Circulation structures in the atmospheric boundary layer: helicity estimations and spectral characteristics at various scales

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• Natalia Vazaeva (A.M. Obukhov Institute of Atmospheric Physics of Russian Academy of Sciences)
• Tuesday 16 January 2024, 14:15-14:45
• Seminar Room 1, Newton Institute.

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Vazaeva N.V.1,2, Chkhetiani O.G.1, Kurgansky M.V. 1, Kramar V.F.1  1 A.M. Obukhov Institute of Atmospheric Physics, RAS , Moscow, Russia 2 Bauman Moscow State Technical University, Moscow, Russia   key-words: Atmospheric Boundary Layer, Helicity, Spectrum, Turbulence, Circulation Structures, Acoustic Sounding, High-Resolution Minisodar            Abstract: Circulation structures with various scales such as mesoscale roll circulations, polar lows (PLs), wind gusts, squalls, thermals, streaks permanently exist in the atmospheric boundary layer (ABL). Investigation of the integral characteristics of these structures has been carried out using the analysis of the global helicity fields in the ABL according to re-analysis data, WRF -ARW model results (version 4.2) and experimental values. Two-scale model of ABL has been applied in the calculation of nonlinear modes of the coherent structures development. Reasonably good correspondence between numerical simulation findings obtained by this model, open nonhydrostatic mesoscale model WRF -ARW, eddy-resolving model WRF -LES and observable vortices with centers lying about 1200-1300 meters high has been received. The development of roll circulation with considerable asymmetry in the positive and negative components of the helicity field has been recorded. The layer-averaged helicity in the ABL from the WRF -ARW model results is close to the two-scale model findings (0.01-0.16 m/s2) and theoretical and empirical estimates of helicity and is equal to about 0.02-0.08 m/s2. There is a good correlation between the integral helicity and the square of the wind velocity at the higher sounding levels (400-800 m, measurements by sodars) in slightly unstable or neutral stratification conditions. The layer average helicity in the ABL is close to theoretical and empirical estimates of turbulent helicity and is equal to about 0.02-0.12 m/s2. The local variations of helicity are neighbor with the front part of a PL. Here we observed typical case studies of PLs in 2013, 2015, 2021. We use different scales of circulations in ABL to estimate the helicity density, and to compare the estimation results with the helicity of large formations of the PLs. At a time of PLs activity, the local maxima of the geopotential field align with the local minima of the integral helicity field. During the PLs occlusion, the helicity decreases in the period of the PLs generation the integral helicity increases. The mean value for the large-scale motions helicity density were obtained (0.3-0.6 m/s2), an order of magnitude greater than its independently measured turbulent value. The square-averaged helicity density of PLs in the ABL according to the analysis of ECMWF data is equal to about 0.04-0.14 m/s2, and according to results of WRF -ARW modeling: 0.1-0.45 m/s2 After modelling of wind gusts in Moscow, squalls in Obninsk, by WRF -ARW model, the banded structures have also been received. Its arrangement and helicity field are comparable to the roll’s one. A time delay between modeling results and in situ data has been detected. The layer-averaged helicity density of streaks in the ABL according to experimental results is equal to about 0.5-4.0 m/s2. Streaks – the submesoscale vortex structures with the temporal scale from tens of seconds to minutes and with the spatial scale from tens to hundreds of meters – permanently exist in the ABL in the form of longitudinal bands, elongated in the direction of the average wind velocity. In the opinion of the observers, it is with these structures that the observed energy spectra are associated with the exponent -1. We have analyzed data from the field campaigns at the Tsimlyansk scientific station (the South of Russia) in the summer period. The experimental studies of the turbulence characteristics, spatio-temporal scales and helicity of streaks and the visualization of the fine structure of the velocity field were carried out by the acoustic sounding using a high-resolution Doppler three-component minisodar (HRMS). The presence of helicity in the ABL can be one of the sources of vorticity intensification in such structures. The results show that at low frequencies the spectrum has a slope close to -1, and with increasing frequency the slope become close to -5/3. Thus, we can consider helicity as the indicator of structure formed in the ABL , and the velocity spectra show us the areas of such a formation. Results may be used to make the description of ABL more accurate in the atmospheric models and contribute to the understanding of the processes of generation of circulation structures in geostrophic flows. This work was supported by Russian Science Foundation (project No. 22-77-00098).

This talk is part of the Isaac Newton Institute Seminar Series series.

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