Choosing a Boundary Layer Parameterization for Tropical Cyclone Modeling

2012 ◽  
Vol 140 (5) ◽  
pp. 1427-1445 ◽  
Author(s):  
Jeffrey D. Kepert
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Surface Wind ◽  
Informed Choice ◽  
Difficult Problem ◽  
Modeling Framework ◽  
Near Surface ◽  
Closure Scheme ◽  
Boundary Layer Scheme ◽  
Simplified Modeling

Abstract The boundary layer in a tropical cyclone is in some respects unlike that elsewhere in the atmosphere. It is therefore necessary to evaluate boundary layer parameterizations for their suitability for use in tropical cyclone simulation. Previous work has shown substantial sensitivity to the choice of scheme and identified specific shortcomings in some schemes, but without recommending which schemes are most suitable. Here, several schemes, representative of those available in popular modeling systems, are reviewed and applied in a simplified modeling framework. Based on a comparison with observations and on theoretical grounds, one popular class of schemes is shown to be badly flawed in that it incorrectly predicts the near-surface wind profile, and therefore should not be used. Another is shown to be sensitive to diagnosis of the boundary layer depth, a difficult problem in the core of the tropical cyclone, and caution is advised. The Louis boundary layer scheme and a higher-order closure scheme are, so far as can be discerned, without major problems, and are recommended. The recommendations and discussion herein should help users make a more informed choice of boundary layer parameterization, and to better understand the results that they obtain.

A New Time-dependent Theory of Tropical Cyclone Intensification

2021 ◽  
Author(s):  
Yuqing Wang ◽  
Yuanlong Li ◽  
Jing Xu
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Numerical Simulations ◽  
Strong Dependence ◽  
Surface Wind ◽  
Time Dependent ◽  
Quasi Equilibrium ◽  
Free Atmosphere ◽  
Near Surface ◽  
Tangential Wind

AbstractIn this study, the boundary-layer tangential wind budget equation following the radius of maximum wind, together with an assumed thermodynamical quasi-equilibrium boundary layer is used to derive a new equation for tropical cyclone (TC) intensification rate (IR). A TC is assumed to be axisymmetric in thermal wind balance with eyewall convection becoming in moist slantwise neutrality in the free atmosphere above the boundary layer as the storm intensifies as found recently based on idealized numerical simulations. An ad-hoc parameter is introduced to measure the degree of congruence of the absolute angular momentum and the entropy surfaces. The new IR equation is evaluated using results from idealized ensemble full-physics axisymmetric numerical simulations. Results show that the new IR equation can reproduce the time evolution of the simulated TC intensity. The new IR equation indicates a strong dependence of IR on both TC intensity and the corresponding maximum potential intensity (MPI). A new finding is the dependence of TC IR on the square of the MPI in terms of the near-surface wind speed for any given relative intensity. Results from some numerical integrations of the new IR equation also suggest the finite-amplitude nature of TC genesis. In addition, the new IR theory is also supported by some preliminary results based on best-track TC data over the North Atlantic and eastern and western North Pacific. Compared with the available time-dependent theories of TC intensification, the new IR equation can provide a realistic intensity-dependent IR during weak intensity stage as in observations.

scholarly journals Tornado-scale vortices in the tropical cyclone boundary layer: numerical simulation with the WRF–LES framework

2019 ◽  
Vol 19 (4) ◽  
pp. 2477-2487 ◽  
Author(s):  
Liguang Wu ◽  
Qingyuan Liu ◽  
Yubin Li
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Large Scale ◽  
Surface Wind ◽  
Wrf Model ◽  
Surface Wind Speed ◽  
Horizontal Scale ◽  
Lower Altitude ◽  
Near Surface ◽  
Research Aircraft

Abstract. A tornado-scale vortex in the tropical cyclone (TC) boundary layer (TCBL) has been observed in intense hurricanes and the associated intense turbulence poses a severe threat to the manned research aircraft when it penetrates hurricane eyewalls at a lower altitude. In this study, a numerical experiment in which a TC evolves in a large-scale background over the western North Pacific is conducted using the Advanced Weather Research and Forecast (WRF) model by incorporating the large-eddy simulation (LES) technique. The simulated tornado-scale vortex shows features similar to those revealed with limited observational data, including the updraft–downdraft couplet, the sudden jump of wind speeds, the location along the inner edge of the eyewall, and the small horizontal scale. It is suggested that the WRF–LES framework can successfully simulate the tornado-scale vortex with grids at a resolution of 37 m that cover the TC eye and eyewall. The simulated tornado-scale vortex is a cyclonic circulation with a small horizontal scale of ∼1 km in the TCBL. It is accompanied by strong updrafts (more than 15 m s−1) and large vertical components of relative vorticity (larger than 0.2 s−1). The tornado-scale vortex favorably occurs at the inner edge of the enhanced eyewall convection or rainband within the saturated, high-θe layer, mostly below an altitude of 2 km. In nearly all the simulated tornado-scale vortices, the narrow intense updraft is coupled with the relatively broad downdraft, constituting one or two updraft–downdraft couplets, as observed by the research aircraft. The presence of the tornado-scale vortex also leads to significant gradients in the near-surface wind speed and wind gusts.

