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Bulk Turbulence Characteristics of M161 Flight 12

Appendix 3.B Supplementary Figures

4.1.3 Bulk Turbulence Characteristics of M161 Flight 12

stationary if all statistics are independent of a shift in time. Here, a less strict criterion has to be applied because the mean velocity of the flow is not accurately known due to time-dependent platform motions as a result of ship motions. Therefore, I regard u1(t) asstatistically stationary if changes of⟨u′21(t)⟩τ under time shifts of the averaging window τ are smaller than the random error of the variance estimate q2⟨u′212T11 [56]. In the following, I will refer to ⟨u′2165T11 as the reference for estimating errors because τ = 65T11 is significantly shorter than τ = 78T11 and still agrees with the prediction, Eq. (1.61), as shown in Fig. 4.2A. In doing so, the random error ⟨u′21τ is 18% for τ = 1800 s and 32% for τ = 600 s. The systematic errors are 4% and 15%, respectively, in comparison to ⟨u′2165T11.

Figure 4.3 shows ⟨u′21(t)⟩τ as a function of time indicating that ⟨u′21(t)⟩τ fluctuates significantly. This might hint at the fact that the stationarity assumption is not fulfilled globally. As an example, ⟨u′21(t)⟩τ drops by 60% on February 16 2020 at 04:30 UTC (dotted line in Fig. 4.3), which is larger than the random error of the variance estimate q2⟨u′212T11 for both τ = 600 s and τ = 1800 s. However, there are local time intervals (e.g. February 15 2020 at 23:45 UTC to February 16 2020 at 01:25 UTC, the gray shaded region in Fig. 4.3) over whichu1(t) appears to be approximately statistically stationary. In that time interval,⟨u′21(t)⟩1800 sis constant within 12%, which is smaller than the random error of ≈ 18%. Furthermore, ⟨u′21(t)⟩τ fluctuates about 17% for τ = 600 s, which is lower than the random error, too. Remarkably, ⟨u′21(t)⟩600 s tends to be lower than ⟨u′21(t)⟩1800 s, which is due to the underestimated variance as illustrated in Fig. 4.2. To summarize, the choice of an averaging window τ = 1800 s compromises between statistical convergence and still being temporally shorter than characteristic time scales of non-stationarities of ∼1 h. This is further consistent with Risius et al. [54] and Stull [40] as mentioned in Sec. 1.2.6.

Feb 15 20:19 Feb 16 04:09 Feb 16 11:59 UTC time

0.0 0.5 1.0

h u

02 122

( t ) i [m s ]


τ=1800 s τ=600 s

Figure 4.3 The variance of longitudinal velocity fluctuations⟨u′21(t)⟩τ based on an averaging windowτ as a function of UTC time. Two averaging windows τ = 1800 s and τ = 600 s are chosen to visualize the effect of time shifting the averaging window. If the change in⟨u′21(t)⟩τ

is smaller than the random errorq2⟨u′212T11 for a specific time interval, the flow can be regarded as stationary for this time window (e.g. grey region). The dotted line indicates 04:30 UTC.

in Chapter 3 that the most reliable estimate of the energy dissipation rate is ϵI2 for airborne measurements in the atmosphere as it is least affected by misalignment as well as finite turbulence intensityI and as it converges reasonably fast. Estimating the mean energy dissipation rate by ϵI2 requires mapping the longitudinal velocity time-record onto one-dimensional spatial lags, which invokes Taylor’s hypothesis (Sec. 3.2.1). As the mean turbulence intensity for M161 Flight 12 is I = 9% (Imax= 12%), Taylor’s hypothesis can be safely applied. Hence, the energy dissipation rate can be obtained following the procedure as illustrated in Fig. 3.1. The resulting estimates for ϵI2 and turbulence quantities relying on ϵI2 are presented by histograms in bulk ignoring the diurnal cycle or other changes in synoptic conditions.

ϵI2 is obtained by fitting the second-order longitudinal structure function DLL(r) in the inertial range. Fig. 4.4A shows the second-order longitudinal structure functions DτLL(r) estimated for each averaging window τ (blue lines). Under the assumption of stationarity,⟨DτLL(r)⟩N is obtained by ensemble-averagingDτLL(r) (red line). ⟨DLLτ (r)⟩N

reveals a pronounced inertial range with a r2/3+1/36-scaling as predicted by K62 (gray dashed line). This is supported by the local scaling exponent ζ2(r) being defined by [188]:

ζ2(r) = d logDLL(r)

d logr . (4.1)

