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  • View in gallery

    Meridional–vertical cross section of the zonal wind (contour, m s−1) and relative humidity (shaded, %) of the initial vortex (across the vortex center) specified in (left) NOSH_CTL and (right) NOSH_DRY.

  • View in gallery

    Background vertical shear profiles specified in the (left) westerly shear and (right) easterly shear experiments.

  • View in gallery

    Time evolution of MSLP from all experiments.

  • View in gallery

    Evolution of 850-hPa wind (vector, m s−1) and RH (shaded, %) fields in (top) NOSH_CTL and (bottom) NOSH_DRY for (left to right) time = 0, 24, 48, and 96 h.

  • View in gallery

    Time–height cross section of (top) RH (%) and (bottom) relative vorticity (1 × 10−4 s−1) fields averaged over a 50-km radius of surface vortex center in (left) NOSH_CTL and (right) NOSH_DRY.

  • View in gallery

    As in Fig. 4, but for WSH_CTL and ESH_CTL.

  • View in gallery

    As in Fig. 5, but for WSH_CTL and ESH_CTL.

  • View in gallery

    As in Fig. 4, but for WSH_DRY and ESH_DRY.

  • View in gallery

    Tracks of the vortex centers at 850- (red) and 400-hPa (purple) levels in a 3-hourly time interval. The hurricane symbols indicate the timing of the vertical alignment: (top) CTL and (bottom) DRY; (left) ESH and (right) WSH.

  • View in gallery

    Schematic summarizing the impact of background vertical shear on the FSC and the subsequent vertical alignment process: (a) how the shear-induced FSC is established and how the convection reinforced FSC helps restore the vertical alignment in WSH_DRY and (b) how the FSC fails to offset the shear advective effect owing to dry air intrusion in ESH_DRY.

  • View in gallery

    Evolution of the forced secondary circulation represented by asymmetric component of vertical velocity (m s−1) at 500 hPa along the east–west direction across the vortex center: (top) CTL and (bottom) DRY; (left) WSH and (right) ESH.

  • View in gallery

    Evolution of simulated radar reflectivity (shaded, dBZ) and upper- (400 hPa; black contour) and lower- (850 hPa; red contour) level vorticity fields (1 × 10−5 s−1) at (left to right) hours 12, 18, 36, and 72 in (top to bottom) WSH_CTL, ESH_CTL, WSH_DRY, and ESH_DRY. Only vorticity fields associated with wavelengths > 200 km are shown.

  • View in gallery

    Zonal–height cross sections of the meridional wind (contour, m s−1) and RH (shaded, %) fields across the surface vortex center in (left) ESH_DRY and (right) WSH_DRY for (top to bottom) time = 24, 48, 72, and 96 h.

  • View in gallery

    Evolution of 850-hPa RH (shaded, %) and equivalent potential temperature (contour, K) fields in (top) ESH_DRY and (bottom) WSH_DRY for (left to right) time = 12, 24, 48, and 96 h.

  • View in gallery

    Evolution of column-averaged normalized OW parameter (shaded) in (top) ESH_DRY and (bottom) WSH_DRY. The red solid (black dashed) contours denote 850 (400)-hPa relative vorticity fields (1 × 10−5 s−1; only signals with wavelengths > 200 km are shown): (left to right) time = 24, 36, 72, and 96 h. The red (black) vectors show 850- (400-) hPa wind fields.

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Effects of Vertical Shears and Midlevel Dry Air on Tropical Cyclone Developments

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  • 1 International Pacific Research Center, University of Hawai‘i at Mānoa, Honolulu, Hawaii
  • 2 Naval Research Laboratory, Monterey, California
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Abstract

A set of idealized experiments using the Weather Research and Forecasting model (WRF) were designed to investigate the impacts of a midlevel dry air layer, vertical shear, and their combined effects on tropical cyclone (TC) development. Compared with previous studies that focused on the relative radial position of dry air with no mean flow, it is found that the combined effect of dry air and environmental vertical shear can greatly affect TC development. Moreover, this study indicates the importance of dry air and vertical shear orientations in determining the impact. The background vertical shear causes the tilting of an initially vertically aligned vortex. The shear forces a secondary circulation (FSC) with ascent (descent) in the downshear (upshear) flank. Hence, convection tends to be favored on the downshear side. The FSC reinforced by the convection may overcome the shear-induced drifting and “restore” the vertical alignment. When dry air is located in the downshear-right quadrant of the initial vortex, the dry advection by cyclonic circulation brings the dry air to the downshear side and suppresses moist convection therein. Such a process disrupts the “restoring” mechanism associated with the FSC and thus inhibits TC development. The sensitivity experiments show that, for a fixed dry air condition, a marked difference occurs in TC development between an easterly and a westerly shear background.

School of Ocean and Earth Science and Technology Contribution Number 8949 and International Pacific Research Center Contribution Number 988.

