Instrumentation

As mentioned above, the mini-MPCK is designed primarily for the characterization of cloud microphysics and atmospheric turbulence. Table 2.1 gives an overview of the sensors installed in the mini-MPCK. All measured quantities except cloud droplet diameter dp, droplet number Np, droplet interval time ∆tp, acoustic temperature Tacoustic, wind vector u, and liquid water content LWC are measured with at least two different sensors. Here, we give an overview of our choice of scientific sensors. The main requirements were serial interface to be compatible with the Max-Planck-Cloudkite Protocol (Sec. 2.3.4), voltage range 3 V to 23 V, a compact design and low weight and power device.

Wind vector and speed

The three-dimensional wind vectoru =vp−v relative to the platform, where vp is the platform velocity and v the flow, is required by a sonic anemometer (hereafter referred to as “sonic”). Here, we use the uSonic3 Class A MP manufactured by Metek GmbH at a sampling rate of 30 Hz. Advantageously, it measures the vertical velocity in the platform frame of reference directly by 3 independent measurement paths, hence attaining a high accuracy of the vertical velocity. The accuracy of the wind measurement decreases with the angle of attack due to shadow effects of the transducers [109]. We also shortened the sonic in order to save weight and to shrink the mini-MPCK. In consequence, we had to sacrifice orientation data by the sonic.

Furthermore, the electronics were rearranged so that they occupied less space.

The relative wind speed u = |vp −v| is measured by the Pitot Static System PSS-8™manufactured by Simtec AG (hereafter called “PSS8”) with a sampling rate of 100 Hz. Assuming that the platform is always aligned with the predominant wind direction, which is chosen to be the e1-direction, u = ^qu²₁+u²₂+u²₃ ≈ u1 in the platform frame of reference if u1 ≪ u2,3.The measurement error of the dynamic pressure due to misalignment between the sensor and the mean wind direction is below 2.5% for angles of attack (or side slip) between±25°. The relative wind speed is further measured by a hot-wire anemometer (55P16 miniature wire probe, mini-CTA, Dantec Dynamics) at a sampling rate of 8×10³Hz. The analog signals are converted to digital signals by a LabJack T7 OEM.

9 m

~ 4 m 50 m

~ 1000 m

MCO winch ground

ground station

mini-MPCK

helikite

altitude above ground main tether

35° - 50°

Figure 2.3 Overview of operating the mini-MPCK in the field. The winch of the Max Planck Observatory (MCO) is anchored to the ground, e.g. the deck of a research vessel in case of remote measurements from a research vessel on the ocean. The helikite (75 m³ or 250 m³) is held by the main tether, which is rolled up by the winch. At maximal pull, the line is inclined by about 45° as drag and lift of the helikite balance tension of the main tether.

The mini-MPCK carries all scientific instruments, records data inside clouds and cloud-free atmosphere in the platform frame of reference (e^′_i where i∈ {1,2,3}) and is supposed to orient itself to the mean wind U in the earth frame of reference (ei wherei∈ {1,2,3}). The mini-MPCK is attached to the main tether 50 m below the helikite. The mini-MPCK is connected to the ground station via two radio links (433 MHz and 2.4 GHz). The sketch is unrealistically scaled for better visibility. The wifi symbol is copied from Keynote.

CategoryInstrumentManufacturerQuantityRangeAcquisitionrate[Hz]Annotations ClouddropletsCloudDropletProbe2UAVDropletMeasurementTechnologies,Inc.dp2µmto50µm2Ufrom10m/sto250m/s Np2lowerboundofLWC ∆tpupto256particles1×106 windmini-CTADantecDynamicsu′>0.2m/s8×103kmaxηK≳1,bandwidth10kHz uSonic3ClassAMPMetekGmbHu0m/sto40m/s30max.samplingrate50Hz Tacoustic−40°Cto60°C30 PitotStaticSystemPSS-8™SimtecAGu0m/sto89.5m/s100 T−60°Cto70°Cτ∼4s@U10m/s p238hPato1080hPa RelativehumidityandHMP7VaisalaT−70°Cto180°C0.2τ∼15s airtemperatureRH0%to100%0.2 AM2315AOSONGT−70°Cto180°C1τ∼5s RH0%to100%1 BMP388AdafruitT−40°Cto85°C1τ∼0.005s p300hPato1250hPa1samplingrate≤200Hz positionandorientationEllipse-NSBG-Systemss,θ,ϕ0°to360°200bandwidthmagnetometer110Hz lat,lon,alt200 ZED-F9Pu-bloxlat,lon,a1samplingrate≤25Hz BNO055Adafruitψ,θ,ϕ0°to360°100bandwidthmagnetometer20Hz Table2.1Instrumentationofthemini-MPCK.Themeasurementquantitiesare:clouddropletdiameterdp,clouddropletnumber countNp,clouddropletinter-arrivaltime∆tp,windvectoru,acoustictemperatureTacoustic,windspeedu(andwindspeedfluctuations u′ ),temperatureT,relativehumidityRH,pressurep,platformorientationangles(rollψ,pitchθ,yawϕ),platformposition(latitude lat,longitudelon,GPSaltitude,a)andliquidwatercontentLWC.BoththewebcamandtheLWCprobewerenotoperated(n.o.) duringEUREC4 Afieldcampaign.kmaxisthelargestresolvedwavenumber,ηKistheKolmogorovlengthscale,τisthetimeconstant (i.e.theresponsetimeforreaching63%oftheactualairtemperatureT),Uthemeanwindspeed.

