + 12, t + 18, t + 24, t + 30, and t + 36 h. These are represented in the tables and graphs as DP(6), DP(12), etc. For consistency with other studies only the results for central pressure change will be presented. Table 1 shows correlation coefficients for the raw data with no cyclones or other data points removed, for with and without vortex shear values. The results show correlations of approximately 0.30–0.40 between vertical wind shear and change in central pressure (CP). Table 2 shows correlation coefficients for the filtered dataset, for with and without vortex shear values. The results show correlations of approximately 0.35–0.50 between vertical wind shear and change in CP. That is, the correlations are only marginally increased with the use of the filtered dataset or with the use of the vortex extraction. The results show higher correlations of approximately 0.40–0.55 between vertical wind shear and change in wind speed for the filtered data. Based on the increasing correlations with time, it appears that there is a time lag of 12–36 h between the onset of increased vertical wind shear and the onset of intensity change. This lag will be further addressed in section 4c.

Significance testing showed that the wind shear parameter was significant to 95% for all time periods for both central pressure and wind speed data. The correlation figures show that wind shear can account for up to a quarter of the variance in intensity change of a tropical cyclone. The remainder is likely accounted for by other factors such as internal vortex processes, changes in SSTs, land interaction, inward advection of dry air, and errors in analysis data and best-track estimates. Further, the fact that there is a time lag between the onset of increased vertical wind shear and the onset of intensity change means that there is some spread in the values of wind shear associated with the same value of pressure change. For example, if the wind shear increases to above 10 m s−1, the central pressure may continue to fall for a further period, implying that a large value of wind shear is then still associated with negative values of pressure changes or TC intensification. That is, there is variability in response time of the vortex to the imposed environmental wind shear, which may be due to variability in vortex structure (Reasor et al. 2004).

4. Wind shear and intensity change, critical value of wind shear, and time lags

a. Wind shear and intensity change

To evaluate the wind shear versus intensity change relationship, and to determine the critical value of vertical wind shear above (below) which a TC weakened (intensified), the shear data both with and without the vortex were partitioned into 1 m s−1 intervals. Table 3 shows wind shear ranges and subsequent mean central pressure change every 6–36 h for data without the vortex. Also shown is the number of data points in each partition.

From Table 3 and Fig. 2 it can be seen that as the vertical wind shear reached 9 m s−1 the central pressure increased within approximately 24 h. It also showed that as the magnitude of the shear increased above 10 m s−1 the time lag decreased between the onset of increased wind shear and central pressure increase. The results clearly indicate a relationship between wind shear and intensity change. Table 3 suggests that vertical wind shear values of less than ∼9 m s−1 favor intensification, with values between ∼2 and 4 m s−1 favoring significant intensification. Shear values greater than ∼9 m s−1 are associated with weakening, with values greater than 11 m s−1 favoring significant weakening.

The data with the vortex included were analyzed and showed similar results but with a slightly higher critical vertical wind shear value of between 10 and 11 m s−1. This data also showed a much greater variation in change in pressure associated with wind shear values than the data with the vortex removed.

An interesting result, but for a small sample, is a class of storms that intensify only very slowly when the shear is small. In a case study we will illustrate that weak shear is a necessary but not sufficient condition for intensification. This suggests that there exists a class of storms that do not significantly intensify under weak shear conditions. We speculate that reasons for this may be one of the following: 1) there is an apparent need for some additional, dynamical influence, perhaps an interacting upper trough; 2) the thermodynamic environment may be unfavorable; and 3) internal storm structure may in some way inhibit intensification.

b. Further partitioning of the wind shear data

In an attempt to smooth out some of the variations in the dataset and provide a table that may be applied in a forecasting situation, the data were partitioned into larger intervals. The new set of partition intervals was selected to match the trends of intensity change that we can see in Table 3: less than 6.0 m s−1 showed average pressure changes that were large and negative; 6.0–9.0 m s−1 showed average pressure changes that were small though still negative; 9.0–12 m s−1 showed average pressure changes that were small and mostly positive; and greater than 12 m s−1 showed average pressure changes that were large and positive. The results (Table 4 and Fig. 3) show a near linear relationship between wind shear and average pressure change for shear values in the critical 3–15 m s−1 range.

