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John B. Cunning Jr.

Abstract

By means of an extremely sensitive pressure transducer and a digital recording system, atmospheric pressure measurements were made for a two-week period in October 1971. A three-station array, roughly triangular in shape, was set up within the Miami, Fla., area so as to obtain the phase speed and direction of pressure perturbations moving through the array. Single-station and multiple-station analysis was limited to the range of frequencies from 8 × 10−3 to 4 × 10−2 cycles min−1.

By analysing the band-pass filtered time series and power spectrum for a two-week period, dominant frequencies were observed within the power spectrum. Analyzing the time series for both fair and disturbed conditions, the amplitude of the pressure perturbations ranged from 0.4 mb for fair weather up to 1.0 mb for disturbed weather conditions.

The analysis of the power spectrum and cross spectrum for all three stations for a three-day period indicates that for this period there were three predominant frequencies. From the phase spectrum these waves were shown to move from the east, or approximately in the direction of the low-level flow, and in each case moved with a speed of about 60 kt.

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John B. Cunning Jr.

Abstract

Raindrop size distributions were measured at cloud base during the 1971–73 Florida Area Cumulus Experiment to study the effect of cloud seeding on the Z-R relationship. The Z-R points derived from the “seeded” distributions appear within the 95% confidence limits for the Z-R relationship and well within the natural variability of Z-R points on a day-to-day basis.

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John B. Cunning

During May and June 1985, the Oklahoma-Kansas Preliminary Regional Experiment for STORM-Central (the Oklahoma-Kansas PRE-STORM Program) was conducted to investigate the structure and dynamics of mesoscale convective systems (MCSs). As the name implies, the program was conducted over the Oklahoma and Kansas regions and emulated to some degree the β-scale network array proposed for the full-scale STORM-Central program. A number of sensing systems, including Doppler radars, digitized radars, surface mesonetwork stations, supplemental and National Weather Service (NWS) rawinsondes, wind profilers, a lightning location system, satellite products, and research aircraft were brought together to collect the data necessary to begin the investigations of MCSs. At the same time, testing and evaluation of new sensing systems, such as profilers and airborne Doppler radar, were carried out to determine how best to operate them in a coordinated observing system. Sixteen operational missions were conducted in which one or more MCSs were investigated. Because of the diurnal nature of these systems, most operational missions continued into a second day. Examples of the data collected from several of the observational systems that operated in the program are shown for one of the operational days.

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John B. Cunning
and
Mark Demaria

Abstract

This paper investigates the interactions between two developing cumulonimbus systems and the boundary layer (surface winds, pressure, divergence and temperature fields) on a case study day, 25 August 1975. These two systems formed within 45 min of each other in similar locations and environments. Analysts indicate that in the case of rapid convective development, a surface pressure low develops below the convection and in the environment surrounding the convective system. The pressure low beneath the convection is hypothesized to be formed by hydrostatic and nonhydrostatic (dynamic) effects, and the extension of the low in the environment surrounding the system is hypothesized to be induced by hydrostatic adjustment resulting from subsidence warming. The environmental flow responds to the induced pressure field, causing the convergence to increase in both the convective region and the mesoscale region surrounding the convection. Once rainfall occurs at the surface, the pressure begins to rise in the convective area, reducing or reversing the pressure gradient, causing the convergence in the mesoscale and convective-sale areas to decrease. Once the precipitation stage is reached, the interaction of the outflows from the convection with the surrounding flow appears to become the dominant mechanism for enhancing convergence in the boundary layer.

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John B. Cunning
and
Robert I. Sax

Abstract

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John B. Cunning
and
Robert I. Sax

Abstract

A better understanding of how the precipitation budget operates in tropical convective systems is a prime objective of the GATE research effort. Measurement of rainfall rate with shipboard radar is the principal method by which precipitation from tropical clouds that develop within the GATE B-scale array will be determined. Knowledge of the relationship between radar reflectivity (Z) and rainfall rate (R) is essential for an accurate interpretation of precipitation data derived through the use of radar technology. The Z-R relationship is determined through application of a least-squares linear regression to data points derived by appropriate integration of the third and sixth moments of a series of raindrop size spectra.

Drop spectra measurements were obtained during GATE by means of a foil impactor operated at cloud-base level on board the NOAA DC-6 aircraft. A total of 107 Z-R data points are available, representing showers occurring on 12 days. The best-fit Z-R relationship for the cloud-base aircraft foil data showed little variability from day to day or on the basis of stratification by rain rate. For all foil data combined, the best-fit Z-R relationship was found to have the form Z=170 R 1.52, which gives, for example, rain rates of 66, 15 and 3 mm h−1 for Z values of 50, 40 and 30 dBZ, respectively.

