Abstract
Abstract. New comparisons between the square of the generalized potential refractive index gradient M2, estimated from the very high-frequency (VHF) Middle and Upper Atmosphere (MU) Radar, located at Shigaraki, Japan, and unmanned aerial vehicle (UAV) measurements are presented. These comparisons were performed at unprecedented temporal and range resolutions (1–4 min and ∼ 20 m, respectively) in the altitude range ∼ 1.27–4.5 km from simultaneous and nearly collocated measurements made during the ShUREX (Shigaraki UAV-Radar Experiment) 2015 campaign. Seven consecutive UAV flights made during daytime on 7 June 2015 were used for this purpose. The MU Radar was operated in range imaging mode for improving the range resolution at vertical incidence (typically a few tens of meters). The proportionality of the radar echo power to M2 is reported for the first time at such high time and range resolutions for stratified conditions for which Fresnel scatter or a reflection mechanism is expected. In more complex features obtained for a range of turbulent layers generated by shear instabilities or associated with convective cloud cells, M2 estimated from UAV data does not reproduce observed radar echo power profiles. Proposed interpretations of this discrepancy are presented.
Highlights
A very high-frequency (VHF) stratosphere–troposphere radar is mainly sensitive to clear-air refractive index fluctuations on Fourier scales of a length equal to half the radar wavelength (Bragg scale)
Horizontal dashed red lines show the cloud top related to the convective boundary layer (CBL) given by unmanned aerial vehicle (UAV) relative humidity sensor measurements, and the altitude of the interface between the KH and ST regions is defined by the mean position of the steep negative humidity gradient at the top of the KH layer
A higher-level cloud layer detected by micro-pulse lidar (MPL) from ∼ 14:00 Launch time (LT) modified the three-region structure during UAV9 and UAV10
Summary
A very high-frequency (VHF) stratosphere–troposphere radar is mainly sensitive to clear-air refractive index (humidity and temperature) fluctuations on Fourier scales of a length equal to half the radar wavelength (Bragg scale). Under certain conditions, presuming the existence of an inertial subrange, a VHF radar can be employed for retrieving the refractive index structure constant Cn2 of atmospheric turbulence (e.g., Nästrom and Eaton, 2001) Fresnel backscatter is another mechanism that occurs preferentially when the beam is oriented vertically due to pronounced horizontal coherency of some irregularities (e.g., Gage and Balsley, 1980; Gage et al, 1985). Fresnel and turbulent scatter can alternately dominate in altitude depending on the local stability conditions, making it difficult, in principle, to reveal the proportionality between Pv and M2 Despite these theoretical difficulties, all the aforementioned studies concluded that Pv ∼ M2 without considering the nature of the backscattering mechanism.
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