c. Time lag between changes in vertical wind shear and central pressure

The filtered data were examined to determine on average how long after the wind shear increased to 10 m s−1 (or greater) that central pressure began to rise. The value of 10 m s−1 was selected because at this value the average change in central pressure at each time step in Table 3 was positive. The average time for central pressure to begin rising was 24 h from the onset of the increase, with a range from −12 (one case) to 54 h (1 case; refer to Fig. 4). The vertical wind shear data for some TCs showed a degree of variability and did not always increase to above 10 m s−1 and consistently remain there. In these cases if subsequent values remained near 10 m s−1 the first highest value was chosen as the initial point in determining the time lag.

The data with the vortex showed an average of 8 h between an increase in wind shear above 11 m s−1 and a rise in the central pressure. These data showed more variability than the data without the vortex and there were two TCs in the set where no time lag could be assigned. Examination of the time change in vertical shear and storm intensity showed no obvious explanation of the variable lag times. This suggests that the variability in the response of the vortex may be related to variability in vortex structure. This line of research and the time lags are generally worthy of further investigation.

5. Synoptic patterns associated with wind shear change and case studies

A major aim of this study is to document and understand how the environmental wind shear alters during intensification and dissipation phases. An examination of the synoptic patterns at 850 and 200 hPa was thus made for each of the 23 TCs that were included in the filtered database. A remarkable 20 out of 23 showed a similar pattern of a moderate or strong high located south of the TC at 850 hPa with an approaching upstream 200-hPa trough near the time the wind shear increased above 10 m s−1. We further note that as each system weakened, 21 cases were associated with westerly shear, one with easterly shear, and one with northerly shear. Two systems showed the TC influenced by the approach of a trough at 200 hPa with a low pressure system to the south and one system (TC Pancho in 1986) was located north of a low-level monsoon trough with a ridge over the system at 200 hPa. All TCs had some southward component in their direction of movement. TC Rewa (in 1994) went through two intensifying and weakening episodes, the first weakening stage appeared to be influenced by a 200-hPa trough, while the second weakening stage was influenced by an 850-hPa high to the south with a 200-hPa trough approaching from the west.

For the case studies of TCs Gwenda and Rewa presented below, examination of available SST analyses based on weekly collections of observations (Smith 1995) suggests that unusually warm or cold SSTs were not a major factor in the rapid intensification and decay of these systems. During Gwenda’s life cycle, analyzed SSTs varied between 30° and 31°C. For Rewa, analyzed SSTs varied between 28° and 30°C.

a. Case study 1: TC Gwenda in April 1999

Track and CP information for Gwenda are shown in Fig. 5. A low in the monsoon trough north of Darwin, Australia, on 2 April 1999 slowly developed into TC Gwenda by 4 April 1999. From this point Gwenda underwent very rapid intensification, the central pressure deepened to an estimated 900 hPa by 1000 UTC 6 April 1999. During this time Gwenda moved on a generally southerly track toward the Western Australia coastline (Fig. 5). All the communities within the forecast warning area were prepared for the possibility of the impact of an intense tropical cyclone.

Late on the 6 April 1999, and immediately following its peak intensity, Gwenda began to weaken. Based on Dvorak analysis of satellite imagery it took just 30 h for the central pressure to increase from 900 to 995 hPa. Figure 6 shows the weakening of the TC during this period. Note the disappearance of the eye and the decrease in organization of the inner-core convection, even while the storm was over the ocean. Gwenda crossed the Pilbara coastline 40 km east of the town of Port Hedland with a central pressure of 980 hPa and a maximum recorded wind gust of 28 m s−1 (55 kt). Comparison of the best-track data with the 12- and 24-h forecast intensities gave errors of 31 and 43 hPa, respectively, suggesting the forecasters were aware of the consequence of the storm’s interaction with increasing shear (i.e., weakening) but were unable to confidently predict the degree of filling that occurred.

