Wen-Chau Lee

Hurricane Intensity and Eyewall Replacement

Observations made during the historic 2005 hurricane season document a case of “eyewall replacement.” Clouds outside the hurricane eyewall coalesce to form a new eyewall at a greater radius from the storm center, and the old eyewall dies. The winds in the new eyewall are initially weaker than those in the original eyewall, but as the new eyewall contracts, the storm reintensifies. Understanding this replacement mechanism is vital to forecasting variations in hurricane intensity. Processes in the “moat” region between the new and old eyewall have been particularly unclear. Aircraft data now show that the moat becomes dynamically similar to the eye and thus is converted into a region inimical to survival of the inner eyewall. We suggest that targeting aircraft to key parts of the storm to gain crucial input to high-resolution numerical models can lead to improvements in forecasting hurricane intensity.

The Bow Echo and MCV Experiment: Observations and Opportunities

The Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX) is a research investigation using highly mobile platforms to examine the life cycles of mesoscale convective systems. It represents a combination of two related investigations to study (a) bow echoes, principally those that produce damaging surface winds and last at least 4 h, and (b) larger convective systems that produce long-lived mesoscale convective vortices (MCVs). The field phase of BAMEX utilized three instrumented research aircraft and an array of mobile ground-based instruments. Two long-range turboprop aircraft were equipped with pseudo-dual-Doppler radar capability, the third aircraft was a jet equipped with dropsondes. The aircraft documented the environmental structure of mesoscale convective systems (MCSs), observed the kinematic and thermodynamic structure of the convective line and stratiform regions (where rear-inflow jets and MCVs reside), and captured the structure of mature MCVs. The ground-based instruments augmented sou...

Assimilation of Doppler Radar Observations with a Regional 3DVAR System: Impact of Doppler Velocities on Forecasts of a Heavy Rainfall Case

Abstract In this paper, the impact of Doppler radar radial velocity on the prediction of a heavy rainfall event is examined. The three-dimensional variational data assimilation (3DVAR) system for use with the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) is further developed to enable the assimilation of radial velocity observations. Doppler velocities from the Korean Jindo radar are assimilated into MM5 using the 3DVAR system for a heavy rainfall case that occurred on 10 June 2002. The results show that the assimilation of Doppler velocities has a positive impact on the short-range prediction of heavy rainfall. The dynamic balance between atmospheric wind and thermodynamic fields, based on the Richardson equation, is introduced to the 3DVAR system. Vertical velocity (w) increments are included in the 3DVAR system to enable the assimilation of the vertical velocity component of the Doppler radial velocity observation. The forecast of the hydrometeor variables of cloud water (qc...

The hurricane rainband and intensity change experiment : Observations and modeling of hurricanes katrina, ophelia, and rita

The Hurricane Rainband and Intensity Change Experiment (RAINEX) used three P3 aircraft aided by high-resolution numerical modeling and satellite communications to investigate the 2005 Hurricanes Katrina, Ophelia, and Rita. The aim was to increase the understanding of tropical cyclone intensity change by interactions between a tropical cyclones inner core and rainbands. All three aircraft had dual-Doppler radars, with the Electra Doppler Radar (ELDORA) on board the Naval Research Laboratorys P3 aircraft, providing particularly detailed Doppler radar data. Numerical model forecasts helped plan the aircraft missions, and innovative communications and data transfer in real time allowed the flights to be coordinated from a ground-based operations center. The P3 aircraft released approximately 600 dropsondes in locations targeted for optimal coordination with the Doppler radar data, as guided by the operations center. The storms were observed in all stages of development, from tropical depression to category ...

Diagnosed Three-Dimensional Axisymmetric Structure of the Mulhall Tornado on 3 May 1999

On 3 May 1999, an unusually large tornado that caused F4-level damage and killed several people was intercepted by the Doppler on Wheels (DOW) mobile radar near Mulhall, Oklahoma, from a range of 4 to 9 km, resulting in high-resolution volumetric data every 55 s up to 1.5-km altitude over a period of 14 min. For the first time, the evolution and three-dimensional structure of a tornado were deduced using the ground-based velocity track display (GBVTD) technique. After the circulation center was determined, the tangential wind and radial wind were derived from the GBVTD technique at each radius and height. In addition, the axisymmetric vertical velocity, angular momentum, vorticity, and perturbation pressure were deduced from the tangential and radial wind fields. This study focuses on the axisymmetric aspects of this tornado. The primary circulation of the Mulhall tornado consisted of an 84 m s 1 peak axisymmetric tangential wind with the radius of maximum wind (RMW) ranging from 500 to 1000 m. The secondary circulation exhibited a two-cell structure characterized by a central downdraft surrounded by an annular updraft near the RMW. The calculated maximum pressure deficit from a 3-km radius to the tornado center at 50-m altitude was 80 hPa. The maximum vorticity during the first 8 min of observation was located inside the RMW away from the tornado center. This vorticity profile satisfied the necessary condition of barotropic instability. As the tornado weakened afterward, the vorticity monotonically increased toward the center. The computed swirl ratios were between 2 and 6, consistent with the observed multiple vortex radar signatures and the vorticity pattern. Swirl ratios were generally smaller during the weakening phase.

