Abstract

On 31 May 2003, a front-fed convective line with leading stratiform precipitation (FFLS) was observed during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX). The high-resolution BAMEX measurements provided one of the first opportunities to thoroughly observe the characteristics of an FFLS system. The 31 May system had an overturning updraft during its early stages, and produced leading stratiform precipitation. As the system matured, a jump updraft developed and the system began to produce trailing stratiform precipitation. It appears that this transition was facilitated by a local decrease in the low-level line-perpendicular vertical wind shear over time, as well as an increase in the surface cold pool’s strength. The BAMEX data further help to address the question of how FFLS systems can be long lived when their inflow passes through the line-leading precipitation: preline soundings suggest a destabilization mechanism resulting from the vertical profile of cooling within the leading stratiform precipitation. This destabilization also helps to explain the 31 May convective system’s persistence in an environment with very low CAPE.

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 internally generated surface fluxes and stresses on squall line evolution. Neither surface fluxes and stresses nor ice microphysics are necessary to simulate the salient features of the squall line. Their inclusion, however, results in differences in the timing of squall line evolution and greater realism of certain structural characteristics. Significant differences in the convectively induced cold pool strength occur between the early stages of simulations that included ice microphysics and a simulation that contained only warm-rain microphysical processes. The more realistic strength and depth of the cold pool in the simulations that contained ice processes is consistent with an updraft tilt that more closely resembles observations. The squall-line-induced surface fluxes also influence the strength but, more dramatically, the areal extent of the surface cold pool. For the majority of the 6-h simulation, this influence on the cold pool strength is felt only within several hundred meters of the surface. Significant impact of squall-line-induced surface, fluxes on the evolving deep convection at the leading edge of the cold pool is restricted to the later stages (t ≥ 4 h) of simulations and is most substantial in regions where the ground-relative winds are strong and the convectively induced cold pool is initially weak and shallow.

Abstract

Mobile weather radars at high frequencies (C, X, K, and W bands) often collect data using staggered pulse repetition time (PRT) or dual pulse repetition frequency (PRF) modes to extend the effective Nyquist velocity and mitigate velocity aliasing while maintaining a useful maximum unambiguous range. These processing modes produce widely dispersed “processor” dealiasing errors in radial velocity estimates. The errors can also occur in clusters in high shear areas. Removing these errors prior to quantitative analysis requires tedious manual editing and often produces “holes” or regions of missing data in high signal-to-noise areas. Here, data from three mobile weather radars were used to show that the staggered PRT errors are related to a summation of the two Nyquist velocities associated with each of the PRTs. Using observations taken during a mature mesoscale convective system, a landfalling tropical cyclone, and a tornadic supercell storm, an algorithm to automatically identify and correct staggered PRT processor errors has been developed and tested. The algorithm creates a smooth profile of Doppler velocities using a Savitzky–Golay filter independently in radius and azimuth and then combined. Errors are easily identified by comparing the velocity at each range gate to its smoothed counterpart and corrected based on specific error characteristics. The method improves past dual PRF correction methods that were less successful at correcting “grouped” errors. Given the success of the technique across low, moderate, and high radial shear regimes, the new method should improve research radar analyses by affording the ability to retain as much data as possible rather than manually or objectively removing erroneous velocities.

Between 1962 and 1983, research in hurricane modification centered on an ambitious experimental program, Project STORMFURY. The proposed modification technique involved artificial stimulation of convection outside the eye wall through seeding with silver iodide. The artificially invigorated convection, it was argued, would compete with the convection in the original eye wall, lead to reformation of the eye wall at larger radius, and thus produce a decrease in the maximum wind.

Since a hurricane's destructive potential increases rapidly as its maximum wind becomes stronger, a reduction as small as 10% would have been worthwhile. Modification was attempted in four hurricanes on eight different days. On four of these days, the winds decreased by between 10 and 30%. The lack of response on the other days was interpreted to be the result of faulty execution of the experiment or poorly selected subjects.

These promising results have, however, come into question because recent observations of unmodified hurricanes indicate: 1) that cloud seeding has little prospect of success because hurricanes contain too much natural ice and too little supercooled water, and 2) that the positive results inferred from the seeding experiments in the 1960s probably stemmed from inability to discriminate between the expected effect of human intervention and the natural behavior of hurricanes.

