First observations of elevated ducts associated with intermittent turbulence in the stable boundary layer over Bosten Lake, China

Abstract

Nocturnal radiative cooling is a main driver for atmospheric duct formation. Within this atmospheric process, the impacts of intermittent turbulence on ducting have seldom been studied. In this paper, we reported two confusing ducting events observed in the early morning in August 2014 over Bosten Lake, China, when a stable boundary layer (SBL) still survived, by using tethered high‐resolution GPS radiosondes. Elevated ducts with strong humidity inversions were observed during the balloon ascents but were absent during observations made upon the balloon descents several minutes later. This phenomenon was initially hypothesized to be attributable to turbulence motions in the SBL, and the connection between the turbulence event and the radar duct was examined by the statistical Thorpe method. Turbulence patches were detected from the ascent profiles but not from the descent profiles. The possible reasons for the duct formation and elimination were discussed in detail. The turbulent transport of moisture in the SBL and the advection due to airflows coming from the lake are the most probable reasons for duct formation. In one case, the downward transport of moisture by turbulence mixing within a Kelvin‐Helmholtz billow at the top of the low‐level jet resulted in duct elimination. In another case, the passage of density currents originating from the lake may have caused the elimination of the duct. Few studies have attempted to associate intermittent turbulence with radar ducts; thus, this work represents a pioneering study into the connection between turbulent events and atmospheric ducts in a SBL.

Characteristics of Atmospheric Turbulence Retrieved From High Vertical‐Resolution Radiosonde Data in the United States

“…The mean bulk-tnr of the current study is 0.83 and 5.60 in the troposphere and stratosphere, respectively. Because the bulk-tnr value of 0.83 in the troposphere is larger than 0.80, which is the minimum bulk-tnr in previous studies (Kantha & Hocking, 2011;Muhsin et al, 2016;Sun et al, 2016;Sunilkumar et al, 2015), an additional noise reduction process is not performed. The minimum detectable size of the turbulent layer can be obtained from the bulk-tnr in the troposphere and stratosphere of the individual profiles (Wilson et al, 2010, Figure 5), and only turbulent layers with sizes larger than this minimum detectable size are selected as the true turbulent layers.…”

“…Given that the Thorpe analysis cannot distinguish between convectively induced turbulence (CIT) and shear‐induced turbulence (SIT; Thorpe, ), the turbulence can be overestimated near the surface during the daytime. With this in mind, for example, Sun et al () calculated the turbulence in the boundary layer only for nighttime. In order to use all profiles launched both in daytime and nighttime, in the present study, we show L T and ε exclusively in the free atmosphere, from 3 km above the station height to the top of the radiosonde profile.…”

“…They showed a clear correlation between the observed turbulence and the diurnal variation in the mesoscale convective systems and pointed to mesoscale convective systems as a major source of turbulence in the upper troposphere. Using radiosonde data from July to August 2014 at the Bosten Lake (41.89°N, 87.22°E), Sun et al () reported two radar duct events, which were detected during the radiosonde ascent but disappeared during radiosonde descent. They concluded that turbulent mixing caused by Kelvin‐Helmholtz instability (KHI) in the upper part of the low‐level jet and by density flow advection are the main causes of each radar duct dissipation event.…”

Estimation of Surface Duct Using Ground-Based GPS Phase Delay and Propagation Loss

“…The electromagnetic waves are trapped within the ducting layer with weak propagation loss and enhanced propagation range, usually known as atmospheric ducting [1,2]. Turton et al [3] presented a classification of different radio-propagation conditions and corresponding atmospheric ducting effects.…”

“…The unit of dM/dr is M-units/km. Equations (2) and (3) are applied for all radio frequencies with little error. Referring to Almond and Clarke [39], dN/dr-which is the gradient of refractivity N with respect to altitude r-is used to classify the profiles and its unit is N-units/km.…”