Warm Cores, Eyewall Slopes, and Intensities of Tropical Cyclones Simulated by a 7-km-Mesh Global Nonhydrostatic Model

2016 ◽  
Vol 73 (11) ◽  
pp. 4289-4309 ◽  
Author(s):  
Tomoki Ohno ◽  
Masaki Satoh ◽  
Yohei Yamada
Keyword(s):  
Tropical Cyclones ◽  
Inner Core ◽  
Grid Spacing ◽  
Satellite Measurements ◽  
Warm Core ◽  
Nonhydrostatic Model ◽  
Proposed Model ◽  
Tangential Wind ◽  
Balanced Model ◽  
Horizontal Grid

Abstract Based on the data of a 1-yr simulation by a global nonhydrostatic model with 7-km horizontal grid spacing, the relationships among warm-core structures, eyewall slopes, and the intensities of tropical cyclones (TCs) were investigated. The results showed that stronger TCs generally have warm-core maxima at higher levels as their intensities increase. It was also found that the height of a warm-core maximum ascends (descends) as the TC intensifies (decays). To clarify how the height and amplitude of warm-core maxima are related to TC intensity, the vortex structures of TCs were investigated. By gradually introducing simplifications of the thermal wind balance, it was established that warm-core structures can be reconstructed using only the tangential wind field within the inner-core region and the ambient temperature profile. A relationship between TC intensity and eyewall slope was investigated by introducing a parameter that characterizes the shape of eyewalls and can be evaluated from satellite measurements. The authors found that the eyewall slope becomes steeper (shallower) as the TC intensity increases (decreases). Based on a balanced model, the authors proposed a relationship between TC intensity and eyewall slope. The result of the proposed model is consistent with that of the analysis using the simulation data. Furthermore, for sufficiently strong TCs, the authors found that the height of the warm-core maximum increases as the slope becomes steeper, which is consistent with previous observational studies. These results suggest that eyewall slopes can be used to diagnose the intensities and structures of TCs.

scholarly journals The Experimental HWRF System: A Study on the Influence of Horizontal Resolution on the Structure and Intensity Changes in Tropical Cyclones Using an Idealized Framework

2011 ◽  
Vol 139 (6) ◽  
pp. 1762-1784 ◽  
Author(s):  
Sundararaman G. Gopalakrishnan ◽  
Frank Marks ◽  
Xuejin Zhang ◽  
Jian-Wen Bao ◽  
Kao-San Yeh ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclones ◽  
Horizontal Resolution ◽  
Grid Resolution ◽  
Weather Research And Forecasting ◽  
Maximum Wind ◽  
Tangential Wind ◽  
Intensity Changes ◽  
Horizontal Grid ◽  
Weather Research

