Numerical Simulations Of Tropical Cyclones Research Articles

Assessment of the impact of ocean mean temperature on numerical simulations of tropical cyclones

Assessment of the impact of ocean mean temperature on numerical simulations of tropical cyclones

High-Wind Drag Coefficient Based on the Tropical Cyclone Simulated With the WRF-LES Framework

The numerical simulation of tropical cyclones has been increasingly conducted using the advanced Weather Research and Forecast (WRF) model with the large-eddy simulation (LES) technique. Given the importance of the boundary wind profile for the vertical exchange of horizontal momentum between the atmosphere and the ocean, the drag coefficient was evaluated in the numerical simulation with the WRF-LES framework at the finest horizontal grid spacing of 37 m. In the absence of the TC–ocean interaction, the drag coefficient derived from the simulated wind profile does not show the leveling off or decrease in the strong wind conditions. The drag coefficient increases with the increasing near-surface wind speed and agrees well with the extrapolation of the Large and Pond formula in the strong wind conditions. It is suggested that the boundary wind structure simulated with the LES technique may be unrealistic when the TC–ocean interaction is not fully considered.

A New Horizontal Mixing-Length Formulation for Numerical Simulations of Tropical Cyclones

AbstractA new physically based horizontal mixing-length formulation is introduced and evaluated in the Hurricane Weather and Research Forecasting (HWRF) Model. Recent studies have shown that the structure and intensity of tropical cyclones (TCs) simulated by numerical models are sensitive to horizontal mixing length in the parameterization of horizontal diffusion. Currently, many numerical models including the operational HWRF Model formulate the horizontal mixing length as a fixed fraction of grid spacing or a constant value, which is not realistic. To improve the representation of the horizontal diffusion process, the new formulation relates the horizontal mixing length to local wind and its horizontal gradients. The resulting horizontal mixing length and diffusivity are much closer to those derived from field measurements. To understand the impact of different mixing-length formulations, we analyze the evolutions of an idealized TC simulated by the HWRF Model with the new formulation and with the current formulation (i.e., constant values) of horizontal mixing length. In two real-case tests, the HWRF Model with the new formulation produces the intensity and track forecasts of Hurricanes Harvey (2017) and Lane (2018) that are much closer to observations. Retrospective runs of hundreds of forecast cycles of multiple hurricanes show that the mean errors in intensity and track simulated by HWRF with the new formulation can be reduced approximately by 10%.

On the Correlation between Total Condensate and Moist Heating in Tropical Cyclones and Applications for Diagnosing Intensity

Abstract While the transfer of moist enthalpy from the ocean to the atmosphere is the fundamental energy source for tropical cyclones, the release of latent heat in moist convection is the mechanism by which this energy is converted into the kinetic and potential energy of these storms. Most observational estimates of this heat release rely on satellite estimates of rain rate. Here, examination of five high-resolution numerical simulations of tropical cyclones reveals that there is a close correlation between the total condensate and the total heating, even though the former quantity is an amount and the latter is a rate. This relationship is due to the fact that for condensate to be sustained at large values, it must be rapidly replaced by new condensate and associated latent heating. Total condensate and total rain rate within fixed radial distances such as 111 km also show good correlations with the current intensity of the storm, but surprisingly, high values of condensate at high altitudes and close to the storm center are not good predictors of imminent intensification. These relationships are confirmed with an additional ensemble of 270 idealized simulations of tropical cyclones with varying sizes and intensities. Finally, simulated measurements of total condensate are computed from narrow swaths modeled after the cloud profiling radar on the CloudSat satellite. Despite their narrow footprint and the fact that they rarely cut through the exact center of the cyclone, these estimates of total condensate also show a useful correlation with current intensity.

Tropical cyclones over the North Indian Ocean: experiments with the high-resolution global icosahedral grid point model GME

In this paper, an attempt has been made to conduct a numerical experiment with the high-resolution global model GME to predict the tropical storms in the North Indian Ocean during the year 2007. Numerical integrations using the icosahedral hexagonal grid point global model GME were performed to study the evolution of tropical cyclones, viz., Akash, Gonu, Yemyin and Sidr over North Indian Ocean during 2007. It has been seen that the GME model forecast underestimates cyclone’s intensity, but the model can capture the evolution of cyclone’s intensity especially its weakening during landfall, which is primarily due to the cutoff of the water vapor supply in the boundary layer as cyclones approach the coastal region. A series of numerical simulation of tropical cyclones have been performed with GME to examine model capability in prediction of intensity and track of the cyclones. The model performance is evaluated by calculating the root mean square errors as cyclone track errors.