Figure 7 shows simultaneous vertical wind shear values and central pressures for TC Gwenda. Small values of wind shear and slow intensification are evident for at least 3 days prior to the commencement of rapid intensification. The intensification begins under weak shear conditions, but just prior to the onset of increased wind shear. We will show that very similar conditions occur with TC Rewa. This suggests that the intensification and decay may be linked to the same evolving environment. During the early stages, the environment may be conducive to intensification but later is associated with increasing shear and weakening. The wind shear for Gwenda continued to increase (Fig. 7) even as the storm rapidly intensified. But eventually and some 24 h after the wind shear increased to more than 10 m s−1, the central pressure began to rise equally rapidly and the storm dissipated. That is, there was approximately a 24-h time lag between the shear increasing to more than 10 m s−1 and the onset of weakening.

Figure 8 shows the 850–200-hPa environmental wind shear fields (i.e., without the vortex) for TC Gwenda at 24-h intervals. Although these figures are of wind shear they are similar to the 200-hPa flow and will be interpreted in this dual way. The figure shows the storm initially located in a low shear region (less than 5 m s−1). As it moved around the ridge in the wind shear, the storm outflow gained access to the midlatitude westerlies, and it rapidly intensified (Figs. 8c,d). Also evident is the amplifying (and decaying) broadscale trough, located near 100°E. Similar features are common to all events we have studied. It is also interesting to note the weak trough extending toward the storm from the southwest (Fig. 8d) during the time of rapid intensification. The development of this weak trough seemed to indicate the start of the relaxation of the main trough. It is also consistent with the potential vorticity analysis of tropical cyclone intensification by Molinari et al. (1998). The storm continued to move poleward, while the region of high wind shear also contracted southward. Eventually Gwenda moved into a region of wind shear of greater than 10 m s−1 and 24 h later it began to rapidly weaken.

An examination of mean sea level pressure and 850-hPa wind fields (not shown) indicates a strengthening and then maintenance in the environmental southeast-to-northeast anticyclonic flow over northern West Australia during Gwenda’s intensification and decay. The contribution to the shear from the low-level winds was quite small up until ∼5 April, and then increased as the storm moved toward the coast (see also Fig. 8). This increase was in part responsible for the increase in shear shown in Fig. 7 that was occurring during Gwenda’s intensification and eventual decay.

b. Case study 2: TC Rewa in December 1993–January 1994

Track and CP information for Rewa are shown in Fig. 9. Tropical Cyclone Rewa formed from a tropical depression north of Vanuatu. It was a long-lived system, which affected several Pacific Island nations and was responsible for nine deaths. The system was also unusual in that it underwent two intensification phases. During each phase it reached a central pressure of 920 hPa.

Figure 10 shows vertical wind shear and central pressure values. The first phase of intensification coincided with average shear values (without vortex) of less than 10 m s−1. In fact, even though the shear increased, the values did not increase above 10 m s−1 until 12 h after the central pressure began to rise. Tropical Cyclone Rewa weakened significantly from 3 January 1994 to 6 January 1994 when the average wind shear over the system remained high. The wind shear again was weak during the period 7 January–12 January, but no intensification was observed. Then, similar to Gwenda’s intensification phase, a second intensification phase commenced under weak wind shear but just prior to a period of increasing shear. Although the shear data are somewhat erratic during the period when Rewa was in its most intense phase, the average wind shear increased to above 10 m s−1 between 1200 UTC 15 January and 1200 UTC 16 January. The central pressure began to rise from 920 hPa at 0000 UTC 17 January, a time lag of 42 h, though the shear values fluctuated above and below 10 m s−1 for the first 24 h until finally increasing above 10 m s−1 and remaining there.