scholarly journals Tornado-Scale Vortices in the Tropical Cyclone Boundary Layer: Numerical Simulation with WRF-LES Framework

2018 ◽  
Author(s):  
Liguang Wu ◽  
Qingyuan Liu ◽  
Yubing Li
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Large Scale ◽  
Surface Wind ◽  
Wrf Model ◽  
Surface Wind Speed ◽  
Horizontal Scale ◽  
Lower Altitude ◽  
Near Surface ◽  
Research Aircraft

Abstract. The tornado-scale vortex in the tropical cyclone (TC) boundary layer (TCBL) has been observed in intense hurricanes and the associated intense turbulence poses a severe threat to the manned research aircraft when it penetrates hurricane eyewalls at a lower altitude. In this study, a numerical experiment in which a TC evolves in a large-scale background over the western North Pacific is conducted using the Advanced Weather Research and Forecast (WRF) model by incorporating the large eddy simulation (LES) technique. The simulated tornado-scale vortex shows the similar features as revealed with the limited observational data, including the updraft/downdraft couplet, the sudden jump of wind speeds, the favorable location, and the horizontal scale. It is suggested that the WRF-LES framework can successfully simulate the tornado-scale vortex with the grids at the resolution of 37 m that cover the TC eye and eyewall. The simulated tornado-scale vortex is a cyclonic circulation with a small horizontal scale of ~ 1 km in the TCBL. It is accompanied by strong updrafts (more than 15 m s−1) and large vertical components of relative vorticity (larger than 0.2 s−1). The tornado-scale vortex favorably occurs at the inner edge of the enhanced eyewall convection or rainband within the saturated, high-θe layer, mostly below the altitude of 2 km. Nearly in all the simulated tornado-scale vortices, the narrow intense updraft is coupled with the relatively broad downdraft, constituting one or two updraft/downdraft couplets or horizontal rolling vortices, as observed by the research aircraft. The presence of the tornado-scale vortex also leads to significant gradients in the near surface wind speed and wind gusts.

Evaluation of the Surface Wind Field over Land in WRF Simulations of Hurricane Wilma (2005). Part II: Surface Winds, Inflow Angles, and Boundary Layer Profiles

2021 ◽  
Author(s):  
David S. Nolan ◽  
Brian D. McNoldy ◽  
Jimmy Yunge ◽  
Forrest J. Masters ◽  
Ian M. Giammanco
Keyword(s):  
Boundary Layer ◽  
Wind Field ◽  
Initial Conditions ◽  
Surface Wind ◽  
Storm Track ◽  
South Florida ◽  
Modeling Framework ◽  
Near Surface ◽  
Hurricane Wilma ◽  
Wind Events

AbstractThis is the second of a two-part study that explores the capabilities of a mesoscale atmospheric model to reproduce the near-surface wind fields in hurricanes over land. The Weather Research and Forecasting Model (WRF) is used with two planetary boundary layer parameterizations: the Yonsei University (YSU) and the Mellor-Yamada-Janjić (MYJ) schemes. The first part presented the modeling framework and initial conditions used to produce simulations of Hurricane Wilma (2005) that closely reproduced the track, intensity, and size of its wind field as it passed over South Florida. This part explores how well these simulations can reproduce the winds at fixed points over land by making comparisons to observations from airports and research weather stations. The results show that peak wind speeds are remarkably well reproduced at several locations. Wind directions are evaluated in terms of the inflow angle relative to the storm center, and the simulated inflow angles are generally smaller than observed. Localized peak wind events are associated with vertical vorticity maxima in the boundary layer with horizontal scales of 5-10 km. The boundary layer winds are compared to wind profiles obtained by velocity-azimuth display (VAD) analyses from National Weather Service Doppler radars at Miami and Key West; results from these comparisons are mixed. Nonetheless the comparisons to surface observations suggest that when short-term hurricane forecasts can sufficiently predict storm track, intensity, and size, they will also be able to provide useful information on extreme winds at locations of interest.

