Latitudinal dependence of the dry air effect on tropical cyclone development (original) (raw)

Highlights

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The impacts of dry air on TC development at different latitudes with no mean flows are investigated.
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The effective radius of the dry air is sensitive to its vertical distribution and the background earth rotation.
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The effect of low-level dry air layer in inhibiting TC development decreases with increasing latitude.

Abstract

The impacts of dry air on tropical cyclone (TC) development at different latitudes with no mean flows are investigated with idealized simulations. It is found that the effective radius of the dry air is sensitive to its vertical distribution and the background earth rotation. The effect of low-level dry-air layer in inhibiting TC development decreases with increasing latitude. At lower latitudes, the greater boundary layer gradient wind imbalance results in a strong low-level inflow, and the dry air can easily penetrate into the TC inner-core region. The intruding dry air inhibits the inner-core deep convection and leads to marked asymmetric convective structure, which significantly suppresses TC development. In contrast, at higher latitudes, the dry air gets moistened before reaching the TC inner-core region due to a weaker radial inflow but can suppress the development of the outer spiral rainbands. The suppressed outer spiral rainbands lead to a weaker barrier effect to the boundary layer inflow and help TC development. Furthermore, the lower the altitude of dry-air layer resides, the greater the impact on TC intensification. The low-level pathway associated with the boundary layer inflow plays an important role on how dry-air layer acts on a TC without considering the mean flow effects. Through examining the climatological distribution of the moisture field, we expect that the intrusion of dry air can be more frequent in the North Atlantic area and therefore has more effects on TC development than in the western North Pacific.

Introduction

Prediction of tropical cyclone (TC) intensity remains a great challenge. Among environmental parameters, relative humidity (RH) is important in controlling TC formation and intensification with high RH generally deemed favorable for TC development (Gray, 1975; McBride, 1981; Kaplan and DeMaria, 2003; Nolan, 2007; Wang, 2012; Wu et al., 2015; Alland et al., 2017). A widely investigated problem is the effect of the midlevel Saharan air layer, which is a layer of warm, dry, and dusty air frequently present over the tropical eastern Atlantic Ocean (Dunion and Velden, 2004; Dunion and Marron, 2008). Numerous studies (Wu, 2007; Fritz and Wang, 2013) suggested that the intrusion of dry Saharan air can potentially inhibit TC development. Using the backward trajectory analysis and water vapor budget analysis, Fritz and Wang (2013) compared two tropical depressions developing under the influence of Saharan air layer, one of which intensified into a tropical storm while the other failed. They found that the non-developed case is characterized by a persistent entrainment of dry air into a 100-km radius from the center of wave pouch, a region of closed circulation within the tropical wave critical layer that protects the tropical cyclone protovortex from the hostile environment (Dunkerton et al., 2009), which suppressed convection therein and thus the tropical depression development. In contrast, the dry air stays away from the pouch center in the developing case.

Dry-air intrusions can have a negative influence on TC intensification as dry air ingestion promotes the formation of cold downdrafts, which transport low equivalent potential temperature air into the sub-cloud layer and storm inflow region (Emanuel, 1989; Tang and Emanuel, 2010, 2012). Shu and Wu (2009) found that TCs weaken when the distance between the intruding dry air and the TC center is less than 380 km. Nevertheless, using satellite data and the NCEP Global Forecast System (GFS) data, Braun (2010) found that the composite characteristics of the Saharan air layer for the strengthening and the weakening TCs have insignificant differences once the TCs reach tropical storm strength, suggesting that the dry air has a larger impact on TC formation than in TC intensification. Based on idealized simulations in the absence of mean flows, Braun et al. (2012) suggested that the dry-air layer inhibits TC intensification only when it is located close to the vortex center at the early stage. As convection eventually moistens the free troposphere, all TCs reach the same intensity despite the different initial dry air distributions. Ge et al. (2013) showed that the effect of midlevel dry-air intrusion on TC intensification is enhanced in the presence of an environmental vertical wind shear. Meanwhile, it has been suggested that TCs are usually subjected to low-level dry-air intrusion just before making landfall (Powell, 1987; Kimball, 2006), which leads to salient convective asymmetries and intensity weakening.