Role of Radiative–Convective Feedbacks in Spontaneous Tropical Cyclogenesis in Idealized Numerical Simulations

AbstractThe authors perform 3D cloud-resolving simulations of radiative–convective equilibrium (RCE) in a rotating framework, with interactive radiation and surface fluxes and fixed sea surface temperature. A tropical cyclone is allowed to develop spontaneously from a homogeneous environment, rather than initializing the circulation with a weak vortex or moist bubble (as is often done in numerical simulations of tropical cyclones). The resulting tropical cyclogenesis is compared to the self-aggregation of convection that occurs in nonrotating RCE simulations. The feedbacks leading to cyclogenesis are quantified using a variance budget equation for the column-integrated frozen moist static energy. In the initial development of a broad circulation, feedbacks involving longwave radiation and surface enthalpy fluxes dominate, which is similar to the initial phase of nonrotating self-aggregation. Mechanism denial experiments are also performed to determine the extent to which the radiative feedbacks that are essential to nonrotating self-aggregation are important for tropical cyclogenesis. Results show that radiative feedbacks aid cyclogenesis but are not strictly necessary.

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.

Sensitivities of eyewall replacement cycle to model physics, vortex structure, and background winds in numerical simulations of tropical cyclones

AbstractA series of sensitivity experiments by the Weather Research and Forecasting (WRF) model is used to investigate the impact of model physics, vortex axisymmetric radial structure, and background wind on secondary eyewall formation (SEF) and eyewall replacement cycle (ERC) in three‐dimensional full physics numerical simulations. It is found that the vertical turbulent mixing parameterization can substantially affect the concentric ring structure of tangential wind associated with SEF through a complicated interaction among eyewall and outer rainband heating, radial inflow in the boundary layer, surface layer processes, and shallow convection in the moat. Large snow terminal velocity can substantially change the vertical distribution of eyewall diabatic heating to result in a strong radial inflow in the boundary layer, and thus, favors the development of shallow convection in the moat allowing the outer rainband convection to move closer to the inner eyewall, which may leave little room both temporally and spatially for a full development of a secondary maximum of tangential wind. Small radius of maximum wind (RMW) of a vortex and small potential vorticity (PV) skirt outside the RMW tend to generate double‐eyewall replacement and may lead to an ERC without a clean secondary concentric maximum of tangential wind. A sufficiently large background wind can smooth out an ERC that would otherwise occur without background wind for a vortex with a small or moderate PV skirt. However, background wind does not appear to have an impact on an ERC if the vortex has a sufficiently large PV skirt.

The role of outer rainband convection in governing the eyewall replacement cycle in numerical simulations of tropical cyclones

Eyewall replacement cycles (ERCs) are frequently observed during the evolution of intensifying tropical cyclones (TCs). Although intensely studied in recent years, the underlying mechanisms of ERCs are still not well understood, and a timely accurate forecast of ERCs remains to be a challenge. To advance our understanding of ERCs and provide insights into the improvement of numerical forecast of ERCs, a series of three-dimensional full physics simulations is performed using the Weather Research and Forecasting (WRF) model to investigate ERCs in TC-like vortices on an f plane. The simulated ERC in the control experiment possesses key features similar to those observed in real TCs including the formation and development of a secondary tangential wind maximum associated with the outer eyewall. The Sawyer-Eliassen diagnoses and tangential momentum budget analyses are performed to investigate the mechanisms underlying the secondary eyewall formation (SEF) and ERC. The simulations show the crucial roles of outer rainband heating in governing the formation and development of the secondary tangential wind maximum and demonstrate that the outer rainband convection must reach a critical strength relative to the eyewall convection before SEF and ERC can occur. The diagnoses reveal a positive feedback among low-level convection, acceleration of tangential winds and convergence of radial flow in the upper boundary layer, and surface evaporation that leads to the development of outer rainband convection and formation of secondary eyewall, and a mechanism for the demise of inner eyewall that involves the interaction between the transverse circulations induced by eyewall and outer rainband convection.

