Inflow Layer Research Articles

Numerical Study of Boundary Layer Structure and Rainfall after Landfall of Typhoon Fitow (2013): Sensitivity to Planetary Boundary Layer Parameterization

The boundary layer structure and related heavy rainfall of Typhoon Fitow (2013), which made landfall in Zhejiang Province, China, are studied using the Advanced Research version of the Weather Research and Forecasting model, with a focus on the sensitivity of the simulation to the planetary boundary layer parameterization. Two groups of experiments—one with the same surface layer scheme and including the Yonsei University (YSU), Mellor–Yamada–Nakanishi–Niino Level 2.5, and Bougeault and Lacarrere schemes; and the other with different surface layer schemes and including the Mellor–Yamada–Janjic and Quasi-Normal Scale Elimination schemes—are investigated. For the convenience of comparative analysis, the simulation with the YSU scheme is chosen as the control run because this scheme successfully reproduces the track, intensity and rainfall as a whole. The maximum deviations in the peak tangential and peak radial winds may account for 11% and 33% of those produced in the control run, respectively. Further diagnosis indicates that the vertical diffusivity is much larger in the first group, resulting in weaker vertical shear of the tangential and radial winds in the boundary layer and a deeper inflow layer therein. The precipitation discrepancies are related to the simulated track deflection and the differences in the simulated low-level convergent flow among all tests. Furthermore, the first group more efficiently transfers moisture and energy and produces a stronger ascending motion than the second, contributing to a deeper moist layer, stronger convection and greater precipitation.

Concentric Eyewall Formation in Typhoon Sinlaku (2008). Part III: Horizontal Momentum Budget Analyses

This is a follow-up work to two prior studies examining secondary eyewall formation (SEF) in Typhoon Sinlaku (2008). This study shows that, in the SEF region, the majority of the elevated winds are supergradient. About two-thirds of the rapid increase in tangential wind tendencies immediately prior to SEF are attributed to agradient wind tendencies. This suggests the importance of nonlinear, unbalanced dynamical processes in SEF in addition to the classical axisymmetric balanced response to forcings of heating and momentum. In the SEF region, analyses show two distinct responsible processes for the increasing azimuthal tangential wind in two vertical intervals. Within the boundary inflow layer, the competing effect between the mean radial influx of absolute vorticity and deceleration caused by surface friction and subgrid diffusion yields a secondary maximum of positive tendency. Analyses further demonstrate the major impact of the mean radial influx of absolute vorticity on SEF. Above the boundary inflow layer, the vertical advection acts to vertically extend the tangential wind jet via the lofting of the enhanced tangential momentum farther upward. The roles of the nonlinear unbalanced dynamics in these two processes are discussed in this paper. From a Lagrangian perspective, the persistently increasing agradient force outweighs the frictional loss, effectively decelerating boundary layer inflowing air across the SEF region. This explains the sharpening of the radial gradient of boundary layer inflow, which is shown to be responsible for the buildup of a zone with concentrated boundary layer convergence. The previously proposed unbalanced dynamical pathway to SEF is elaborated upon and supported by the current results and discussion.

Sensitivity of Tropical Cyclone Intensity and Structure to Planetary Boundary Layer Parameterization

The advanced weather research and forecasting model is used to investigate the influence of planetary boundary layer (PBL) processes on intensity and structure of the storm Phailin (2013). Five simulations are conducted with five PBL schemes; Yonsei University (YSU), Mellor−Yamada−Nakanishi−Niino order2.5 (MYNN2), Assymetric Convective Model2 (ACM2), Medium Range Forecast (MRF), and Bougeault and Lacarrere (BouLac). The simulation duration includes the pre − intensification and rapid intensification phase of Phailin before landfall. Results indicate that during the pre − intensification phase, storm’s track and intensity are not much sensitivity to PBL but structural changes are noted. A significant sensitivity of track and intensity to PBL parameterizations are found during rapid intensification phase. BouLac and MRF produced two extremes with 39 hPa intense and 16 km compact storm for BouLac than MRF. Further analysis reveals an outward movement of air parcel just above the boundary layer which causes spin-down for YSU and MYNN2. BouLac is associated with stronger eddy diffusivity and moisture fluxes within the boundary layer and stronger cyclonic vorticity just above the boundary layer than other experiments. Stronger cyclonic vorticity above the boundary layer in BouLac favors intense updraft, facilitating more moisture transport from the boundary layer to upper layers aiding stronger secondary circulation and robustly intensifying the storm. A relatively deeper and drier inflow layer associated with weaker cyclonic vorticity just above the boundary layer reduces the moisture transport and weaken the secondary circulation for MRF than others.

