BeCOOL is embedded inside Zephyr gondola, a platform designed by the CNRS DT-INSU. This platform drifts at an atmospheric pressure level of 50 hPa (altitude of about 20 km in the Tropics) below a stratospheric pressurized balloon operated by CNES. It is supposed to fly up to 3 months. For safety reason the Zephyr gondola is constrained to be small and light (less than 1 m high and about 0.6 m wide). This drastically limits the mass and the volume of the microlidar. Inside the gondola, room is left for other instruments and for the Zephyr OnBoard Computer (OBC) in charge of communications with the ground and of the management of the full payload. The Zephyr gondola is equipped with solar panels recharging embedded batteries. Zephyr provides power continuously, for a total average power lower than 25 W. Regular Iridium communications make it possible to receive telecommand and to send the data.

Operating from the stratosphere prevents the microlidar from any icing or condensation issues on optics, but the low atmospheric pressure make it difficult to obtain an efficient thermal regulation for components requiring it for optimal efficiency. Taking into account all these constrains drove the design of BeCOOL and led to the block diagram shown in Fig. . This microlidar is composed of three main subsystems: the optical head (OH, microlidar lenses), the optical box (OB) and the electronic box (EB). The total weight of BeCOOL is less than 6 kg and is roughly split between OH (3 kg), EB (1 kg) and OB (1.8 kg). To perform nadir measurements a specific aperture was made in a separate lower part of the gondola where the lenses are located. This arrangement is possible thanks to the use of optical fibers. This way, the thermal sensitive parts of the microlidar (laser diode, optical filter) are located in a more insulated upper part of the gondola as shown in Fig. . BeCOOL main features values are shown in Table .

The laser source is located in the EB. It is a compact and spectrally narrow (<0.4 nm) laser diode module emitting around 802 nm provided by DILAS. It is optically fibered (105 µm; NA 0.22) and linked to the OH at the emission lens focal point, so that the emission divergence is about 660 µrad. This laser module is powered by a laser driver board (LDB) designed and provided by the French company CIMEL Electronique to obtain a pulsed emission at a frequency of about 4.7 kHz with pulse energy of about 10 µJ. The LDB receives commands from the BeCOOL OBC to start or stop the laser emission, and sends housekeeping data such as temperatures (including laser diode temperature), voltage and current, as well as synchronization signal to synchronize emission and photon counting electronics. The LDB is also in charge of the laser diode thermal regulation via a heating resistance since the emitted spectrum is sensitive to thermal variation. A shift in temperature may impact the microlidar global efficiency. Indeed, the emission wavelength must stay within the spectral window of the optical filter of the reception channel.

The backscattered signal is collected by the OH where an optical fiber (105 µm; NA 0.22) is located at the reception lens focal point. This optical fiber links the OH to the filtering optic located in the OB, where two interferential filters reject the background signal from the sky. The spectral width of the interferential filter is larger than the emitted spectrum. This prevents the overall optical transmission from dropping because of a too important thermal shift impacting either the laser diode or the interferential filter. The APD is linked to the output of the filtering optic by an optical fiber with the same characteristic. Each photon detected by the APD produces an electronic pulse which is then accumulated by the OBC to obtain an atmospheric profile of backscattered signal with a 15-m spatial resolution up to 21.2 km below the microlidar. The highest time resolution of BeCOOL is about 1 s. Iridium communications used to recover the atmospheric profiles on ground are not efficient enough to transfer continuous measurements with such a resolution. The time resolution has been decreased by averaging 60 highly resolved atmospheric profiles. When operated onboard Zephyr, the ultimate time resolution of BeCOOL is therefore about 1 min. During the campaign, in order to prevent overheating of the laser diode (lack of convection prevents from using active cooling with a Peltier module), BeCOOL only measuring half of the time during a duty cycle (10 min ON mode, to 10 min OFF). Since BeCOOL average consumption is about 8 W, with peak up to 13 with the laser heating resistance, this 50 % duty cycle led to a global in-flight consumption of 4 W.

The temperature differs significantly between the ground-based environment, where BeCOOL is mechanically assembled and aligned, and the in-flight environment. This thermal difference distorts the optomechanical structure of the OH, resulting in an in-flight misalignment between the emission and reception optical axes. To mitigate this in-flight misalignment, an empirical strategy is implemented: an “in-lab controlled misalignment” is introduced on the ground, such that BeCOOL becomes correctly aligned once it reaches its operational altitude. To determine the appropriate “in-lab controlled misalignment”, the microlidar is placed in a climatic chamber whose temperature and pressure match those expected during the campaign. Inside the chamber, BeCOOL is mounted on a thermally insensitive optical retroreflector that enables a parallel optical injection from the emission axis into the reception axis. The optical power injected into the reception fiber is used as a proxy to assess the optical alignment. The optomechanical adjusters of the microlidar are equipped with small motors whose rotation compensates for the induced optomechanical distortion. The motor rotation is monitored quantitatively in order to determine the appropriate “in-lab controlled misalignment” that will optimize the performance of BeCOOL when operated in the stratosphere.

