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James E. Bossert, John D. Sheaffer, and Elmar R. Reiter

Abstract

Mountaintop data from remote stations in the central Rocky Mountains have been used to analyze terrain-induced regional (meso-β to meso-α) scale circulation patterns. The circulation consists of a diurnally oscillating wind regime, varying between daytime inflow toward, and nocturnal outflow from, the highest terrain. Both individual case days and longer term averages reveal these circulation characteristics. The persistence and broadscale organization of nocturnal outflow at mountaintop, well removed from valley drainage processes, demonstrates that this flow is part of a distinct regime within the hierarchy of terrain-induced wind systems.

The diurnal cycle of summertime convective storm development imparts a strong influence upon regional-scale circulation patterns. Subcloud cooling processes, associated with deep moist convection, alter the circulation by producing early and abrupt shifts in the regional winds from an inflow to outflow direction. These wind events occur frequently when moist conditions prevail over the central Rocky Mountains. Atmospheric soundings suggest that significant differences occur in the vertical profile of the topographically influenced layer, depending upon the dominant role of either latent or radiative forcing.

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Daniel N. Shoemaker, William M. Gray, and John D. Sheaffer

Abstract

This report examines the impact of synoptic reconnaissance by United States Air Force aircraft on the accuracy of tropical cyclone motion forecasts. Synoptic reconnaissance missions were requested for the purpose of collecting data on atmospheric conditions in proximity to developed cyclones at levels and locations which were assumed to govern the future motion of each storm. The results presented here suggest that synoptic reconnaissance data contributed to improved JTWC motion forecasts. Data include results for cyclone-motion forecasts aided by 63 synoptic missions in the western North Pacific during the 1983–86 seasons. Position errors were analyzed for approximately 200 motion forecasts for 24-, 48-, and 72-h movements following the synoptic missions as well as for forecast errors for predictions without synoptic reconnaissance data. Inspection of the storm tracks for which synoptic missions were flown indicated that the reconnaissance-assisted forecasts were for forecasting situations which were typically as difficult as those which occurred for the average forecast.

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Elmar R. Reiter, John D. Sheaffer, James E. Bossert, Richard C. Fleming, William E. Clements, J. T. Lee, Sumner Barr, John A. Archuleta, and Donald E. Hoard

During the late summer of 1985 a field experiment was conducted to investigate mountaintop winds over a broad area of the Rocky Mountains extending from south central Wyoming through northern New Mexico. The principal motivation for this experiment was to further investigate an unexpectedly strong and potentially important wind cycle observed at mountaintop in north central Colorado during August 1984. These winds frequently exhibited nocturnal maxima of 20 to 30 m · s−1 from southeasterly directions and often persisted for eight to ten hours. It appears that these winds originate as outflow from intense mesoscale convective systems that form daily over highland areas along the Continental Divide. However, details of the spatial extent and variability of these winds could not be determined from “routine” regional weather data that are mostly collected in valleys. Although synoptic conditions during much of the 1985 experiment period did not favor diurnally recurring convection over the study area, sufficient data were obtained to verify the regional-scale organization of strong convective outflow at mountaintop elevations. In addition, the usefulness and feasibility of a mountain-peak weather-data network for routine synoptic analysis is demonstrated.

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