A

B

fins Power Box

Instrument Box Teather mount

main line

1

2 3

7 7a

7b

6 5

4

main line

main line 8

7a 7b

2 1 3

9 10

12

7

C

6 aluminium 5

tube

air flow

11

Figure 2.4 The mini - Max Planck CloudKite (mini-MPCK). (A) The mini-MCPK is on RV Maria S. Merian during the EUREC⁴A - ATOMIC field campaign in the Caribbean January to February 2020. Posters showing the members of the ship’s and scientific crew are blurred to protect privacy. (B) Schematic view of the mini-MPCK consisting of an Instrument Box, Power box, fins, a centered aluminum tube, a tether moint and the main line. (C) Front perspective to visualize the sensor arrangement. The mini-MPCK is equipped with a sonic anemometer (1), a hot-wire anemometer (2), a pitot tube (3), a webcam (5), a liquid-water-content probe (6), three temperature and relative humidity sensors (8, 9, 11), an orientation sensor (10) and a UFT probe (12). Temperature and relative humidity sensors are shielded by the nose (4). The cloud droplet probe (7) is supported by aluminum structures (7a) and protected by a frame (7b).

Temperature and Relative Humidity

Here, we use the HMP 7 manufactured by Vaisala to record the air temperature (PT 100) and relative humidity (capacitive measurement) at a sampling frequency of 0.2 Hz.

Supplementary RHT measurement is conducted with AOSONG AM2315 at 1 Hz. The AOSONG RHT sensors are calibrated as explained in [37]. The air temperature is further measured by the PSS8 temperature sensor (PT-100) at 100 Hz and by the BMP388 (Bosch, Adafruit) at 1 Hz. The uSonic3 Class A MP manufactured by Metek GmbH (“sonic”) is able to measure the air temperature with a sampling frequency of 30 Hz indirectly via the measurement of the speed of sound in air as explained in Sec. 2.B.

Cloud Droplets

The Cloud Droplet Probe 2 UAV (CDP2) from Droplet Measurement Technologies Inc. is used to measure the size and number density of cloud droplets (2 µm to 50 µm in diameter) with a sampling rate of 2 Hz. The CDP2 also provides the cloud droplet inter-arrival time of up to 256 particles per sampling interval with a 1 µs temporal resolution. Due to the low relative wind speed, more than 99% of the totally measured particles is also recorded as particle-by-particle data (PbP-data) in typical shallow cumulus conditions with a cloud droplet number concentration of ∼ 100/cm³ and mean relative wind U ∼10 m/s. The CDP2 requires a wind speed between 10 m/s to 250 m/s. The measurement principle of the CDP2 is based on the forward scattering of a laser beam (wavelength 658 nm) by crossing cloud particles. The scattered light is projected through a 50/50 beam splitter onto two photodetectors (a qualifier and a sizer), where the detected intensity is used to size the particles. A disadvantage is that the CDP2 is not able to measure the velocity of the particles. However, the CDP2 is lightweight (≈1.8 kg including power and data cables) and consumes≲48 W.

Combining the droplet counts with the relative wind speed measurements, we obtain the droplet density n = Np/(⟨u⟩A∆t), where ⟨u⟩ is the average relative wind speed, A = 0.24 mm² is the laser beam cross-section, and ∆t is the sampling interval. It is possible to calculate the LWC using Np and dp. However, the LWC will most likely be underestimated since droplets smaller than 2 µm and larger than 50 µm will not be detected.

Position and Orientation

In addition to the time of measurement, it is essential to determine the platform position, orientation and rotation. To meet this demand, the mini-MPCK is equipped with an INS (Inertial Navigation System), which is a combination of a GNSS (Global Navigation Satellite System) and IMU (Inertial Measurement Unit). Here, we use an SBG Ellipse-N (manufactured by SBG Systems) containing an accelerometer, a gyro, a magnetometer, a pressure sensor and a single-band GNSS. Due to its sensor fusion logic, the SBG Ellipse-N is able to calculate angular velocities in real-time.

The SBG position,velocity, orientation and angular velocities are sampled at 200 Hz.

Advantageously, it is very light (47 g) and small (46 x 45 x 24 mm). The SBG is the origin of the platform frame of reference which is why all momentum arms are given relative to the SBG Ellipse-N.

The SBG Ellipse-N GNSS position and velocity is backed up by the SparkFun GPS-RTK2 (sampling rate 1 Hz), which is equipped with a u-blox ZED-F9P together with a u-blox multiband antenna (ANN-MB Multi-band ANN-MB-00-00). Platform orientation and acceleration are additionally acquired by the BNO055 (Adafruit) at 100 Hz. The barometric altitude is redundantly recorded by the PSS8 (at 100 Hz)and the BMP388 (at 1 Hz).