Search Results

You are looking at 1 - 10 of 10 items for

  • Author or Editor: Jeffrey B. Halverson x
  • All content x
Clear All Modify Search
Biswadev Roy, Jeffrey B. Halverson, and Junhong Wang

Abstract

Hundreds of Vaisala sondes with an RS80-H Humicap thin-film capacitor humidity sensor were launched during the Tropical Rainfall Measuring Mission (TRMM) field campaigns (1999) Large Scale Biosphere–Atmosphere (LBA) experiment held in Brazil and the Kwajalein Experiment (KWAJEX) held in the Republic of the Marshall Islands. Six humidity error correction algorithms were used primarily for applying system-bias correction to RS80-H humidity data. All TRMM field campaign Vaisala humidity soundings were corrected for dry bias using this algorithm. An overall improvement of 3% RH for daytime and 5% RH for nighttime soundings was achieved. Sonde age was ascertained using respective serial numbers (in this case the range is 0.06–2.07 yr) and used in the algorithm for calculation of sensor aging error and chemical contamination errors. Chemical contamination error is also found to be a dominant error source. Daytime sensor-arm heating for the first 50 s of the sonde launch is found to bear a cosine variation with sonde age. Surface reference temperature and sonde registered surface temperature are both used for calculating surface saturation vapor pressure, which in turn is used for sensor-arm-heating error estimation during the first 50 s. Site-mean CAPE values are found to increase significantly after correction. It is suggested that sonde surface temperature error must also be corrected for sonde age while using the present RS80-H correction algorithm. An age–height plot of the differences between the uncorrected and corrected specific humidity value for all Vaisala soundings shows an age-dependent increase (approximately 3.4 g kg−1 for 2-yr-old sondes). Variation of specific humidity difference was not found to be very significant for the upper levels when the sensor is less than 1.25 yr old.

Full access
Haiyan Jiang, Jeffrey B. Halverson, and Joanne Simpson

Abstract

It has been well known for years that the heavy rain and flooding of tropical cyclones over land bear a weak relationship to the maximum wind intensity. The rainfall accumulation history and rainfall potential history of two North Atlantic hurricanes during 2002 (Isidore and Lili) are examined using a multisatellite algorithm developed for use with the Tropical Rainfall Measuring Mission (TRMM) dataset. This algorithm uses many channel microwave data sources together with high-resolution infrared data from geosynchronous satellites and is called the real-time Multisatellite Precipitation Analysis (MPA-RT). MPA-RT rainfall estimates during the landfalls of these two storms are compared with the combined U.S. Next-Generation Doppler Radar (NEXRAD) and gauge dataset: the National Centers for Environmental Prediction (NCEP) hourly stage IV multisensor precipitation estimate analysis. Isidore produced a much larger storm total volumetric rainfall as a greatly weakened tropical storm than did category 1 Hurricane Lili during landfall over the same area. However, Isidore had a history of producing a large amount of volumetric rain over the open gulf. Average rainfall potential during the 4 days before landfall for Isidore was over a factor of 2.5 higher than that for Lili. When using the TRMM-based MPA-RT rainfall estimate, results are consistent with a previous study, which analyzed just the infrared-based rain estimation; that is, the rain potential history could be used as a predictor for the storm’s potential for inland flooding 3–4 days in advance of landfall.

Full access
Sim D. Aberson and Jeffrey B. Halverson

Abstract

A photograph of vertically aligned Kelvin–Helmholtz billows in the eastern eyewall of Hurricane Erin on 10 September 2001 is presented. The vertical shear instability in the horizontal winds necessary to produce the billows is confirmed with a high-altitude dropwindsonde observation. This shear instability is not known to be common in tropical cyclone eyewalls and is likely only in cases with a very large eyewall tilt. However, research and reconnaissance aircraft pilots need to be aware of the possibility of their existence, along with other types of hazardous conditions, in such rare circumstances.

Full access
Haiyan Jiang, Jeffrey B. Halverson, Joanne Simpson, and Edward J. Zipser

Abstract

Part I of this two-part paper examined the satellite-derived rainfall accumulation and rain potential history of Hurricanes Isidore and Lili (2002). This paper (Part II) uses analyses from the Navy Operational Global Atmospheric Prediction System (NOGAPS) to examine the water budget and environmental parameters and their relationship to the precipitation for these two storms. Factors other than storm size are found to account for large volumetric differences in storm total rainfall between Lili and Isidore. It is found that the horizontal moisture convergence was crucial to the initiation and maintenance of Isidore’s intense rainfall before and during its landfall. When the storm was over the ocean, the ocean moisture flux (evaporation) was the second dominant term among the moisture sources that contribute to precipitation. During Isidore’s life history, the strong horizontal moisture flux convergence corresponded to the large storm total precipitable water. The large difference in budget-derived stored cloud ice and liquid water between Isidore and Lili is corroborated from Tropical Rainfall Measuring Mission (TRMM) measurements. During Isidore’s landfall, the decrease in environmental water vapor contributed to rainfall in a very small amount. These results indicate the importance of the environmental precipitable water and moisture convergence and ocean surface moisture flux in generating Isidore’s large rainfall volume and inland flooding as compared with Lili’s water budget history. Both the moisture convergence and ocean flux were small for Lili.

