Search Results

You are looking at 1 - 10 of 29 items for

  • Author or Editor: Shuyi S. Chen x
  • All content x
Clear All Modify Search
Falko Judt and Shuyi S. Chen

Abstract

Eyewall replacements in mature tropical cyclones usually cause intensity fluctuations. One reason for eyewall replacements remaining a forecasting challenge is the lack of understanding of how secondary eyewalls form. This study uses high-resolution, full-physics-model forecast fields of Hurricanes Katrina and Rita (2005) to better understand potential vorticity (PV) generation in the rainbands and the role that convectively generated PV played in the formation of a secondary eyewall in Hurricane Rita. Previous studies have focused on dynamic processes in the inner core and/or the effects of certain specified PV distributions. However, the initial development of a concentric PV ring in the rainband region has not been fully addressed. Katrina and Rita were extensively observed by three research aircraft during the Hurricane Rainband and Intensity Change Experiment (RAINEX), which was designed to study the interaction of the rainbands and the inner core. Rita developed a secondary eyewall and went through an eyewall replacement cycle, whereas Katrina maintained a single primary eyewall during the RAINEX observation period before landfall. These distinct features observed in RAINEX provide a unique opportunity to examine the physical and dynamical processes that lead to formation of concentric eyewalls. A triply nested high-resolution model with 1.67-km resolution in the innermost domain, initialized with operational model forecasts in real time during RAINEX, is used in this study. Analyses of wind, vorticity, PV, and vortex Rossby wave (VRW) activity in the inner-core region were conducted using both RAINEX airborne observations and model output. The results show that a higher PV generation rate and accumulation in the rainband region in Rita leads to a secondary PV/vorticity maximum, which eventually became the secondary eyewall. A strong moat area developed between the primary eyewall and the concentric ring of convection in Rita, prohibiting VRW activity. In contrast, VRWs propagated radially outward from the inner core to the rainband region in Katrina. The VRWs were not a contributing factor in the initial formation of the secondary eyewall in Rita since the moat region with near-zero PV gradient did not allow for radial propagation of VRWs. The large accumulation of convectively generated PV in the rainband region was the key factor in the formation of the secondary eyewall in Rita.

Full access
Chia-Ying Lee and Shuyi S. Chen
Full access
Chia-Ying Lee and Shuyi S. Chen

Abstract

It is widely accepted that air–sea interaction is one of the key factors in controlling tropical cyclone (TC) intensity. However, the physical mechanisms for connecting the upper ocean and air–sea interface with storm structure through the atmospheric boundary layer in TCs are not well understood. This study investigates the air–sea coupling processes using a fully coupled atmosphere–wave–ocean model, especially the coupling-induced asymmetry in surface winds, sea surface temperature, air–sea fluxes, and their impacts on the structure of the hurricane boundary layer (HBL). Numerical experiments of Hurricane Frances (2004) with and without coupling to an ocean model and/or a surface wave model are used to examine the impacts of the ocean and wave coupling, respectively. Model results are compared with the airborne dropsonde and surface wind measurements on board the NOAA WP-3D aircraft. The atmosphere–ocean coupling reduces the mixed-layer depth in the rear-right quadrant due to storm-induced ocean cooling, whereas the wind–wave coupling enhances boundary inflow outside the radius of maximum wind. Storm motion and deep tropospheric inflow create a significant front-to-back asymmetry in the depth of the inflow layer. These results are consistent with the dropsonde observations. The azimuthally averaged inflow layer and the mixed layer, as documented in previous studies, are not representative of the asymmetric HBL. The complex, three-dimensional asymmetric structure in both thermodynamic and dynamic properties of the HBL indicates that it would be difficult to parameterize the effects of air–sea coupling without a fully coupled model.

Full access
Falko Judt and Shuyi S. Chen

Abstract

Rapid intensification (RI) of tropical cyclones (TCs) remains one of the most challenging issues in TC prediction. This study investigates the predictability of RI, the uncertainty in predicting RI timing, and the dynamical processes associated with RI. To address the question of environmental versus internal control of RI, five high-resolution ensembles of Hurricane Earl (2010) were generated with scale-dependent stochastic perturbations from synoptic to convective scales. Although most members undergo RI and intensify into major hurricanes, the timing of RI is highly uncertain. While environmental conditions including SST control the maximum TC intensity and the likelihood of RI during the TC lifetime, both environmental and internal factors contribute to uncertainty in RI timing. Complex interactions among environmental vertical wind shear, the mean vortex, and internal convective processes govern the TC intensification process and lead to diverse pathways to maturity. Although the likelihood of Earl undergoing RI seems to be predictable, the exact timing of RI has a stochastic component and low predictability.

Despite RI timing uncertainty, two dominant modes of RI emerged. One group of members undergoes RI early in the storm life cycle; the other one later. In the early RI cases, a rapidly contracting radius of maximum wind accompanies the development of the eyewall during RI. The late RI cases have a well-developed eyewall prior to RI, while an upper-level warm core forms during the RI process. These differences indicate that RI is associated with distinct physical processes during particular stages of the TC life cycle.

