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Flight Properties and Platform Motion

2.4 Performance assessment during in-situ measurements

2.4.2 Flight Properties and Platform Motion

Airborne measurements with an aerostat like the helikites are flexible in operation. Not only are they capable of constant altitude flights, which is necessary when sampling mainly clouds, but they are also capable of atmospheric profiling. A combination of the two capabilities results in the staircase altitude profile. This flight strategy is advantageous for measuring (vertical) fluxes in the atmospheric boundary layer. Since the fluxes decrease linearly with altitude, two sufficiently long flight segments with constant altitude in the sub-cloud layer are sufficient to determine the flux profile [56, 110]. Since the cloud base is variable in height, it is safe to choose the highest flight distance in the sub-cloud layer at least 100 m below the cloud base. As mentioned earlier, the cloud base was known a priori based on the remote sensing instruments (radar) during our ship-based measurements during the EUREC4A field campaign.

The primary purpose of the mini-MPCK on RV Meteor was to characterize atmo- spheric turbulence and measure vertical fluxes in the sub-cloud layer. Clouds were sampled whenever they were present and the dynamic lift of the helikite was sufficient to reach altitudes of 800 m above mean sea level (MSL). Therefore, we performed staircase flights, as exemplified by the barometric altitude record in Fig. 2.7A. The mean altitude of the constant altitude ⟨zPSS8τ flights is shown by the black dashed lines, where PSS8 refers to the pitot tube. The standard deviation of barometric altitude during constant altitude flight segments increases from 5 m for low altitudes

1 A

2

1 C

2 5

B

1

2

3 4

Figure 2.6 The Helikite launched aboard the RV Meteor during the EUREC4A - ATOMIC field campaign in the Caribbean January to February 2020. (A) Side view of the 75 m3 Helikite (1) in its parking position at 50 m altitude above MSL. The Helikite is anchored to the rear of RV Meteor by the main line (2). (B) Perspective from the deck of RV Meteor.

The Helikite (1) is connected to the winch (4) via the main line (2). The line guiding system (3) prevents the line from entangling in the ship’s structures such as the A-frame or various

cranes. (C) The Helikite (1) lifts the mini-MPCK being attached to the main line (2).

≈200 m above MSL to 25 m for high altitudes of ≈ 1 km above MSL. Cloud events, i.e., when the number of cloud particles Nd is greater than 30 0.5 s, are represented by blue dots. The inset of Fig. 2.7A shows a map where the position of the mini-MPCK is represented by red dots in terms of latitude and longitude. An overview of the flight time per altitude is shown in Fig. 2.7B. The lowest flight segment is at about 200 m above MSL, while the highest flight segment is concentrated at about 800 m above MSL, since this was the maximum altitude we could reach at the beginning of EUREC4A in low wind conditions. Between flight segments, the mean ascent speeds are in the range of 0.18 m to 0.28 m, as shown in the left panel of Fig. 2.17. The mean descent rate is in the range of −0.15 m/s to−0.35 m/s, with the slower descent rates due to the lower spooling speed of the RV Meteor mooring winch (dashed line) compared to the MCO winch (dashed line) in the right panel of Fig. 2.17.

The mini-MPCK on RV Maria S. Merian concentrated on cloud microphysical measurements. Therefore, the flight altitude was adjusted to be inside the cloud layer based on information gained by remote sensing. As an example, the altitude profile of flight 19 on RV Maria S. Merian (MSM89) is shown in Fig. 2.7C where the first clouds were encountered at∼600 m and ∼1000 m above MSL. As the flight strategy was different, the total flight time per altitude accumulates at ∼800 m and ∼1200 m above MSL as shown in Fig. 2.7D.

We conducted airborne measurements with a kite-stabilized, helium-filled balloon, which orients itself to the mean wind direction with the help of a keel. Hence, the balloon is moving in the turbulent flow. As the mini-MPCK is not mounted on, e.g., a fixed mast, it is also prone to platform motions. If the mini-MPCK is mounted on the pre-tensioned main tether due to the aerodynamic drag, the system corresponds mechanically to a damped and driven pendulum. If the damping due to the tension of the main tether is low, the driving force exerted by balloon motions can excite pendulum motions of the mini-MPCK. The pendulum motion of the mini-MPCK on RV Meteor is visualized in Fig. 2.8A by the power spectral density of the roll rate PSD( ˙ψ) which corresponds to angular accelerations. Notably, the tension on the main tether was low due to the limited lift of the 75 m3 helikite which is why the damping of the pendulum motion was small. We suggest that the pendulum motion of the mini-MPCK results in an oscillation around its equilibrium position (i.e. the connection to the tether-mount) at a frequency of ∼0.1 Hz due to the aerodynamic drag of the fins. The tether-mount itself allows for horizontal and vertical rotation as explained above so that oscillations are generally not dampened. The inset of Fig. 2.8A shows the correlation offmax on the mean wind speed U per flight where fmax is the frequency of the global maximum of PSD( ˙ψ) as indicated by dots.

In contrast, the tension of the main tether of the 250 m3 was much higher (<1 t) on RV Maria S. Merian. On MSM 89, the mini-MCPCK was attached to the main tether only on flight 7. For all the other flights on MSM89, the mini-MPCK was mounted below the balloon on more than 4 attachment points. This is why PSD( ˙ψ) for flight 7 also exhibits higher frequency modes as shown in Fig. 2.8B. All other flights (flights 10, 13, 15, 17, 19) decay after their global maximum at roughly 1 Hz. Thus, mounting the

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.