Gravity currents propagating into two-layer stratified fluids: vorticity-based models

The vorticity-based modelling approach originally introduced by Borden & Meiburg (J. Fluid Mech., vol. 726, 2013b, R1) is extended to gravity currents propagating into two-layer stratified ambients. Vorticity models are developed for three different flow configurations: no upstream-propagating wave, an upstream-propagating expansion wave only and an upstream-propagating expansion wave and a bore. For a given gravity current height and stratification strength, along with ambient inflow layer thicknesses and velocities, the models yield the gravity current velocity, the bore and expansion wave properties and the ambient outflow layer thicknesses and velocities. We furthermore establish which of the three configurations will occur in a given parameter regime. Since energy-related closure assumptions are not required for any of the configurations, we can determine the dissipation as a function of the gravity current height, for a given set of flow parameters. To investigate which gravity current height is selected in real flows, we carry out two-dimensional Navier–Stokes simulations for comparison. These yield gravity current heights close to the vorticity model solutions for energy-conserving flows. Hence we adopt these energy-conserving solutions as the vorticity model predictions. We subsequently discuss these predictions in the context of earlier models by other authors, and of two-layer stratified flows over obstacles.

The role of inflow boundary layer thickness for numerical modelling of flow around bluff bodies of built-up areas

The paper presents the numerical results obtained with the use of the ANSYS FLUENT commercial code for analysing the flow structure around a two rectangles in line surface-mounted bluff bodies immersed in a boundary layer. The simulation of a configuration with a flow around objects has been done using both the steady and unsteady approaches. Effects of the inflow boundary layer thickness on the flow field, wall-shear stress and the grid resolution accuracy in predicting periodic vortex shedding from two tandem arrangement bodies are examined. It has been observed that the periodic vortex shedding is considerably reduced with an increase in the δ/H parameter.

Dramatic Inner-Core Tropopause Variability during the Rapid Intensification of Hurricane Patricia (2015)

Abstract Dropsondes with horizontal spacing as small as 4 km were released from the stratosphere in rapidly intensifying Hurricane Patricia (2015) during the Office of Naval Research Tropical Cyclone Intensity experiment. These observations provide cross sections of unprecedented resolution through the inner core of a hurricane. On 21 October, Patricia exhibited a strong tropopause inversion layer (TIL) across its entire circulation, with a maximum magnitude of 5.1 K (100 m)−1. This inversion weakened between 21 and 22 October as potential temperature θ increased by up to 16 K just below the tropopause and decreased by up to 14 K in the lower stratosphere. Between 22 and 23 October, the TIL over the eye weakened further, allowing the tropopause to rise by 1 km. Meanwhile over Patricia’s secondary eyewall, the TIL restrengthened and bulged upward by about 700 m into what was previously the lower stratosphere. These observations support many aspects of recent modeling studies, including eyewall penetration into the stratosphere during rapid intensification (RI), the existence of a narrow inflow layer near the tropopause, and the role of subsidence from the stratosphere in developing an upper-level warm core. Three mechanisms of inner-core tropopause variability are hypothesized: destabilization of the TIL through turbulent mixing, weakening of the TIL over the eye through upper-tropospheric subsidence warming, and increasing tropopause height forced by overshooting updrafts in the eyewall. None of these processes are seen as the direct cause of RI, but rather part of the RI process that includes strong increases in boundary layer moist entropy.

Observational study on thermodynamic and kinematic structures of Typhoon Vicente (2012) at landfall