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John B. Cunning
and
Mark DeMaria

Abstract

No abstract available.

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John B. Cunning
,
Henry W. Poor
, and
Mark Demaria

Abstract

The in-cloud structure of radar reflectivity and vertical velocity from Doppler radar measurements are described for two cumulonimbus systems that developed over the FACE-1975 surface mesonetwork area on a case study day, 25 August 1975. Results imply a strong interrelationship between updraft velocity and water loading in convective storm development and show the importance of boundary layer forcing in the development of convective clouds in the Florida environment. Analysis of the temporal evolution of cloud mass at 600-m intervals in the vertical shows that below 3 km the two cumulonimbus systems appeared to evolve to about the same size, with significant differences occurring above that level. The increase cloud mass at some levels above 3 km in the second convective system (System II) were more than twice as large as the increase in cloud mass for the first convective system (System I). The difference in the vertical structure of cloud mass in these systems appears to be caused by the differences in the magnitude of the low-level updraft velocities. Approximately 7–10% of the area in the lower portions of System II had updrafts ≥5 m s−1 compared with only 2–3% for System I. These differences in low-level updraft velocity are then related to the differences in boundary layer forcing determined in Part I.

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Andrew I. Watson
,
JoséG. Meitín
, and
John B. Cunning

Abstract

The relationship of vertical motion to the occurrence of precipitation from the convective and stratiform regions of a mesoscale convective system (MCS) is presented. On 20–21 May 1979, an MCS developed over portions of Oklahoma, Texas, and Arkansas. The uniqueness of this system was its lack of squall-line characteristics and development of a large stratiform precipitation region. The evolution of the system is detailed by rawinsonde observations, radar cross sections, 15-min composite analyses of six NWS WSR-57 radars, and by raingages. The genesis stage of the MCS was described by strong convection along an east-west cold front that was reinforced by outflow generated by two mesoscale convective complexes (MCCS) that formed tile night before in Kansas and Missouri. The mature stage of the MCS was characterized by the development of a large stratiform precipitation region while convection was limited to the southern and eastern flanks of the system. Finally, in the dissipative stage, a moderate north-south squall line that developed over west Texas in the afternoon moved rapidly to the cast apparently associated with a short-wave aloft and appeared to sweep the entire system out of Oklahoma.

A modified Cheng and Houze technique is applied to the radar composites to determine stratiform and convective regions utilizing temporal as well as areas considerations. For the system as a whole, the stratiform region generated 30–50% of the total precipitation. The vertical-motion profiles hold the key to the precipitation characteristics over the storm-scale network. The genesis period was characterized by a strongly convective profile. As the system matured, low-level upward motion cased, while middle-level upward motion was sustained. A large area of stratiform rain developed as the deep convection weakened. Water-budget considerations suggest that the stratiform region was maintained by a combination of mesoscale middle-level updraft and by horizontal transfer of convective debris.

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John B. Cunning
,
Ronald L. Holle
,
Patrick T. Gannon
, and
Andrew I. Watson

Abstract

This paper investigates the interactions between the various scales of motion and, specifically, the inter-actions between convection and the surface boundary layer in the development of a mesoscale convective system within the Florida Area Cumulus Experiment (FACE) experimental area. Data used in the analysis are from a surface mesonetwork covering a 1500 km2 area which consisted of wind measuring stations, raingages, hygrothermographs, microbarographs and temperature, humidity and pressure transducers.

Surface convergence was shown to exist up to 2 h before the development of precipitation over the convergence area within the mesonetwork. Convergence was not being balanced by divergence within the network, which implies mesoscale and/or synoptic-scale forcing.

The subsidence warming and drying in the near environment of the mesoscale convective system appeared to play an important role in its evolution from the mature to the dissipating stage. Soundings taken in the near environment of the convective system showed a 0.8 km lowering in the depth of the moist layer in time. It is hypothesized that the entrainment of this drier air into the convective system, particularly the new convective elements, helps accelerate the mesoscale outflow away from the convective system, which causes the system to evolve into the dissipating stage.

Surface pressure perturbations are shown to be very important in the feedback between developing convection and the boundary layer. Two types of pressure responses are shown, a mesoscale response which appears to affect the entire mesonetwork area and a convective-scale response which affects only a relatively small area beneath the developing cells. The surface pressure perturbations are shown to increase the surface convergence into the area, and as shown in previous papers, this increased surface convergence should then produce a larger and more intense convective system.

The merger between the two convective systems which developed the mesoscale system on this day is described in terms of the radar, visual cloud and raingage characteristics. It is shown that new cloud development and differential motion between convective elements were the two main causes of merger.

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