Abstract Forecasting intensity changes in tropical cyclones (TCs) is a complex and challenging multiscale problem. While cloud-resolving numerical models using a horizontal grid resolution of 1–3 km are starting to show some skill in predicting the intensity changes in individual cases, it is not clear at this time what may be a reasonable horizontal resolution for forecasting TC intensity changes on a day-to-day-basis. The Experimental Hurricane Weather Research and Forecasting System (HWRFX) was used within an idealized framework to gain a fundamental understanding of the influence of horizontal grid resolution on the dynamics of TC vortex intensification in three dimensions. HWFRX is a version of the National Centers for Environmental Prediction (NCEP) Hurricane Weather Research and Forecasting (HWRF) model specifically adopted and developed jointly at NOAA’s Atlantic Oceanographic and Meteorological Laboratory (AOML) and Earth System Research Laboratory (ESRL) for studying the intensity change problem at a model grid resolution of about 3 km. Based on a series of numerical experiments at the current operating resolution of about 9 km and at a finer resolution of about 3 km, it was found that improved resolution had very little impact on the initial spinup of the vortex. An initial axisymmetric vortex with a maximum wind speed of 20 m s−1 rapidly intensified to 50 m s−1 within about 24 h in either case. During the spinup process, buoyancy appears to have had a pivotal influence on the formation of the warm core and the subsequent rapid intensification of the modeled vortex. The high-resolution simulation at 3 km produced updrafts as large as 48 m s−1. However, these extreme events were rare, and this study indicated that these events may not contribute significantly to rapid deepening. Additionally, although the structure of the buoyant plumes may differ at 9- and 3-km resolution, interestingly, the axisymmetric structure of the simulated TCs exhibited major similarities. Specifically, the similarities included a deep inflow layer extending up to about 2 km in height with a tangentially averaged maximum inflow velocity of about 12–15 m s−1, vertical updrafts with an average velocity of about 2 m s−1, and a very strong outflow produced at both resolutions for a mature storm. It was also found in either case that the spinup of the primary circulation occurred not only due to the weak inflow above the boundary layer but also due to the convergence of vorticity within the boundary layer. Nevertheless, the mature phase of the storm’s evolution exhibited significantly different patterns of behavior at 9 and 3 km. While the minimum pressure at the end of 96 h was 934 hPa for the 9-km simulation, it was about 910 hPa for the 3-km run. The maximum tangential wind at that time showed a difference of about 10 m s−1. Several sensitivity experiments related to the initial vortex intensity, initial radius of the maximum wind, and physics were performed. Based on ensembles of simulations, it appears that radial advection of the tangential wind and, consequently, radial flux of vorticity become important forcing terms in the momentum budget of the mature storm. Stronger convergence in the boundary layer leads to a larger transport of moisture fluxes and, subsequently, a stronger storm at higher resolution.

scholarly journals Sensitivity of Fine-Scale Structure in Tropical Cyclone Boundary Layer to Model Horizontal Resolution at Sub-Kilometer Grid Spacing

2021 ◽  
Vol 9 ◽  
Author(s):  
Hongxiong Xu ◽  
Yuqing Wang
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Inner Core ◽  
Mesoscale Model ◽  
Horizontal Resolution ◽  
Scale Structure ◽  
Fine Scale ◽  
Grid Spacing ◽  
Large Eddies ◽  
Horizontal Grid

In view of the increasing interest in the explicit simulation of fine-scale features in the tropical cyclone (TC) boundary layer (TCBL), the effects of horizontal grid spacing on a 7–10 h simulation of an idealized TC are examined using the Weather Research and Forecast (ARW-WRF) mesoscale model with one-way moving nests and the nonlinear backscatter with anisotropy (NBA) sub-grid-scale (SGS) scheme. In general, reducing the horizontal grid spacing from 2 km to 500 m tends to produce a stronger TC with lower minimum sea level pressure (MSLP), stronger surface winds, and smaller TC inner core size. However, large eddies cannot be resolved at these grid spacings. In contrast, reducing the horizontal grid spacing from 500 to 166 m and further to 55 m leads to a decrease in TC intensity and an increase in the inner-core TC size. Moreover, although the 166-m grid spacing starts to resolve large eddies in terms of TCBL horizontal rolls and tornado-scale vortex, the use of the finest grid spacing of 55 m tends to produce shorter wavelengths in the turbulent motion and stronger multi-scale turbulence interaction. It is concluded that a grid spacing of sub-100-meters is desirable to produce more detailed and fine-scale structure of TCBL horizontal rolls and tornado-scale vortices, while the relatively coarse sub-kilometer grid spacing (e.g., 500 m) is more cost-effective and feasible for research that is not interested in the turbulence processes and for real-time operational TC forecasting in the near future.

scholarly journals A Latent Heat Retrieval and Its Effects on the Intensity and Structure Change of Hurricane Guillermo (1997). Part II: Numerical Simulations

