On the estimation of vertical air velocity and detection of atmospheric turbulence from the ascent rate of balloon soundings

Hubert Luce (1) , Hiroyuki Hashiguchi

1 MIO - Institut méditerranéen d'océanologie

Vertical ascent rate V_B of meteorological balloons is sometimes used for retrieving vertical air velocity W, an important parameter for meteorological applications, but at the cost of crude hypotheses on atmospheric turbulence and without the possibility of formally validating the models from concurrent measurements. From simultaneous radar and unmanned aerial vehicle (UAV) measurements of turbulent kinetic energy dissipation rates ɛ, we show that V_B can be strongly affected by turbulence, even above the convective boundary layer. For "weak" turbulence (here ɛ≲10^-4 m² s^-3), the fluctuations of V_B were found to be fully consistent with W fluctuations measured by middle and upper atmosphere (MU) radar, indicating that an estimate of W can indeed be retrieved from V_B if the free balloon lift is determined. In contrast, stronger turbulence intensity systematically implies an increase in V_B, not associated with an increase in W according to radar data, very likely due to the decrease in the turbulence drag coefficient of the balloon. From the statistical analysis of data gathered from 376 balloons launched every 3 h at Bengkulu (Indonesia), positive V_B disturbances, mainly observed in the troposphere, were found to be clearly associated with Ri≲0.25, usually indicative of turbulence, confirming the case studies. The analysis also revealed the superimposition of additional positive and negative disturbances for Ri≲0.25 likely due to Kelvin-Helmholtz waves and large-scale billows. From this experimental evidence, we conclude that the ascent rate of meteorological balloons, with the current performance of radiosondes in terms of altitude accuracy, can potentially be used for the detection of turbulence. The presence of turbulence complicates the estimation of W, and misinterpretations of V_B fluctuations can be made if localized turbulence effects are ignored.

Domaines: Planète et Univers [physics]

Hubert Luce, Hiroyuki Hashiguchi. On the estimation of vertical air velocity and detection of atmospheric turbulence from the ascent rate of balloon soundings. Atmospheric Measurement Techniques, 2020, 13, pp.1989-1999. ⟨10.5194/amt-13-1989-2020⟩. ⟨insu-03668100⟩