Tang and Emanuel (2010) proposed two possible pathways through which the low-entropy air can penetrate into the eyewall region and inhibit the TC intensification. The first is a low-level pathway where the low-entropy air related to the convective downdrafts outside the eyewall is advected inwards by the radial inflow in the subcloud layer and the second pathway is the direct ventilation of low-entropy midlevel air into the eyewall through turbulent entrainment. Riemer et al. (2011, 2013) suggested that the low-entropy air brought into the boundary layer from above by shear-induced downdrafts may significantly reduce the equivalent potential temperature in boundary layer and subsequently constrain the storm intensity when the lower-entropy air enters the eyewall updrafts.

Meanwhile, the dynamics of the planetary boundary layer varies perceivably with the latitude (Li et al., 2012; Smith et al., 2015; Bi et al., 2018). Li et al. (2012) demonstrated that a stronger boundary layer gradient wind imbalance in a lower planetary vorticity environment favors a faster establishment of the boundary layer inflow. The strong boundary layer inflow then advects relatively large environmental absolute angular momentum inward to spin up the vortex. Meanwhile, there is a positive feedback between the boundary layer inflow and inner-core convection. Namely, stronger inner-core convection promotes greater diabatic heating and forces a greater secondary circulation. Using a steady-state slab boundary layer model, Smith et al. (2015) indicated that a TC at lower latitudes is characterized by stronger boundary layer inflow and higher diabatic heating rate, accounting for a faster vortex spin-up.

Although the individual impact of dry-air intrusion or background earth rotation on TC development is extensively studied, the relationship between latitude, dry air intrusions, and TC intensity change is unclear. One possibility is that the strong boundary layer inflow at lower latitudes might enhance the negative effect of dry air through greater radial advection toward the TC center. On the other hand, since a stronger TC is less susceptible to dry-air intrusion than a weaker TC (Braun, 2010; Sippel et al., 2011), a TC at lower latitudes might be less affected by the dry air thanks to its greater intensification. In Braun et al. (2012), all the simulations were performed on a certain latitude and thus the impacts of latitude are not examined, which motivates us to perform the current work. Hence, the objective of this study is to examine the latitudinal dependence of the impact of a simulated dry-air layer on TC development and the underlying processes responsible for a potential latitudinal dependence. To focus on the combined effect of planetary vorticity and dry air, no mean flow is included in our simulations. Moreover, the sensitivity of the impacts of dry air to its vertical distribution is investigated. The paper is organized as follows. In Section 2, the model configuration and experiment design are presented. Section 3 compares the simulated results, and examines the processes accounting for the latitudinal dependence of the dry-air impact on the TC. A summary and discussion are given in Section 4.

Section snippets

Model and experiment design

The Advanced Research Weather Research and Forecasting model (ARW-WRF) version 3.3.1 (Skamarock et al., 2008) is used for idealized simulations. It is triple-nested with the inner two domains following the TC center. The sizes of the three domains are 6480 × 6480 km, 2700 × 2700 km and 630 × 630 km, with horizontal grid spacing of 27, 9, and 3 km, respectively. The model has 28 vertical levels with the model top at 50 hPa. The Lin scheme (Lin et al., 1983) is used for microphysics

TC evolution

In the current study, the maximum azimuthal-mean tangential wind speed is utilized to represent the TC intensity. At a specified latitude, all the simulations eventually reach very similar intensities (not shown) despite the initially different configurations of the dry-air layer. This is consistent with previous studies, which showed that a different moisture profile could change the onset timing of TC intensification but weakly impacted the TC maximum intensity (Kieu et al., 2013; Kieu and

Summary and discussion

In this study, the latitudinal dependence of the effect of dry air on TC development is investigated using idealized numerical simulations. A series of experiments with different settings of the ambient RH at different latitudes are conducted. The result shows that the effect of low-level dry air on TC intensification is sensitive to the latitude, since the strength of low-level inflow is different with different planetary vorticity. At lower latitudes, the stronger boundary layer gradient wind

Acknowledgements

This work was jointly sponsored by the National Key R& D Program of China (2017YFC1502000), the National Science Foundation of China (Grant No.41575056, 41730961,41775058), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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