Figures 11, 12 and 13 illustrate the respective synoptic conditions under which Rewa’s periods of intensification and decay, no intensification and reintensification, occurred. Figures 11a–e show a succession of amplifying and decaying troughs affecting the region traversed by Rewa. During the initial period, wind shear was weak and outflow into the tropical easterlies and midlatitude westerlies was clearly possible, and Rewa intensified. These environmental structure changes are almost identical to those described by Davidson and Kar (2002) for some other rapidly intensifying storms. Eventually, however, Rewa moved to a region of high wind shear (Figs. 11d,e), associated with the same evolving environment that initially was linked to the intensification. During this time, Rewa began to rapidly decay.

Figure 12 illustrates the environment during the period when Rewa was weak. It shows the weak wind shear described in Fig. 10. During this period, Rewa was embedded in an elongated upper ridge, far removed from the upstream trough and no intensification was observed even though the wind shear was weak.

Figure 13 illustrates the environment during Rewa’s reintensification. Again, a succession of active troughs, with wind maxima on their eastern flanks, amplify and decay in the near vicinity of Rewa. In these conditions and while Rewa was only influenced by weak shear, it reintensified. Eventually however it moved into a region of high shear (Figs. 13c,d) and from then on rapidly dissipated.

During this final period the contribution to the shear from the low-level winds can be described as follows. From Fig. 9 (Rewa’s track) and Fig. 13, the storm was moving to the southeast as it reintensified and then changed its direction of motion to the west as it weakened. Charts of low-level winds (with and without vortex) strongly suggest that this change in direction of motion was associated with a developing large-scale low- to midlevel anticyclone to the south of Rewa. That is the wind shear during weakening was a combination of increasing upper-level northwesterlies and strengthening low-level easterlies. At times, the contribution to the shear from the 850-hPa environmental wind was of the order of 30%. This environmental structure of upper trough to the west and low-level ridge to the south is a common characteristic of the intensification-weakening cycle of TCs in our dataset.

We suggest that the intensification under weak wind shear and just prior to increasing wind shear is no coincidence. These conditions are normally associated with amplifying and decaying upper troughs. What may be quite critical is the location of the storm in these evolving environments. Low (high) shear zones would seem to favor intensification (decay), but there appears to be an additional dynamical requirement, perhaps the ascent or descent regions associated with nearby upper troughs. Following similar suggestions by Hanley et al. (2001), to illustrate this influence and the competing effects of shear and large-scale ascent, Fig. 14 shows the 200-hPa wind (Fig. 14a), potential vorticity (Fig. 14b) and horizontal divergence (Fig. 14c) from the analysis with the vortex removed (i.e., the large-scale environment) while Rewa was rapidly intensifying. The 24-h pressure change centered at this time was estimated to be 40 hPa. Note that the trough is characterized by a dome of cyclonic PV with extensive lobes of divergence and convergence to the east and west, respectively. This structure appears to belong to the “favorable distant interaction” category described in Hanley et al. (2001). The remote influence of the PV anomaly via the divergence extends over the location of Rewa. Note as well that these structures are large scale and unlikely to be determined by the presence of the storm, although we cannot eliminate the possibility of the actual TC intensification either reducing or enhancing the environmental wind shear. However, the presence of structures similar to those illustrated in Fig. 11 suggests that the changes are more related to environmental processes. We propose that in the case of Rewa the dynamical influence of the environmental divergence has offset the negative influence of the increasing shear. This dynamical influence may innoculate the storm against the effects of the shear (K. Emanuel 2003, personal communication) and allow the intensification to proceed uninhibited, until the shear exceeds 10 m s−1. We wish to emphasize however that the process described here is just one of many that are likely to influence intensification.

6. Summary and conclusions

A special method of removing a symmetric vortex from objective analyses to reveal the large-scale environment has been used to investigate the impact of environmental wind shear on the intensity of hurricane-strength tropical cyclones in the Australian region. A relationship between wind shear and intensity change is documented. Correlations between wind shear at some time t, and change in intensity at 6-h intervals to 36 h are of the order of 0.4. Other processes (SST changes, motion, inward advection of dry air, time lags between onset of wind shear and CP change, etc.) are thought to account for the rest of the variability.

