David P. Jorgensen

Vertical Velocity Characteristics of Oceanic Convection

Abstract Oceanic cumulonimbus updraft and downdraft events observed in the Western Pacific during the TAMEX program by NOAA P-3 research aircraft are analyzed and discussed. The basic dataset consists of flight-level data from 10 missions in the Taiwan region during May and June 1987. The 1 Hz time series of vertical velocity is used to define convective updrafts using the criteria that the velocity must be continuously positive for at least 0.5 km and exceed 0.5 m s−1 for 1 s. A subset of the strongest drafts, termed cores, are defined as events that exceed 1 m s−1 for 0.5 km. Downdrafts and downdraft cores are defined analogously. The statistics are from a total of 12 841 km of flight legs and consist of 359 updrafts and 466 downdrafts at altitudes from 150 m to 6.8 km MSL. The populations of average vertical velocity, maximum vertical velocity, diameter, and mass transport for both drafts and cores are approximately log-normally distributed, consistent with the results of previous studies of convective...

Mesoscale and Convective Structure of a Hurricane Rainband

Abstract The mesoscale thermodynamic, kinematic, and radar structure of a Hurricane Floyd rainband observed on 7 September 1981 is presented. Data are from 26 aircraft passes through the rainband from 150 to 6400 m. A composite technique which presents rainband structure as a function of distance from the storm circulation center reveals inflow from the outer edge of the band and a partial barrier to this flow below 3 km. In the direction parallel to rainband orientation, radar reveals cellular reflectivity structure on the upwind and central portions of the rainband; the frequency of cellular precipitation decreases in favor of stratiform precipitation further downwind as the band spirals gradually towards the eyewall. In the radial direction, a decrease of 12 K in θe, is observed across the rainband in the subcloud layer. Convective scale up- and downdrafts that are associated with cellular reflectivity structure are hypothesized to be responsible for the thermodynamic modification of the cloud and subc...

Vertical Motions in Intense Hurricanes

Abstract Hurricane vertical motion properties are studied using aircraft-measured 1 Hz time series of vertical velocity obtained during radial penetrations of four mature hurricanes. A total of 115 penetrations from nine flight sorties at altitudes from 0.5 to 6.1 km are included in the data set. Convective vertical motion events are classified as updrafts (or downdrafts) if the vertical velocity was continuously positive (or negative) for at least 500 m and exceed an absolute value of 0.5 m s−1. Over 3000 updrafts and nearly 2000 downdrafts are included in the data set. A second criteria was used to define stronger events, called cores. This criteria required that upward (or downward) vertical velocity be continuously greater than an absolute value of 1 m s−1 for at least 500 m. The draft and core properties are summarized as distributions of average and maximum vertical velocity, diameter, and vertical mass transport in two regions: eyewall and rainband. In both regions updrafts dominated over downdraft...

Mesoscale and Convective-Scale Characteristics of Mature Hurricanes. Part II. Inner Core Structure of Hurricane Allen (1980)

Abstract The eyewall structure of Hurricane Alien is examined from analyses of multiple aircraft data on two days, 5 and 8 August 1980. These data sets are unique in that, for the first time, three instrumented aircraft executed coordinated radial penetrations of the eyewall at multiple levels. The data collected on 5 August illustrate the persistence of various features on horizontal scales > 10 km over several hours. Composite cross sections constructed from the 8 August data show similar structure, although the eye diameter had decreased to less than half that of 5 August. The convergence of air in the eyewall was highly two-dimensional. This convergence supported organized ascent that was along the inner edge of the high reflectivity region and displaced inward several kilometers from the radius of maximum wind (RMW). A mean eyewall updraft of 5–6 m s−1 is computed from integration of the two-dimensional continuity equation. Embedded within the two-dimensional eyewall were cores of high reflectivity t...

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...

