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Michael M. Bell and Michael T. Montgomery

Abstract

Observations from the Pre-Depression Investigation of Cloud Systems in the Tropics (PREDICT), Genesis and Rapid Intensification Processes (GRIP), and Intensity Forecast Experiment (IFEX) field campaigns are analyzed to investigate the mesoscale processes leading to the tropical cyclogenesis of Hurricane Karl (2010). Research aircraft missions provided Doppler radar, in situ flight level, and dropsonde data documenting the structural changes of the predepression disturbance. Following the pre-Karl wave pouch, variational analyses at the meso-β and meso-α scales suggest that the convective cycle in Karl alternately built the low- and midlevel circulations leading to genesis episodically rather than through a sustained lowering of the convective mass flux from increased stabilization. Convective bursts that erupt in the vorticity-rich environment of the recirculating pouch region enhance the low-level meso-β- and meso-α-scale circulation through vortex stretching. As the convection wanes, the resulting stratiform precipitation strengthens the midlevel circulation through convergence associated with ice microphysical processes, protecting the disturbance from the intrusion of dry environmental air. Once the column saturation fraction returns to a critical value, a subsequent convective burst below the midlevel circulation further enhances the low-level circulation, and the convective cycle repeats. The analyses suggest that the onset of deep convection and associated low-level spinup were closely related to the coupling of the vorticity and moisture fields at low and midlevels. Our interpretation of the observational analysis presented in this study reaffirms a primary role of deep convection in the genesis process and provides a hypothesis for the supporting role of stratiform precipitation and the midlevel vortex.

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Michael M. Bell and Michael T. Montgomery

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Unprecedented observations of Hurricane Isabel (2003) at category 5 intensity were collected from 12 to 14 September. This study presents a detailed analysis of the inner-core structure, atmospheric boundary layer, sea surface temperature, and outflow layer of a superintense tropical cyclone using high-resolution in situ flight-level, NCAR GPS dropwindsonde, Doppler radar, and satellite measurements. The analysis of the dropwindsonde and in situ data includes a comprehensive discussion of the uncertainties associated with this observational dataset and provides an estimate of the storm-relative axisymmetric inner-core structure using Barnes objective analysis. An assessment of gradient and thermal wind balance in the inner core is also presented. The axisymmetric data composites presented in this study suggest that Isabel built a reservoir of high moist entropy air by sea-to-air latent heat flux inside the low-level eye that was utilized as an additional energy source to nearly maintain its extreme intensity even after crossing the cool wake of Hurricane Fabian. It is argued here that the combined mean and asymmetric eddy flux of high moist entropy air from the low-level eye into the eyewall represents an additional power source or “turbo boost” to the hurricane heat engine. Recent estimates of the ratio of sea-to-air enthalpy and momentum exchange at high wind speeds are used to suggest that Isabel utilized this extra power to exceed the previously assumed intensity upper bound for the given environmental conditions on all three days. This discrepancy between a priori potential intensity theory and observations may be as high as 35 m s−1 on 13 September.

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Annette M. Foerster and Michael M. Bell

Abstract

Thermodynamic retrievals can derive pressure and temperature information from kinematic measurements in regions where no in situ observations are available. This study presents a new retrieval technique called SAMURAI-TR (Spline Analysis at Mesoscale Utilizing Radar and Aircraft Instrumentation–Thermodynamic Retrieval) that derives three-dimensional fields of pressure and density potential temperature from multiple-Doppler radar data using a variational approach. SAMURAI-TR advances existing methods by 1) allowing for a horizontal variation in the reference-state definition and 2) representing the retrieved quantities of pressure and temperature as three-dimensional functions consisting of a series of finite-element cubic B-splines. The first advancement enables the retrieval to explicitly account for the large radial gradient of the mean thermodynamic state in tropical cyclones and other rapidly rotating vortices. The second advancement allows for specification of the three-dimensional pressure and temperature gradients as pseudo-observations from Doppler-derived winds, effectively linking the vertical levels without the use of the thermodynamic equation or a microphysical closure. The retrieval uses only the horizontal and vertical momentum equations, their derivatives, and low-pass filters. The accuracy and sensitivity of the retrieval are assessed using a WRF simulation of a tropical cyclone. SAMURAI-TR has good accuracy compared to prior techniques and retrieves pressure to within 0.25 hPa and temperature to within 0.7 K RMSE. The application of the method to real data is demonstrated using multiple-Doppler data from Hurricane Rita (2005).

