2.4 Performance assessment during in-situ measurements
2.4.3 Comparison to Radiosonde measurements
mini-MPCK below the main balloon not only shifted the global maximum of PSD( ˙ψ) to higher frequencies, it also hindered higher-frequency oscillations, e.g. at ≈10 Hz.
The inset of Fig. 2.8B shows no correlation between fmax and the mean wind speed per flight.
The platform motion also affects the angle of attack α = arcsinuu3 where u =
qu21+u22+u23 and angle of sideslipβ = arcsinu2/qu21+u22
, whereu1,2,3 are given in the platform frame of reference with the platform North (1), platform East (2) and platform Down (3). Both α and β are ideally close to 0°. Figure 2.9 shows α and β for M161 (A, B) and MSM89 (C,D) for each flight of the mini-MPCK. Considering M161 on RV Meteor, the angle of attack α (Fig. 2.9A) is reasonable for all flights but flights 1, 5 and 6. Flight 1 was a test flight where the static lift of the helikite was insufficient in wind-still conditions. As a result, the mini-MPCK could not be lifted out of the wake of RV Meteor and did not achieve to follow the turbulent flow. In flight 5, the CDP2 exerted a torque that the clamping mechanism on the rod could not withstand. As a result, the instrument was turned by ≈45°. Similarly, the box was still not stable against rotation on the rod in flight 6 even without CDP2. From flight 7 on, we strengthened the clamping mechanism with an additional improvised clamp. This attempt was successful and was even stable when the CDP2 was flown as well in flight 9 and 10. The angle of sideslipβ (Fig. 2.9) differs significantly from 0°
for each flight. This is due to the twist in the main tether and the limited horizontal rotational freedom of 270° only. It happened that the mini-MPCK was blocked on one end of the tether-mount even though we tried to clamp the tether-mount to the main tether with the largest dynamical range possible. Considering MSM89, α is only distributed around 0° in the case of flight 7, when the mini-MPCK was attached to the main line, and flights 10 and 13. The angle of sideslip is non-ideal in any flight despite that the mini-MPCK is mounted to the helikite. The angle of sideslip β is also affected by the mounting configuration (Fig. 2.9D) where flight 7 (tether-mount) differs from the other flights (10-19) where the mini-MPCK is hung from the main spare of the 250 m3 helikite. We attribute these differences to both the mounting configuration and the different dimensions of the helikite and the mini-MPCK. Due to its larger size, the helikite reacts to larger scales of the turbulent flow compared to the mini-MPCK.
Hence, the helikite is more inertial in reaction to the main flow and is advected by much larger scales only. During the advection motions, the mini-MPCK does not necessarily point into the mean wind.
0 1 2 3 4 5 flight time [h]
0 200 400 600 800 1000
zPSS8aboveMSL[m]
basics CDP2 HW
13:39 14:58 16:16 17:34 18:53
UTC time 0
200 400 600 800 1000
zPSS8aboveMSL[m] mini-MPCKzPSS8τ
Nd>30
0 150 300km 8◦N 10◦N 12◦N 14◦N
61◦W59◦W57◦W
A B
0 2 4 6 8 10 12 14 16
flight time [h]
0 300 600 900 1200 1500
zPSS8aboveMSL[m]
basics CDP2
D
12:42 14:03 15:24 16:44 18:05
UTC time 0
250 500 750 1000 1250 1500
zPSS8aboveMSL[m] mini-MPCKNd>30
0 150 300km 8◦N 10◦N 12◦N 14◦N
61◦W59◦W57◦W
C
Figure 2.7 Flight strategies visualized by time series of barometric altitudes of the mini- MPCK (A) for flight 10 during M161 on RV Meteor and (C) flight 19 during MSM89 on RV Maria S. Merian. The barometric altitude is recorded as a function of time by the PSS8 (zPSS8, grey line). The measurement location in terms of latitude and longitude is upstream of Barbados as shown by the red dots in the inset map of (A) and (C). Constant altitude legs with mean altitude ⟨zPSS8⟩τ are highlighted by the black dashed line in (A) where τ is the duration of each flight leg. The mini-MPCK traversed several clouds as shown by the number counts of cloud droplets measured by the CDP2 exceeding 30 counts per 0.5 s. (B) and (D) present the overview of total flight time per altitude for M161 on RV Meteor and MSM89 on RV Maria S. Merian, respectively. The bin size is chosen to be 50 m. “basics” comprise the Metek, PSS8, HMP7, AM2315, BMP388, SBG, BNO055 and ZED-F9P. HW is the hot-wire.
