Tropical Cyclone Structure Research Articles

Tropical Cyclone Data Assimilation with Coyote Uncrewed Aircraft System Observations, Very Frequent Cycling, and a New Online Quality Control Technique

Abstract A unique dataset obtained from the Coyote small uncrewed aircraft system (sUAS) in the inner-core boundary layer of Hurricane Maria (2017) is assimilated using NOAA’s Hurricane Ensemble Data Assimilation System (HEDAS) for data assimilation and Hurricane Weather Research and Forecasting (HWRF) system for model advances. The case of study is 1800 UTC 23 September 2017 when Maria was a category-3 hurricane. In addition to the Coyote observations, measurements collected by the NOAA Lockheed WP-3D Orion and U.S. Air Force C-130 aircraft were also included. To support the assimilation of this unique dataset, a new online quality control (QC) technique in HEDAS scales the observation–background difference by the total uncertainty during data assimilation and uses the interquartile range outlier method to identify outlier observations. Experimental setup includes various very frequent cycling scenarios for a control that does not assimilate Coyote observations, assimilation of Coyote observations in addition to the control observations, and the application of online QC. Findings suggest progressively improved analyses with more-frequent cycling, Coyote assimilation, and application of online QC. This applies to verification statistics computed at the locations of both Coyote and non-Coyote observations. In terms of the storm structure, only experiments that assimilated the Coyote observations were able to reproduce the double-eyewall structure that was observed at the time of the analysis, which is more consistent with the intensity of the storm according to the observations that were collected. Limitations of the study and future plans are also discussed. Significance Statement Findings from this study illustrate the significant impact difficult-to-obtain, near-surface observations can have on improving the accuracy of tropical cyclone structure and intensity. Adding these novel measurements in a way that also includes advanced cycling and quality control techniques in data assimilation has the potential to improve public forecasts that are reliant upon detailed depictions of storm strength and boundary layer structure prior to landfall. The results speak to the importance of parallel and consistent advancements in modeling, data assimilation, and observational capabilities to improve the depiction of the tropical cyclone inner-core structure in numerical models.

Impacts of planetary boundary layer parameterization in RegCM4.7 on the intensity and structure of simulated tropical cyclones over the Philippines

Impacts of planetary boundary layer parameterization in RegCM4.7 on the intensity and structure of simulated tropical cyclones over the Philippines

On the Two Types of Tropical Cyclone Eye Formation: Clearing Formation and Banding Formation

Abstract Two types of eye formation are proposed: “clearing formation (CF)” and “banding formation (BF)”. The objectives are to identify the tropical cyclone (TC) characteristics associated with these two types of eye formation and to clarify how the environment, pattern of synoptic systems and TC structure during initial and development stages affect the eye formation. The satellite imagery and best-track data are used to classify and analyze the TCs named in the western North Pacific from 2007 to 2020. The results show that TCs with CF have significantly higher intensity and intensification rate during the period of the first eye presence, smaller size after genesis and prior to eye formation, smaller eye size when the eye forms and a more westward track. It is noted that CF and BF TCs tend to occur in autumn and summer, respectively. Meanwhile, the composite results using reanalysis data show that the CF TCs are generally characterized by easterly-wave features with a dryer environment, smaller initial size and larger radial gradient of vorticity, while the BF TCs feature a monsoon-depression structure with a wetter environment, larger initial size and flatter vorticity profile. A conceptual hypothesis is thus proposed, as compared to BF TCs, the smaller size and weaker outer wind in CF storms associated with the easterly-wave disturbance are facilitated by inactive outer convection, leading to larger radial gradient of inertial stability. The low-level inflow can penetrate inward close to the center, resulting in more diabatic heating inside the radius of maximum wind with much higher heating efficiency, and also a higher intensification rate.

