Cross-valley flow and its impact on the boundary-layer structure investigated by multiple Doppler lidar measurements
Final Report Abstract
The CROSSINN field campaign was designed (i) to monitor the vertical structure of the MoBL with high temporal resolution by continuously measuring wind, temperature, and humidity profiles in the valley center and (ii) to capture the spatial variability in the MoBL under different large-scale conditions. For the latter, the two-dimensional kinematic flow in a vertical plane across the valley was retrieved from continuous coplanar Doppler lidar scans, and in situ aircraft measurements of temperature, humidity, and wind were performed on stacked legs across and along the valley. Thermally driven flows and associated cross-valley flow patterns regularly developed in the Inn Valley during all IOPs. However, their strength, depth, and onset time varied considerably just like the large-scale wind speed and direction, cloud coverage and atmospheric stratification. The analysis clearly demonstrates that the combination of different sources of information allows to gain detailed insight into the spatial variability of the MoBL and to link the vertical MoBL structure to the three-dimensional flow. Some unexpected features were identified in the data, such as a single cross-valley circulation with upward motion in the outer part of the slightly curved valley. Denoted as a cross-valley vortex (CVV), the circulation was observed to be tightly linked to the onset of the upvalley flow. CVVs were found to occur primarily during the afternoon, on 23 upvalley flow days during the 74-day campaign period. During these occurrences, the strength and shape of the vertical upvalley flow profile was found to be the major determinant for CVV prevalence. Quantification of vorticity-based parameters using coplanar retrievals, revealed a positive correlation between vorticity and upvalley flow strength, and an overturning period on the order of 30 to 80 min. Lastly, ongoing data analysis is now focusing on utilizing Doppler spectrum width to obtain spatially rich information on MoBL depth, to this date with highest temporal and spatial resolution. An interesting aspect of the former analysis is the tentative impact of the CVV on the MoBL structure downwind of the CROSSINN area of investigation, considering that no CBL is present for the majority of the CVV occurrence given the negative surface sensible heat flux at this time. The instrument setup and measurement strategy of the CROSSINN field campaign will facilitate the planning and conduction of future campaigns, such as the upcoming large international program and experiment on multiscale transport and exchange processes over mountains (TE- AMx; Serafin et al., 2020), a major experimental part of which will partly take place in the Inn Valley. When planning to use Doppler lidars to observe the flow field in mountainous terrain, for example, the backscatter capability of the atmosphere has to be considered. It may be limited above mountain ridges or in regions with subsidence, e.g., in the valley center, due to the impact of dry, clean air from the free troposphere. The retrieval of the cross-valley flow in a vertical plane from coplanar Doppler lidar scans performed during CROSSINN is very promising and provides profound insights into the flow structure.
Publications
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The CROSSINN project: Motivation and preliminary results. ICAM 2019, Riva del Garda, Italy, 2-6 September 2019. Poster presentation.
Babi´ N., Adler B., Kalthoff N., Gohm A., Lehner M., Rotach M.W. & Haid M.
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Boundary layer flows in a deep Alpine Valley investigated with Doppler lidar measurements. Invited seminar talk at the National Renewable Energy Laboratory (NREL) at Boulder CO, USA. 2020.
Adler B., Gohm A., Kalthoff N., Babi´ N., Lehner M., Rotach M.W. & Haid M.
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Boundary layer flows in a deep Alpine valley investigated with in situ and remote sensing measurements. Talk at NOAA’s Physical Sciences Laboratory Seminars in Boulder CO, USA. 2020.
Adler B., Gohm A., Kalthoff N., Babi´ N., Lehner M., Rotach M.W. & Haid M.
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First findings from the CROSSINN campaign on the structure and variability of cross-valley circulations in the Inn Valley, Austria. Challenges in Meteorology 7, Zagreb, Croatia, 4-5 November 2020. Oral presentation.
Babi´ N.
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Coupling multiple Doppler LiDARs for studying wind dynamics in complex terrain. ICAM 2021 Online Event, 8-10 June 2021. Oral presentation.
Babi´ N.
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CROSSINN: A Field Experiment to Study the Three-Dimensional Flow Structure in the Inn Valley, Austria. Bulletin of the American Meteorological Society, 102(1), E38-E60.
Adler, Bianca; Gohm, Alexander; Kalthoff, Norbert; Babić, Nevio; Corsmeier, Ulrich; Lehner, Manuela; Rotach, Mathias W.; Haid, Maren; Markmann, Piet; Gast, Eckhard; Tsaknakis, George & Georgoussis, George
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Cross‐valley vortices in the Inn valley, Austria: Structure, evolution and governing force imbalances. Quarterly Journal of the Royal Meteorological Society, 147(740), 3835-3861.
Babić, Nevio; Adler, Bianca; Gohm, Alexander; Kalthoff, Norbert; Haid, Maren; Lehner, Manuela; Ladstätter, Paul & Rotach, Mathias W.
