Jeffrey B. Halverson

ER-2 Doppler Radar Investigations of the Eyewall of Hurricane Bonnie during the Convection and Moisture Experiment-3

Abstract A persistent, mesoscale region of intense eyewall convection contained within Hurricane Bonnie on 23 August 1998 is examined from multiple observations synthesized from the National Aeronautics and Space Administration ER-2 and DC-8 aircraft. The intense convection occurred late in the day as Bonnie was attaining its minimum central pressure and during a stage when the inner core featured a markedly asymmetric structure. The internal structure of this convective burst and its relationship to the warm core are presented using a synthesis of high-resolution satellite, aircraft radar, and in situ data. An exceptionally vigorous eyewall tower within the burst and penetrating to nearly 18 km is described. A second intense eyewall tower, adjacent to the eye, is shown to be associated with a mesoscale subsiding current of air, with vertical velocities on the order of several meters per second that descends at least 9 km and extends horizontally nearly 25 km into the eye interior. The subsidence is a muc...

Amazon Coastal Squall Lines. Part I: Structure and Kinematics

Abstract Mesoscale to synoptic-scale squall lines that form along the northeastern coast of South America as sea-breeze-induced instability lines and propagate through the Amazon Basin are investigated using data collected during the April–May 1987 Amazon Boundary Layer Experiment (ABLE 2B). These systems, termed “Amazon coastal squall lines” (ACSL), have been noted by others, but details of the structure and evolution of the ACSL are limited. The present paper uses Geostationary Operational Environmental Satellite, radar, upper-air rawinsonde, and surface Portable Automated Mesonet data to describe the structure, dynamics, and life cycle of the ACSL. Twelve ACSL were sampled during ABLE 2B, and three cases are discussed in detail. The ACSL are discontinuous lines of organized mesoscale cloud clusters that propagate across the central Amazon Basin at speeds of 50–60 km h−1. The ACSL undergo six possible life cycle stages: coastal genesis, intensification, maturity, weakening, reintensification, and dissip...

On the role of “hot towers” in tropical cyclone formation

SummaryThe probabilistic approach to tropical cyclogenesis is advanced here by examining the role of convection in the early stages. The development of “hot towers”, that is tall cumulonimbus towers which reach or penetrate the tropopause, and their role in tropical cyclogenesis is investigated in two well-documented cases of formation. namely hurricane Daisy (1958) in the Atlantic and Tropical Cyclone Oliver (1993) in the Coral Sea. The hot towers in Daisy had been intensively studied by Malkus and Riehl three decades ago but remained mainly unpublished. The dynamics of Oliver genesis by merging mesoscale vortices has been recently reported, but much of the aircraft data remained. This paper adds the evolving contribution of cumulus-scale events and their associated electrification, which was made possible by the addition of an electric field mill, a numerical cloud model and other remote sensors.In their genesis stages, Daisy and Oliver appeared very different because Daisy resulted from a deepening tropical wave in the Atlantic and the pre-Oliver vortex emerged eastward from the Australian monsoon trough. However, the vertical profiles of θE in the rain areas were nearly identical, with the characteristic concave shape showing substantial midlevel minima. Therefore, both required increasing upflux of high θE subcloud air in order to accomplish the formation stage, with about two hot towers each in the nascent eyewall. In both cases, partial eyewalls developed at the edge of the convection, permitting subsidence in the forming eye, which was shown to contribute to the pressure fall. The probabilistic concept proposes that any contribution to early pressure fall raises the probability of success. When the incipient storm goes through those fragile phases more rapidly, the risk of death by the onset of unfavorable large-scale factors such as wind shear or upper-level subsidence is reduced. Daisy developed in an inactive, moist environment with light, variable winds throughout the troposphere while in Oliver, strong divergent upper outflow apparently outweighed strong wind shear, although the latter was responsible for a slow and messy development of a closed, circular eye.In both storms, the hot towers in the major rainband were taller and stronger than those in the early eyewall. Onedimensional time-dependent model runs were used to simulate both in Oliver with two important results: 1) the taller rainband clouds permitted greater high level heating, if it could be retained; and 2) greater electrification and more lighting occurred in the rainband although the partial eyewall clouds also showed strong electrification. Airborne radar, electrification measurements and models are fitted together to understand their relationship. An important result is the clear inference that fairly deep mixed phase regions existed in both eyewall and rainband, in which the DC-8 aircraft experienced liquid water at temperatures colder than −40°C below freezing. These results show that the claims of no supercooled liquid water in tropical cyclones require re-examination with the proper measurements of electricification that are now feasible.

