Abstract
Abstract. Vertical ascent rate VB 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 ε, we show that VB can be strongly affected by turbulence, even above the convective boundary layer. For “weak” turbulence (here ε≲10−4 m2 s−3), the fluctuations of VB 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 VB if the free balloon lift is determined. In contrast, stronger turbulence intensity systematically implies an increase in VB, 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 VB 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 VB fluctuations can be made if localized turbulence effects are ignored.
Highlights
The vertical ascent rates VB of meteorological balloons are mainly the combination of the free lift and fluctuations due to vertical air velocities and variations in atmospheric turbulence drag effects
Despite balloons’ frequent use all over the world, few studies have tried to extract information from VB. Most of these studies have focused on the estimation of the vertical air velocity because this parameter is very important for many meteorological applications (e.g., Wang et al, 2009) and for the characterization of internal gravity waves (e.g., McHugh et al, 2008)
This is primarily because (1) the datasets were originally processed for comparisons with data from unmanned aerial vehicle (UAV), which did not fly above altitudes of a few kilometers; (2) a limited height range makes the description of individual turbulent events less tedious; (3) the increasing horizontal distance between the radar and balloons with height due to strong horizontal winds becomes an important factor of uncertainty when doing comparisons; and (4) the signal-to-noise ratio (SNR) of radar measurements statistically decreases with height in the troposphere and low SNR values produce additional uncertainties
Summary
The vertical ascent rates VB of meteorological balloons are mainly the combination of the free lift and fluctuations due to vertical air velocities and variations in atmospheric turbulence drag effects. Statistics on the occurrence of atmospheric turbulence could be made from balloon ascent rates if the contribution of air motion is accurately taken into account This alternative purpose seems to be more achievable than retrieving W , except at stratospheric heights and during very calm tropospheric conditions, as shown by earlier studies, and likely during deep convective storms during which strong vertical motions are expected. 3, we show comparison results between VB, vertical velocity measured by MU radar, energy dissipation rate, and Richardson number profiles from three case studies selected from ShUREX2017 These comparisons clearly indicate that turbulence effects dominate the balloon ascent rate.
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