Tropical Cyclone Impact and Forest Resilience in the Southwestern Pacific

Tropical cyclones (TCs) can have profound effects on the dynamics of forest vegetation that need to be better understood. Here, we analysed changes in forest vegetation induced by TCs using the normalized difference vegetation index (NDVI). We used an accurate historical database of TC tracks and intensities, together with the Willoughby cyclone model to reconstruct the 2D surface wind speed structure of TCs and analyse how TCs affect forest vegetation. We used segmented linear models to identify significant breakpoints in the relationship between the reconstructed maximum sustained wind speed (Wmax) and the observed changes in NDVI. We tested the hypothesis that the rate of change in damage caused by TCs to forest and recovery time would increase according to Wmax thresholds as defined in the widely used Saffir–Simpson hurricane wind scale (SSHWS). We showed that the most significant breakpoint was located at 50 m/s. This breakpoint corresponds to the transition between categories 2 and 3 TCs in the SSHWS. Below this breakpoint, damages caused to forest vegetation and the time needed to recover from these damages were negligable. We found a second breakpoint, with a sharp increase in damages for winds >75 m/s. This suggested that extremely intense tropical cyclones, which might be more frequent in the future, can cause extreme damages to forest vegetation. Nevertheless, we found high variation in the observed damages and time needed to recover for a given Wmax. Further studies are needed to integrate other factors that might affect the exposure and resistance to TCs as well as forests’ capacity to recover from these disturbances.

High-Definition Hurricanes: Improving Forecasts with Storm-Following Nests

Abstract To forecast tropical cyclone (TC) intensity and structure changes with fidelity, numerical weather prediction models must be “high definition,” i.e., horizontal grid spacing ≤ 3 km, so that they permit clouds and convection and resolve sharp gradients of momentum and moisture in the eyewall and rainbands. Storm-following nests are computationally efficient at fine resolutions, providing a practical approach to improve TC intensity forecasts. Under the Hurricane Forecast Improvement Project, the operational Hurricane Weather Research and Forecasting (HWRF) system was developed to include telescopic, storm-following nests for a single TC per model integration. Subsequently, HWRF evolved into a state-of-the-art tool for TC predictions around the globe, although its single-storm nesting approach does not adequately simulate TC–TC interactions as they are observed. Basin-scale HWRF (HWRF-B) was developed later with a multistorm nesting approach to improve the simulation of TC–TC interactions by producing high-resolution forecasts for multiple TCs simultaneously. In this study, the multistorm nesting approach in HWRF-B was compared with a single-storm nesting approach using an otherwise identical model configuration. The multistorm approach demonstrated TC intensity forecast improvements, including more realistic TC–TC interactions. Storm-following nests developed in HWRF and HWRF-B will be foundational to NOAA’s next-generation hurricane application in the Unified Forecast System.

Assessing the Forecast Impact of a Geostationary Microwave Sounder Using Regional and Global OSSEs

Abstract Forecast observing system simulation experiments (OSSEs) are conducted to assess the potential impact of geostationary microwave (GeoMW) sounder observations on numerical weather prediction forecasts. A regional OSSE is conducted using a tropical cyclone (TC) case that is very similar to Hurricane Harvey (2017), as hurricanes are among the most devastating of weather-related natural disasters, and hurricane intensity continues to pose a significant challenge for numerical weather prediction. A global OSSE is conducted to assess the potential impact of a single GeoMW sounder centered over the continental United States versus two sounders positioned at the current locations of the National Oceanic and Atmospheric Administration Geostationary Operational Environmental Satellites (GOES) East and West. It is found that assimilation of GeoMW soundings result in better characterization of the TC environment, especially before and during intensification, which leads to significant improvements in forecasts of TC track and intensity. TC vertical structure (warm core thermal perturbation and horizontal wind distribution) is also substantially improved, as are the surface wind and precipitation extremes. In the global OSSE, assimilation of GeoMW soundings leads to slight improvement globally and significant improvement regionally, with regional impact equal to or greater than nearly all other observation types. Significance Statement This work seeks to determine the impact of a new geostationary microwave (GeoMW) sounder on tropical cyclone forecasts in particular, and on weather forecasts in general. It does so by assimilating simulated GeoMW sounder data into two different forecast models: one global and one regional. The data have a small positive impact globally, and a significant positive impact over the region viewed by the GeoMW instrument. In particular, assimilation of GeoMW data has a significant and positive impact on forecasts of tropical cyclone track, strength, and structure.