Environmental Characteristics of Convective Systems during TRMM-LBA

Abstract In this paper, data collected from 51 days of continual upper-atmospheric soundings and the Tropical Ocean Global Atmosphere (TOGA) radar at Anglo–Brazilian Amazonian Climate Observation Study (ABRACOS) Hill during the Tropical Rainfall Measuring Mission component of the Brazilian Large Scale Biosphere–Atmosphere (TRMM-LBA) experiment are used to describe the mean thermodynamic and kinematic airmass properties of wet season convection over Rondonia, Brazil. Distinct multiday easterly and westerly lower-tropospheric wind regimes occurred during the campaign with contrasting airmass characteristics. Westerly wind periods featured modest CAPE (1000 J kg−1), moist conditions (>90% RH) extending through 700 mb, and shallow (900 mb) speed shear on the order of 10−4 s−1. This combination of characteristics promoted convective systems that featured a relatively large fraction of stratiform rainfall and weak convection nearly devoid of lightning. In contrast, easterly regime convective systems were more s...

Warm Core Structure of Hurricane Erin Diagnosed from High Altitude Dropsondes during CAMEX-4

Abstract A combination of multiaircraft and several satellite sensors were used to examine the core of Hurricane Erin on 10 September 2001, as part of the Fourth Convection and Moisture Experiment (CAMEX-4) program. During the first set of aircraft passes, around 1700 UTC, Erin was still at its maximum intensity with a central pressure of 969 hPa and wind speed of 105 kt (54 m s−1). The storm was moving slowly northwestward at 4 m s−1, over an increasingly colder sea surface. Three instrumented aircraft, the National Oceanic and Atmospheric Administration (NOAA) P3 with radar, the National Aeronautics and Space Administration (NASA) ER-2 at 19 km, newly equipped with GPS dropwindsondes, and the NASA DC-8 with dropwindsondes flew in formation across the eye at about 1700 UTC and again 2.5 h later around 1930 UTC. The storm had weakened by 13 m s−1 between the first and second eye penetrations. The warm core had a maximum temperature anomaly of only 11°C, located at 500 hPa, much weaker and lower than activ...

Factors Affecting the Evolution of Hurricane Erin (2001) and the Distributions of Hydrometeors: Role of Microphysical Processes

Fine-resolution simulations of Hurricane Erin are conducted using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) to investigate roles of thermodynamic, boundary layer, and microphysical processes on Erin’s structure and evolution. Choice of boundary layer scheme has the biggest impact on simulations, with the minimum surface pressure (Pmin) averaged over the last 18 h (when Erin is relatively mature) varying by over 20 hPa. Over the same period, coefficients used to describe graupel fall speeds (Vg) affect Pmin by up to 7 hPa, almost equivalent to the maximum 9-hPa difference between microphysical parameterization schemes; faster Vg and schemes with more hydrometeor categories generally give lower Pmin. Compared to radar reflectivity factor (Z ) observed by the NOAA P-3 lower fuselage radar and the NASA ER-2 Doppler radar (EDOP) in Erin, all simulations overpredict the normalized frequency of occurrence of Z larger than 40 dBZ and underpredict that between 20 and 40 dBZ near the surface; simulations overpredict Z larger than 25 to 30 dBZ and underpredict that between 15 and 25 or 30 dBZ near the melting layer, the upper limit depending on altitude. Brightness temperatures (Tb) computed from modeled fields at 37.1- and 85.5-GHz channels that respond to scattering by graupel-size ice show enhanced scattering, mainly due to graupel, compared to observations. Simulated graupel mixing ratios are about 10 times larger than values observed in other hurricanes. For the control run at 6.5 km averaged over the last 18 simulated hours, Doppler velocities computed from modeled fields (Vdop) greater than 5 m s 1 make up 12% of Erin’s simulated area for the base simulation but less than 2% of the observed area. In the eyewall, 5% of model updrafts above 9 km are stronger than 10 m s 1 , whereas statistics from other hurricanes show that 5% of updrafts are stronger than only 5 m s 1 . Variations in distributions of Z, vertical motion, and graupel mixing ratios between schemes are not sufficient to explain systematic offsets between observations and models. A new iterative condensation scheme, used with the Reisner mixedphase microphysics scheme, limits unphysical increases of equivalent potential temperature associated with many condensation schemes and reduces the frequency of Z larger than 50 dBZ, but has minimal effect on Z below 50 dB Z, which represent 95% of the modeled hurricane rain area. However, the new scheme changes the Erin simulations in that 95% of the updrafts are weaker than 5 m s 1 and Pmin is up to 12 hPa higher over the last 18 simulated hours.

