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SEVERE WIND STORMS AT SPRINGFIELD, ILL.

STORM OF SEPTEMBER 6, 1911

J. C. JENSEN

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J. C. JENSEN

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J. C. Jensen
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J. C. Jensen
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Prof. J. C. JENSEN

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Deborah J. Abbs and Jørgen B. Jensen

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A nonhydrostatic mesoscale model is used to simulate the dynamics and microphysics of postfrontal flow in the mountainous region of southeastern Australia. The aim of the paper is to determine if it is possible to use 2D models to simulate the characteristics of the liquid water field upstream from Baw Baw Plateau under postfrontal conditions. Results from both 2D and 3D simulations are compared with aircraft and surface observations taken during the Australian Winter Storms Experiment I, conducted during July and August 1988. The observations and both the 2D and 3D simulations show that under postfrontal conditions, the main feature of the flow is a series of standing lee waves downstream from Baw Baw Plateau. The microphysical fields are characterized by a cap cloud over Baw Baw Plateau and a region of high liquid water content extending at least 50 km upstream from the plateau. Convective elements form upstream from the plateau and are subsequently advected to the northeast. As the convective elements cross Baw Baw Plateau, they precipitate and subsequently evaporate in the drier subsidence region to the lee of the plateau. The main features of the airflow and cloud fields are well simulated by the 2D model runs; however, the 2D runs overestimate the precipitation amounts as compared with the surface observations and the 3D model results.

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Christopher J. Nowotarski and Anders A. Jensen

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The self-organizing map (SOM) statistical technique is applied to vertical profiles of thermodynamic and kinematic parameters from a Rapid Update Cycle-2 (RUC-2) proximity sounding dataset with the goal of better distinguishing and predicting supercell and tornadic environments. An SOM is a topologically ordered mapping of input data onto a two-dimensional array of nodes that can be used to classify large datasets into meaningful clusters. The relative ability of SOMs derived from each parameter to separate soundings in a way that is useful in discriminating between storm type, location, and time of year is discussed. Sensitivity to SOM configuration is also explored. Simple skill scores are computed for each SOM to evaluate the relative potential of each variable for future development as a method of probabilistic forecasting. It is found that variance in SOM nodes is reduced compared to the overall dataset, indicating that this is a viable classification method. SOMs of profiles of wind-derived variables are more effective in discriminating between storm type than thermodynamic variables. The SOM method also identifies meteorological, geographic, and temporal regimes within the dataset. In general, conditional probabilities of storm-type occurrence generated using SOMs have higher skill when wind-derived variables are considered and when forecasting nonsupercell events. Storm-relative wind variables tend to have better skill than ground-relative wind variables when forecasting nonsupercells, whereas ground-relative variables become more important when forecasting tornadoes.

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H. Dean Parry, Melvin J. Sanders Jr., and Hans P. Jensen

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A monostatic acoustic sounding system and its capabilities in providing information about the temperature structure of the lower atmosphere are described. By means of the sounder record the meteorologist can 1) detect stable atmospheric strata including inversion layers, 2) determine the height of base and top of such strata, 3) detect convections, 4) determine height of base and top of convections, 5) determine time rate of change of all heights because the record is continuous and in real time, and 6) infer the form of the temperature profile. Numerous examples of applications of the sounder information to actual weather situations are discussed.

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E. Stratmann, D. Atlas, J. H. Richter, and D. R. Jensen

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A method of calibrating a fixed vertically pointing radar is presented. The technique involves the firing of B-B shot of known radar cross section through the beam while making successive trajectory corrections until the absolute maximum signal is attained. The results agree closely with an independent calibration of antenna gain. The approach is particularly suited to an FM-CW radar with high range resolution because the pellets reach heights well in excess of the minimum range and errors in range are negligible. Corrections are presented for the reduction in maximum two-way gain resulting from intersecting beams whose full gain is attained only at the point of intersection. It is also shown that Probert-Jones’ k 2 factor is significantly smaller for this system, and possibly for others, than the generally accepted value of unity. The method can be readily extended to any sufficiently sensitive pulsed radar by using small elevation angles and direct measurements of range rather than those obtained from the echoes.

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Matthew Hayman, Katie J. McMenamin, and Jørgen B. Jensen

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Two-dimensional optical array probes are commonly used for imaging raindrops and ice particles on research aircraft. The ability of these probes to accurately measure particle concentration and size partly depends on the response characteristics of the detection system. If the response characteristics are too slow, then small particles are less likely to be detected and the associated effective sample volume decreases. In an effort to better understand the sample volumes of optical array probes at the National Center for Atmospheric Research, the temporal response of the Fast-2D optical array probe detector board from optical input on the detector to digitization was characterized. The analysis suggests that the board electronics have a response time constant consistently near 50 ns. However, there is also a slow decay term that conforms to a decay rate. The amplitude of this slow function can impact the probe response, varying the minimum detectable pulse width between 60 and 150 ns. Also, the amplitude of the slow function is largely dictated by the illumination angle of incidence. The effects of the response time characteristics are analyzed using a simulator for a 2D cloud (2D-C) probe with 25-μm photodiode spacing. The results show the greatest sensitivity to response time characteristics when particles are smaller than 150 μm, where 10% uncertainty in the slow fraction is likely to produce sample volume uncertainties near 10%. Ignoring response time effects may bias sample volume estimates in the small size regime by as much as 25%.

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