Full access
Haiyan Jiang, Jeffrey B. Halverson, Joanne Simpson, and Edward J. Zipser

Abstract

The Tropical Rainfall Measuring Mission–based National Aeronautics and Space Administration Goddard Multisatellite Precipitation Analysis (MPA) product is used to quantify the rainfall distribution in tropical cyclones that made landfall in the United States during 1998–2004. A total of 37 tropical cyclones (TC) are examined, including 2680 three-hourly MPA precipitation observations. Rainfall distributions for overland and overocean observations are compared. It is found that the TC rainfall over ocean bears a strong relationship with the TC maximum wind, whereas the relationship for overland conditions is much weaker. The rainfall potential is defined by using the satellite-derived rain rate, the satellite-derived storm size, and the storm translation speed. This study examines the capability of the overocean rainfall potential to predict a storm’s likelihood of producing heavy rain over land. High correlations between rain potentials before landfall and the maximum storm total rain over land are found using the dataset of the 37 landfalling TCs. Correlations are higher with the average rain potential on the day prior to landfall than with averages over any other time period. A TC overland rainfall index is introduced based on the rainfall potential study. This index can be used to predict the storm peak rainfall accumulation over land. Six landfalling storms during the 2005 Atlantic Ocean hurricane season are examined to verify the capability of using this index to forecast the maximum storm total rain over land in the United States. The range of the maximum storm overland rain forecast error for these six storms is between 2.5% and 24.8%.

Full access
Jeffrey B. Halverson, Brad S. Ferrier, Thomas M. Rickenbach, Joanne Simpson, and Wei-Kuo Tao

Abstract

An active day during the Coupled Ocean–Atmosphere Response Experiment (COARE) Intensive Observation Period (IOP) is examined in which nine convective systems evolved and moved eastward across the region of shipboard radar coverage in the Intensive Flux Array (IFA) within westerly wind burst conditions. The detailed genesis, morphology, and interactions between these cloud systems are documented from a radar and satellite perspective. One of these systems was a large and complex elliptical cluster, among the largest observed during the Tropical Ocean Global Atmosphere COARE. Multiple, parallel deep convective lines spaced 20–30 km apart and embedded within this system were initially oriented from north-northwest to south-southeast, oblique to the storm motion. Furthermore, the lines underwent counterclockwise realignment as the system moved eastward. The influence of strong lower-tropospheric directional and speed shear on these convective system properties is examined in the context of a dynamic, large-scale near-equatorial trough/transequatorial flow regime. A daily analysis of flow conditions during the 119-day IOP revealed that this type of synoptic regime was present in the IFA at least 40% of the time.

Radar-derived rainfall statistics are examined throughout the life cycles of each individual convective system. Spatial mapping of accumulated rainfall reveals long, linear swaths produced by the most intense cells embedded within convective lines. The evolution of rainfall properties includes an increase in the stratiform rainfall fraction and areal coverage in later generations of systems, with a peak in total rainfall production after local midnight. These trends can be explained by anvil cloud interactions originating within the sequence of closely spaced disturbances, including the effects of both enhanced midtropospheric moisture and also strong reversing (easterly) shear. The issue of boundary layer recovery between the frequent, intense convective systems on this day is also examined.

Full access
Jeffrey B. Halverson, Thomas Rickenbach, Biswadev Roy, Harold Pierce, and Earle Williams

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 strongly electrified and featured larger convective rain rates and reduced stratiform rainfall fraction. These systems formed in an environment with larger CAPE (1500 J kg−1), drier lower- and midlevel humidities (<80% RH), and a wind shear layer that was both stronger (10−3 s−1) and deeper (700 mb).

The time series of low- and midlevel averaged humidity exhibited marked variability between westerly and easterly regimes and was characterized by low-frequency (i.e., multiday to weekly) variations. In addition to its importance in stratiform rain formation, the humidity content directly influenced cloud cover and, thus, the degree of thermal instability present during regimes. The synoptic-scale origins of these moisture fluctuations are examined. The results reported herein provide an environmental context for ongoing dual-Doppler analyses and numerical modeling case studies of individual TRMM-LBA convective systems.

Full access
Gerald M. Heymsfield, Jeffrey B. Halverson, Joanne Simpson, Lin Tian, and T. Paul Bui

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 much deeper and broader-scale feature than the convectively induced, symmetric overturning that commonly occurs on the upper-level flanks of convective towers in other tropical environments. The air supplying the deep current probably originates both at tropopause height and also from air detrained out of the adjacent updraft at midlevels. Strong downdrafts within the eye could not be associated with every hot tower. Whether this result was due to undersampling by aircraft or whether deep eye downdrafts are indeed sporadic, it is plausible that up to 3°C of midlevel eye warming observed in Bonnie may arise from one or more of these convectively induced episodes rather than as a result of a gradual sinking motion applied uniformly throughout the eye.

Full access
Bart Geerts, Gerald M. Heymsfield, Lin Tian, Jeffrey B. Halverson, Anthony Guillory, and Mercedes I. Mejia

Current understanding of landfalling tropical cyclones is limited, especially with regard to convective-scale processes. 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-level updraft magnitudes over 20 m s−1 and microwave brightness temperatures below 100 K at 89 GHz (implying large ice concentrations), occurred when the eye moved over the Cordillera Central, the island's main mountain chain. The location, intensity and evolution of this convection indicate that it was coupled to the surface orography. The authors speculate that orographic lifting released potential energy, which had been trapped beneath the eye's subsidence inversion.

It is likely that surface rain rates increased during landfall, both in the convective and in the more widespread stratiform rainfall areas over the island. Evidence for this is the increase in radar reflectivity below the bright band down to ground level. Such increase was absent offshore. This low-level rain enhancement must be due to the ascent of boundary layer air over the topography.

Full access
Greg M. McFarquhar, Henian Zhang, Gerald Heymsfield, Jeffrey B. Halverson, Robbie Hood, Jimy Dudhia, and Frank Marks Jr.

Abstract

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 (P min) 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 P min 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 P min. 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 (V dop) 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 mixed-phase 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 dBZ, 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 P min is up to 12 hPa higher over the last 18 simulated hours.

Full access