The fit region r ∈ [2 m,20 m] is visually identified based on Fig. 4.4A and shaded

in gray. The best fitting range for ⟨DτLL(r)⟩N would be r ∈[2 m,6 m] although ζ2(r) scatters strongly for individual DτLL(r) in that range. The longer fit range is chosen to reduce the influence of that scatter on ϵI2. However, as ζ2(r) = 0.71 on average (standard deviation of 0.09) for 2 m ≤r ≤ 20 m, it is acceptable to use ϵI2 invoking r2/3-scaling according to K41. The resulting estimates ofϵI2 obtained from eachDτLL(r) are cumulatively shown by the histogram of the energy dissipation rate estimatesϵI2

(Fig. 4.4B). The most frequent mean energy dissipation rate during M161 Flight 12 is ϵI2 ≈8 mW/kg. This value is in accordance with energy dissipation rate estimates in mountainous terrain where a range between 10−4−10−2W/kg is observed [54]. More precisely, Fig. 4.4B shows a double peak. In the first half of M161 Flight 12, the time-resolved ϵI2 (τ = 1800 s) strongly fluctuates by a factor of 2 while ϵI2 is more uniform in the second half of M161 Flight 12. The strong fluctuations in the first half might be due to the advection of more turbulent patches. However, the presence of the double peak lacks detailed understanding. For completeness, the averaging window is chosen in time to be τ = 1800 s, which is converted into the spatial averaging window R= ⟨u1(t)⟩ττ as shown in Fig. 4.4D. The most frequent spatial averaging windowR is 17 km.

The estimate of the mean energy dissipation rateϵI2 serves as a basis for estimating the Kolmogorov length scale ηK, Eq. (1.12), the longitudinal Taylor micro-scale λf, Eq. (1.34), the integral length scale L11, Eq. (1.29), and the Taylor-scale Reynolds number Rλ, Eq. (1.38). The corresponding histograms are shown in Fig. 4.5A-D. Start- ing with ηK, the most frequent Kolmogorov length scale isηK = 0.75 mm. Moreover, λf is most frequently 0.16 m whereas the most likely integral length scale L11 is 21 m.

Using the relation for the transverse Taylor micro-scale λg = √

10ηK2/3L1/3 [5], one can estimate the energy injection scale from ηK andλg: L= λg



≈81 m with ηK = 0.75 mm and λg = 0.11 m. In homogeneous isotropic turbulence with L11=L/2 at high Rλ, the integral scale is≈40 m which is close to the average integral scale of

L11N ≈38 m but twice as large as the most frequent L11 in Fig. 4.5C. As the most likely Rλ is ≈ 4300 and ⟨RλN ≈ 5100 (4.5D), the assumption on high Rλ is valid.

Despite this discrepancy and taking into account that the integral scale isestimated by a scaling argument, the estimated length scale across the entire range of turbulent scales are consistent.

To evaluate the accuracy of the Taylor-scale Reynolds number and the turbulence length scales, the accuracy of ϵI2 is critical. As mentioned before, the accuracy ofϵI2

can be captured by the systematic and random error based on the averaging window R and integral length scale L11. The random error is estimated by Eq. (3.45). The relative systematic error δsysI2 (R) can be derived similarly to δI2(R) (Sec. 3.3.4):

δI2sys(R) = 1−2L11



−1, (4.2)

where the histograms of δI2sys(R) andδI2(R) are shown in Fig. 4.6. The systematic error δsysI2 (R) is <2% and can be neglected in the following. The most frequent random error

Lorem ipsumLorem ipsumLorem ipsum

10−1 100 101 102


0 1 2 ζ2(r)

DτLL(r) DτLL(r)N

K62 K41

10−1 100 101 102

r [m]

10−3 10−1 101

DLL(r)[m2 s2 ]

DLLτ (r) DτLL(r)N


0.00 0.01 0.02 0.03

I2 [W kg−1] 0

5 10 15 20


10.0 12.5 15.0 17.5 20.0


0 5 10 15 20





Figure 4.4Energy dissipation rate estimateϵI2 for M161 Flight 12 derived from second-order longitudinal structure functions. A: The second-order longitudinal structure function DτLL(r) is estimated for each averaging windowsτ (blue lines). Under the assumption of stationarity, the ensemble average ⟨DτLL(r)⟩N is shown by the red line. The expected K62 scaling in the inertial range is indicated by the gray dashed line. The fit region is visually identified and shaded in gray. B: Histogram of the energy dissipation rate estimateϵI2 obtained from each DτLL(r). The fit range corresponds to the gray-shaded region in (A) or (C). C: Local scaling exponentζ2(r) ofDτLL(r) (blue lines) according to Eq. (4.1) and ⟨DτLL(r)⟩N (red line). The expected scaling exponents in the inertial range are drawn for K41 (gray dashed line) and K62 (gray dotted line). The fit region is gray-shaded. D: The averaging window is chosen in time τ = 1800 s converting to the spatial averaging window R=⟨u1(t)⟩ττ, which is shown in the histogram.

