Tilted refractive‐index layers possibly caused by Kelvin–Helmholtz instability and their effects on the mean vertical wind observed with multiple‐receiver and multiple‐frequency imaging techniques

Abstract

Clear‐air VHF/UHF radars worldwide have observed that long‐term mean vertical winds are downward in the middle troposphere and usually slightly upward above the jet‐stream wind maximum. Kelvin–Helmholtz instability (KHI), which can tilt quasi‐specular layers in opposite orientations above and below the jet‐stream wind maximum, has been postulated to be one important contributing factor to the radar‐measured mean vertical wind (r). This factor is examined here using simultaneous radar interferometric observations of echo centers and layer structures. The altitude of layer structure and the incident angle of echo center were estimated, respectively, with multiple‐frequency and multiple‐receiver techniques. Radar data were collected with the Japanese MU radar, between 3 km and 22.2 km and over 33 h. The observations of r showed downward tendency in the middle troposphere, with a maximum of ∼10 cm/s at the height of ∼8 km. However the reversal height of r was at ∼15 km, which is higher than the jet‐stream wind maximum observed (∼12 km). Positive correlations between the vertical velocities (wr) and the incident angles of echo centers were found in the region of downward r, and moreover, the mean vertical velocities derived from the incident angles of echo centers below ∼10 km were close to r. Statistical distributions of layer slopes, incident angles of echo centers, and echo power imbalance between two symmetrically oblique radar beams provide evidence of asymmetrically tilted layer structures in the region of downward r, suggesting that wind‐shear tilted/KHI layers contributed a significant part of r in the middle troposphere.