Full access
Brandon W. Kerns and Shuyi S. Chen

Abstract

Dynamics of the Madden–Julian oscillation (DYNAMO) was conducted over the equatorial Indian Ocean (IO) from October 2011 to March 2012. During mid- to late November, a strong Madden–Julian oscillation (MJO) event, denoted MJO-2, initiated in the western IO and passed through the DYNAMO observation array. Dry air intrusions associated with synoptic variability in the equatorial region played a key role in the evolution of MJO-2. First, a sharp dry air intrusion surging from the subtropics into the equatorial region suppresses convection in the ITCZ south of the equator. This diminishes subsidence on the equator associated with the ITCZ convection, which leads to an equatorward shift of convection. It is viewed as a contributing factor for the onset of equatorial convection in MJO-2. Once the MJO convection is established, a second type of dry air intrusion is related to synoptic gyres within the MJO convective envelope. The westward-propagating gyres draw drier air from the subtropics into the equatorial region on the west side of the MJO-2. This dry air intrusion contributes to a 1–2-day break in the rainfall during the active phase of MJO-2. Furthermore, the dry air intrusion suppresses convection in the westerlies of the MJO in the IO. This favors the abrupt shutdown of MJO convection during transition to the suppressed phase in DYNAMO. The two types of dry air intrusions can redistribute convection from the ITCZ to the equator and favor the eastward propagation of the MJO convection. Further study of multiple MJO events is necessary to determine the generality of these findings.

Full access
Shuyi S. Chen and William M. Frank

Abstract

The purpose of this study is to understand the genesis of extratropical convective mesovortices and the large-scale environmental features that influence the vortex formation. A hypothesis is proposed that mesovortices form in the stratiform rain regions of mesoscale convective systems (MCSs) due to the reduction of static stability that reduces the effective local Rossby radius in such regions. A conceptual model of the mesoscale convective cyclogenesis is introduced, which describes the three stages of the mesovortex formation.

A modified version of the Pennsylvania State University/National Center for Atmospheric Research three-dimensional hydrostatic mesoscale model is used to simulate mesovortex genesis in analytically generated pre-MCS large-scale environments. The model simultaneously incorporates parameterized convection and a grid-resolvable convective scheme containing the effects of hydrostatic water loading, condensation (evaporation), freezing (melting), and sublimation.

A control simulation is performed with a specified pre-MCS environment that is characterized by a midtro-pospheric short wave, a low-level jet ahead of the short-wave trough, a large area of conditionally unstable air, a deep layer of moisture, and small vertical wind shear. A mesovortex forms within a stratiform region behind a leading convective line. The evolution and structure of the mesovortex are similar to observations of the mesovortices associated with MCSs over land at midlatitudes. The results show that the mesovortex is produced by localized warming in a region of locally reduced Rossby radius, which induces convergence and, hence, creates rotational momentum via geostrophic adjustment.

Full access
Chia-Ying Lee and Shuyi S. Chen

Abstract

The atmospheric boundary layer (BL) in tropical cyclones (TCs) connects deep convection within rainbands and the eyewall to the air–sea interface. Although the importance of the BL in TCs has been widely recognized in recent studies, how physical processes affect TC structure and intensity are still not well understood. This study focuses on a particular physical mechanism through which a TC-induced upper-ocean cooling within the core circulation of the TC can affect the BL and TC structure. A coupled atmosphere–ocean model forecast of Typhoon Choi-Wan (2009) is used to better understand the physical processes of air–sea interaction in TCs. A persistent stable boundary layer (SBL) is found to form over the cold wake within the TC’s right-rear quadrant, which influences TC structure by suppressing convection in rainbands downstream of the cold wake and enhancing the BL inflow into the inner core by increasing inflow angles over strong SST and pressure gradients. Tracer and trajectory analyses show that the air in the SBL stays in the BL longer and gains extra energy from surface heat and moisture fluxes. The enhanced inflow helps transport air in the SBL into the eyewall. In contrast, in the absence of a TC-induced cold wake and an SBL in an uncoupled atmosphere model forecast, the air in the right-rear quadrant within the BL tends to rise into local rainbands. The SBL formed over the cold wake in the coupled model seems to be a key feature that enhances the transport of high energy air into the TC inner core and may increase the storm efficiency.

Full access
Brandon W. Kerns and Shuyi S. Chen

Abstract

The development of a compact warm core extending from the mid-upper levels to the lower troposphere and related surface pressure falls leading to tropical cyclogenesis (TC genesis) is not well understood. This study documents the evolution of the three-dimensional thermal structure during the early developing stages of Typhoons Fanapi and Megi using aircraft dropsonde observations from the Impact of Typhoons on the Ocean in the Pacific (ITOP) field campaign in 2010. Prior to TC genesis, the precursor disturbances were characterized by warm (cool) anomalies above (below) the melting level (~550 hPa) with small surface pressure perturbations. Onion-shaped skew T–logp profiles, which are a known signature of mesoscale subsidence warming induced by organized mesoscale convective systems (MCSs), are ubiquitous throughout the ITOP aircraft missions from the precursor disturbance to the tropical storm stages. The warming partially erodes the lower-troposphere (850–600 hPa) cool anomalies. This warming results in increased surface pressure falls when superposed with the upper-troposphere warm anomalies associated with the long-lasting MCSs/cloud clusters. Hydrostatic pressure analysis suggests the upper-level warming alone would not result in the initial sea level pressure drop associated with the transformation from a disturbance to a TC. As Fanapi and Megi intensify into strong tropical storms, aircraft flight-level (700 hPa) and dropsonde data reveal that the warm core extends down to 850–600 hPa and has some characteristics of subsidence warming similar to the eyes of mature TCs.

Full access