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Annette M. Boehm and Michael M. Bell

Abstract

The newly developed Spline Analysis at Mesoscale Utilizing Radar and Aircraft Instrumentation–Thermodynamic Retrieval (SAMURAI-TR) is used to estimate three-dimensional temperature and pressure perturbations in Hurricane Rita on 23 September 2005 from multi–Doppler radar data during the RAINEX field campaign. These are believed to be the first fully three-dimensional gridded thermodynamic observations from a TC. Rita was a major hurricane at this time and was affected by 13 m s−1 deep-layer vertical wind shear. Analysis of the contributions of the kinematic and retrieved thermodynamic fields to different azimuthal wavenumbers suggests the interpretation of eyewall convective forcing within a three-level framework of balanced, quasi-balanced, and unbalanced motions. The axisymmetric, wavenumber-0 structure was approximately in thermal-wind balance, resulting in a large pressure drop and temperature increase toward the center. The wavenumber-1 structure was determined by the interaction of the storm with environmental vertical wind shear resulting in a quasi balance between shear and shear-induced kinematic and thermodynamic perturbations. The observed wavenumber-1 thermodynamic asymmetries corroborate results of previous studies on the response of a vortex tilted by shear, and add new evidence that the vertical motion is nearly hydrostatic on the wavenumber-1 scale. Higher-order wavenumbers were associated with unbalanced motions and convective cells within the eyewall. The unbalanced vertical acceleration was positively correlated with buoyant forcing from thermal perturbations and negatively correlated with perturbation pressure gradients relative to the balanced vortex.

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Michael M. Bell and Wen-Chau Lee

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This study presents an extension of the ground-based velocity track display (GBVTD)-simplex tropical cyclone (TC) circulation center–finding algorithm to further improve the accuracy and consistency of TC center estimates from single-Doppler radar data. The improved center-finding method determines a TC track that ensures spatial and temporal continuities of four primary characteristics: the radius of maximum wind, the maximum axisymmetric tangential wind, and the latitude and longitude of the TC circulation center. A statistical analysis improves the consistency of the TC centers over time and makes it possible to automate the GBVTD-simplex algorithm for tracking of landfalling TCs. The characteristics and performance of this objective statistical center-finding method are evaluated using datasets from Hurricane Danny (1997) and Bret (1999) over 5-h periods during which both storms were simultaneously observed by two coastal Weather Surveillance Radar-1988 Doppler (WSR-88D) units. Independent single-Doppler and dual-Doppler centers are determined and used to assess the absolute accuracy of the algorithm. Reductions of 50% and 10% in the average distance between independent center estimates are found for Danny and Bret, respectively, over the original GBVTD-simplex method. The average center uncertainties are estimated to be less than 2 km, yielding estimated errors of less than 5% in the retrieved radius of maximum wind and wavenumber-0 axisymmetric tangential wind, and ~30% error in the wavenumber-1 asymmetric tangential wind. The objective statistical center-finding method can be run on a time scale comparable to that of a WSR-88D volume scan, thus making it a viable tool for both research and operational use.

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Benjamin C. Trabing and Michael M. Bell

Abstract

A growing body of work has documented the existence of diurnal oscillations in the tropical cyclone outflow layer. These diurnal pulses have been examined primarily using satellites or numerical models, and detailed full tropospheric observations or case study analyses of diurnal pulses are lacking. Questions remain on the vertical extent of diurnal pulses and whether diurnal pulses are coupled to convective bands or constrained to the outflow layer. During the Propagation of Intraseasonal Tropical Oscillations (PISTON) field campaign, diurnal oscillations in the upper-level clouds were observed during Typhoon Kong-rey’s (2018) rapid intensification. Over a 3.5-day period where a broad distribution of cold upper-level clouds was overhead, detailed observations of Typhoon Kong-rey’s rainbands show that convection had reduced echo tops but enhanced reflectivity and differential reflectivity aloft compared to other observations during PISTON. Shortwave heating in the upper levels increased the stability profile in an overall favorable thermodynamic environment for convection during the day, which could help to explain the diurnal differences in convective structure. Under the cirrus canopy, nocturnal convection was deeper and daytime convection shallower in contrast to the rest of the PISTON dataset. Diurnal oscillations in the brightness temperatures were found to be coupled to radially outward propagating convective rainbands that were preceded ~6 h by outflow jets. The cooling pulses occurred earlier than found in previous studies. The pulses were asymmetric spatially, which is likely due to a combination of the vertical wind shear and storm intensity.