Structure and Evolution of the 22 February 1993 TOGA COARE Squall Line: Aircraft Observations of Precipitation, Circulation, and Surface Energy Fluxes

Abstract This study documents the precipitation and kinematic structure of a mature, eastward propagating, oceanic squall line system observed by instrumented aircraft during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). Doppler radar and low-level in situ observations are used to show the evolution of the convection from an initially linear NNW–SSE-oriented convective line to a highly bow-shaped structure with an embedded low- to midlevel counterclockwise rotating vortex on its northern flank. In addition to previously documented features of squall lines such as highly upshear-tilted convection on its leading edge, a channel of strong front-to-rear flow that ascended with height over a “rear-inflow” that descended toward the convective line, and a pronounced low-level cold pool apparently fed from convective and mesoscale downdrafts from the convective line; rearward, the observations of this system showed distinct multiple maxima in updraft strength with...

A Radar Study of Convective Cells in Mesoscale Systems in GATE. Part I: Vertical Profile Statistics and Comparison with Hurricanes

Abstract This is Part I of a two-paper describing the structure of the vertical profile of radar reflectivity in convective cells which are of part of mesoscale convective systems in GATE. Earlier work has established that characteristic mean vertical velocities in such convective clouds in GATE were rather weak, <3–5 m s−1. The microphysical implications of the weak updrafts have been proposed to include the rarity of large particles above the freezing level. As a working hypothesis for these papers it is proposed that cells with weak updrafts have characteristic vertical reflectivity profiles exhibiting modest reflectivities at low levels, and decreasing rapidly with height above the freezing level. Vertical radar reflectivity profiles are compiled from 296 convective cells having at least 40 dBZ surface reflectivity, and it is found that the profiles are consistent with the above reasoning. The, mean profile is of modest strength—45 dBZ echo at the surface and a 20 dBZ echo top of 8.2 km, and the refle...

Structure and evolution of the 22 February 1993 TOGA COARE squall line : Numerical simulations

Abstract In this study a numerical cloud model is used to simulate the three-dimensional evolution of an oceanic tropical squall line observed during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment and investigate the impact of small-scale physical processes including surface fluxes and ice microphysics on its structure and evolution. The observed squall line was oriented perpendicular to a moderately strong low-level jet. Salient features that are replicated by the model include an upshear-tilted leading convective region with multiple updraft maxima during its linear stage and the development of a 30-km scale midlevel vortex and associated transition of the line to a pronounced bow-shaped structure. In this modeling approach, only surface flukes and stresses that differ from those of the undisturbed environment are included. This precludes an unrealistically large modification to the idealized quasi-steady base state and thus allows us to more easily isolate effects of ...

Modification of Fronts and Precipitation by Coastal Blocking during an Intense Landfalling Winter Storm in Southern California: Observations during CALJET

Abstract The California Land-falling Jets Experiment (CALJET) was carried out along the California coast, and up to 1000-km offshore, during the winter of 1997/98 to study the underlying physical processes that cause flooding rains and high winds in the orographically complex coastal zone and to explore the impact of potential future observing systems on short-term (<24 h) quantitative precipitation and wind forecasts during the landfall of winter storms from the data-sparse eastern Pacific Ocean. Using the suite of experimental and operational observing systems that were available during CALJET, this study documented the mesoscale modification of an intense landfalling cyclone by the steep coastal orography on 3 February 1998. This storm heavily impacted the populous and highly vulnerable coastal zone of southern California with flooding rains, strong winds, and major beach erosion. A pair of landfalling cold-frontal zones produced most of the damaging weather, while the primary cyclone circulation remai...

Mesovortex circulations seen by airborne Doppler radar within a bow-echo mesoscale convective system

Abstract During the spring of 1991, scientists from the National Severe Storms Laboratory conducted a field observational program to obtain a better understanding of the processes responsible for organizing and maintaining the dynamical and electrical structure of mesoscale convective systems (MCSs), as well as mechanisms acting to organize and propagate the dryline. Extensive use was made of a relatively new observing tool, the airborne Doppler radar installed on one of the NOAA P-3 research aircraft, to map the precipitation and kinematic structure of large convective systems. The radar was operated in an innovative scanning mode in order to collect pseudo dual-Doppler wind data from a straightline flight path. This scanning method, termed the fore/aft scanning technique (FAST), effectively maps out the three-dimensional wind field over mesoscale domains (e.g., 80 km× 100 km) in ∼15 min with horizontal data spacing of 1–2 km. Several MCSs were observed over central Oklahoma during May and June of 1991, ...