2012 ◽  
Vol 69 (11) ◽  
pp. 3128-3146 ◽  
Author(s):  
Stephen R. Guimond ◽  
Jon M. Reisner
Keyword(s):  
Numerical Simulations ◽  
Tropical Cyclones ◽  
Latent Heat ◽  
Inner Core ◽  
Doppler Radar ◽  
Model Parameters ◽  
Saturation State ◽  
Wind Structure ◽  
Tangential Wind ◽  
Airborne Doppler Radar

Abstract In Part I of this study, a new algorithm for retrieving the latent heat field in tropical cyclones from airborne Doppler radar was presented and fields from rapidly intensifying Hurricane Guillermo (1997) were shown. In Part II, the usefulness and relative accuracy of the retrievals is assessed by inserting the heating into realistic numerical simulations at 2-km resolution and comparing the generated wind structure to the radar analyses of Guillermo. Results show that using the latent heat retrievals as forcing produces very low intensity and structure errors (in terms of tangential wind speed errors and explained wind variance) and significantly improves simulations relative to a predictive run that is highly calibrated to the latent heat retrievals by using an ensemble Kalman filter procedure to estimate values of key model parameters. Releasing all the heating/cooling in the latent heat retrieval results in a simulation with a large positive bias in Guillermo’s intensity that motivates the need to determine the saturation state in the hurricane inner-core retrieval through a procedure similar to that described in Part I of this study. The heating retrievals accomplish high-quality structure statistics by forcing asymmetries in the wind field with the generally correct amplitude, placement, and timing. In contrast, the latent heating fields generated in the predictive simulation contain a significant bias toward large values and are concentrated in bands (rather than discrete cells) stretched around the vortex. The Doppler radar–based latent heat retrievals presented in this series of papers should prove useful for convection initialization and data assimilation to reduce errors in numerical simulations of tropical cyclones.

scholarly journals Observations of the Structure and Evolution of Hurricane Edouard (2014) during Intensity Change. Part II: Kinematic Structure and the Distribution of Deep Convection

2016 ◽  
Vol 144 (9) ◽  
pp. 3355-3376 ◽  
Author(s):  
Robert F. Rogers ◽  
Jun A. Zhang ◽  
Jonathan Zawislak ◽  
Haiyan Jiang ◽  
George R. Alvey ◽  
...  
Keyword(s):  
Inner Core ◽  
Deep Convection ◽  
Convective Available Potential Energy ◽  
Structural Evolution ◽  
Local Environment ◽  
Intensity Change ◽  
Satellite Measurements ◽  
Warm Core ◽  
Moist Environment ◽  
Azimuthal Asymmetries

The structural evolution of the inner core and near-environment throughout the life cycle of Hurricane Edouard (2014) is examined using a synthesis of airborne and satellite measurements. This study specifically focuses on differences in the distribution of deep convection during two periods: when Edouard intensified toward hurricane status, and when Edouard peaked in intensity and began to weaken. While both periods saw precipitation maximized in the downshear-left and upshear-left quadrants, deep convection was only seen from the aircraft during the intensifying period. Deep convection was located farther inside the radius of maximum winds (RMW) during the intensifying period than the weakening period. This convection is traced to strong updrafts inside the RMW in the downshear-right quadrant, tied to strong low-level convergence and high convective available potential energy (CAPE) as the storm remained over warm water in a moist environment. Strong updrafts persisted upshear left and were collocated with high inertial stability in the inner core. During weakening, no deep convection was present, and the precipitation that was observed was associated with weaker convergence downshear right at larger radii, as CAPE was reduced from lower sea surface temperatures, reduced humidity from subsidence, and a stronger warm core. Weak updrafts were seen upshear left, with little coincidence with the high inertial stability of the inner core. These results highlight the importance of the azimuthal coverage of precipitation and the radial location of deep convection for intensification. A more symmetrical coverage can occur despite the presence of shear-driven azimuthal asymmetries in both the forcing and the local environment of the precipitation.

scholarly journals How Do Outer Spiral Rainbands Affect Tropical Cyclone Structure and Intensity?*