Is There an Outward Propagating Diurnal Signal in the Precipitation of Tropical Cyclones?

AbstractDiurnal pulse (outward propagating signal) is a prominent feature in tropical cyclones (TCs) and closely related to TC structure and intensity. It is elusive whether the diurnal pulse is deep convectively coupled and what is the influence on (daily) mean TC structure. This study addresses these outstanding problems using long‐term satellite precipitation, infrared, and microwave data. Diurnal pulse of precipitation occurs in ∼50% TC days, and the frequency increases with TC intensity. The precipitation, cold clouds, and microwave‐based deep convection of active precipitation pulses show significant outward‐propagating or phase‐delaying diurnal features, indicating deep convective coupling in these pulsing events. Nearly half of the diurnal pulses have cloud and precipitation coupled, but the coupling frequency is reduced (20%–30%) at the later stage (12–15 hr) of long‐duration pulses. Daily mean precipitation, cloud top height, and microwave ice scattering are markedly enhanced in TCs with precipitation pulse, compared to TCs without precipitation pulse.

Tropical Cyclone Intensity Estimation Using Himawari-8 Satellite Cloud Products and Deep Learning

This study develops an objective deep-learning-based model for tropical cyclone (TC) intensity estimation. The model’s basic structure is a convolutional neural network (CNN), which is a widely used technology in computer vision tasks. In order to optimize the model’s structure and to improve the feature extraction ability, both residual learning and attention mechanisms are embedded into the model. Five cloud products, including cloud optical thickness, cloud top temperature, cloud top height, cloud effective radius, and cloud type, which are level-2 products from the geostationary satellite Himawari-8, are used as the model training inputs. We sampled the cloud products under the 13 rotational angles of each TC to augment the training dataset. For the independent test data, the model shows improvement, with a relatively low RMSE of 4.06 m/s and a mean absolute error (MAE) of 3.23 m/s, which are comparable to the results seen in previous studies. Various cloud organization patterns, storm whirling patterns, and TC structures from the feature maps are presented to interpret the model training process. An analysis of the overestimated bias and underestimated bias shows that the model’s performance is highly affected by the initial cloud products. Moreover, several controlled experiments using other deep learning architectures demonstrate that our designed model is conducive to estimating TC intensity, thus providing insight into the forecasting of other TC metrics.

Resolution dependence of tropical cyclones simulated by a spectral cumulus parameterization

The resolution dependence of tropical cyclones (TCs) simulated by a spectral cumulus parameterization (spectral scheme) was investigated using an atmospheric general circulation model with three ensemble members and atmosphere-only experiments. The focus of the analysis was the cause of resolution dependence of TCs which has not been clarified in previous studies. The results showed that underestimation of the TC track density and TC intensity was improved by an increase in the horizontal resolution as reported in previous studies. A mean state analysis revealed that the mean states were qualitatively similar regardless of model resolution although TC statistics were different; this implies that local differences in the atmospheric condition may have a strong impact on the TC statistics. Analyses of the horizontal structure indicated that the local TC structure better approximated observations as the resolution increased, but this was not due to enhanced atmospheric instability because the TC center location coincided with the highly unstable region only in the early stage of TC development. Influential differences for TC statistics were found in the vertical TC structure. A temperature anomaly with a large radial gradient induced by diabatic heating within the eyewall of a TC first impacted the vertical–radial circulation and then caused strong boundary layer inflow toward the eyewall. If a low resolution was adopted, the radial gradient of the temperature anomaly with strong low-level inflow was not well simulated, resulting in a weaker TC intensity and lower TC genesis frequency. Some TC property changes associated with resolution in the current spectral scheme were found to be similar to those of existing convection scheme, but there were some improvements that were specific to this spectral scheme. The cause of the improvements introduced by the present spectral scheme is briefly discussed.