To assist the National Science Foundation in meeting the needs of the community of scientists by providing them with the instrumentation and platforms necessary to conduct their research successfully, a meeting was held in late November 2012 with the purpose of defining the problems of the next generation that will require radar technologies and determining the suite of radars best suited to help solve these problems. This paper summarizes the outcome of the meeting: (i) Radars currently in use in the atmospheric sciences and in related research are reviewed. (ii) New and emerging radar technologies are described. (iii) Future needs and opportunities for radar support of high-priority research are discussed. The current radar technologies considered critical to answering the key and emerging scientific questions are examined. The emerging radar technologies that will be most helpful in answering the key scientific questions are identified. Finally, gaps in existing radar observing technologies are listed.

Abstract

The radar network deployed in southern France during the first special observing period (SOP 1) of the Hydrological Cycle in the Mediterranean Experiment (HyMeX) was designed to precisely document the 3D structure of moist upstream flow impinging on complex terrain as a function of time, height, and along-barrier distance, and surface rainfall patterns associated with orographic precipitation events. This deployment represents one of the most ambitious field experiments yet, endeavoring to collect high-quality observations of thunderstorms and precipitation systems developing over and in the vicinity of a major mountain chain.

Radar observations collected during HyMeX represent a valuable, and potentially unique, dataset that will be used to improve our knowledge of physical processes at play within coastal orographic heavy precipitating systems and to develop, and evaluate, novel radar-based products for research and operational activities. This article provides a concise description of this radar network and discusses innovative research ideas based upon preliminary analyses of radar observations collected during this field project with emphasis on the synergetic use of dual-polarimetric radar measurements collected at multiple frequencies.

The multination, high-resolution field study of Meteorology And Diffusion Over Non-Uniform Areas (MADONA) was conducted by scientists from the United States, the United Kingdom, Germany, Denmark, Sweden, and the Netherlands at Porton Down, Salisbury, Wiltshire, United Kingdom, during September and October 1992. The host of the field study was the Chemical and Biological Defence Establishment (CBDE, now part of Defence Evaluation and Research Agency) at Porton Down. MADONA was designed and conducted for high-resolution meteorological data collection and diffusion experiments using smoke, sulphurhexaflouride (SF₆), and propylene gas during unstable, neutral, and stable atmospheric conditions in an effort to obtain terrain-influenced meteorological fields, dispersion, and concentration fluctuation measurements using specialized sensors and tracer generators. Thirty-one days of meteorological data were collected during the period 7 September–7 October and 27 diffusion experiments were conducted from 14 to 23 September 1992. Puffs and plumes of smoke and SF₆ were released simultaneously for most of the experiments to gauge the resultant diffusion and concentration behavior. Some 44 meteorological and aerosol sensors and four source generators were used during each day of the field study. This array of sensors included 14 towers of wind cups and vanes, 10 sonic anemometer/thermometers, one boundary layer sonde, two lidar, one ion sensor, the CBDE Weather Station, and several one-of-a-kind sensors. Simulations of airflow and diffusion over the MADONA topography (a 9 km by 7.5 km area) were made with a variety of models. Wind fields and wind-related parameters were simulated with several high-resolution (microalpha scale) wind flow models. A tally of the various data-gathering activities indicates that the execution of MADONA was highly successful. Preliminary use of the datasets shows the high quality and depth of the MADONA database. This well-documented database is suitable for the evaluation and validation of short-range/near-field wind and diffusion models/codes. The database was originally placed on CD-ROM in a structured way by CBDE, Porton Down. The database is now available from the Risø National Laboratory, Denmark.

The Fronts and Atlantic Storm-Track Experiment (FASTEX) will address the life cycle of cyclones evolving over the North Atlantic Ocean in January and February 1997. The objectives of FASTEX are to improve the forecasts of end-of-storm-track cyclogenesis (primarily in the eastern Atlantic but with applicability to the Pacific) in the range 24 to 72 h, to enable the testing of theoretical ideas on cyclone formation and development, and to document the vertical and the mesoscale structure of cloud systems in mature cyclones and their relation to the dynamics. The observing system includes ships that will remain in the vicinity of the main baroclinic zone in the central Atlantic Ocean, jet aircraft that will fly and drop sondes off the east coast of North America or over the central Atlantic Ocean, turboprop aircraft that will survey mature cyclones off Ireland with dropsondes, and airborne Doppler radars, including ASTRAIA/ELDORA. Radiosounding frequency around the North Atlantic basin will be increased, as well as the number of drifting buoys. These facilities will be activated during multiple-day intensive observing periods in order to observe the same meteorological systems at several stages of their life cycle. A central archive will be developed in quasi-real time in Toulouse, France, thus allowing data to be made widely available to the scientific community.