Abstract
Diurnal valley winds frequently form over complex topography, particularly under fair weather conditions, and have a significant impact on the local weather and climate. Since diurnal valley winds result from complex and multi-scale interactions, their representation in numerical weather prediction models is challenging. Better understanding of these local winds based on observations is crucial to improve the accuracy of the forecasts. This study investigates the diurnal evolution of the three-dimensional mean wind structure in a deep Alpine valley, the Rhone valley at Sion, using data from a radar wind profiler and a surface weather station operated continuously from 1 September 2016 to 17 July 2017. In particular, the wind profiler data was analyzed for a subset of days on which fair weather conditions allowed for the full development of thermally driven winds. A pronounced diurnal cycle of the wind speed, as well as a reversal of the wind direction twice per day is documented for altitudes up to about 2 km above ground level (AGL) in the warm season and less than 1 km AGL in winter. The diurnal pattern undergoes significant changes during the course of the year. Particularly during the warm-weather months of May through to September, a low-level wind maximum occurs, where mean maximum up-valley velocities of 8–10 m s−1 are found between 15–16 UTC at altitudes around 200 m AGL. In addition, during nighttime, a down-valley jet with maximum wind speeds of 4–8 m s−1 around 1 km AGL is found. A case study of a three-day period in September 2016 illustrates the occurrence of an elevated layer of cross-valley flow around 1–1.5 km AGL.
Highlights
Diurnal valley winds are local thermally driven wind systems, which typically develop under fair weather conditions and are characterized by a pronounced daily periodicity with there being a reversal of the wind direction twice a day (e.g., [1,2,3,4])
We refer to these days with little or no cloud cover, a large diurnal range of global radiation and which are typically characterized by a periodic pattern of the diurnal evolution of the wind field ([4]), as valley wind days (VWDs)
Since relatively similar results were found for the other seasons, in Figure 5, only the frequency distribution of the wind direction and wind speed at the 224 m above ground level (AGL) level of the low-mode wind profiler data is compared with weather-station data for winter, spring (b and e), and summer (c and f)
Summary
Diurnal valley winds are local thermally driven wind systems, which typically develop under fair weather conditions and are characterized by a pronounced daily periodicity with there being a reversal of the wind direction twice a day (e.g., [1,2,3,4]) They are a frequent phenomenon over mountainous terrain and play an important role in the atmospheric exchange of mass, momentum, and heat. In different valleys, the strength, depth, and timing of the valley flows are highly dependent on climatic and local factors (e.g., [20]), it is important to sample a wide variety of valleys to further our knowledge of the diurnal valley wind system in complex terrain Having such a climatology available allows, for example, the quantitative evaluation of the representation of the local thermally driven winds in numerical weather prediction and climate models.
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