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William C. Skamarock and Joseph B. Klemp

physicalinterest often are of much lower frequency than thehighest-frequency modes admitted by the equations,such as high-frequency gravity and acoustic modes,that are meteorologically unimportant. Numerical techniques used to integrate the equations are often timestep limited by these irrelevant modes, and a popularintegration approach is to integrate the high-frequencycomponents with a smaller time step, or with an implicit technique, while integrating the lower-frequencycomponents with an explicit scheme

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Bruce W. Hootman and William Blumen

arrival of the flow at theBoulder Atmospheric Observatory tower east ofBoulder. The turbulent nature of the leading edge ofthe flow could be delineated with high resolution (10second average) wind and temperature data in addition to data obtained from an acoustic sounder located at the tower. Finally, the approximate steadystate of the flow that tends to develop soon after passage of the leading edge is compared with the modelresults obtained by both Prandtl (1952) and Rao andSnodgrass (1981). The

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, Nev., Dee. 1969,49 pp.Baker, D. M., and Davies, K., "F-2 Region Acoustic Waves FromSevere Weather," Journal of Atmospheric and Terrestrial Physics,Vol. 31, Pergamon Press, London, 1969, pp. 1345-1352.Baker, Donald M., and Davies, Kenneth, "Use of the Doppler Tech-nique to Study the Fading of High-Frequency Ionospheric RadioSignals," Proceedings of the AGARD Cmference on Oblique Imlo-spheric Radio-wave Propagation at Frequencies Near the LowestUsable High Freauency, Leicester, England, July 25

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JANUARY, 1901. MONTHLY WEATHER REVIEW. 26much-discussed question of the abnormal aberration of fogsignals. It will be remembered that Prof. Joseph Henry,who for twelve years served as chairman of the LighthouseBoard, thought that the wind played a more important partin the abnormal aberration of sound waves than the so-called acoustic clouds described by Professor Tyndall. It is prob- able that up to a certain point both explanations may hold,but the wind is seemingly the more active factor in

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JANUARY, 1901. MONTHLY WEATHER REVIEW. 26much-discussed question of the abnormal aberration of fogsignals. It will be remembered that Prof. Joseph Henry,who for twelve years served as chairman of the LighthouseBoard, thought that the wind played a more important partin the abnormal aberration of sound waves than the so-called acoustic clouds described by Professor Tyndall. It is prob- able that up to a certain point both explanations may hold,but the wind is seemingly the more active factor in

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M. A. Shapiro, Tamara Hampel, Doris Rotzoll, and F. Mosher

source of additionalinformation during frontal passage. The NationalCenter for Atmospheric Research (NEAR) Sabrelinerresearch aircraft provided 1 s measurements of horizontal wind velocity, vertical motion, pressure, temperature and moisture during frontal penetration overwest Texas. Satellite. visible imagery of narrow (ropelike) cloud lines and of mesoconvective cloud systems, ACOUSTIC SOUNDER PROFILE600 ~ 60010 KM-- 300195019552000200520102015 TIME

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ChemicalPhysics, T'ol. 54, No. 1, Jan. 1, 1971, pp. 438-439.Fehsenfeld, F. C., Ferguson, E. E., and Bohme, D. K., "AdditionalFlowing Afterglow Measurements of Negative Ion Reactions ofD-Region Interest.," Planetary and Space Science, Vol. 17, Per-gamon Press, Northern Ireland, 1969, pp. 1759-1762.Ferguson, E. E., "Laboratory Measurements of F-Region ReactionRates," Annales de Gdophysiquc, 5'01. 25, No. 4, Centre Nationalde la Recherche Scientifique, Paris, France, 1969, pp. 819-823.Flowers, Edwin C., "New

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A. Shelby Frisch, Brad W. Orr, and Brooks E. Martner

the antenna controller software, the 90.0-elevation was not available. The slight difference from true zenithpointing has negligible effects on the resulting measurements.degree of precision in the wind estimations. Using asimilar number of beams, Kropfli (1986) comparedestimates made by two collocated radars scanning atthe same elevation angle and obtained an estimate ofabout 0.1 m s-i for the precision of the measurements. The largest scales or turbulence wavelengths sampledin the harmonic

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Wayne M. Angevine and Kenneth Mitchell

arranged around the perimeter of the profiler antenna. The profiler measures the speed of sound, and the virtual temperature is derived therefrom. The profiler was operated in RASS mode for 3 min every half hour and the rest of the time ran in the normal wind mode described above. Data are available approximately 50% of the time up to 800 m AGL (Angevine et al. 1998b). The height coverage of RASS at 915 MHz is limited by acoustic attenuation, in addition to the effects of horizontal wind that blows the

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Frédéric Fabry

both model fields and measurements. As instruments and model initialization improve, these results will change. They are also based on the assumption that the 4-km resolution runs used in our identical-twin simulations are realistic enough not to miss crucial effects or phenomena that would significantly alter forecast outcomes. In the end, the most valuable aspect of this study is the approach we used and what information it can provide, but the exercise needs

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