2009 ◽  
Vol 66 (5) ◽  
pp. 1250-1273 ◽  
Author(s):  
Yuqing Wang
Keyword(s):  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Surface Pressure ◽  
Diabatic Heating ◽  
Inner Core ◽  
Phase Changes ◽  
Wave Radiation ◽  
Atmospheric Column ◽  
Tangential Wind ◽  
Spiral Rainbands

Abstract A long-standing issue on how outer spiral rainbands affect the structure and intensity of tropical cyclones is studied through a series of numerical experiments using the cloud-resolving tropical cyclone model TCM4. Because diabatic heating due to phase changes is the main driving force of outer spiral rainbands, their effect on the tropical cyclone structure and intensity is evaluated by artificially modifying the heating and cooling rate due to cloud microphysical processes in the model. The view proposed here is that the effect of diabatic heating in outer spiral rainbands on the storm structure and intensity results mainly from hydrostatic adjustment; that is, heating (cooling) of an atmospheric column decreases (increases) the surface pressure underneath the column. The change in surface pressure due to heating in the outer spiral rainbands is significant on the inward side of the rainbands where the inertial stability is generally high. Outside the rainbands in the far field, where the inertial stability is low and internal atmospheric heating is mostly lost to gravity wave radiation and little is left to warm the atmospheric column and lower the local surface pressure, the change in surface pressure is relatively small. This strong radially dependent response reduces the horizontal pressure gradient across the radius of maximum wind and thus the storm intensity in terms of the maximum low-level tangential wind while increasing the inner-core size of the storm. The numerical results show that cooling in the outer spiral rainbands maintains both the intensity of a tropical cyclone and the compactness of its inner core, whereas heating in the outer spiral rainbands decreases the intensity but increases the size of a tropical cyclone. Overall, the presence of strong outer spiral rainbands limits the intensity of a tropical cyclone. Because heating or cooling in the outer spiral rainbands depends strongly on the relative humidity in the near-core environment, the results have implications for the formation of the annular hurricane structure, the development of concentric eyewalls, and the size change in tropical cyclones.

scholarly journals An Investigation of Spiral Gravity Waves Radiating from Tropical Cyclones Using a Linear, Nonhydrostatic Model

2020 ◽  
Vol 77 (5) ◽  
pp. 1733-1759
Author(s):  
David S. Nolan
Keyword(s):  
Tropical Cyclones ◽  
Gravity Waves ◽  
Static Stability ◽  
Small Scale ◽  
Heat Sources ◽  
Pressure Oscillations ◽  
Nonhydrostatic Model ◽  
Tangential Wind ◽  
Aircraft Observations ◽  
The Waves

Abstract A recent study showed observational and numerical evidence for small-scale gravity waves that radiate outward from tropical cyclones. These waves are wrapped into tight spirals by the radial and vertical shears of the tangential wind field. Reexamination of the previously studied tropical cyclone simulations suggests that the dominant source for these waves are convective asymmetries rotating along the eyewall, modulated in intensity by the preferred convection region on the left side of the environmental wind shear vector. A linearized, nonhydrostatic model for perturbations to a balanced vortex is used to study the waves. Forcing the linear model with rotating and pulsing asymmetric heat sources generates radiating gravity waves with multiple vertical and horizontal structures. The pulsation of the rotating heat source generates two types of waves: fast, deep waves with larger radial wavelengths, and slower, secondary waves with shorter radial and vertical wavelengths. The deeper waves produce surface pressure oscillations that have time scales consistent with surface observations, whereas the shorter waves have little surface indication but produce oscillations in vertical velocity with shorter radial wavelengths that are consistent with aircraft observations. Convective forcing that is either not pulsing or not rotating produces gravity waves but they are not as similar to the observed or simulated waves. The effects of varying the intensity of the cyclone, the asymmetry of the forcing, and the static stability of the surrounding atmosphere are explored.