Diurnal Variations in Contraction of the Radius of Maximum Tangential Wind in Tropical Cyclones

AbstractDiurnal variations in contraction of the radius of maximum tangential wind (RMW) of tropical cyclones (TCs) were assessed using best‐track data for 2001–2018. For RMW contractions before TC intensification, the largest RMW contraction rate in 6 hr at 0300–0900 local solar time (LST) was associated with the maximum coverage of very deep convective clouds with infrared brightness temperatures <208 K at 0300–0600 LST, which were found mostly outside the mean RMW. The maximum RMW contraction rate and maximum intensification rate occurred simultaneously in 6 hr at 0300–0900 LST for rapidly intensifying storms associated with the greatest coverage of very deep convective cloud at 0300–0600 LST at storm center. This was not the case for nonrapidly intensifying storms. The time‐dependence of RMW contraction indicates the importance of radiative forcing on TC structure.

The Effects of Environmental Wind Shear Direction on Tropical Cyclone Boundary Layer Thermodynamics and Intensity Change from Multiple Observational Datasets

Abstract The relationship between deep-layer environmental wind shear direction and tropical cyclone (TC) boundary layer thermodynamic structures is explored in multiple independent databases. Analyses derived from the tropical cyclone buoy database (TCBD) show that when TCs experience northerly component shear, the 10-m equivalent potential temperature θe tends to be more symmetric than when shear has a southerly component. The primary asymmetry in θe in TCs experiencing southerly component shear is radially outward from 2 times the radius of maximum wind speed, with the left-of-shear quadrants having lower θe by 4–6 K than the right-of-shear quadrants. As with the TCBD, an asymmetric distribution of 10-m θe for TCs experiencing southerly component shear and a symmetric distribution of 10-m θe for TCs experiencing northerly component shear was found using composite observations from dropsondes. These analyses show that differences in the degree of symmetry near the sea surface extend through the depth of the boundary layer. Additionally, mean dropsonde profiles illustrate that TCs experiencing northerly component shear are more potentially unstable between 500- and 1000-m altitude, signaling a more favorable environment for the development of surface-based convection in rainband regions. Analyses from the Statistical Hurricane Intensity Prediction Scheme (SHIPS) database show that subsequent strengthening for TCs in the Atlantic Ocean basin preferentially occurs in northerly component deep-layer environmental wind shear environments whereas subsequent weakening preferentially occurs in southerly component wind shear environments, which further illustrates that the asymmetric distribution of boundary layer thermodynamics is unfavorable for TC intensification. These differences emphasize the impact of deep-layer wind shear direction on TC intensity changes that likely result from the superposition of large-scale advection with the shear-relative asymmetries in TC structure. Significance Statement This research investigates how the direction of the winds surrounding the storm impacts the strength of a tropical cyclone. Analyses from this study illustrate that when the winds come from the south the atmospheric boundary layer has a cool and dry side along with a warm and moist side. When the large-scale winds come from the north, temperature and moisture conditions are more uniform throughout the boundary layer. Consequently, results from tropical cyclone climatology show that winds observed to come from the north favor subsequent intensification. These relationships illustrate that tropical cyclone structure and intensity are directly influenced by their surrounding environments and that knowledge of the wind environment could help to improve future forecasts of tropical cyclone intensity change.