Severe Typhoon Vicente was the strongest tropical cyclone (TC) that has attacked Hong Kong (HK) since 1999. This paper presents a comprehensive observational study on this windstorm during its landfall, based on in-situ measurements from weather radars, radiosonde balloons, wind profilers and other ground-based devices at various meteorological stations in HK. Both its thermodynamic and kinematic structures are investigated to explore the TC characteristics at landfall. The results of this study reflected that the vortex extended to the tropopause around 16 km. Vertical profiles of pressure deficit below TC outflow layer depicted a linear distribution pattern with respect to isobar-expressed altitude, with a slope increasing almost linearly with increasing surface pressure deficit. The inner and outer rainband regions were respectively dominated by stratiform and convective precipitations, and the inner region was vertically separated into two discriminatory layers by the melting layer located at 5.4 km. The sea-air contrast temperature varied noticeably during the landfall process, with an increase of 1–1.5 °C in the TC inner region where sea-air flux exchange strengthened distinctly compared to that at outer areas.Vertical wind profiles from balloons showed that horizontal wind speed and wind direction experienced a sharp change around the tropopause, above which flow motions were dominated by the anti-cyclone of Tibetan high. The inflow layer depth was found on the order of 2 km, compared to 1–2 km for the atmospheric mixing layer depth. The gradient height, recognized via the maximum horizontal wind, was in the range of 0.5–2.5 km which decreased with decreasing radii prior to landfall. Examination of the results from wind profilers over marine, urban and rugged terrain showed that the wind structures in the TC boundary layer (TCBL) depended upon storm-relative position and underlying terrain features. Vertical profiles of marine winds in the inner rainband region demonstrated a jet feature, with mean speed being logarithmical with height in the lowest 1 km. TCBL winds above rugged areas were influenced severely by topographic effects. The low-level wind maximum, or the gradient speed, changed insignificantly above different sites.

Impacts of the Lower Stratosphere on the Development of Intense Tropical Cyclones

This study examines potential impacts of the lower stratosphere on the development and the inner-core structure of intense tropical cyclones (TCs). By initializing the Hurricane Weather Research and Forecasting (HWRF) model with different monthly averaged sounding profiles in the Northwestern Pacific and the North Atlantic basins, it is shown that the lower stratosphere layer (LSL) can impose a noticeable influence on the TC structure and development via formation of an extra warm core near the tropopause along with a thin layer of inflow in the LSL at the high-intensity limit. Specifically, a lower tropopause level allows for higher TC intensity and a more distinct double warm core structure. Likewise, a weaker LSL stratification also corresponds to a warmer upper-level core and higher intensity. Of further significance is that the double warm core formation is more sensitive to tropopause variations in the Northwestern Pacific basin than those in the North Atlantic basin, given the same sea surface temperature. The results suggest that variations in tropopause level and LSL stratification could be an important factor that is responsible for the long-term variability of TC intensity.

Effects of Midlevel Dry Air on Development of the Axisymmetric Tropical Cyclone Secondary Circulation

Abstract Idealized experiments conducted with an axisymmetric tropical cyclone (TC) model are used to assess the effects of midlevel dry air on the axisymmetric TC secondary circulation. Moist entropy diagnostics of convective parcels are used to determine how midlevel dry air affects the distribution and strength of convection. Analyzing upward and downward motions in the Eulerian radius–height coordinate system shows that the moistest simulation has stronger vertical motions and a wider overturning circulation compared to drier simulations. A Lagrangian entropy framework further analyzes convective motions by separating upward higher-entropy streams from downward lower-entropy streams. Results show that the driest simulation has a weaker mean overturning circulation with updrafts characterized by lower mean entropy compared to moister simulations. Turbulent entrainment of dry air into deep convection at midlevels is small, suggesting that the influence of midlevel dry air on convective strength and the structure of the secondary circulation are through modification of the inflow layer. Backward trajectories show low-entropy air subsiding into the subcloud layer from low to midlevels of the atmosphere between radii of 200 and 400 km. Surface fluxes increase the entropy of these parcels before they rise in convective updrafts, but the increased recovery time, combined with descending motion closer to the inner core, decreases the width of the TC secondary circulation in the driest simulation.

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

AbstractThis 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.

Coyote unmanned aircraft system observations in Hurricane Edouard (2014)

AbstractHorizontal wind, temperature, and moisture observations are presented from two Coyote unmanned aircraft system (UAS) flights in the boundary layer of Hurricane Edouard (2014). The first flight sampled the meteorological conditions in the eye and eyewall at altitudes from 900 to 1500 m while Edouard was a major hurricane (105 kt) on 16 September 2014. The following day, a second Coyote sampled the inflow layer outside of the storm core at ~760 m altitude, when Edouard had weakened to an 80‐kt hurricane. These flights represent the first deployments of a UAS from an airborne manned aircraft into a tropical cyclone. Comparisons between the Coyote data and the Lockheed WP‐3D Orion (WP‐3D) flight‐level measurements and analyses constructed from dropsonde data are also provided. On 16 September 2014, the Coyote‐measured horizontal wind speeds agree, on average, to within ~1 m s−1 of the wind speeds observed by the WP‐3D and reproduce the shape of the radial wind profile from the WP‐3D measurements. For the inflow layer experiment on 17 September, the mean wind speeds from the Coyote and the dropsonde analysis differ by only 0.5 m s−1, while the Coyote captured increased variability (σ = 3.4 m s−1) in the horizontal wind field compared to the dropsonde analysis (σ = 2.2 m s−1). Thermodynamic data from the Coyote and dropsondes agree well for both flights, with average discrepancies of 0.4°C and 0.0°C for temperature and 0.7°C and 1.3°C for dew point temperature on 16 and 17 September, respectively