scholarly journals Impact of the Tropopause Temperature on the Intensity of Tropical Cyclones: An Idealized Study Using a Mesoscale Model

2014 ◽  
Vol 71 (11) ◽  
pp. 4333-4348 ◽  
Author(s):  
Shuguang Wang ◽  
Suzana J. Camargo ◽  
Adam H. Sobel ◽  
Lorenzo M. Polvani
Keyword(s):  
Tropical Cyclones ◽  
Mesoscale Model ◽  
Three Dimensional ◽  
Grid Spacing ◽  
Stratospheric Temperature ◽  
Similar Dependence ◽  
Temperature Equilibrium ◽  
The Impact ◽  
Horizontal Grid ◽  
Tropopause Temperature

Abstract This study investigates the impact of the tropopause temperature on the intensity of idealized tropical cyclones (TCs) superimposed on background states of radiative–convective equilibrium (RCE) in a three-dimensional (3D) mesoscale model. Simulations are performed with constant sea surface temperature and an isothermal stratosphere with constant tropopause temperature. The potential intensity (PI) computed from the thermodynamic profiles of the RCE state (before the TCs are superimposed on it) increases by 0.4–1 m s−1 for each 1 K of tropopause temperature reduction. The 3D TC experiments yield intense tropical cyclones whose intensities exceed the PI value substantially. It is further shown that the discrepancy may be largely explained by the supergradient wind in the 3D simulations. The intensities of these 3D TCs increase by ~0.4 m s−1 per 1 K of cooling in the tropopause temperature in RCE, on the low end of the PI dependence on the tropopause temperature. Sensitivity experiments with a larger horizontal grid spacing of 8 km produce less intense TCs, as expected, but similar dependence (~−0.5 m s−1 K−1) on tropopause temperature. Equilibrium TC solutions are further obtained in 200-day experiments with different values of constant stratospheric temperature. Similar relationships between TC intensity and tropopause temperature are also found in these equilibrium TC solutions.

Different Climatological Characteristics, Inner-Core Structures, and Intensification Processes of Simulated Intense Tropical Cyclones between 20-km Global and 5-km Regional Models

2017 ◽  
Vol 30 (5) ◽  
pp. 1583-1603 ◽  
Author(s):  
Sachie Kanada ◽  
Akiyoshi Wada
Keyword(s):  
Tropical Cyclones ◽  
General Circulation ◽  
Large Scale ◽  
Atmospheric General Circulation Model ◽  
Inner Core ◽  
Circulation Model ◽  
Maximum Intensity ◽  
Rapid Intensification ◽  
Nonhydrostatic Model ◽  
Before And After

Abstract Climatological characteristics of simulated intense tropical cyclones (TCs) in the western North Pacific were explored with a 20-km-mesh atmospheric general circulation model (AGCM20) and a 5-km-mesh regional atmospheric nonhydrostatic model (ANHM5). From the AGCM20 climate runs, 34 intense TCs with a minimum central pressure (MCP) less than or equal to 900 hPa were sampled. Downscaling experiments were conducted with the ANHM5 for each intense TC simulated by the AGCM20. Only 23 developed into TCs with MCP ≤ 900 hPa. Most of the best-track TCs with an MCP ≤ 900 hPa underwent rapid intensification (RI) and attained maximum intensities south of 25°N. The AGCM20 simulated a similar number of intense TCs as the best-track datasets. However, the intense AGCM20 TCs tended to intensify longer and more gradually; only half of them underwent RI. The prolonged gradual intensification resulted in significant northward shifts of the location of maximum intensity compared with the location derived from two best-track datasets. The inner-core structure of AGCM20 TCs exhibited weak and shallow eyewall updrafts with maxima below an altitude of 6 km, while downscaling experiments revealed that most of the intense ANHM5 TCs underwent RI with deep and intense eyewall updrafts and attained their maximum intensity at lower latitudes. The altitudes of updraft maxima simulated by the AGCM20 descended rapidly during the phase of greatest intensification as midlevel warming markedly developed. The change in major processes responsible for precipitation in AGCM20 TCs before and after maximum intensification suggests close relationships between the large-scale cloud scheme and midlevel warming and prolonged gradual intensification.