0.0 0.5 1.0 1.5 2.0 ηK [mm]

0 10 20 30


0.0 0.1 0.2 0.3

λf [m]

0 5 10 15 20


0 25 50 75 100


0 5 10 15 20





2000 4000 6000 8000 10000 Rλ

0 5 10 15 20



Figure 4.5 Bulk turbulence characteristics of M161 Flight 12 for averaging window of τ = 1800 s. Histograms are shown for the Kolmogorov length scaleηK (A), the longitudinal Taylor micro-scale λf (B), the integral length scale L11 (C) and the Taylor micro-scale Reynolds number Rλ (D). Histograms in B, C and D are limited up to 20 counts (#) for better visibility.

0.000 0.005 0.010 0.015 0.020

δsysI2(R) 0

10 20 30


0.00 0.05 0.10 0.15 0.20

δI2(R) 0

5 10 15 20



Figure 4.6 Histograms of the systematic error δI2sys(R) (A) and the random error δI2(R) (B) of the energy dissipation rate estimate ϵI2 for M161 Flight 12. R =U τ is the spatial averaging window withτ = 1800 s. Both errors are estimated fromL11 and ⟨u′21R based on the sameR and, hence, have to be regarded rather as statistical estimates.

is δI2(R) = 8% based on the estimated R and L11.

While the plausibility of the length scale has been demonstrated above, the signifi- cance of the Rλ-estimate is assessed in the following. Estimating Rλ by Rλuλg and choosingu =qu′2165T11 = 0.81 m/s,ν = 1.552×10−5m2/s andλg = 0.12 m, one obtains Rλ ≈6300. This is 15% higher than⟨RλN ≈5100 (Fig. 4.5A) which has been derived from ϵI2 andL11. The expected errore(Rλ) due to the random error in ϵI2 can be calculated by Gaussian error propagation and by the chain rule to

e(Rλ) =






6ϵI2 − 2 3





= 1





(4.3) with


L2/3 =−2 3

1 ϵI2

L2/3, (4.4)

Equation (4.3) yields e(Rλ = 5100)≈200 and e(Rλ = 6300) ≈250 assuming that the random error is 10% (compare the most frequent value in Fig. 4.6B). Using the maximum value of δI2(R) observed during M161 Flight 12 ( δI2(R)≈15%),e(Rλ = 5100)≈380 and e(Rλ = 6300) ≈ 470. These values suggest that the uncertainty in Rλ cannot be fully captured by the random error of ϵI2. Presumably, it is further affected by the uncertainty in L11 of about 50% as illustrated by the deviation between the most frequent and mean value of L11. Notably,δI2(R) is proportional toL3/211 . Thus, δI2(R) potentially changes by a factor significantly larger than 1. However, Rλ is less amplified by ν in Eq. (1.38) which scales only as ν−1/2 in contrast to Rλuλg. A similar range in Rλ has been observed in the surface layer in mountainous terrain with a most likely Rλ ≈3000 and maximal Rλ ∼104 [54].

At last, these measurements are compared to VDTT experiments [188]. The first point to note is that the PSS8 cannot resolve the dissipative scales in flows with ηK ∼1 mm. In addition, due to recovering the PSS8 velocity time measurement and the filtering in the frequency domain at 12 Hz (see Sec. 2.A), the local scaling exponent should not be trusted for scales below ∼ 1 m. The strong oscillation at those scales might be also due to platform motion. Despite minor oscillations of ζ2(r) around 2/3, the agreement of ζ2(r) derived from⟨DLLτ (r)⟩N for r∈[2 m,6 m] in the inertial range with the K62-prediction is remarkable. Assuming the energy injection scale to be L∼80 m as estimated above, r/L∼10−2 in the range 2 m≤r≤6 m. In the decade from 10−2r/L≤10−1, ζ2(r) shows a comparable plateau in the VDTT experiments with Rλ > 1000, which is in accordance with the observed Rλ for M161 Flight 12.

For r >20 m, a uniform scaling according to K41 or K62 is not expected anymore as rL. Large scales atr >20 m take longer to converge due to the complex nature of atmospheric flows and are not expected to exhibit the same scaling behavior due to the

non-universality of turbulence at scales comparable to or larger than L11. Furthermore, the agreement of ζ2(r) in the inertial range with Kolmogorov’s prediction at least in the ranger ∈[2 m,6 m] suggests that the micro-MPCK measures statistically isotropic turbulence. Hence, the flow distortion of the helikite is supposedly small in that range.

To summarize, the micro-MPCK is able to characterize turbulence in terms of the mean energy dissipation rate, length scales and Taylor micro-scale Reynolds number.

The same holds for all other configurations that are keel-mounted. Tether-mounted instruments measure a flow that is less distorted by the helikite but the dynamics of platform motion are more complex, too.

4.2 Atmospheric Turbulence Characteristics of the