Corresponding author address: Dr. Tim Li, IPRC and Department of Meteorology, University of Hawai‘i at Mānoa, 1680 East-West Rd., Honolulu, HI 96825. E-mail: timli@hawaii.edu

Abstract

A set of idealized experiments using the Weather Research and Forecasting model (WRF) were designed to investigate the impacts of a midlevel dry air layer, vertical shear, and their combined effects on tropical cyclone (TC) development. Compared with previous studies that focused on the relative radial position of dry air with no mean flow, it is found that the combined effect of dry air and environmental vertical shear can greatly affect TC development. Moreover, this study indicates the importance of dry air and vertical shear orientations in determining the impact. The background vertical shear causes the tilting of an initially vertically aligned vortex. The shear forces a secondary circulation (FSC) with ascent (descent) in the downshear (upshear) flank. Hence, convection tends to be favored on the downshear side. The FSC reinforced by the convection may overcome the shear-induced drifting and “restore” the vertical alignment. When dry air is located in the downshear-right quadrant of the initial vortex, the dry advection by cyclonic circulation brings the dry air to the downshear side and suppresses moist convection therein. Such a process disrupts the “restoring” mechanism associated with the FSC and thus inhibits TC development. The sensitivity experiments show that, for a fixed dry air condition, a marked difference occurs in TC development between an easterly and a westerly shear background.

School of Ocean and Earth Science and Technology Contribution Number 8949 and International Pacific Research Center Contribution Number 988.

Corresponding author address: Dr. Tim Li, IPRC and Department of Meteorology, University of Hawai‘i at Mānoa, 1680 East-West Rd., Honolulu, HI 96825. E-mail: timli@hawaii.edu

1. Introduction

The environmental midlevel moisture condition is often regarded as an important controlling factor for tropical cyclone (TC) genesis (Gray 1975; McBride and Zehr 1981; Nolan 2007; Li 2012; Wang 2012; Tang and Emanuel 2012). High values of midlevel relative humidity (RH) generally favor the development of a TC. It was also noted that the environmental moisture condition may determine the TC size (Hill and Lackmann 2009). Dry air intrusion is considered as one of the potential inhibiting factors for TC development (e.g., Dunion and Velden 2004; Wu et al. 2006; Wu 2007; Kimball 2006; Braun 2010; Wang 2012). Most of previous studies focused on the suppression of Atlantic hurricanes due to the intrusion of the Saharan air layers (SAL). It has been shown that dry air can negatively impact a TC by fostering cold downdrafts (Emanuel 1989) and lowering the convective available potential energy (CAPE) near the TC core region (Dunion and Velden 2004). Wang (2012) noted that the major impact of dry air is through the reduction of updraft buoyancy. The observational study by Shu and Wu (2009) found that a storm tends to weaken when dry air is located within 380 km from the storm center. This result seems contradictory to idealized numerical simulation by Braun et al. (2012), who showed that the dry air can only effectively affect TC development when it is placed near the TC inner-core region (e.g., a distance comparable to the radius of maximum wind).

It is worth noting that the numerical experiments by Braun et al. (2012) were conducted under a resting environment. It is not clear to what extent the presence of an environmental flow, particularly a background vertical shear, may modulate the result. The vertical shear effects on TC intensity and structure changes have been extensively studied (Frank and Ritchie 2001; Rappin and Nolan 2012; Tang and Emanuel 2012). It is generally accepted that the vertical shear may inhibit TC development through “venting” moisture and energy out of the TC core region (Gray 1968), inducing midlevel warming (DeMaria 1996), and transporting low-entropy air into the atmospheric boundary layer (Riemer et al. 2009). The objective of this study is to expand the work of Braun et al. (2012) by investigating the combined effect of midlevel dry air and vertical shear on TC development. In particular, we are interested in the effect of dry air intrusion with a background easterly and westerly shear, respectively.

The rest of this paper is organized as follows. Section 2 describes the model and experiment design. Section 3 presents the model simulation results from various experiments with a focus on the effect of dry air intrusion for flows with easterly and westerly shear. A concluding summary and discussion are given in section 4.

2. Model and experiments design

In this study, the Advanced Research Weather Research and Forecasting model (ARW-WRF) system version 2.2 is utilized. Four nested grids with the finest resolution of 2 km are employed to resolve explicitly the convection in the core region. The outermost domain has a grid spacing of 54 km and contains 121 × 121 grid points, which is large enough to avoid the vortex approaching to the lateral boundary during the entire integration period. Other three inner meshes have the following gird spacing and dimensions: 18 km and 121 × 121, 6 km and 301 × 301, and 2 km and 361 × 361, respectively. The model has 35 vertical levels. The physics include the Yonsei University boundary layer scheme (Hong et al. 2006), microphysics scheme (Lin et al. 1983), Dudhia shortwave scheme (Dudhia 1989), and Rapid Radiative Transfer Model (RRTM) longwave radiation (Mlawer et al. 1997).