An Approach of Radar Reflectivity Data Assimilation and Its Assessment with the Inland QPF of Typhoon Rusa (2002) at Landfall

Abstract A radar reflectivity data assimilation scheme was developed within the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) three-dimensional variational data assimilation (3DVAR) system. The model total water mixing ratio was used as a control variable. A warm-rain process, its linear, and its adjoint were incorporated into the system to partition the moisture and hydrometeor increments. The observation operator for radar reflectivity was developed and incorporated into the 3DVAR. With a single reflectivity observation, the multivariate structures of the analysis increments that included cloud water and rainwater mixing ratio increments were examined. Using the onshore Doppler radar data from Jindo, South Korea, the capability of the radar reflectivity assimilation for the landfalling Typhoon Rusa (2002) was assessed. Verifications of inland quantitative precipitation forecasting (QPF) of Typhoon Rusa (2002) showed positive impacts of assi...

Convective Contribution to the Genesis of Hurricane Ophelia (2005)

Abstract The convection occurring in the tropical depression that became Hurricane Ophelia (2005) was investigated just prior to tropical storm formation. Doppler radar showed a deep, wide, intense convective cell of a type that has been previously thought to occur in intensifying tropical depressions but has not heretofore been documented in detail. The updraft of the cell was 10 km wide, 17 km deep, had updrafts of 10–20 m s−1 throughout its mid- to upper levels, and contained a cyclonic vorticity maximum. The massive convective updraft was maintained by strong positive buoyancy, which was maximum at about the 10-km level, probably aided by latent heat of freezing. Evaporative cooling and precipitation drag occurred in the rain shower of the cell but were insufficient to produce a strong downdraft or gust front outflow to force the updraft. The convective updraft was fed by a layer of strong inflow that was several kilometers deep. Wind-induced turbulence, just above the ocean surface, enriched the equi...

Tropical Cyclone Kinematic Structure Retrieved from Single-Doppler Radar Observations. Part I: Interpretation of Doppler Velocity Patterns and the GBVTD Technique

Abstract Deducing the three-dimensional primary circulation of landfalling tropical cyclones (TCs) from single ground-based Doppler radar data remains a difficult task. The evolution and structure of landfalling TCs and their interactions with terrain are left uncharted due to the lack of dual-Doppler radar observations. Existing ground-based single-Doppler radar TC algorithms provide only qualitative information on axisymmetric TC center location and intensity. In order to improve understanding of the wind structures of landfalling TCs using the widely available WSR-88D data along the U.S. coastal region, a single ground-based radar TC wind retrieval technique, the ground-based Velocity Track Display (GBVTD) technique, is developed. Part I of this paper presents 1) single-Doppler velocity patterns of analytic, asymmetric TCs, 2) derivation of the GBVTD technique, and 3) evaluation of the GBVTD-retrieved winds using analytic TCs. The Doppler velocity patterns of asymmetric TCs display more complex structu...

Analysis of a Heavy Rainfall Event during TAMEX

Abstract A heavy rainfall event during the Taiwan Area Mesoscale Experiment intensive observing period 13 has been studied using upper-air, surface mesonet, and dual-Doppler radar data. The heavy rainfall (≥231 mm day−1) occurred over northwestern Taiwan with the maximum rainfall along the northwestern coast and was caused by a long-lived, convective rainband in the prefrontal atmosphere. It occurred in an upper-level divergence region and along the axis of the maximum equivalent potential temperature at the 850-hPa level. As a Mei-Yu front advanced southeastward, the postfrontal cold air in the lowest levels was retarded by the hilly terrain along the southeastern China coast. As a result, a low-level wind-shift line associated with a pressure trough at the 850-hPa level moved over the Taiwan Strait before the arrival of the surface front. The westerly flow behind the trough interacted with a barrier jet along the northwestern coast of Taiwan. The barrier jet is caused by the interaction between the pref...

Mobile doppler radar observations of a tornado in a supercell near Bassett, Nebraska, on 5 June 1999. Part II: Tornado-vortex structure

Abstract This is Part II of a paper detailing an analysis of high-resolution wind and reflectivity data collected by a mobile, W-band Doppler radar; the analysis depicts the near-surface life history of a tornado in a supercell in north-central Nebraska on 5 June 1999. The structure of the tornado vortex near the ground is described from a sequence of sector scans at 10–15-s intervals during much of the lifetime of the tornado. The formation of the tornado vortex near the ground is described in Part I. The wind and reflectivity features in the tornado evolved on timescales of 10 s or less. A time history of the azimuthally averaged azimuthal and radial wind profiles and the asymmetric components of the azimuthal and radial wind fields in the tornado were estimated by applying the ground-based velocity track display (GBVTD) technique to the Doppler wind data. If the magnitude of the asymmetric part of the radial wind component were indeed much less than that of the azimuthal wind component (a necessary req...