A Sampling‐Based Path Planning Algorithm for Improving Observations in Tropical Cyclones

AbstractLack of high‐resolution observations in the inner‐core of tropical cyclones remains a key issue when constructing an accurate initial state of the storm structure. The major implication of an improper initial state is the poor predictability of the future state of the storm. The size and associated hazard from strong winds at the inner‐core make it impossible to sample this region entirely. However, targeting regions of the inner‐core where forecasted atmospheric measurements have high uncertainty can significantly improve the accuracy of measurements for the initial state of the storm. This study provides a scheme for targeted high‐resolution observations for small Unmanned Aircraft Systems (sUAS) platforms (e.g., Coyote sUAS) to improve the estimates of the atmospheric measurement in the inner‐core structure. The benefit of observation is calculated based on the high‐fidelity state‐of‐the‐art hurricane ensemble data assimilation system. Potential locations with the most informative measurements are identified through exploration of various simulation‐based solutions depending on the state variables (e.g., pressure, temperature, wind speed, relative humidity) and a combined representation of those variables. A sampling‐based sUAS path planning algorithm considers energy usage when locating the regions of highly uncertain prediction of measurements, allowing sUAS to maximize the benefit of observation. Robustness analysis of our algorithm for multiple scenarios of sUAS drop and goal locations shows satisfactory performance against benchmark similar to current NOAA field campaign. With optimized sUAS observations, a data assimilation analysis shows significant improvements of up to 4% in the tropical cyclone structure estimates after resolving uncertainties at targeted locations.

Scale Analysis of Infrared Water Vapor Brightness Temperatures for Tropical Cyclone All‐Sky Radiance Assimilation

AbstractWe analyzed the scale features of satellite infrared (IR) water vapor (WV) brightness temperature observations of tropical cyclones (TCs). This is to characterize the storm information at dominate scales in all‐sky radiance assimilation for TC numerical weather prediction. This paper presents the results from the study of Hurricane Patricia (2015). Our study shows that IR WV brightness temperatures have the ability to observe multiscale structures of TCs, ranging from a size of above 1,000 km that covers the entire storm and its surrounding areas to a scale resolving individual convective clouds embedded in the TC. The atmospheric moisture for TC development is mainly represented by large scales covering the storm and surrounding areas while the storm structures are characterized basically by all scales. The large‐scale moisture and small‐scale convection demonstrate strong correlation and are closely related to the TC development, suggesting the need for all‐sky radiance assimilation at multiple scales.

Long-Term Effect of Barotropic Instability across the Moat in Double-Eyewall Tropical Cyclone–Like Vortices in Forced and Unforced Shallow-Water Models

Abstract Secondary eyewall formation and the ensuing eyewall replacement cycles may take place in mature tropical cyclones (TCs) during part of their lifetime. A better understanding of the underlying dynamics is beneficial to improving the prediction of TC intensity and structure. Previous studies suggested that the barotropic instability (BI) across the moat (aka type-2 BI) can make a substantial contribution to the inner-eyewall decay through the associated eddy radial transport of absolute angular momentum (AAM). Simultaneously, the type-2 BI can also increase the AAM of the outer eyewall. While the previous studies focused on the early stage of the type-2 BI, this paper explores the long-term effect of the type-2 BI and the underlying processes in forced and unforced shallow-water experiments. Under the long-term effect, it will be shown that the inner eyewalls repeatedly weaken and strengthen (while the order is reversed for the outer eyewalls). Sensitivity tests are conducted to examine the sensitivity of the long-term effect of the type-2 BI to different vortex parameters and the strength of the parameterized diabatic heating. Implication of the long-term effect for the intensity changes of the inner and outer eyewalls of real TCs are also discussed.

An Investigation of the Impact of Different Turbulence Schemes on the Tropical Cyclone Boundary Layer at Turbulent Gray‐Zone Resolution