The details for the designed experiments are given in Table 1. In the control experiment (NOSH_CTL), we exclude both the dry air intrusion and the vertical shear. For all experiments, the initial fields contain a weak symmetric cyclonic vortex, with a maximum surface wind speed of 8 m s−1 at a radius of 150 km and the wind speed decreasing with height. The control experiment represents TC development under the favorable conditions with no vertical shear or a dry air layer. The second experiment (NOSH_DRY) is designed to examine the impact of the midlevel dry air layer with no mean flow. The specification of dry air layer follows Braun et al. (2012). A dry air layer is specified between 850 and 650 hPa, in which RH is set as 25% to mimic the dry SAL. This dry air layer is embedded at all grid points 150 km north of the initial vortex center. Figure 1 shows the vertical profile of the zonal wind and initial relative humidity in the north–south direction across the center of the vortex for the second experiment. The mass and thermodynamic fields are obtained by solving the nonlinear balance equation for the given tangential wind field (Wang 1995). The initial background temperature and moisture profiles are from the mean tropical sounding (Jordan 1958). The lateral boundary conditions in the outermost domain are fixed, and three inner meshes use vortex-center-following technique. The model is run on an f plane with the background Coriolis parameter at 15°N and with a constant sea surface temperature (SST) of 29°C.

Table 1.

The experiment descriptions and symbols.

Table 1.
Fig. 1.
Fig. 1.

Meridional–vertical cross section of the zonal wind (contour, m s−1) and relative humidity (shaded, %) of the initial vortex (across the vortex center) specified in (left) NOSH_CTL and (right) NOSH_DRY.

Citation: Journal of the Atmospheric Sciences 70, 12; 10.1175/JAS-D-13-066.1

Four additional experiments with different vertical shears are designed. WSH_CTL and ESH_CTL are to examine the impact of different vertical shear profiles on TC development without midlevel dry air layer. These idealized vertical shear profiles are displayed in Fig. 2. In the flow with an easterly (westerly) wind shear, the zonal wind decreases (increases) linearly from 4 (−4) m s−1 at the surface to −4 (4) m s−1 at the top of the model, and 0 wind speed at 500 hPa. WSH_DRY and ESH_DRY have the same shear as in WSH_CTL and ESH_CTL, but with dry air located to the north of the vortex initially as shown in Fig. 1.

Fig. 2.
Fig. 2.

Background vertical shear profiles specified in the (left) westerly shear and (right) easterly shear experiments.

Citation: Journal of the Atmospheric Sciences 70, 12; 10.1175/JAS-D-13-066.1

3. Results of the simulations

Figure 3 displays the time evolution of the vortex intensity represented by the minimum sea level pressure (MSLP) for all experiments. It is not surprising that NOSH_CTL attains the greatest intensity among all the cases. The initial weak vortex develops into a strong TC with MSLP of 920 hPa after 108-h integration. In the absence of vertical shear, NOSH_DRY has a MSLP evolution close to NOSH_CTL, except with a slightly slower intensification rate. Braun et al. (2012) pointed out that, in the absence of mean flow, dry air can effectively affect TC development only when it is placed near the storm center. The results from our bench run such as NOSH_DRY agree with Braun et al. (2012) in the sense that dry air placed at a radius of 150 km is ineffective in a quiescent environment.

Fig. 3.
Fig. 3.

Time evolution of MSLP from all experiments.

Citation: Journal of the Atmospheric Sciences 70, 12; 10.1175/JAS-D-13-066.1

Without the midlevel dry air effect, ESH_CTL and WSH_CTL have almost identical evolution features on the f plane, consistent with Frank and Ritchie (2001). Compared with NOSH_CTL and NOSH_DRY, both ESH_CTL and WSH_CTL have a slower intensification rate, indicating that the environmental vertical shear is indeed one of detrimental factors on TC development.

The most interesting evolution features appear in cases when both a vertical shear and midlevel dry air effect are considered. There is a marked difference between the westerly and easterly shear cases. In the former, a TC-like vortex, although developed slowly, finally forms after 108-h integration; whereas there is no TC formation under the easterly shear case. The results show that, with the existence of vertical shear, the effect of midlevel dry air on TC development is significantly enhanced. Moreover, this study suggests that the TC development is highly sensitive to the orientation of vertical shear in the presence of midlevel dry air layer. In the following sections, we will investigate in detail these experiments.