10−2 10−1 100 101 102 f[Hz]
10−9 10−8 10−7 10−6 10−5 10−4 10−3 10−2 10−1 100
PSD(˙ψ)[rad2s−1]
4 7 10 13
U[m s−1] 0.00
0.33 0.67 1.00
fmax[s−1]
7 13 15
17 19 10
10−2 10−1 100 101 102
f[s−1] 10−9
10−8 10−7 10−6 10−5 10−4 10−3 10−2 10−1 100
PSD(˙ψ)[rad2s−1]
4 7 10 13
U[m s−1] 0.14
0.17 0.21 0.24
fmax[s−1]
1 3 4
5 6 7
8 9 10
A B
Figure 2.8 Spectral properties of the platform motion are captured by the power spectral density (PSD) of platform roll rate ˙ψin frequency space for all flights on RV Meteor (A) and on RV Maria S. Merian (B). The PSD( ˙ψ) is block-averaged for time windows of 60 s. fmax
corresponds to the location of the global maximum of PSD( ˙ψ) being illustrated by the dots.
The inset shows the frequency of maximal PSD( ˙ψ) against the global mean velocityU.
−20 0 20 40 60
α 0.00
0.05 0.10 0.15 0.20
PDF(α)
1 3 4 5 6
7 8 9 10
−60 −40 −20 0 20
β 0.00
0.05 0.10 0.15 0.20
PDF(β)
1 3 4 5 6
7 8 9 10
−20 0 20 40 60
α 0.00
0.05 0.10 0.15 0.20 0.25
PDF(α)
15 13 7
19 17 10
−60 −40 −20 0 20 β
0.00 0.05 0.10 0.15 0.20
PDF(β)
15 13 7 19 17 10
A
C
B
D
Figure 2.9Angle of attackαand angle of sideslipβmeasured by the Metek sonic anemometer for the mini-MPCK on RV Meteor (A, B) and the mini-MPCK on RV Maria S. Merian (C, D).
Ideally, i.e. in case of alignment to the mean wind direction, both PDF(α) and PDF(β) are distributed around 0 m/s. Theα-axis is clipped from −20° to 60° and the β-axis is clipped from −60° to 20° to optimize visibility.
mounted in plastic feed throughs due to the lower thermal conductivity compared to the aluminum housing, the thermal mass of the instrument biases the air temperature and relative humidity measurement. While the relative humidity is a monotonic func- tion of air temperature, the absolute humidity is not. Hence, the absolute humidity measurement is accurate provided that the ventilation of the RHT sensors is sufficient and radiative errors are negligible and no droplets hit the RHT sensors. Furthermore, the acoustic temperature measurement is the least affected temperature measurement of the mini-MPCK. Neglecting radiative errors, the air temperature is derived from first principles via the speed of sound of air which can be converted into air temperature for a given absolute humidity [111] as explained in Sec. 2.B.
Here, we use altitude profiles of the air temperature and relative humidity measured by the mini-MPCK to compare with three radiosonde measurements during flight 10 on RV Meteor (M161) as shown in Fig. 2.10A. On average, the mini-MPCK air temperature is 0.06 K higher than the radiosonde (compare Fig. 2.10B). The accuracy of the radiosonde is 0.3 K in soundings (below 16 km altitude above MSL) and the accuracy of the AM2315 is between 0.1 K to 1 K. Thus, the altitude profile of the mini-MPCK and all three radiosondes agrees well in the limit of accuracy of the measurements.
Between 50 m to 700 m altitude above MSL, the vertical air temperature gradient measured by the mini-MPCK is −0.95 K per 100 m and the air temperature gradient measured by the radiosondes is −0.98 K per 100 m. The vertical gradient of the air temperature is close to the adiabatic lapse-rate of air temperature, which suggests a well-mixed boundary layer. The reverse conversion is shown in Figs. 2.10 where the speed of sound cis obtained from radiosonde measurements. As the mini-MPCK air temperature was slightly higher than the radiosonde air temperature, the opposite is true for the speed of sound. At last, the comparison of the relative humidity is shown in Figs. 2.10E and F. On average, the mini-MPCK and radiosonde relative humidity profiles deviate by 0.04% (absolute). However, the longer flight duration enables the mini-MPCK to measure more variability in relative humidity as illustrated by altitudes from 750 m to 900 m where two radiosondes measure a 10% change and the mini-MPCK 20%-30% change in relative humidity. This significant change in relative humidity is coincidental with a drop in air temperature. This is usually a sign of entrainment of cold and dry air from the free troposphere. Further analysis is needed to explore this event. In summary, the mini-MPCK compares statistically very well with measurements from radiosondes for flight 10 on M161.