Impact of period and timescale of FDDA analysis nudging on the numerical simulation of tropical cyclones in the Bay of Bengal

In this study, the impact of four-dimensional data assimilation (FDDA) analysis nudging is examined on the prediction of tropical cyclones (TC) in the Bay of Bengal to determine the optimum period and timescale of nudging. Six TCs (SIDR: November 13–16, 2007; NARGIS: April 29–May 02, 2008; NISHA: November 25–28, 2008; AILA: May 23–26, 2009; LAILA: May 18–21, 2010; JAL: November 04–07, 2010) were simulated with a doubly nested Weather Research and Forecasting (WRF) model with a horizontal resolution of 9 km in the inner domain. In the control run for each cyclone, the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) analysis and forecasts at 0.5° resolution are used for initial and boundary conditions. In the FDDA experiments available surface, upper air observations obtained from NCEP Atmospheric Data Project (ADP) data sets were used for assimilation after merging with the first guess through objective analysis procedure. Analysis nudging experiments with different nudging periods (6, 12, 18, and 24 h) indicated a period of 18 or 24 h of nudging during the pre-forecast stage provides maximum impact on simulations in terms of minimum track and intensity forecasts. To determine the optimum timescale of nudging, two cyclone cases (NARGIS: April 28–May 02, 2008; NISHA: November 25–28, 2008) were simulated varying the inverse timescales as 1.0e−4 to 5.0e−4 s−1 in steps of 1.0e−4 s−1. A positive impact of assimilation is found on the simulated characteristics with a nudging coefficient of either 3.0e−4 or 4.0e−4 s−1 which corresponds to a timescale of about 1 h for nudging dynamic (u,v) and thermodynamical (t,q) fields.

Impacts of reconnaissance flight data on numerical simulation of tropical cyclones over South China Sea

AbstractIn 2011, the Hong Kong Observatory (HKO) and the Government Flying Service (GFS) of the Hong Kong Government conducted reconnaissance data collection flights for two tropical cyclones over the South China Sea. It was the first time in the region that flight measurements of this kind had been carried out specifically for tropical cyclones. Horizontal wind velocity components, temperature and relative humidity at selected flight levels were recorded at the temporal resolution of 20 times per second to delineate the wind flow and other meteorological conditions. They were found to be consistent with other synoptic station data sparsely located over the sea areas, as well as in good agreement with the ocean surface wind data from the satellite microwave scatterometer. In this study, aircraft measurements for the two tropical cyclones in 2011 and one in July 2009 were assimilated into the HKO mesoscale Non‐Hydrostatic Model (NHM) using its three‐dimensional variational data assimilation (3DVAR) to investigate the impact of the data on the model analysis and forecast. Model sensitivity experiments with and without the flight measurements were conducted. The flight observations were found to improve model analysis and forecasts of wind circulation, temperature and humidity in the vicinity of the tropical cyclones. Improvement in track forecast was obtained although an insignificant effect was seen in predicting the central pressure of the storms. The inclusion of flight measurements also improved the forecast of the surface wind distribution and precipitation associated with the tropical cyclones.

Improving simulation of a tropical cyclone using dynamical initialization and large-scale spectral nudging: A case study of Typhoon Megi (2010)

In this study, an approach combining dynamical initialization and large-scale spectral nudging is proposed to achieve improved numerical simulations of tropical cyclones (TCs), including track, structure, intensity, and their changes, based on the Advanced Weather Research and Forecasting (ARW-WRF) model. The effectiveness of the approach has been demonstrated with a case study of Typhoon Megi (2010). The ARW-WRF model with the proposed approach realistically reproduced many aspects of Typhoon Megi in a 7-day-long simulation. In particular, the model simulated quite well not only the storm track and intensity changes but also the structure changes before, during, and after its landfall over the Luzon Island in the northern Philippines, as well as after it reentered the ocean over the South China Sea (SCS). The results from several sensitivity experiments demonstrate that the proposed approach is quite effective and ideal for achieving realistic simulations of real TCs, and thus is useful for understanding the TC inner-core dynamics, and structure and intensity changes.