a. Impact of midlevel dry air layer with no mean flow

As aforementioned, NOSH_CTL and NOSH_DRY experiments are designed to investigate the role of the midlevel dry air layer with no mean flow. As a result, NOSH_CTL and NOSH_DRY experiments are taken as the control runs for comparison with the other four experiments. As seen from Fig. 3, the MSLP evolutions in NOSH_CTL and NOSH_DRY are quite similar, suggesting that the midlevel dry air layer away from the vortex center only has a very limited effect on TC development without mean flows. Figure 4 displays the time evolution of 850-hPa vortex circulation and RH fields in these two experiments. In NOSH_DRY, the dry air gets increasingly wrapped around the vortex center and becomes axisymmetrized over time. This lowers the overall RH surrounding the core of the vortex compared with NOSH_CTL. However, during the entire integration period, the dry air (with RH value less than 40%) remains 150 km away from the vortex center and does not penetrate into the inner core region. Fritz and Wang (2013) pointed out that, since the midlevel inflow is very weak, the entrainment time scale is much longer than the moistening time scale such that particles undergo several resolutions as moving to the circulation center and get moistened on their paths. As a result, the impact of the dry air on storm intensity is weak. This result also qualitatively agrees with Hill and Lackmann (2009), who noted that lower-RH air outside a 100-km radius had a very limited impact on storm intensity.

Fig. 4.
Fig. 4.

Evolution of 850-hPa wind (vector, m s−1) and RH (shaded, %) fields in (top) NOSH_CTL and (bottom) NOSH_DRY for (left to right) time = 0, 24, 48, and 96 h.

Citation: Journal of the Atmospheric Sciences 70, 12; 10.1175/JAS-D-13-066.1

Next, we examine the vortex evolution features in the core region. Figure 5 depicts the time–vertical cross section of relative vorticity and RH averaged within a radius of 50 km from the surface vortex center. A salient feature is a close relationship between the setup of a deep moist layer and the rapid increase of the vorticity. For instance, in both NOSH_CTL and NOSH_DRY (Figs. 5a,b), a steady deepening of the moist layer appears in the core region during the first 24 h. The 90% RH contour reaches to about 500 hPa at hour 30 in both cases. The development of the vorticity in NOSH_DRY in the early stage is slightly behind the development in NOSH_CTL, following the buildup of the deep moist layer. It is the establishment of this near-saturation air column that sets up the stage for the next development: a rapid drop of minimum sea level pressure (as seen in Fig. 3). The importance of this moisture preconditioning feature has been pointed out by several previous studies (e.g., Emanuel 1995; Bister and Emanuel 1997; Li et al. 2006; Nolan 2007; Ge et al. 2013).

Fig. 5.
Fig. 5.

Time–height cross section of (top) RH (%) and (bottom) relative vorticity (1 × 10−4 s−1) fields averaged over a 50-km radius of surface vortex center in (left) NOSH_CTL and (right) NOSH_DRY.

Citation: Journal of the Atmospheric Sciences 70, 12; 10.1175/JAS-D-13-066.1

Another interesting characteristic is the development of a midlevel vorticity maximum started at 30 h before the overall enhanced vorticity in a deep layer (Li 2012). This midlevel vorticity enhancement is observed in both cases around hour 30.

In summary, similarities between NOSH_CTL and NOSH_DRY suggest that the existence of midlevel dry air layer 150 km away from a vortex center does not affect TC development in a resting environment. This implies that the distance of 150 km is ineffective without mean flows.

b. Impact of the vertical shear without the midlevel dry air

It has been shown that environmental vertical shear has a detrimental effect on the TC development (e.g., DeMaria 1996; Frank and Ritchie 2001; Riemer et al. 2009; among many others). As shown in Fig. 3, TCs indeed develop at a slower rate in the presence of vertical shears. Figure 6 depicts the evolution patterns of 850 hPa wind and RH fields in ESH_CTL and WSH_CTL. Note that the RH maximum initially placed at the center of the vortex lags behind the wind circulation center at 48 h but then lines up with the wind circulation again by 96 h. Meanwhile, the vortex core region is much drier compared NOSH_CTL as shown in Fig. 4. The overall TC development is very similar in ESH_CTL and WSH_CTL. In both cases, the final TC centers are located to the downshear flank of the initial vortex center. With the westerly (easterly) shear, the low-level cyclone shifts westward (eastward) while the upper-level circulation shifts eastward (westward). The final TC location follows the low-level vortex center.

Fig. 6.
Fig. 6.

As in Fig. 4, but for WSH_CTL and ESH_CTL.

Citation: Journal of the Atmospheric Sciences 70, 12; 10.1175/JAS-D-13-066.1

The time–vertical cross sections of the relative vorticity and RH averaged over a 50-km radius of the surface vortex center for ESH_CTL and WSH_CTL are given in Fig. 7. Compared with the no-shear cases, the timing of establishment of a full deep moist layer in the core region is delayed in the presence of the vertical shears. Again there is a close relationship between the deepening of the moist layer and rapid development of the vorticity (and the pressure drop). Note that the column moistening leads the rapid development of vorticity by about 24 h. To summarize, the evolutions of the MSLP and the vertical distribution of the area-averaged vorticity show similar characteristics in WSH_CTL and ESH_CTL. Nevertheless, the final TC intensities in both the cases are weaker than the case without the vertical shear (NOSH_CTL). The result is consistent with the previous studies that the vertical shears have a detrimental effect on TC genesis.

