Abstract. This article deals with the detection of small-scale turbulence from in situ meteorological measurements performed under superpressure balloons (SPBs). These balloons allow long-duration flights (several months) at a prerequisite height level. The data set is gathered from the Strateole-2 probationary campaign during which eights SPBs flew in the tropical tropopause layer at altitudes of around 19 and 20.5 km from November 2019 to March 2020. Turbulence is not directly measured by the instrument set onboard the SPBs. Nonetheless, there is the potential to derive information about the occurrence of turbulence from the temporally well-resolved measurements of pressure, temperature, and position. It constitutes a challenge to extract the aforementioned information from a measurement set that was not designed for quantifying turbulence, and the paper explains the methodology developed to overcome this difficulty. It is observed that SPBs oscillate quasi-periodically around their equilibrium positions. The oscillation periods, which are 220 s on average with a range of 130 to 500 s, are close to but noticeably smaller than the Brunt–Väisälä period (∼300 s). The amplitude of these vertical motions is ∼±15 m, inducing large fluctuations in all quantities, whether measured (e.g., pressure, temperature and position) or inferred (e.g., density and potential temperature). The relationships between the changes in these quantities and the vertical displacements of the balloons are used to infer properties of the flow in which the SPBs drift. In the case of active turbulence, the vertical stratification as well as the wind shear are likely to be reduced by mixing. Hence, the increments of potential temperature, δθ, and of the vertical displacements of the balloon, δzB, are expected to be uncorrelated because ∂θ/∂z→0. Moreover, the local vertical gradients of measured quantities, temperature (T) and horizontal velocities (u and v), are estimated from the covariance of the increments of the considered quantity with δzB. The Richardson number of the flow is deduced. Several binary indexes (true or false) to describe the state of the flow, laminar or turbulent, are evaluated. These turbulence indexes, based either on correlations between δθ and δzB or on estimates of the local Richardson number, are found to be consistent, as they differ in less than 3 % of cases. The flow is observed to be turbulent for about 5 % of the time, with strong inhomogeneities along the longitude.
<p>The role played by turbulent mixing in the vertical transport of constituents in the UTLS is still poorly understood: there is a lack of knowledge of turbulence due to the limited number of observations in this region as well as to the limitations of current observation techniques.</p> <p>The first part of the present work deals with the detection of small-scale turbulence in the tropical upper troposphere - lower stratosphere from in-situ meteorological measurements collected under super-pressure balloons (SPBs). Eight SPBs were launched during the first Strateole-2 campaign, from November 2019 to March 2020 and flying for several weeks (&#8764; 3 months).&#160;</p> <p>Turbulence detection methods relies on the quasi-periodic vertical oscillations (&#8764; &#177;15 m) of the SPBs around their equilibrium positions, such oscillations inducing large fluctuations of measured quantities (pressure, temperature, positions) and inferred quantities (density, potential temperature). A first method of detection is based on correlations between the increments of potential temperature &#948;&#952; and the vertical displacements of the balloons (i.e. of the sensors) &#948;z. Such correlations are expected to be null as &#8706;&#952;/&#8706;z &#8594; 0 in case of turbulent mixing. A second method relies on the Richardson number criterion, Ri < 0.25. Ri is deduced from the vertical gradients of measured quantities (T , u, v), estimated from covariances between the increments of the considered quantities and the vertical displacements &#948;z.&#160;</p> <p>Turbulence indexes (true of false) to describe the different states of the flow encountered by the SPBs during their flights (laminar or turbulent), are evaluated. These different indexes, based on independent measurements and on various methods, correlations or linear regressions, are found to be consistent: they differ for less than 3% of the cases. The flow is observed to be turbulent for about 5% of the time, with strong inhomogeneities along the longitude.</p> <p>The second part of the present work aims to improve our understanding of turbulence, and its impacts, in the tropical UTLS by studying small- to meso-scale processes, i.e. atmospheric waves, deep convection and associated observed turbulence. These are all key processes of the dynamics of the equatorial UTLS. One can evaluate the probability of turbulence occurrences as a function of the distance to deep convection. Such a distance seems to be a good proxy of wave activity generated by deep convection. The occurrence frequency of turbulence is significantly larger when the distance to deep convection is small, i.e. smaller than ~ 200 km.</p> <p>This research should contribute to a better parametrization of these processes in climate models, and to a better estimation of their impact to vertical transport in the tropical UTLS.</p>
Abstract. This article deals with the detection of small-scale turbulence from in-situ meteorological measurements performed under superpressure balloons (SPBs). These balloons allow long duration flights (several months) at a prerequisite height level. The dataset is gathered from the Strateole-2 probationary campaign during which eights SPBs flew in the tropical tropopause layer at around 19 and 20.5 km altitudes, from November 2019 to March 2020. Turbulence is not directly measured by the instrument set onboard the SPBs. Nonetheless, there is a potential to derive information about the occurrence of turbulence from the well-resolved in time measurements of pressure, temperature and position. It constitutes a challenge to extract that information from a measurement set that was not designed for quantifying turbulence, and the manuscript explains the methodology developed to overcome this difficulty. It is observed that SPBs oscillate quasi-periodically around their equilibrium positions. The oscillation periods, 220 s in the average, range from 130 to 500 s, close to, but noticeably smaller than, the Brunt-Väisälä period (∼ 300 s). The amplitude of these vertical motions is ∼ ±15 m, inducing large fluctuations in all quantities, whether measured (pressure, temperature, positions) of inferred (density, potential temperature). The relationships between the changes in these quantities and vertical displacements of the balloons are used to infer properties of the flow in which the SPBs drift. In case of active turbulence, the vertical stratification as well as the wind shear are likely to be reduced by mixing. Hence, the increments of potential temperature, δθ, and of the vertical displacements of the balloon, δzB, are expected to be uncorrelated since ∂θ / ∂z → 0. Also, the local Richardson number is expected to be less than ∼ 0.25. Several binary indexes (true of false) to describe the state of the flow, laminar or turbulent, are evaluated. They are based either on correlations between δθ and δzB, or on estimations of the local Richardson number. Correlation coefficients are computed and compared, by using different measures of δθ and δzB and by estimating either the Pearson or the Spearman coefficients. Turbulence indexes based on a null-correlation are built using a randomisation test to check whether these correlations are significantly non-positive. It is also shown than a linear regression between the increments of a quantity and the increments of vertical displacements allows to estimate the vertical gradient of this quantity. Least square fit and Theil-Sen fit are used to estimate time series of vertical gradients ∂T / ∂z, ∂u / ∂z, ∂v / ∂z. Related quantities such as the Brunt-Väisälä frequency, or the local Richardson number, Ri, are inferred, allowing to establish turbulence indexes from Ri. These different indexes, based on independent measurements and on various methods, correlations or linear regressions, are found to be consistent: they differ for less than 3 % of the cases. The flow is observed to be turbulent for about 5 % of the time, with strong inhomogeneities along the longitude.
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