scholarly journals Observed Boundary Layer Wind Structure and Balance in the Hurricane Core. Part I: Hurricane Georges

2006 ◽  
Vol 63 (9) ◽  
pp. 2169-2193 ◽  
Author(s):  
Jeffrey D. Kepert
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Tropical Cyclone ◽  
Radial Profile ◽  
Lower Altitude ◽  
Near Surface ◽  
Wind Structure ◽  
The Right ◽  
Tropical Cyclone Boundary Layer ◽  
Upper Boundary

Abstract The GPS dropsonde allows observations at unprecedentedly high horizontal and vertical resolution, and of very high accuracy, within the tropical cyclone boundary layer. These data are used to document the boundary layer wind field of the core of Hurricane Georges (1998) when it was close to its maximum intensity. The spatial variability of the boundary layer wind structure is found to agree very well with the theoretical predictions in the works of Kepert and Wang. In particular, the ratio of the near-surface wind speed to that above the boundary layer is found to increase inward toward the radius of maximum winds and to be larger to the left of the track than to the right, while the low-level wind maximum is both more marked and at lower altitude on the left of the storm track than on the right. However, the expected supergradient flow in the upper boundary layer is not found, with the winds being diagnosed as close to gradient balance. The tropical cyclone boundary layer model of Kepert and Wang is used to simulate the boundary layer flow in Hurricane Georges. The simulated wind profiles are in good agreement with the observations, and the asymmetries are well captured. In addition, it is found that the modeled flow in the upper boundary layer at the eyewall is barely supergradient, in contrast to previously studied cases. It is argued that this lack of supergradient flow is a consequence of the particular radial structure in Georges, which had a comparatively slow decrease of wind speed with radius outside the eyewall. This radial profile leads to a relatively weak gradient of inertial stability near the eyewall and a strong gradient at larger radii, and hence the tropical cyclone boundary layer dynamics described by Kepert and Wang can produce only marginally supergradient flow near the radius of maximum winds. The lack of supergradient flow, diagnosed from the observational analysis, is thus attributed to the large-scale structure of this particular storm. A companion paper presents a similar analysis for Hurricane Mitch (1998), with contrasting results.

The Evolution of Asymmetries in the Tropical Cyclone Boundary Layer Wind Field during Landfall

2021 ◽  
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Wind Field ◽  
Surface Wind ◽  
Frictional Stress ◽  
Homogeneous Surface ◽  
Sudden Decrease ◽  
The Right ◽  
Tropical Cyclone Boundary Layer

Abstract The evolution of the tropical cyclone boundary layer (TCBL) wind field before landfall is examined in this study. As noted in previous studies, a typical TCBL wind structure over the ocean features a supergradient boundary layer jet to the left of motion and Earth-relative maximum winds to the right. However, the detailed response of the wind field to frictional convergence at the coastline is less well known. Here, idealized numerical simulations reveal an increase in the offshore radial and vertical velocities beginning once the TC is roughly 200 km offshore. This increase in the radial velocity is attributed to the sudden decrease in frictional stress once the highly agradient flow crosses the offshore coastline. Enhanced advection of angular momentum by the secondary circulation forces a strengthening of the supergradient jet near the top of the TCBL. Sensitivity experiments reveal that the coastal roughness discontinuity dominates the friction asymmetry due to motion. Additionally, increasing the inland roughness through increasing the aerodynamic roughness length enhances the observed asymmetries. Lastly, a brief analysis of in-situ surface wind data collected during the landfall of three Gulf of Mexico hurricanes is provided and compared to the idealized simulations. Despite the limited in-situ data, the observations generally support the simulations. The results here imply that assumptions about the TCBL wind field based on observations from over horizontally-homogeneous surface types - which have been well-documented by previous studies - are inappropriate for use near strong frictional heterogeneity.

scholarly journals Dynamics of Lower-Tropospheric Vorticity in Idealized Simulations of Tropical Cyclone Formation

2019 ◽  
Vol 76 (3) ◽  
pp. 707-727 ◽  
Author(s):  
Yaping Wang ◽  
Christopher A. Davis ◽  
Yongjie Huang
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Mass Flux ◽  
Cloud Model ◽  
Vertical Wind Shear ◽  
Lower Troposphere ◽  
Cold Pool ◽  
Surface Drag ◽  
Near Surface ◽  
Cold Pools