AbstractAccurately resolving turbulence in the tropical‐cyclone boundary layer (TCBL) is crucial for realistic simulations of tropical cyclones (TCs), but how well the fine‐scale structure of the simulated TCBL can be reproduced at gray‐zone resolutions by state‐of‐the‐art planetary boundary layer (PBL) parameterization schemes, and why the simulated fine‐scale structure is so sensitive to the PBL scheme, are yet to be evaluated and understood. To address these issues, a series of numerical experiments under idealized conditions were conducted using the advanced research weather research and forecasting (WRF‐ARW) model at 500 and 166 m grid spacing. An 18‐hr WRF‐ARW large‐eddy simulation (LES) with a 55‐m grid spacing based on the nonlinear backscatter and anisotropy subfilter‐scale (NBA‐SFS) stress scheme was used as reference (RLES). Results from experiments using five different PBL schemes, that is, two conventional schemes—the Yonsei University (YSU) and the original MYNN (termed MYNN1)—two scale‐aware schemes—the Shin‐Hong (S‐H) and the revised MYNN (termed MYNN2)—and the coarse‐resolution LES (CLES), were subsequently compared with the RLES. The YSU and S‐H not only produced better simulations of TC intensity and structure than the other three PBL schemes, but also produced a fine‐scale turbulent structure reasonably well compared with the RLES. Discrepancies in the simulated fine‐scale structure among the PBL schemes result primarily from the different strength of vertical mixing, which could be reduced by the scale‐aware option. Results from a series of sensitivity experiments with a 166‐m grid spacing demonstrate that the scale‐aware S‐H can significantly improve the finer resolution simulations than the conventional YSU at relatively finer grid spacing at gray zone.

Storm‐Scale and Fine‐Scale Boundary Layer Structures of Tropical Cyclones Simulated With the WRF‐LES Framework

AbstractPrevious studies demonstrated that the tropical cyclone (TC) boundary layer (TCBL) contains small‐scale coherent structures, such as roll vortices and tornado‐scale vortices (TSVs), and they play important roles in energy transport and intensity changes in TCs. However, little is known about how horizontal resolutions can affect storm‐scale and fine‐scale structures in the TCBL with the grid spacing decreasing from the turbulent gray zone (100 m–1 km) to the large‐eddy scale (<100 m). In this study, numerical experiments with the large eddy simulation technique are used to investigate the effect of the model horizontal resolution on the simulated TC‐scale and fine‐scale structures in the TCBL. The simulated TC tends to have a shallower BL, a lower BL jet associated with stronger near‐surface vertical wind shear in the TCBL when the grid spacing decreases from 333 to 111 m or 37 m. When the grid spacing decreases to 111 m or 37 m, the fine‐scale coherent structures and near‐surface wind streaks associated with roll vortices and TSVs can be simulated, and the characteristics of the simulated turbulent kinetic energy in the TCBL are in good agreement with observational studies. With the grid spacing of 111 and 37 m, no significant differences in TC intensity are found in terms of instantaneous maximum and azimuthal‐mean maximum wind speeds. This study suggests that simulations with the horizontal grid spacing of 100 m or less can simulate the small coherent structures in the TCBL and their effect on TC intensity.

Tropical Cyclone Winds and Inflow Angle Asymmetry From SAR Imagery

AbstractThis study developed a morphological model for Tropical Cyclone (TC) wind and inflow angle asymmetry based on sea surface wind fields derived from spaceborne synthetic aperture radar (SAR) images. The model extracts the standard TC morphological information (center, intensity, and radius of the maximum wind) and decomposes the SAR‐derived winds into vortex rotation winds and motion vector, making the reconstruction of the entire TC structure reliable, even in areas not mapped by SAR. The derived wind speeds and inflow angles are verified with aircraft measurements by stepped‐frequency microwave radiometer and dropsondes, obtaining root‐mean‐square errors of 4.32 m/s and 16.04°, respectively. A systematic analysis of 130 SAR TCs images, collected by RADARSAT‐2 and SENTINEL‐1, reveals that the model can capture the main asymmetrical TC structure. Both TC asymmetry and eye size decrease as TC intensity increases.

