Showing papers on "Winds aloft published in 2004"

Episodes of Strong Flow down the Western Slope of the Subtropical Andes

Abstract: Nocturnal flows down the narrow Andean valleys within the western slope of the subtropical Andes (central Chile) are episodically enhanced by easterly downslope winds that flow into the Santiago basin over the radiatively cooled air above the surface. Local, regional, and large-scale data have been used here to characterize the mean features of these episodes. About 80% of easterly downslope flow episodes in austral winter are forced by a reversal in the sea level pressure gradient along the coast of south-central Chile, when a midlatitude cold high migrates from southern Chile eastward across the Andes under midtroposphere SW winds associated with a warm ridge aloft. Under these circumstances low-level, easterly (offshore) flow sets in, producing a compensating downslope flow that subsides over central Chile. The remaining cases are associated with prefrontal conditions under a midlatitude trough with NW winds aloft. Since in most of these cases the easterly low-level flow occurs beneath westerly flow higher above, these episodes classify as strong windward downslope flows. Within the Andean valleys and canyons, the near-surface air experiences a sensible warming and drying at night and early morning during these episodes, as the strong downvalley winds tend to destroy the surfacebased radiative inversion and mix down warmer air from aloft. At the exit region of these valleys into the central basin, these downslope flows in austral winter are not able to flush the cold air pool there. Hence, dawn surface temperatures over the basin tend to be lower than average as clear skies and dry subsiding air aloft favor surface radiative cooling. The resulting enhancement of the near-surface static stability hampers the subsequent development of the mixed layer, leading to severe air pollution events in Santiago and other cities in central Chile. A comparative discussion on governing mechanisms with respect to apparently similar phenomena, as gap flow and shallow foehn, is included.

Detecting winds aloft from water vapour satellite imagery in the vicinity of storms

Abstract: 2 Introduction The most extensive use of water vapour imagery in recent years has been to identify upper-level wind features such as shortwave troughs and to compare their location and intensity to those produced by numerical forecast models. For example, Weldon and Holmes (1991) compiled a catalogue of water vapour imagery for use in identifying a wide range of upper air features. The identification of these features arises from spatial patterns of brightness in the water vapour images (related to the variation of radiance with moisture content in the upper troposphere). Subjective adjustments to forecasts can be made from observed differences in the location and intensity of troughs, jets, and other features in satellite imagery to those in model analyses and forecasts. In addition to the subjective use of water vapour imagery in forecasting, the imagery has been used quantitatively to provide estimates of the wind through movement of clouds and moisture features between successive images, where direct measures from weather balloons are lacking. Winds were determined by manually tracking clouds between successive images and calculating the wind from the displacement and the time interval between images, in the early years of this type of research (Stewart et al., 1985). The height of the wind estimate was derived from the cloud top temperature and a vertical profile of temperature. This presupposes that the cloud moves with the speed of the wind, an assumption that is not strictly valid. Comparisons with independent balloon measurements suggest an underestimation of wind speed and the need for an empirical adjustment. In recent years, the technique has been automated and is based on cross correlation of the patterns within boxes which are displaced according to the winds from a forecast model 'guess' field (Velden et al., 1997). With this approach, winds cannot only be estimated from tracking clouds, but also from the displacement of small-scale structures present in clear areas of the imagery. These structures occur due to 3 humidity fluctuations in the middle and upper layers of the troposphere. While winds obtained from tracking cloud features in the imagery are prone to greater uncertainty than balloon measurements, they have been used to improve analyses and et al., 2002). In order to explore the use of satellite winds over land, an automated method for calculating water vapour winds, developed at the Cooperative Institute for Meteorological Satellite Studies, CIMSS (Velden et al., 1997), is being applied …

Tropospheric Wind Monitoring During Day-of-Launch Operations for National Aeronautics and Space Administration's Space Shuttle Program

Abstract: The Environments Group at the National Aeronautics and Space Administration's Marshall Space Flight Center (NASA/MSFC) monitors the winds aloft at Kennedy Space Center (KSC) during the countdown for all Space Shuttle launches. Assessment of tropospheric winds is used to support the ascent phase of launch. Three systems at KSC are used to generate independent tropospheric wind profiles prior to launch; 1) high resolution Jimsphere balloon system, 2) 50-MHz Doppler Radar Wind Profiler (DRWP) and 3) low resolution radiosonde system. Data generated by the systems are used to assess spatial and temporal wind variability during launch countdown to ensure wind change observed does not violate wind change criteria constraints.

Tropospheric Wind Monitoring During Day-of-Launch Operations for NASA's Space Shuttle Program

Abstract: The Environments Group at the National Aeronautics and Space Administration's Marshall Space Flight Center monitors the winds aloft above Kennedy Space Center (KSC) in support of the Space Shuttle Program day-of-launch operations. Assessment of tropospheric winds is used to support the ascent phase of launch. Three systems at KSC are used to generate independent tropospheric wind profiles prior to launch; 1) high resolution jimsphere balloon system, 2) 50-MHz Doppler Radar Wind Profiler (DRWP) and 3) low resolution radiosonde system. All independent sources are compared against each other for accuracy. To assess spatial and temporal wind variability during launch countdown each jimsphere profile is compared against a design wind database to ensure wind change does not violate wind change criteria.