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

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.

Sensitivity of tropical cyclone Jal simulations to physics parameterizations

In this study, the sensitivity of numerical simulations of tropical cyclones to physics parameterizations is carried out with a view to determine the best set of physics options for prediction of cyclones originating in the north Indian Ocean. For this purpose, the tropical cyclone Jal has been simulated by the advanced (or state of science) mesoscale Weather Research and Forecasting (WRF) model on a desktop mini super computer CRAY CX1 with the available physics parameterizations. The model domain consists of one coarse and two nested domains. The resolution of the coarse domain is 90 km while the two nested domains have resolutions of 30 and 10 km, respectively. The results from the inner most domain have been considered for analyzing and comparing the results. Model simulation fields are compared with corresponding analysis or observation data. The track and intensity of simulated cyclone are compared with best track estimates provided by the Joint Typhoon Warning Centre (JTWC) data. Two sets of experiments are conducted to determine the best combination of physics schemes for track and intensity and it is seen that the best set of physics combination for track is not suitable for intensity prediction and the best combination for track prediction overpredicts the intensity of the cyclone. The sensitivity of the results to orography and level of nesting has also been studied. Simulations were also done for the cyclone Aila with (i) best set of physics and (ii) randomly selected physics schemes. The results of the Aila case show that the best set of physics schemes has more prediction skill than the randomly selected schemes in the case of track prediction. The cumulus (CPS), planetary boundary layer (PBL) and microphysics (MP) parameterization schemes have more impact on the track and intensity prediction skill than the other parameterizations employed in the mesoscale model.

The Influence of Airborne Doppler Radar Data Quality on Numerical Simulations of a Tropical Cyclone

Abstract The impact of airborne Doppler radar data assimilation on improving numerical simulations of tropical cyclones (TCs) has been well recognized. However, the influence of radar data quality on the numerical simulation of tropical cyclones has not been given much attention. It is commonly assumed that higher quality radar data would be more beneficial to numerical simulations of TCs. This study examines the impact of the radar data quality control on assimilation of the airborne Doppler radar reflectivity and radial velocity observations in a numerical simulation of Typhoon Jangmi (2008). It is found that the quality of radar data has a strong influence on the numerical simulation of Typhoon Jangmi in terms of its track, intensity, and precipitation structures. Specifically, results suggest that a trade-off between the data quality and data coverage is necessary for different purposes in practical applications, as the higher quality data contribute to intensity forecast improvements, whereas data of lower quality but having better coverage are more beneficial to accurate track forecasting.

An Extended Model for the Potential Intensity of Tropical Cyclones

AbstractEmanuel’s theory of hurricane potential intensity (E-PI) makes use of the assumption that slantwise convective instability vanishes in a steady-state vortex of a tropical cyclone. In the framework of an extended mathematical potential intensity model it is shown that relaxing this assumption and including an eye results in a larger maximum wind speed compared to that of the predictions made by E-PI. Previous studies by Bryan and Rotunno demonstrate that the effect of unbalanced flow considerably contributes to maximum winds in excess of E-PI (“superintensity”). The authors argue that the proposed mechanism induced by convective instability provides another possible explanation for simulated and observed tropical cyclones exceeding E-PI in addition to flow imbalance. Further evidence for the relevance of conditional instability in mature tropical cyclones to superintensity is given by the fact that convective available potential energy arises in numerical simulations of tropical cyclones. This is demonstrated by means of an axisymmetric cloud model that is in qualitative agreement with the analytical model. These simulations reveal a dependence of superintensity on the amount of CAPE outside the eyewall and also reproduce the decrease in superintensity with increased horizontal diffusion as found in previous studies.