Typically a critical wind shear value of ∼10 m s−1 represents a changeover from intensification to weakening. Wind shear values of less than ∼10 m s−1 favor intensification, with values between ∼2 and 4 m s−1 favoring rapid intensification. Shear values greater than ∼10 m s−1 are associated with weakening, with values greater than 12 m s−1 favoring rapid weakening. An interesting result, but for a small sample, is the very weak intensity change when the shear is extremely small. There appears to be a class of storms that do not intensify even when the wind shear is weak. We suggest that weak shear is a necessary but not a sufficient condition for intensification. There appears to be a requirement for other dynamical influences. One hypothesis is the presence of an interacting upper trough (Molinari et al. 1998; Hanley et al. 2001; Davidson and Kar 2002).

The data suggest that there is a time lag between the onset of increased vertical wind shear and the onset of weakening of a tropical cyclone, typically between 12 and 36 h. The cause of the variability in the response time of the vortex cannot be determined here, but may be related to the vortex structure (Reasor et al. 2004), or to the competing influences of wind shear and the dynamical effects of interacting troughs.

Common environmental structures associated with intensification and decay of TCs have been documented. A remarkable 21 out of 23 events showed a low-level anticyclone on the poleward side of the storm and an approaching 200-hPa trough to the west. Under these conditions, environmental wind shear will normally eventually increase as the storm and upper trough move closer together. In the cases described in detail here (and during other events), intensification commences under weak shear conditions as the trough approaches, but just prior to the onset of high wind shear. We suggest that the environmental conditions, which initially are conducive to intensification, eventually evolve into high wind shear environments, which contribute to dissipation. That is, the low-level anticyclone with upper trough to the west are initially associated with intensification, but evolve to eventually contribute to decay.

Based on the methodology developed here, real-time fields of analyzed and forecast wind shear will be available for operational use. Examination of fields representing the environment of storms provides opportunities for interpreting model behavior and diagnosing motion and intensity scenarios. We have also started to develop an appropriate compositing strategy, similar to Hanley et al. (2001), which we will use to further isolate the critical processes. We hope to also extend the study to numerical simulations of intensity change based on idealized and real data initial conditions.

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Fig. 1.
Fig. 1.

TC Rewa 2300 UTC 2 Jan 1994 (a) 850-hPa wind field with vortex. The TC symbol marks observed location; (b) 850-hPa wind field without vortex; (c) 200-hPa wind field with vortex; (d) 200-hPa wind field without vortex; (e) 200–850-hPa vertical wind shear. The contour interval is 10 m s−1. At 850 hPa, light shading indicates regions with winds greater than 10 m s−1. At 200 hPa, light and dark shading indicates regions with winds greater than 20 and 40 m s−1, respectively.

Citation: Monthly Weather Review 133, 12; 10.1175/MWR3041.1

Fig. 2.
Fig. 2.

Graph of average pressure change over a 6–36-h time period compared to vertical wind shear partitions, without vortex. Here DP(6) is the average change in pressure over 6 h, etc.

Citation: Monthly Weather Review 133, 12; 10.1175/MWR3041.1

Fig. 3.
Fig. 3.

Same as in Fig. 3, but for broader vertical wind shear partitions, without vortex.

Citation: Monthly Weather Review 133, 12; 10.1175/MWR3041.1

Fig. 4.
Fig. 4.

Distribution of the time lag between a rise in vertical wind shear (without vortex) above 10 m s−1 and a rise in central pressure of a TC.

Citation: Monthly Weather Review 133, 12; 10.1175/MWR3041.1

Fig. 5.
Fig. 5.

Best track of TC Gwenda during the period 1–8 Apr 1999. First and final locations are plotted, and then at 24-h intervals in between. The plot shows central pressure in hPa at times DDHH, where DD is day of month and HH is hours UTC, for the times where the symbol is plotted on the track.

Citation: Monthly Weather Review 133, 12; 10.1175/MWR3041.1

Fig. 6.
Fig. 6.

GMS5 IR image for TC Gwenda (a) 0930 UTC 6 Apr 1999, Gwenda at peak intensity; (b) 0930 UTC 7 Apr 1999, Gwenda 24 h after peak intensity; (c) 1540 UTC 7 Apr 1999, Gwenda at coastal crossing.