scholarly journals On the Development of Double Warm-Core Structures in Intense Tropical Cyclones

2016 ◽  
Vol 73 (11) ◽  
pp. 4487-4506 ◽  
Author(s):  
Chanh Kieu ◽  
Vijay Tallapragada ◽  
Da-Lin Zhang ◽  
Zachary Moon
Keyword(s):  
Tropical Cyclones ◽  
Lower Stratosphere ◽  
Inner Core ◽  
Northwestern Pacific ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Warm Core ◽  
Warm Anomaly ◽  
Upper Level ◽  
Almost All

Abstract This study examines the formation of a double warm-core (DWC) structure in intense tropical cyclones (TCs) that was captured in almost all supertyphoon cases during the 2012–14 real-time typhoon forecasts in the northwestern Pacific basin with the Hurricane Weather Research and Forecasting Model (HWRF). By using an idealized configuration of HWRF to focus on the intrinsic mechanism of the DWC formation, it is shown that the development of DWC in intense TCs is accompanied by a thin inflow layer above the typical upper outflow layer. The development of this thin inflow layer in the lower stratosphere (~100–75 hPa), which is associated with an inward pressure gradient force induced by cooling at the cloud top, signifies intricate interaction of TCs with the lower stratosphere as TCs become sufficiently intense, which has not been examined previously. Specifically, it is demonstrated that a higher-level inflow can advect potentially warm air from the lower stratosphere toward the inner-core region, thus forming an upper-level warm core that is separated from a midlevel one of tropospheric air. Such formation of the upper-level warm anomaly in intense TCs is linked to an episode of intensification at the later stage of TC development. While these results are produced by HWRF, the persistent DWC and UIL features in all HWRF simulations of intense TCs suggest that the lower stratosphere may have significant impacts on the inner-core structures of intense TCs beyond the current framework of TCs with a single warm core.

scholarly journals Using Simulated Dropsondes to Understand Extreme Updrafts and Wind Speeds in Tropical Cyclones

2018 ◽  
Vol 146 (11) ◽  
pp. 3901-3925 ◽  
Author(s):  
Daniel P. Stern ◽  
George H. Bryan
Keyword(s):  
High Resolution ◽  
Wind Speed ◽  
Tropical Cyclones ◽  
Grid Spacing ◽  
Wind Speeds ◽  
Eddy Simulation ◽  
Wind Gusts ◽  
Large Eddy ◽  
Horizontal Grid ◽  
Insight Into

Abstract Extreme updrafts (≥10 m s−1) and wind gusts (≥90 m s−1) are ubiquitous within the low-level eyewall of intense tropical cyclones (TCs). Previous studies suggest that both of these features are associated with coherent subkilometer-scale vortices. Here, over 100 000 “virtual” dropsonde trajectories are examined within a large-eddy simulation (31.25-m horizontal grid spacing) of a category 5 hurricane in order to gain insight into the nature of these features and to better understand and interpret dropsonde observations. At such a high resolution, profiles of wind speed and vertical velocity from the virtual sondes are difficult to distinguish from those of real dropsondes. PDFs of the strength of updrafts and wind gusts compare well between the simulated and observed dropsondes, as do the respective range of heights over which these features are found. Individual simulated updrafts can be tracked for periods of up to several minutes, revealing structures that are both coherent and rapidly evolving. It appears that the updrafts are closely associated with vortices and wind speed maxima, consistent with previous studies. The peak instantaneous wind gusts in the simulations (up to 150 m s−1) are substantially stronger than have ever been observed. Using the virtual sondes, it is demonstrated that the probability of sampling such extremes is vanishingly small, and it is argued that actual intense TCs might also be characterized by gusts of these magnitudes.

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