Validation of Atmospheric Infrared Sounder temperature and moisture profiles over tropical oceans and their impact on numerical simulations of tropical cyclones

The quality of the retrieved temperature and moisture profiles acquired from the Atmospheric Infrared Sounder (AIRS) aboard the NASA Aqua spacecraft was evaluated by comparing the data with dropsonde observations obtained from NASA African Monsoon Multidisciplinary Analyses (NAMMA 2006) and international THORPEX Pacific Asian Regional Campaign (T‐PARC 2008) field programs. Results indicate that the AIRS retrieved temperature profiles are in good agreement with dropsonde observations. However, the AIRS retrieved moisture profiles show a larger bias compared with the dropsondes over the tropical oceans where tropical cyclones developed. A series of data assimilation experiments is then performed with an advanced research version of the Weather Research and Forecasting model and its three‐dimensional variational data assimilation system. Results show that the assimilation of the AIRS retrieved temperature and moisture profiles has a significant impact on the numerical simulations of tropical cyclones. However, the overall impacts of the data assimilation on numerical simulations of tropical cyclones are very sensitive to the bias corrections of the data. Specifically, the dry biases of moisture profiles cause the decay of Tropical Storm Debby (2006) in the numerical simulations. Only with bias correction can data assimilation result in a reasonable portrayal of storm development. Compared with the moisture profiles, temperature profiles show a larger impact on the track forecasting. Assimilation of the temperature profiles resulted in significant improvements in the track forecasts for both Debby (2006) and Typhoon Jangmi (2008).

An Observing System Simulation Experiment (OSSE) to Assess the Impact of Doppler Wind Lidar (DWL) Measurements on the Numerical Simulation of a Tropical Cyclone

The importance of wind observations has been recognized for many years. However, wind observations—especially three‐dimensional global wind measurements—are very limited. A satellite‐based Doppler Wind Lidar (DWL) is proposed to measure three‐dimensional wind profiles using remote sensing techniques. Assimilating these observations into a mesoscale model is expected to improve the performance of the numerical weather prediction (NWP) models. In order to examine the potential impact of the DWL three‐dimensional wind profile observations on the numerical simulation and prediction of tropical cyclones, a set of observing simulation system experiments (OSSEs) is performed using the advanced research version of the Weather Research and Forecasting (WRF) model and its three‐dimensional variational (3DVAR) data assimilation system. Results indicate that assimilating the DWL wind observations into the mesoscale numerical model has significant potential for improving tropical cyclone track and intensity forecasts.

High-resolution numerical simulations of Hurricane Isabel (2003) over North Carolina

Two numerical simulations were performed to study the ability of a high-resolution mesoscale model to predict the track and structure of Hurricane Isabel over North Carolina. One simulation (Control) used standard NCEP climatologically-based sea surface temperature (SST) data for the lower boundary condition while another simulation (Experimental) prescribed real-time high-resolution SST data for the lower boundary. Results from this study show that both simulations predict the track of Isabel over North Carolina reasonably well, although the track predicted by the experimental simulation agrees more closely with observations. The experimental simulation more closely agrees with observations of the intensity of Isabel and the amount and spatial distribution of precipitation. These results reinforce the importance of accurate high-resolution SST data on numerical simulations of tropical cyclones.

Steering Weight Concept and its Application to Tropical Cyclones Simulated in a Vertical Shear Experiment

A series of idealized numerical simulations of tropical cyclones is performed to enhance the knowledge on the mechanism of tropical cyclone (TC) motion in vertically sheared environments. The simulations are performed using an f-plane version of the typhoon track prediction model previously run at the Japan Meteorological Agency. A wide variety of TC tracks show up when the model is integrated repeatedly with different formulations of convective parameterization from an identical initial field. To account for the diversity of TC motion in vertically sheared environment, a steering-weight concept is introduced. The steeringweight is a set of weighting factors, mathematically derived from the surface pressure tendency equation, and can be a measure of how sensitive the motion of a TC is, to the steering flow at each vertical level. The experiments with different cumulus parameterizations show that the vertical profile of steering weight tends to strongly depend on the cumulus parameterization scheme used, but much less on the environment specified. This result implies that the axisymmetric thermal structure of modeled TCs, is mostly determined by parameterization schemes rather than environment in the model. And the longterm (say, 72 h mean) TC motion is well explained by the combination of steering weight, and large-scale flow fields. The result suggests that the differences in TC track among numerical prediction models, may be attributable to the differences in steering weight to some degree, given that there is no significant difference in the simulated large-scale flow fields among the models. Moreover, as long as the steering weight concept holds in nature, the steering-weighted deep layer mean flow, instead of the conventional and empirical pressure-weighted one, would work better in accounting for the motion of real tropical cyclones.