Fig. 7.
Fig. 7.

As in Fig. 5, but for WSH_CTL and ESH_CTL.

Citation: Journal of the Atmospheric Sciences 70, 12; 10.1175/JAS-D-13-066.1

c. Combined effects of vertical shear and midlevel dry air

The result above indicates that without the existence of a midlevel dry air layer, the easterly and westerly shear effects on a TC development are quite similar. However, in the presence of dry air, the intensification rate and the final TC state in WSH_DRY and ESH_DRY are very different from each other (Fig. 3). To further illustrate such discrepancies, Fig. 8 shows the evolution of low-level circulation and RH fields for these two cases. In WSH_DRY, dry air (represented by less than 70% RH contour; green colored area) is kept away from the TC core region all the time. In ESH_DRY, dry air is quickly wrapped into the vortex core region during the first 48 h, and covers almost the entire core region by 96 h. Accordingly, in WSH_DRY, a TC is able to form, even though its intensification rate is much weaker than that in WSH_CTL. On the contrary, no TC is formed in ESH_DRY. The results indicate that the midlevel dry air imposed is very effective in hindering the storm development under the shear condition. Furthermore, it indicates that the impact of dry air is highly sensitive to the orientation of vertical shear. In other words, the relative position of dry air and vertical shear is crucial in determining their combined impact on TC development. To verify this hypothesis, we conducted two additional experiments. In these two experiments, the vertical shears are as in WSH_DRY and ESH_DRY but the midlevel dry air layer is placed to the south side of the vortex center. Interestingly, the result is exactly opposite to what occurs in the WSH_DRY and ESH_DRY (i.e., a TC forms in ESH_DRY and no TCs form in WSH_DRY; figure not shown). These experiments support our claim that the combined effects of vertical shear and dry air on TC development indeed depend critically on the relative position of the dry air and the orientation of vertical shear. In short, when located in the downshear-right (-left) quadrant of the initial vortex, dry air tends to exert a significant (insignificant) impact on TC development.

Fig. 8.
Fig. 8.

As in Fig. 4, but for WSH_DRY and ESH_DRY.

Citation: Journal of the Atmospheric Sciences 70, 12; 10.1175/JAS-D-13-066.1

Of a particular interest is what causes this asymmetric shear impact. We attempt to examine possible mechanisms through the diagnosis of the model outputs. In the early stage, the background shear tends to cause the tilting of an initially vertically aligned vortex. Later, the vortex becomes vertically aligned again as a mesoscale core vortex develops. To demonstrate this realignment process, we first to compare these four experiments with vertical shears, either with or without the midlevel dry air layer. Figure 9 displays the tracks of the vortex centers at 850- and 400-hPa levels at a 3-h interval, respectively. The vortex center is defined as the maximum relative vorticity center at each level. To minimize the influence of small-scale convective cells, a spatial filtering (Fang and Zhang 2011) was applied to the vorticity field so that only the signal with wavelength greater than 200 km is retained. The vertical shear leads to the zonal shift of the upper- and lower-level vortices in the early stage. For example, in ESH_CTL, the upper-level (400 hPa) vortex shifts initially toward the west, while the lower-level (850 hPa) center shifts toward the east. After an adjustment period, the upper-level center turns eastward while the lower-level vortex turns westward. Thereafter, both turned southward and eventually vertically aligned with each other. A similar evolution feature is found in WSH_CTL, except the center moved toward north. With this vertical alignment, a TC forms. In the WSH_DRY case, trajectories of the vortices at the levels of interest are in general similar to those in the WSH_CTL, except that the displacements are larger so that the TC forms at a much later stage (Fig. 3). However, no TC forms in ESH_DRY. The vortex at 850 hPa drifts far away to the east and never turns back, whereas the entity at 400 hPa disappears. Hence, these two fail to align with each other.

Fig. 9.
Fig. 9.

Tracks of the vortex centers at 850- (red) and 400-hPa (purple) levels in a 3-hourly time interval. The hurricane symbols indicate the timing of the vertical alignment: (top) CTL and (bottom) DRY; (left) ESH and (right) WSH.

Citation: Journal of the Atmospheric Sciences 70, 12; 10.1175/JAS-D-13-066.1

It has been shown that the vertical shear will lead to the development of a forced secondary circulation (FSC; Zhang and Kieu 2006) with anomalous ascent (descent) in the downshear (upshear) flank. It is worth mentioning that the FSC is asymmetric with respect to the vortex center. In a moist environment, the ascending branch associated with the FSC may reinforce itself through the release of latent heating. The reinforced secondary circulation may overcome the shear-induced tilting and help restore the vertical alignment.