Citation: Monthly Weather Review 133, 12; 10.1175/MWR3041.1

Fig. 7.
Fig. 7.

The average vertical wind shear (without vortex) compared to the central pressure of TC Gwenda, 2–7 Apr 1999, plotted at 6-h intervals.

Citation: Monthly Weather Review 133, 12; 10.1175/MWR3041.1

Fig. 8.
Fig. 8.

The 850–200-hPa vertical wind shear without vortex for TC Gwenda at (a) 1100 UTC 2 Apr 1999, CP 1007 hPa; (b) 1100 UTC 3 Apr 1999, CP 1004 hPa; (c) 1100 UTC 4 Apr 1999, CP 998 hPa; (d) 1100 UTC 5 Apr 1999, CP 967 hPa; (e) 1100 UTC 6 Apr 1999, CP 900 hPa; (f) 1100 UTC 7 Apr 1999, CP 960 hPa. The contour interval is 10 m s−1. Light and dark shading indicate regions with wind shear greater than 10 and 40 m s−1, respectively.

Citation: Monthly Weather Review 133, 12; 10.1175/MWR3041.1

Fig. 9.
Fig. 9.

Same as in Fig. 5, but for TC Rewa during the period 26 Dec 1993–19 Jan 1994. First and final locations are plotted, and then at 48-h intervals in between.

Citation: Monthly Weather Review 133, 12; 10.1175/MWR3041.1

Fig. 10.
Fig. 10.

Average vertical wind shear (without vortex) compared to the central pressure for TC Rewa 28 Dec 1993–20 Jan 1994, plotted at 6-h intervals.

Citation: Monthly Weather Review 133, 12; 10.1175/MWR3041.1

Fig. 11.
Fig. 11.

Same as in Fig. 8, but for TC Rewa at (a) 1100 UTC 28 Dec 1993, CP 995 hPa; (b) 1100 UTC 30 Dec 1993, CP 975 hPa; (c) 1100 UTC 1 Jan 1994, CP 955 hPa; (d) 1100 UTC 3 Jan 1994, CP 930 hPa; (e) 1100 UTC 5 Jan 1994, CP 980 hPa.

Citation: Monthly Weather Review 133, 12; 10.1175/MWR3041.1

Fig. 12.
Fig. 12.

Same as in Fig. 8, but for TC Rewa at (a) 1100 UTC 8 Jan 1994, CP 992 hPa; (b) 1100 UTC 10 Jan 1994, CP 994 hPa.

Citation: Monthly Weather Review 133, 12; 10.1175/MWR3041.1

Fig. 13.
Fig. 13.

Same as in Fig. 8, but for TC Rewa at (a) 1100 UTC 12 Jan 1994, CP 990 hPa; (b) 1100 UTC 14 Jan 1994, CP 975 hPa; (c) 1100 UTC 16 Jan 1994, CP 920 hPa ; (d) 1100 UTC 18 Jan 1994, CP 965 hPa.

Citation: Monthly Weather Review 133, 12; 10.1175/MWR3041.1

Fig. 14.
Fig. 14.

TC Rewa 1100 UTC, 15 Jan 1994 (a) 200-hPa wind; (b) 200-hPa potential vorticity, calculated between levels 150 and 250 hPa; (c) 200-hPa divergence. Contour intervals are, respectively, 10 m s−1, 0.1 PV units, and 2.5 × 10−6 s−1. TC symbol marks the position of TC Rewa.

Citation: Monthly Weather Review 133, 12; 10.1175/MWR3041.1

Table 1.

Correlation figures for raw data. Here DP(6) = change in pressure in hPa over 6 h, etc.

Table 1.
Table 2.

Same as in Table 1, but for filtered data.

Table 2.
Table 3.

Average pressure change over a 6–36-h time period associated with vertical wind shear partitions without vortex. DP(6) = change in pressure in hPa over 6 h, etc.

Table 3.
Table 4.

Same as in Table 3, but without vortex.

Table 4.
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