NASA's tropical cloud systems and processes experiment: Investigating tropical cyclogenesis and hurricane intensity change

In July 2005, the National Aeronautics and Space Administration investigated tropical cyclogenesis, hurricane structure, and intensity change in the eastern North Pacific and western Atlantic using its ER-2 high-altitude research aircraft. The campaign, called the Tropical Cloud Systems and Processes (TCSP) experiment, was conducted in conjunction with the National Oceanic and Atmospheric Administration/Hurricane Research Divisions Intensity Forecasting Experiment. A number of in situ and remote sensor datasets were collected inside and above four tropical cyclones representing a broad spectrum of tropical cyclone intensity and development in diverse environments. While the TCSP datasets directly address several key hypotheses governing tropical cyclone formation, including the role of vertical wind shear, dynamics of convective bursts, and upscale growth of the initial vortex, two of the storms sampled were also unusually strong, early season storms. Highlights from the genesis missions are described in...

Hurricane Georges's landfall in the Dominican Republic : Detailed airborne doppler radar imagery

Abstract Current understanding of landfalling tropical cyclones is limited, especially with regard to convective–scale pro–cesses. On 22 September 1998 Hurricane Georges made landfall on the island of Hispaniola, leaving behind a trail of death and devastation, largely the result of excessive rainfall, not storm surge or wind. Detailed airborne measurements were taken as part of the Third Convection and Moisture Experiment. Of particular interest are the ER–2 nadir X–band Doppler radar data, which provide a first–time, high–resolution view of the precipitation and airflow changes as a hurricane interacts with mountainous terrain. The circulation of Hurricane Georges obviously declined during landfall, evident in the rapid increase in minimum sea level pressure, the subsidence of the eyewall anvil, and the decrease in average ice concentrations in the eyewall. The eye, as seen in satellite imagery, disappeared as deep convection erupted within the eye. The main convective event within the eye, with upper–l...

Structure of Highly Sheared Tropical Storm Chantal during CAMEX-4

Abstract Tropical Storm Chantal during August 2001 was a storm that failed to intensify over the few days prior to making landfall on the Yucatan Peninsula. An observational study of Tropical Storm Chantal is presented using a diverse dataset including remote and in situ measurements from the NASA ER-2 and DC-8 and the NOAA WP-3D N42RF aircraft and satellite. The authors discuss the storm structure from the larger-scale environment down to the convective scale. Large vertical shear (850–200-hPa shear magnitude range 8–15 m s−1) plays a very important role in preventing Chantal from intensifying. The storm had a poorly defined vortex that only extended up to 5–6-km altitude, and an adjacent intense convective region that comprised a mesoscale convective system (MCS). The entire low-level circulation center was in the rain-free western side of the storm, about 80 km to the west-southwest of the MCS. The MCS appears to have been primarily the result of intense convergence between large-scale, low-level easte...

Amazon Coastal Squall Lines. Part II: Heat and Moisture Transports

Abstract The column response to propagating deep convection over the central Amazon Basin is investigated with rawinsonde data from the Amazon Boundary Layer Experiment (ABLE 2B). Heat and moisture budgets are calculated from a relatively small surface network (1000 km2) to determine the distribution of heating within the convective and stratiform regions of three Amazon coastal squall lines (ACSL) in varying degrees of maturity. Portable Automated Mesonet instrumentation, satellite imagery, and radar data are used to partition the large-scale system into distinct cloud and rainfall components. The dimensions of the surface network enable an evaluation of the collective effects of an ensemble of convective elements that are considered to be representative of the synoptic-scale system. Calculations of Q1 and Q2 from the ABLE 2B network follow the methods used by Johnson and Young and Gallus and Johnson. The computations are performed over intervals of 3–6 h using composite soundings derived from a network ...