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Scott W. Powell and Michael M. Bell

Abstract

Hurricane Matthew locally generated more than 400 mm of rainfall on 8–9 October 2016 over the eastern Carolinas and Virginia as it transitioned into an extratropical cyclone. The heaviest precipitation occurred along a swath situated up to 100–200 km inland from the coast and collocated with enhanced low-tropospheric frontogenesis. Analyses from version 3 of the Rapid Refresh (RAPv3) model indicate that rapid frontogenesis occurred over eastern North and South Carolina and Virginia on 8 October, largely over a 12-h time period between 1200 UTC 8 October and 0000 UTC 9 October. The heaviest rainfall in Matthew occurred when and where spiral rainbands intersected the near-surface front, which promoted the lift of conditionally unstable, moist air. Parallel to the spiral rainbands, conditionally unstable low-tropospheric warm, moist oceanic air was advected inland, and the instability was apparently released as the warm air mass rose over the front. Precipitation in the spiral rainbands intensified on 9 October as the temperature gradient along the near-surface front rapidly increased. Unlike in Hurricane Floyd over the mid-Atlantic states, rainfall totals within the spiral rainbands of Matthew as they approached the near-surface front evidently were not enhanced by release of conditional symmetric instability. However, conditional symmetric instability release in the midtroposphere may have enhanced rainfall 200 km northwest of the near-surface front. Finally, although weak cold-air damming occurred prior to heavy rainfall, damming dissipated prior to frontogenesis and did not impact rainfall totals.

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Muhammad Naufal Razin and Michael M. Bell

Abstract

Hurricane Ophelia (2005) underwent an unconventional eyewall replacement cycle (ERC) as it was a category-1 storm located over cold sea surface temperatures near 23°C. The ERC was analyzed using airborne radar, flight-level, and dropsonde data collected during the Hurricane Rainband and Intensity Change Experiment (RAINEX) intensive observation period on 11 September 2005. Results showed that the spinup of the secondary tangential wind maximum during the ERC can be attributed to the efficient convergence of absolute angular momentum by the midlevel inflow of Ophelia’s dominantly stratiform rainbands. This secondary tangential wind maximum strongly contributed to the azimuthal mean tangential wind field, which is conducive for increased low-level supergradient winds and corresponding outflow. The low-level supergradient forcing enhanced convergence to form a secondary eyewall. Ophelia provides a unique example of an ERC occurring in a weaker storm with predominantly stratiform rainbands, suggesting an important role of stratiform precipitation processes in the development of secondary eyewalls.

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Chaehyeon C. Nam and Michael M. Bell

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The impact of vertical wind shear (VWS) on tropical cyclogenesis is examined from the synoptic to mesoscales using airborne Doppler radar observations of predepression Hagupit during the Tropical Cyclone Structure 2008 (TCS08)/THORPEX Pacific Area Regional Campaign (T-PARC) field campaigns. The high temporal and spatial resolution observations reveal complex localized convective and vortical characteristics of a predepression in a sheared environment. Predepression Hagupit interacted with an upper-tropospheric trough during the observation period. The strong deep-layer VWS (>20 m s−1) had a negative impact on the development through misalignment of the low- and midlevel circulations and dry air intrusion. However, the low-level circulation persisted, and the system ultimately formed into a tropical cyclone after it left the high-shear zone. Here we propose that a key process that enabled the predepression to survive through the upper-tropospheric trough interaction was persistent vorticity amplification on the meso-γ scale that was aggregated on the meso-α scale within the wave pouch. Multi-Doppler wind analysis indicates that cumulus congestus tilted the low-level horizontal vorticity into the vertical in the early stage of convective life cycle, followed by stretching from maturing deep convection. Variations in low-level VWS on the meso-β scale affect convective organization and horizontal vorticity generation. The results provide new insights into multiscale processes during TC genesis and the interactions of a predepression with VWS at various spatial scales.

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Benjamin C. Trabing and Michael M. Bell

Abstract

The characteristics of official National Hurricane Center (NHC) intensity forecast errors are examined for the North Atlantic and east Pacific basins from 1989 to 2018. It is shown how rapid intensification (RI) and rapid weakening (RW) influence yearly NHC forecast errors for forecasts between 12 and 48 h in length. In addition to being the tail of the intensity change distribution, RI and RW are at the tails of the forecast error distribution. Yearly mean absolute forecast errors are positively correlated with the yearly number of RI/RW occurrences and explain roughly 20% of the variance in the Atlantic and 30% in the east Pacific. The higher occurrence of RI events in the east Pacific contributes to larger intensity forecast errors overall but also a better probability of detection and success ratio. Statistically significant improvements to 24-h RI forecast biases have been made in the east Pacific and to 24-h RW biases in the Atlantic. Over-ocean 24-h RW events cause larger mean errors in the east Pacific that have not improved with time. Environmental predictors from the Statistical Hurricane Intensity Prediction Scheme (SHIPS) are used to diagnose what conditions lead to the largest RI and RW forecast errors on average. The forecast error distributions widen for both RI and RW when tropical systems experience low vertical wind shear, warm sea surface temperature, and moderate low-level relative humidity. Consistent with existing literature, the forecast error distributions suggest that improvements to our observational capabilities, understanding, and prediction of inner-core processes is paramount to both RI and RW prediction.

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