High Resolution Real-data WRF Modeling and Verification of Tropical Cyclone Tornadoes Associated with Hurricane Ivan 2004

This study used high-resolution real-data WRF simulations of Hurricane Ivan (2004) to document the structure of potentially tornadic supercells embedded in tropical cyclone (TC) rainbands. The simulated TC track and intensity matched well with observations post landfall, and the simulated TC structure closely replicated the observed shape and rainbands, as evident by an assessment of observed composite and simulated radar reflectivity. TC tornado surrogates (TCT-Ss) were identified and calibrated using thresholds based on percentile values of maximum updraft helicity (UHmax) and simulated radar reflectivity. Although the magnitude of UHmax generally decreased as the simulated TC moved inland, sensitivity testing revealed that a threshold based on the 99.95% percentile value of UHmax achieved optimal TCT-S coverage and agreement with observed TC tornado (TCT) tracks on domains with 3-km and 1-km grid spacings. Three cells were identified at three different stages of the TC inland evolution and were found to have hook appendages resembling supercells found in midlatitudes. The cells produced storm tops of 13 km, with rotating cores with depths of 4 km above ground level. Although results in this study are unique to Hurricane Ivan, the authors believe the use of convection-allowing models and UH-based surrogate methodologies can be applied to other TC cases.

Unified ensemble mean forecasting of tropical cyclones based on the feature-oriented mean method

AbstractOperational and research applications generally use the consensus approach for forecasting the track and intensity of tropical cyclones (TCs) due to the spatial displacement of the TC location and structure in ensemble member forecasts. This approach simply averages the location and intensity information for TCs in individual ensemble members, which is distinct from the traditional pointwise arithmetic mean (AM) method for ensemble forecast fields. The consensus approach, despite having improved skills relative to the AM in predicting the TC intensity, cannot provide forecasts of the TC spatial structure. We introduced a unified TC ensemble mean forecast based on the feature-oriented mean (FM) method to overcome the inconsistency between the AM and consensus forecasts. FM spatially aligns the TC-related features in each ensemble field to their geographical mean positions before the amplitude of their features is averaged.We select 219 TC forecast samples during the summer of 2017 for an overall evaluation of the FM performance. The results show that the TC track consensus forecasts can differ from AM track forecasts by hundreds of kilometers at long lead times. AM also gives a systematic and statistically significant underestimation of the TC intensity compared with the consensus forecast. By contrast, FM has a very similar TC track and intensity forecast skill to the consensus approach. FM can also provide the corresponding ensemble mean forecasts of the TC spatial structure that are significantly more accurate than AM for the low- and upper-level circulation in TCs. The FM method has the potential to serve as a valuable unified ensemble mean approach for the TC prediction.

A Framework for Simulating the Tropical-Cyclone Boundary Layer Using Large-Eddy Simulation and Its Use in Evaluating PBL Parameterizations

AbstractBoundary layer turbulent processes affect tropical cyclone (TC) structure and intensity change. However, uncertainties in the parameterization of the planetary boundary layer (PBL) under high-wind conditions remain challenging, mostly due to limited observations. This study presents and evaluates a framework of numerical simulation that can be used for a small-domain [O(5 km)] large-eddy simulation (LES) and single-column modeling (SCM) to study the TC boundary layer. The framework builds upon a previous study that uses a few input parameters to represent the TC vortex and adds a simple nudging term for temperature and moisture to account for the complex thermodynamic processes in TCs. The reference thermodynamic profiles at different wind speeds are retrieved from a composite analysis of dropsonde observations of mature hurricanes. Results from LES show that most of the turbulence kinetic energy and vertical momentum flux is associated with resolved processes when horizontal grid spacing is O(10 m). Comparison to observations of turbulence variables such as momentum flux, effective eddy viscosity, and turbulence length scale show that LES produces reasonable results but highlight areas where further observations are necessary. LES results also demonstrate that compared to a classic Ekman-type boundary layer, the TC boundary layer is shallower, develops steady conditions much quicker, and exhibits stronger wind speed near the surface. The utility of this framework is further highlighted by evaluating a first-order PBL parameterization, suggesting that an asymptotic turbulence length scale of 40 m produces a good match to LES results.