Why does the dry air allow the vertical alignment process under the westerly shear but not the easterly shear? We hypothesize that the difference is attributed to the relative location of dry air in relevance to the downshear quadrant. When it is placed to the right (left) of the downshear region, the dry air is more (less) easily advected into the convective region of the FSC. Figure 10 contains two schematic diagrams showing how the westerly and easterly shear (with the same vertical profiles as in Fig. 2) affect the FSC and the subsequent vertical alignment process in the case when the dry air is placed initially to the north of the vortex center. In the westerly shear case (Fig. 10a), a cyclonic vortex that is initially vertically aligned will gradually tilt eastward with height owing to vorticity advection. While the upper-level vortex shifts eastward from the original position, the lower-level vortex shifts westward. According to the omega equation, positive (negative) vorticity advection to the east of the upper- (lower-) level vorticity maximum would lead to ascending motion. Likewise, the differential vorticity advection will lead to descending motion to the west side of the low-level circulations. Thus, a secondary circulation (green arrows) forms, with maximum vertical velocity near 500 hPa. With the aid of the moist process, the ascending branch of the FSC is strengthened owing to condensational heating associated with enhanced convection. Therefore, the FSC is reinforced to help overcome the shear-induced drifting and “restore” the vertical alignment.

Fig. 10.
Fig. 10.

Schematic summarizing the impact of background vertical shear on the FSC and the subsequent vertical alignment process: (a) how the shear-induced FSC is established and how the convection reinforced FSC helps restore the vertical alignment in WSH_DRY and (b) how the FSC fails to offset the shear advective effect owing to dry air intrusion in ESH_DRY.

Citation: Journal of the Atmospheric Sciences 70, 12; 10.1175/JAS-D-13-066.1

How does the ESH_DRY differ from the WSH_DRY? Recall that in our model configurations, the dry air layer is placed to the north of the vortex center. Because of the combination of cyclonic circulation of the vortex and the mean flow, the dry air originally located north of the vortex will be quickly advected to the west side of the circulation. As a result, there is an asymmetry of the moisture distribution in the zonal direction with dryer air to the west and moister air to the east. This characteristic can be seen in Fig. 8 and the bottom panel in Fig. 4. While the FSC for the easterly shear (top panel in Fig. 10b) is exactly opposite to the one for the westerly shear (top panel in Fig. 10a), the dry air zonal asymmetric distribution comes to play in the next step. As illustrated in the middle panel of Fig. 10b for ESH_DRY, with the dryer air existing in regions where the upward branch of the FSC resides, the FSC in ESH_DRY lacks convective mechanism for enhancing itself as seen in the WSH_DRY case. Therefore, the parted upper-level vortex due to the shear continues drifting away from the low-level vortex so that they never realign again.

To support our hypothesis and to illustrate the “restoring” effect by the FSC, we plotted the evolution of the shear-induced secondary circulation in the four shear experiments. Because 500 hPa is the transition level for the shear-induced shifting, we use the asymmetric component of vertical velocity at this level to represent the strength of the FSC. Figure 11 illustrates the evolution of the asymmetric 500-hPa vertical velocity along the east–west direction across the vortex center. As an initial symmetric vortex is subjected to vertical shear, a counter-shear-forced secondary circulation appears, with rising (sinking) motion downshear (upshear). The FSC acts to reduce the destructive role of the background shear. Notice that after initially drifting away from the center, the FSC in ESH_CTL and WSH_CTL moves back toward the vortex center. The overall evolution of the FSC in these two cases may be viewed as a close mirror image in the zonal direction. The strength of the shear-induced FSC is much weaker when the midlevel dry air layer is presented and the center of the FSC is drifted farther away from the vortex center. Not surprisingly, there is a marked difference between WSH_DRY and ESH_DRY. In WSH_DRY the maximum ascending motion associated with the FSC turns back toward the center after initial drifting away, whereas in ESH_DRY the FSC continues weakening and drifting away from the vortex center.

Fig. 11.
Fig. 11.

Evolution of the forced secondary circulation represented by asymmetric component of vertical velocity (m s−1) at 500 hPa along the east–west direction across the vortex center: (top) CTL and (bottom) DRY; (left) WSH and (right) ESH.

Citation: Journal of the Atmospheric Sciences 70, 12; 10.1175/JAS-D-13-066.1

The role of convection in strengthening the FSC may be inferred from the model generated radar reflectivity. In the four shear-present experiments, maximum radar reflectivity centers tend to collocate with the upper-level vortex center where the FSC associated ascending motion is located (Fig. 12). Because of the vertical shear, the upper- and lower-level vortices initially move in opposite directions. The fact that there is an in-phase relationship between the convection and the upper-level vortex confirms that the shear-induced FSC and associated convection play an important role in restoring the vertical alignment of the vortex. Notice that the convection is much stronger in cases without midlevel dry air. This is consistent with faster TC development in WSH_CTL and ESH_CTL. The weakest convective activity occurs in ESH_DRY (bottom panel in Fig. 12). Although the strength of initial convective activity (i.e., at hour 12) in ESH_DRY is similar to the other three cases, the convective activity decays rapidly with time. By comparing Figs. 11 and 12, one may conclude that the strength of the FSC is closely related to local convective activity. Thus, our results suggest that the convection on the downshear side reinforces the FSC and helps restore the vertical alignment between upper- and lower-level vortices.

