Potential sources of atmospheric turbulence estimated using the Thorpe method and operational radiosonde data in the United States

Abstract

This study estimated the eddy dissipation rate (ε) and the thickness of the turbulence layer (THTL) in the free atmosphere (z = 3–30 km) based on the Thorpe method using high vertical-resolution radiosonde data for 6 years (January 2012–December 2017) at 68 operational stations in the United States and investigated potential sources of turbulence. The results showed that turbulence occurs more frequently in the troposphere than in the stratosphere, with log10ε ranging from −4 to 0 (from −4 to −0.5) m2 s−3 in the troposphere (stratosphere). The mean and median of THTL in the troposphere are 278 m and 205 m, respectively, which are larger than those in the stratosphere of 140 m and 115 m. However, the layer-mean log10ε over 3-km altitude bins is found to be larger in the stratosphere due to there being less turbulence cases than in the troposphere. To better represent the layer-mean turbulence, a new quantity is suggested, named the layer-mean effective ε (EE), by combining ε and THTL. The potential sources of the observed turbulence are examined by analyzing four turbulence indices: squared Brunt-Vaisala frequency (N2), vertical wind shear (VWS), orographic gravity-wave drag (OGWD), and convective precipitation, calculated using the ERA5 reanalysis, which are categorized into cases when there exists at least one turbulence event at the nearby grid and cases without turbulence nearby. Small N2 occurred frequently in the near-turbulence cases at all altitudes, while strong VWS and OGWD occurred more in near-turbulence cases at z = 15–21 km, and strong convective precipitation occurred more in near-turbulence cases at z = 9–15 km. The results demonstrate that the generation of Thorpe-estimated turbulence is favored in conditions of weak static stability at all altitudes, in conditions of strong VWS and OGWD at z = 15–21 km, and in conditions of strong convective precipitation at z = 9–15 km. In addition, the EE and N2 (convective precipitation) are negatively (positively) correlated at all altitudes and in most regions. Contrary, VWS and OGWD are correlated with EE under specific conditions and in specific locations: VWS is positively correlated under the strong static stability condition, and OGWD is positively correlated in western mountainous regions at z = 15–21 km.