Derivation of turbulent energy dissipation rate with the Middle Atmosphere Alomar Radar System (MAARSY) and radiosondes at Andøya, Norway

Abstract

We present the derivation of turbulent energy dissipation rate ϵ from a total of 522 days of observations with the Middle Atmosphere Alomar Radar SYstem (MAARSY) mesosphere-stratosphere-troposphere (MST) radar running tropospheric experiments during the period of 2010-2013 as well as with balloon-borne radiosondes based on a campaign in the summer 2013. Spectral widths are converted to ϵ after the removal of the broadening effects due to the finite beam width of the radar. With the simultaneous in situ measurements of ϵ with balloon-borne radiosondes at the MAARSY radar site, we compare the ϵ values derived from both techniques and reach an encouraging agreement between them. Using all the radar data available, we present a preliminary climatology of atmospheric turbulence in the UTLS (upper troposphere and lower stratosphere) region above the MAARSY site showing a variability of more than 5 orders of magnitude inherent in turbulent energy dissipation rates. The derived ϵ values reveal a log-normal distribution with a negative skewness, and the ϵ profiles show an increase with height which is also the case for each individual month. Atmospheric turbulence based on our radar measurements reveals a seasonal variation but no clear diurnal variation in the UTLS region. Comparison of ϵ with the gradient Richardson number Ri shows that only 1.7% of all the data with turbulence occur under the condition of Ri < 1 and that the values of ϵ under the condition of Ri ϵ 1 are significantly larger than those under Ri > 1. Further, there is a roughly negative correlation between ϵ and Ri that is independent of the scale dependence of Ri. Turbulence under active dynamical conditions (velocity of horizontal wind U > 10ms-1) is significantly stronger than under quiet conditions (U < 10ms-1). Last but not least, the derived ϵ values are compared with the corresponding vertical shears of background wind velocity showing a linear relation with a corresponding correlation coefficient r = 58% well above the 99.9% significance level. This implies that wind shears play an important role in the turbulence generation in the troposphere and lower stratosphere (through the Kelvin-Helmholtz instability).