Abstract Idealized simulations are conducted using the Cloud Model version 1 (CM1) to explore the mechanism of tropical cyclone (TC) genesis from a preexisting midtropospheric vortex that forms in radiative–convective equilibrium. With lower-tropospheric air approaching near saturation during TC genesis, convective cells become stronger, along with the intensifying updrafts and downdrafts and the larger area coverage of updrafts relative to downdrafts. Consequently, the low-level vertical mass flux increases, inducing vorticity amplification above the boundary layer. Of interest is that while surface cold pools help organize lower-tropospheric updrafts, genesis still proceeds, only slightly delayed, if subcloud evaporation cooling and cold pool intensity are drastically reduced. More detrimental is the disruption of near saturation through the introduction of weak vertical wind shear. The lower-tropospheric dry air suppresses the strengthening of convection, leading to weaker upward mass flux and much slower near-surface vortex spinup. We also find that surface spinup is similarly inhibited by decreasing surface drag despite the existence of a nearly saturated column, whereas larger drag accelerates spinup. Increased vorticity above the boundary layer is followed by the emergence of a horizontal pressure gradient through the depth of the boundary layer. Then the corresponding convergence resulting from the gradient imbalance in the frictional boundary layer causes vorticity amplification near the surface. It is suggested that near saturation in the lower troposphere is critical for increasing the mass flux and vorticity just above the boundary layer, but it is necessary yet insufficient because the spinup is strongly governed by boundary layer dynamics.

scholarly journals A Diagnostic Study of the Intensity of Three Tropical Cyclones in the Australian Region. Part II: An Analytic Method for Determining the Time Variation of the Intensity of a Tropical Cyclone*

2010 ◽  
Vol 138 (1) ◽  
pp. 22-41 ◽  
Author(s):  
France Lajoie ◽  
Kevin Walsh
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Surface Pressure ◽  
Time Variation ◽  
Surface Wind ◽  
Ground Truth Data ◽  
Maximum Wind ◽  
Australian Region ◽  
Central Surface

Abstract The observed features discussed in Part I of this paper, regarding the intensification and dissipation of Tropical Cyclone Kathy, have been integrated in a simple mathematical model that can produce a reliable 15–30-h forecast of (i) the central surface pressure of a tropical cyclone, (ii) the sustained maximum surface wind and gust around the cyclone, (iii) the radial distribution of the sustained mean surface wind along different directions, and (iv) the time variation of the three intensity parameters previously mentioned. For three tropical cyclones in the Australian region that have some reliable ground truth data, the computed central surface pressure, the predicted maximum mean surface wind, and maximum gust were, respectively, within ±3 hPa and ±2 m s−1 of the observations. Since the model is only based on the circulation in the boundary layer and on the variation of the cloud structure in and around the cyclone, its accuracy strongly suggests that (i) the maximum wind is partly dependent on the large-scale environmental circulation within the boundary layer and partly on the size of the radius of maximum wind and (ii) that all factors that contribute one way or another to the intensity of a tropical cyclone act together to control the size of the eye radius and the central surface pressure.

scholarly journals An Analysis of the Physical Characteristics of the Summer Low Atmosphere in the Gobi Desert Adjacent to Bosten Lake, Xinjiang, China

2018 ◽  
Vol 2018 ◽  
pp. 1-10 ◽  
Author(s):  
Yan Li ◽  
Xuejin Sun ◽  
Hui Ning ◽  
Hongcai Qin ◽  
Jiuquan Zhao
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Surface Wind ◽  
Physical Characteristics ◽  
Gobi Desert ◽  
Lake Area ◽  
Desert Area ◽  
Local Circulation ◽  
Bosten Lake ◽  
Near Surface

A month-long field observation campaign was conducted, which covered approximately 100 km2 of the Gobi Desert area on the southeast bank of Bosten Lake during the summer of 2016. The purpose of the study was to examine the physical characteristics of the low atmosphere over land-lake nonuniform underlying surfaces in the Gobi Desert of northwestern China. The results of the statistical analysis showed that, during the observational period, the average daytime surface horizontal thermal gradient reached up to −0.2°C/km from the lakeshore to southern Gobi Desert area. The near-surface wind field of the 7 km horizontal extent from the lakeshore was dominated by onshore breezes with average peak wind speeds above 5 m/s. In the atmospheric near-surface layer, an isohumidity layer at a height between 10 and 50 m a.g.l. was observed from 11:00 to 18:00 LST. Also, a case study for the atmospheric boundary layer and local circulation analyses was conducted. The onshore breezes were found to play a major role in the vertical structure of the local atmospheric boundary layer. The numerical simulation results indicated that there was an alternating day-night local circulation in the Bosten Lake area.

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