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Steven E. Koch, Wayne Feltz, Frédéric Fabry, Mariusz Pagowski, Bart Geerts, Kristopher M. Bedka, David O. Miller, and James W. Wilson

Abstract

Families of solitary waves (“solitons”) associated with two atmospheric bores on the same day were observed by an unprecedented number of ground-based and airborne profiling systems during the International H2O Project (IHOP). In addition, a very high-resolution numerical weather prediction model initialized with real data was used with success to simulate one of the bores and the evolving soliton. The predicted wave amplitude, phase speed, wavelength, and structure compared well to these extraordinarily detailed observations. The observations suggest that during the active phase (when turbulent mixing was active, which was prior to bore collapse), the bores and waves vigorously mixed dry air from above a nocturnal boundary layer down to the surface. Refractivity computed from near-surface radar observations showed pronounced decreases due to sudden drying during the passage of the bores in this phase, but refractivity increases appeared during the period of bore collapse. During both phases, the bores wafted aerosol-laden moist air up to the middle troposphere and weakened the capping inversion, thus reducing inhibition to deep convection development. The model results indicate that the refractivity decreases near the surface were due to drying caused by downward turbulent mixing of air by the wave circulations. Turbulent kinetic energy was generated immediately behind the bore head, then advected rearward and downward by the solitary waves. During the dissipation stage, the lifting by the bore head produced adiabatic cooling aloft and distributed the very moist air near the surface upward through the bore depth, but without any drying due to the absence of vigorous mixing. Thus, this study shows that the moist thermodynamic effects caused by atmospheric bores and solitons strongly depend upon the life cycle of these phenomena.

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Richard D. Rosen, David A. Salstein, Thomas Nehrkorn, Margaret R.P. McCalla, Alvin J. Miller, Jean O. Dickey, T. Marshall Eubanks, and J. Alan Steppe

Abstract

Forecasts of zonal wind fields produced by the medium-range forecast (MRF) model of the National Meteorological Center are used to create predictions of the atmosphere's angular momentum at lead times of 1–10 days. Forecasts of this globally integrated quantity are of interest to geodesists and others concerned with monitoring changes in the earth's orientation for navigational purposes. Based on momentum forecasts archived for the period December 1985–November 1986, we find that, on average, the MRF exhibits positive skill relative to persistence-based forecasts at all lead times. Over our entire one-year study period, the improvement over persistence exceeds 20% for 2–6-day forecasts and remains as large as 10% even for 10-day forecasts. On the other hand, skill scores for the MRF momentum predictions vary considerably from month to month, and for a sizeable fraction of our study period the MRF is less skillful than persistence. Thus, although our initial impression of the overall quality of the MRF momentum forecasts is favorable, further improvement is certainly desirable.

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