Why is the Tropical Cyclone Boundary Layer Not “Well Mixed”?

AbstractPlausible diagnostics for the top of the tropical cyclone boundary layer include (i) the top of the layer of strong frictional inflow and (ii) the top of the “well mixed” layer, that is, the layer over which potential temperature θ is approximately constant. Observations show that these two candidate definitions give markedly different results in practice, with the inflow layer being roughly twice the depth of the layer of nearly constant θ. Here, the authors will present an analysis of the thermodynamics of the tropical cyclone boundary layer derived from an axisymmetric model. The authors show that the marked dry static stability in the upper part of the inflow layer is due largely to diabatic effects. The radial wind varies strongly with height and, therefore, so does radial advection of θ. This process also stabilizes the boundary layer but to a lesser degree than diabatic effects. The authors also show that this differential radial advection contributes to the observed superadiabatic layer adjacent to the ocean surface, where the vertical gradient of the radial wind is reversed, but that the main cause of this unstable layer is heating from turbulent dissipation. The top of the well-mixed layer is thus distinct from the top of the boundary layer in tropical cyclones. The top of the inflow layer is a better proxy for the top of the boundary layer but is not without limitations. These results may have implications for boundary layer parameterizations that diagnose the boundary layer depth from thermodynamic, or partly thermodynamic, criteria.

Identification and Climatology of Alpine Pumping from a Regional Climate Simulation

The thermally driven circulation between the European Alps and the alpine foreland - named Alpine pumping – occurs regularly under clear and calm weather conditions. While previous studies focused on the impact of Alpine pumping on moist convection and transport of air pollutants, this study was motivated by its ventilation effect for Munich, located about 50 km north of the Alps in undulating and only slightly inclined terrain, where local thermal circulations are weak. Hourly data from a reanalysis driven regional climate simulation with COSMO-CLM model for the period 1989 to 2008 were analysed to identify days with Alpine pumping and to determine the mean diurnal characteristics of this regional thermal circulation. Four literature derived combinations of meteorological criteria were tested to identify days favorable for Alpine pumping from COSMO-CLM results. The first criterion selects days with a daily sum of solar radiation ≥20 MJ/m2 and has been used in an earlier observational study. On average 60 d/y are fulfilling the criterion in the model simulation, which compares well to the 67 d/y determined from observations. The other three criteria combinations consider a maximum wind velocity at 850 hPa, a maximum daily precipitation sum, and/or a maximum mean cloud cover. The mean annual number of selected days is lower for these criteria combinations and ranges between 20 and 52. Diurnal wind reversals occur on 77 to 81% of the selected days, depending on the criteria combination The daily solar radiation sum of 20 MJ/m2 is only exceeded during April to September, while days satisfying the criteria combinations without the radiation threshold occur all year round. In agreement with observations, the simulated regional thermally driven wind field extends up to ~100 km north of the Alps with average near-surface wind speeds of 0.5-1.5 m/s in the Munich area. With increasing distance from the Alps, the diurnal cycle of Alpine pumping is delayed by up to 3 hours. The simulated mean depth of the daytime inflow layer ranges between 500 m and 1500 m, whereas the depth of the nocturnal outflow layer typically reaches up to a few hundred meters.