Fig. 12.
Fig. 12.

Evolution of simulated radar reflectivity (shaded, dBZ) and upper- (400 hPa; black contour) and lower- (850 hPa; red contour) level vorticity fields (1 × 10−5 s−1) at (left to right) hours 12, 18, 36, and 72 in (top to bottom) WSH_CTL, ESH_CTL, WSH_DRY, and ESH_DRY. Only vorticity fields associated with wavelengths > 200 km are shown.

Citation: Journal of the Atmospheric Sciences 70, 12; 10.1175/JAS-D-13-066.1

Except in ESH_DRY, the upper-level vortex in the other three cases eventually becomes vertically aligned with the lower-level vortex center after initial drifting. In ESH_DRY, on the other hand, the lower- and upper-level vortices continue drifting away from each other. The upper-level circulation vanishes eventually, and only a weak lower-level vortex remnant exists after day 4.

The results above indicate that whether or not the shear-induced FSC can be maintained by convection is critical in determining the subsequent vertical alignment and TC development. To examine how the shear orientation makes a difference, we examined the zonal–vertical cross section of the meridional wind and RH fields (Fig. 13). In WSH_DRY, during the initial 48 h, the vortex tilts eastward with height. At the same time, RH is quite large along the tilting axis. After hour 48, the vortex becomes vertically aligned. Along with the vertical alignment process, the cyclone deepens as it strengthens. Compared to WSH_DRY, much drier air appears right below the upper-level vortex (where the shear-induced ascent appears) at hour 24 in ESH_DRY. The dry air suppresses the convection by cutting off moisture transport from the boundary layer, leading to the weakening of the FSC. As a result, the upper-level vortex drifts farther away from the low-level center, and the westward tilting increases with time. By hour 96, only a weak, near-surface cyclonic vortex remnant can be identified.

Fig. 13.
Fig. 13.

Zonal–height cross sections of the meridional wind (contour, m s−1) and RH (shaded, %) fields across the surface vortex center in (left) ESH_DRY and (right) WSH_DRY for (top to bottom) time = 24, 48, 72, and 96 h.

Citation: Journal of the Atmospheric Sciences 70, 12; 10.1175/JAS-D-13-066.1

The results above suggest that, for a given initial dry air, two processes may account for the asymmetry between easterly and westerly shear. The first is the shear-induced ascending motion that occurs to the east of the initial vortex in the WSH-DRY and to the west in the ESH_DRY. The second process is the moisture advection. While the dry air is initially placed to the north, the advection by cyclonic flows brings dry air to the west while leaving the moisture to the east of the vortex intact. The process leads to the asymmetry of the moisture in the east–west direction as illustrated from the evolution of 850-hPa RH and equivalent potential temperature (θe) fields (Fig. 14). In both WSH_DRY and ESH_DRY, dry low-entropy air is wrapped cyclonically around the vortex. Because the midlevel dry air is initially placed to the north of the vortex center, the dry low-entropy air is primarily confined in the west flank of the vortex, and much less low-entropy air can penetrate into the east flank. Therefore, the dry air mainly affects the convection to the west. In ESH_DRY (WSH_DRY), the FSC-induced ascent is located to the west (east) of the vortex center. As a consequence, the dry air has a greater impact on the shear-induced convection in ESH_DRY. In contrast, in WSH_DRY, the shear-induced FSC has its upward branch to the east of the vortex center, which is far away from the dry air intrusion and allows convection to occur. The FSC is able to be maintained and further enhanced, which restores the shear-induced tilting of the vortex and enables the continuous development of the vortex. In short, in the presence of vertical shear, when dry air is located in the downshear-right quadrant of the initial vortex, advection by the cyclonic circulation brings dry air to the downshear side and suppresses moist convection therein. Such a process disrupts the restoring mechanism associated with the FSC and thus inhibits TC development. It is not clear whether the descent branch of the FSC in the upshear flank also plays a role in the aforementioned vertical alignment process. It is speculated that the descent may induce dry advection, lowering the convective available potential energy. The so-induced low-θe air may be advected into the TC core region by low-level inflow and reduce the TC genesis efficiency. To reveal its role, a more detailed analysis is needed.

Fig. 14.
Fig. 14.

Evolution of 850-hPa RH (shaded, %) and equivalent potential temperature (contour, K) fields in (top) ESH_DRY and (bottom) WSH_DRY for (left to right) time = 12, 24, 48, and 96 h.

