Extensive studies of large-amplitude Kelvin–Helmholtz billows in the lower atmosphere with VHF middle and upper atmosphere radar

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Abstract

In the present work, characteristics of large-amplitude (>150 m) Kelvin–Helmholtz (KH) billows in the lower atmosphere are analysed, with emphasis on the turbulent properties. First, past studies are reviewed. Almost all the studies reported gradient Richardson number (Ri) close to 0.25. Results of observations in the height range 1.32–20.34 km using 46.5 MHz middle and upper atmosphere radar (MUR; Japan, 34.85°N, 136.10°E) are then described. A total of ∼600 h of data at high vertical and range resolutions was used. Except for very few cases, trains of KH braids are not followed by layer splitting, which can eventually result from turbulent mixing. A particular event shows some sort of turbulent plumes at the interface between a nearly neutral layer and a stable layer in the tropopause region. These turbulent plumes likely arose as the result of entrainment at the stable layer interface produced by still active turbulent mixing. Statistics on the characteristics of the KH billows are also presented. Large-amplitude KH billows in the lower atmosphere were only present less than 0.6% of the time and height. The typical depth, horizontal wavelength and depth-to-wavelength ratio of the KH braids are about 600 m, 2600 m and 0.22, respectively. They are thus relatively shallow and the quasi-absence of layer splitting may suggest that they are more frequently generated when Ri falls just below 0.25 (so that the large-amplitude KH billows are not sufficiently energetic for breaking and mixing). Vertical shears of horizontal wind are about 23 ms−1 km−1 on average. Finally, attempts at estimating (maximum) energy dissipation rate ε and (minimum) vertical eddy diffusivity K are made. Mean values of εmax and Kmin are typically ∼1 mW kg−1 (77% in the range 0.15–3.0 mW kg−1) and ∼5 m2 s−1 (70% in the range 1–10 m2 s−1), respectively. Copyright © 2011 Royal Meteorological Society

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