The Influence of Land Surface Heterogeneities on Heavy Convective Rainfall in the Baltimore–Washington Metropolitan Area

Abstract Low-level convergence induced by land surface heterogeneities can have substantial influence on atmospheric convection and rainfall. Analyses of heavy convective rainfall in the Baltimore–Washington metropolitan area are performed using the Weather Research and Forecasting (WRF) Model, coupled with the Princeton Urban Canopy Model (PUCM) that resolves urban subfacet heterogeneity. Analyses center on storms that produced heavy rainfall and record urban flooding in Baltimore on 1 June 2012. The control simulation using PUCM shows a better performance in reproducing the surface energy balance and rainfall than the simulation using a traditional slab model for the urban area. Sensitivity experiments are carried out to identify the role of the land surface heterogeneities, arising from land–water and urban–nonurban contrasts in the Baltimore–Washington metropolitan area, on heavy rainfall from organized thunderstorm systems. The intersection of low-level convergence zones from thunderstorm downdrafts and from the bay breeze from the Chesapeake Bay enhances the upward motion of preexisting convective storms. The larger sensible heating from the urban area modifies the low-level temperature and wind fields, which in turn modifies the bay breeze. The enhanced moisture supply in the deepened bay-breeze inflow layer due to urban heating promotes intense convection and heavy rainfall in conjunction with the enhanced upward motion at intersecting convergence zones. This study suggests that better representations of surface heat and moisture fluxes in urban areas along major water bodies are required to better capture the timing and location of severe thunderstorms and heavy rainfall.

Inflow Turbulence Generation for Eddy-Resolving Simulations of Turbomachinery Flows

A simple variant of recycling and rescaling method to generate inflow turbulence using unstructured grid computational fluid dynamics (CFD) codes is presented. The method has been validated on large eddy simulation (LES) of spatially developing flat plate turbulent boundary layer. The proposed rescaling algorithm is based on the momentum thickness which is more robust and essentially obviates the need of finding the edge of the turbulent boundary layer in unstructured grid codes. Extension of this algorithm to hybrid Reynolds-Averaged Navier-Stokes (RANS) LES type of approaches and for wall-bounded turbomachinery flows is also discussed. Results from annular diffuser with different inflow boundary layer characteristics are presented as an example application to show the utility of such an algorithm.

Secondary Circulation of Tropical Cyclones in Vertical Wind Shear: Lagrangian Diagnostic and Pathways of Environmental Interaction

Abstract This study introduces a Lagrangian diagnostic of the secondary circulation of tropical cyclones (TCs), here defined by those trajectories that contribute to latent heat release in the region of high inertial stability of the TC core. This definition accounts for prominent asymmetries and transient flow features. Trajectories are mapped from the three-dimensional physical space to the (two dimensional) entropy–temperature space. The mass flux vector in this space subsumes the thermodynamic characteristics of the secondary circulation. The Lagrangian diagnostic is then employed to further analyze the impact of vertical wind shear on TCs in previously published idealized numerical experiments. One focus of this analysis is the classification and quantitative depiction of different pathways of environmental interaction based on thermodynamic properties of trajectories at initial and end times. Confirming results from previous work, vertical shear significantly increases the intrusion of low–equivalent potential temperature () air into the eyewall through the frictional inflow layer. In contrast to previous ideas, vertical shear decreases midlevel ventilation in these experiments. Consequently, the difference in eyewall between the no-shear and shear experiments is largest at low levels. Vertical shear, however, significantly increases detrainment from the eyewall and modifies the thermodynamic signature of the outflow layer. Finally, vertical shear promotes the occurrence of a novel class of trajectories that has not been described previously. These trajectories lose entropy at cold temperatures by detraining from the outflow layer and subsequently warm by 10–15 K. Further work is needed to investigate in more detail the relative importance of the different pathways for TC intensity change and to extend this study to real atmospheric TCs.

Sensitivity of the warm core of tropical cyclones to solar radiation

To investigate the impacts of solar radiation on tropical cyclone (TC) warm-core structure (i.e., the magnitude and height), a pair of idealized simulations are conducted by specifying different strengths of solar shortwave radiation. It is found that the TC warm core is highly sensitive to the shortwave radiative effect. For the nighttime storm, a tendency for a more intense warm core is found, with an elevated height compared to its daytime counterpart. As pointed out by previous studies, the radiative cooling during nighttime destabilizes the local and large-scale environment and thus promotes deep moist convection, which enhances the TC’s intensity. Due to the different inertial stabilities, the diabatic heating in the eyewall will force different secondary circulations. For a strong TC with a deeper vertical structure, this promotes a thin upper-level inflow layer. This inflow carries the lower stratospheric air with high potential temperature and descends adiabatically in the eye, resulting in significant upper-level warming. The Sawyer–Eliassen diagnosis further confirms that the height of the maximum temperature anomaly is likely attributable to the balance among the forced secondary circulations.