Citation: Journal of the Atmospheric Sciences 70, 12; 10.1175/JAS-D-13-066.1

A recent work by Dunkerton et al. (2009) proposed a “wave pouch” concept for understanding TC formation from easterly waves. The so-called wave pouch provides a favorable environment for the formation and intensification of a TC protovortex. The pouch center is a region of quasi-closed Lagrangian circulation, where air is repeatedly moistened by convection. An issue arises as to where is the most favorable pouch center in the presence of the midlevel dry air and vertical shears? A measurement of the effect of horizontal flow deformation is the so-called Okubo–Weiss (OW) parameter (Dunkerton et al. 2009; Raymond et al. 2011; Wang et al. 2012), which is an indicator of the potential for cyclone intensification. Following Raymond et al. (2011), a normalized OW parameter is defined as follows:
eq1
where , , and .

This parameter equals 1 when the flow is totally rotational and −1 when it is completely strained. Figure 15 shows the evolution of the normalized vertical-average OW parameter in both ESH_DRY and WSH_DRY. In general, the OW maxima are located at a closed cyclonic circulation center. In ESH_DRY, the values of OW are much weaker compared to its counterpart. In association with the continuous drifting of the upper- and lower-level vortices, associated OW maxima are also separated farther. On the contrary, in WSH_DRY, the OW values in both the upper and lower levels increase and become vertically aligned. The overlapping of the OW maxima between the upper and lower levels implies that this is a favorable region of TC development, which prohibits the entrainment of environmental dry air (Raymond et al. 2011).

Fig. 15.
Fig. 15.

Evolution of column-averaged normalized OW parameter (shaded) in (top) ESH_DRY and (bottom) WSH_DRY. The red solid (black dashed) contours denote 850 (400)-hPa relative vorticity fields (1 × 10−5 s−1; only signals with wavelengths > 200 km are shown): (left to right) time = 24, 36, 72, and 96 h. The red (black) vectors show 850- (400-) hPa wind fields.

Citation: Journal of the Atmospheric Sciences 70, 12; 10.1175/JAS-D-13-066.1

4. Summary and discussion

In this study, a set of idealized numerical experiments with WRF are designed to investigate the effect of midlevel dry air layer, the vertical shear, and their combined effects on TC development. It is found that, compared with the result in a quiescent environment (Braun et al. 2012), a vertical shear and a nearby midlevel dry air layer may significantly enhance the negative impacts on TC development. Furthermore, it suggests the importance of relative position of the nearby dry air and orientation of the shear in determining their combined impacts on TC developments. In the presence of vertical shear, the background shear causes the tilting of an initially vertically aligned vortex. The shear forces a FSC with ascent (descent) in the downshear (upshear) flank. With abundant moisture, this FSC has the capability of enhancing its vertical circulation in its upward motion branch with convective activities. This reinforced circulation can overcome the shear-induced tilting and restore the vertical alignment. When dry air is located in the downshear-right quadrant of the initial vortex, advection of dry air by the cyclonic circulation will bring dry air to the downshear side and suppress moist convection therein, by which disrupting the “resort” mechanism and thus inhibiting TC development. With this regard, a marked difference likely occurs under different orientations of vertical shears.

This study complements the work by Braun et al. (2012), in which the impact of midlevel dry air was examined in a resting environment. In that scenario, the dry air can affect TC development only when it is placed close to the vortex center (e.g., the radius of maximum wind). Accordingly, they suggested that the impact of the dry air on TC intensity is likely overstated. However, the current study indicates that the large-scale environmental flow, particularly with vertical shears, can significantly enhance the negative impact through interactions among the vortex, FSC, and moisture processes. This appears to be consistent with an observational study by Shu and Wu (2009), who noted that the effective distance of dry air can be larger (i.e., 380 km). It is possible that, in nature, the dry air may act in tandem with large-scale environmental flows such as vertical shear to produce more substantial inhibiting effects. With this regard, this study may explain, to a certain degree, the discrepancies between the observational study of Shu and Wu (2009) and the modeling study of Braun et al. (2012). Of course, to better understand the effective radius, sensitivity experiments similar to the current setting of background flow patterns but with different radial locations of dry air should be further studied.

In additional, Braun et al. (2010) and Sippel et al. (2011) found that vortex intensification is sensitive to dry SAL air only during the early development stage. It was speculated that stronger TCs with larger size may have greater resistance to the background vertical shear. This implies that the dry air impact may be sensitive to storm size and/or intensity. Admittedly, in the present study, the specified vertical shear profiles are highly idealized. Previous studies (e.g., Rappin and Nolan 2012) showed that the TC genesis is sensitive to the vertical profile of the shear. Therefore, to better understand the combined effect of dry air and environmental flows, more realistic dry air and environmental conditions should be considered in further studies.

Acknowledgments

This work was supported by ONR Grant N00014-0810256 and by the International Pacific Research Center that is sponsored by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), NASA (NNX07AG53G), and NOAA (NA17RJ1230). The first author is also supported by China NSF Grant 41075037.

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