Typhoon kinematic and thermodynamic boundary layer structure from dropsonde composites

AbstractThe data from 438 Global Positioning System dropsondes in six typhoons are analyzed to investigate the mean atmospheric boundary layer structure in a composite framework. Following a recent study on boundary layer height in Atlantic hurricanes, we aim to quantify characteristics of boundary layer height scales in Western Pacific typhoons including the inflow layer depth (hinflow), height of the maximum tangential wind speed (hvtmax), and thermodynamic mixed layer depth. In addition, the kinematic and thermodynamic boundary layer structures are compared between the dropsonde composites using data in typhoons and hurricanes. Our results show that similar to the hurricane composite, there is a separation between the kinematic and thermodynamic boundary layer heights in typhoons, with the thermodynamic boundary layer depth being much smaller than hinflow and hvtmax in the typhoon boundary layer. All three boundary layer height scales tend to decrease toward the storm center. Our results confirm that the conceptual model of Zhang et al. (2011a) for boundary layer height variation is applicable to typhoon conditions. The kinematic boundary layer structure is generally similar between the typhoon and hurricane composites, but the typhoon composite shows a deeper inflow layer outside the eyewall than the hurricane composite. The thermodynamic structure of the typhoon boundary layer composite is warmer and moister outside the radius of maximum wind speed than the hurricane composite. This difference is attributed to different environmental conditions associated with typhoons compared to the hurricanes studied here.

Tropical cyclones in the North American Regional Reanalysis: An assessment of spatial biases in location, intensity, and structure

AbstractLong‐term reanalysis data sets, such as the North American Regional Reanalysis (NARR) and Climate Forecast System Reanalysis (CFSR), are data‐rich resources for weather and climate research. However, investigations into tropical cyclone (TC) structures are lacking. This study examines position, intensity, and structure of U.S. landfalling TCs in these data sets during 1998–2012. TC positions are determined using three dynamic and thermodynamic parameters. In NARR, positions are problematic near the domain boundary but show statistically significant improvements compared with CFSR in regions over the U.S. and near its coastlines. TC intensity is universally underestimated in both data sets, yet the pressure‐wind relationship is reasonably maintained. NARR and CFSR intensities are highly correlated (r = 0.93), suggesting that these data sets are limited in a similar fashion in their generation of intensity and that advanced data assimilation techniques are unable to overcome the shortcomings of low resolutions. To investigate TC structure, a 10‐storm composite is constructed using the most intense model TCs at peak intensity. Both models develop robust warm cores (T′ ~ 6–7 K), but the radius of maximum winds (>100 km) is too broad compared with storms in nature. Secondary circulation features in NARR are more realistic, particularly the shallow inflow layer, vertical motion in the inner core, and confined outflow in the upper troposphere. Results indicate that, over the U.S. and near its coastline, NARR positions and structure are competitive with or slightly improved upon CFSR. Away from the domain boundary, NARR is a suitable resource for evaluating TC positions and certain aspects of the secondary circulation.

Numerical simulations of Hurricane Katrina (2005) in the turbulent gray zone

AbstractCurrent numerical simulations of tropical cyclones (TCs) use a horizontal grid spacing as small as Δx = 103 m, with all boundary layer (BL) turbulence parameterized. Eventually, TC simulations can be conducted at Large Eddy Simulation (LES) resolution, which requires Δx to fall in the inertial subrange (often <102 m) to adequately resolve the large, energy‐containing eddies. Between the two lies the so‐called “terra incognita” because some of the assumptions used by mesoscale models and LES to treat BL turbulence are invalid. This study performs several 4–6 h simulations of Hurricane Katrina (2005) without a BL parameterization at extremely fine Δx [333, 200, and 111 m, hereafter “Large Eddy Permitting (LEP) runs”] and compares with mesoscale simulations with BL parameterizations (Δx = 3 km, 1 km, and 333 m, hereafter “PBL runs”). There are profound differences in the hurricane BL structure between the PBL and LEP runs: the former have a deeper inflow layer and secondary eyewall formation, whereas the latter have a shallow inflow layer without a secondary eyewall. Among the LEP runs, decreased Δx yields weaker subgrid‐scale vertical momentum fluxes, but the sum of subgrid‐scale and “grid‐scale” fluxes remain similar. There is also evidence that the size of the prevalent BL eddies depends upon Δx, suggesting that convergence to true LES has not yet been reached. Nevertheless, the similarities in the storm‐scale BL structure among the LEP runs indicate that the net effect of the BL on the rest of the hurricane may be somewhat independent of Δx.