The raw signal P(t) measured by the photodiode against time is first corrected from saturation using a set of factory parameters. In most cases, this correction leads to a signal enhancement lower than 1 %, very exceptionally reaching values up to 20 % over strongly backscattering cloud tops. We note P* the signal corrected from saturation. Looking downward, the lidar is operating in a passive mode once the ground has been hit by the laser pulse. We thus average the tail of the signal beyond this ground echo to estimate the constant background value P*(∞) of the sky radiance measured by the lidar when operating like a radiometer. Noting r the distance between the lidar and the atmospheric layer illuminated at time t, with r=ct/2, where c is light velocity, we estimate the range corrected lidar signal Z(r)=[P*(r)-P*(∞)]r2.

Given the classical lidar equation, we know that the range corrected signal is a function of atmospheric properties: 1Z(r)=K×O(r)[βp(r)+βm(r)]exp⁡-2∫0rη[αp(r′)+αm(r′)]dr′=K×O(r)β(r)[T(r)]2 K is the instrumental constant, O(r) the lidar overlap function, βp(r) and βm(r) the particular and molecular backscatter coefficients at range r, αp(r) and αm(r) the particular and molecular extinction coefficients at range r, and η a correction parameter accounting for multiple scattering (η=1 for single scattering). We note β(r)=βp(r)+βm(r) the total backscatter coefficient and T(r)=exp⁡-∫0rηαp(r′)+αm(r′)dr′ the transmission of the atmosphere between the lidar and the atmospheric layer sampled at range r.

We want to estimate the attenuated backscatter coefficient B(r)=β(r)[T(r)]2. According to Eq. (), B(r)=Z(r)O(r)K. We thus need to determine the overlap function of O(r) (Sect. ) and the value of K (Sect. ).

Even if the microlidar was optimized in a thermal chamber in order to be aligned when operated in the stratosphere, (Sect. ), a misalignment of the optical axes of the emission and of the reception channels was expected in real conditions. The overlap is not only a function of the field of views of these two channels but also of the angular tilt between their optical axis. In order to understand what drives the overlap function O(r), we first computed its theoretical shape following . Considering the plane defined by the optical centers of the two lenses and the direction of the two optical axes when perfectly aligned, the angular tilt can be split into two parts, one parallel to this plane, one perpendicular to this plane. Perpendicular misalignment only reduces the overall transmission of the system and has no impact on the shape of the overlap function. Figure a shows simulations of overlap functions for different angles of parallel misalignment, considering converging optical axes. The larger the angle, the smaller the value of overlap once an almost constant value is reached, several kilometers away from the platform. Figure b shows the same functions, arbitrarily normalized at a distance of 1.5 km. It emphasizes the large variability of the overlap function in the first 2 km.

These theoretical functions have been compared to empirical overlap functions. For a given instrument it is possible to compute the shape of O(r) as the ratio between the range-corrected profiles for clear sky conditions (no clouds or aerosol layers) and the average attenuated backscatter profile derived from corresponding ERA5 pressure and temperature fields , following to calculate Rayleigh scattering cross-sections. The black line on Fig. b is an example of such an empirical function, normalized 1.5 km away from the laser source. It turns out that the shape of the overlap function is mostly driven by the parallel misalignment angle between the two optical axes. The misalignment slightly evolves with the temperature of the optical head, which exhibits a diurnal cycle linked with day/night contrast that leads to a small but systematic re-alignment of the optical channels during each night. It turns out that this is a second order effect. Indeed the instrumental constant K strongly depends on the temperature (see Sect. ). It is therefore difficult to disentangle the estimates of K and of O(r). This issue has been investigated. We have found that the uncertainty on the overlap function due to the alignment angle dependence on temperature is smaller than 10 %. For a given instrument we thus consider a constant overlap function over time. Following the procedure described in this section, a smoothed empirical overlap function is estimated for each instrument, making it possible to compute Z(r)/O(r).

Lidar signals have now to be normalized in order to compute the attenuated backscatter profiles. Ideally, the instrumental constant K should be evaluated using a region of the atmosphere where molecular scattering is prevailing. In this region, the expected value of B(r)=β(r)[T(r)]2 can be computed assuming β(r)=βm(r). CALIOP benefits from a region of almost pure molecular atmosphere above 35 km which enables the normalization upon synthetic Rayleigh lidar profiles build from MERRA-2 reanalysis .

Given the flight level of the balloons, it turns out that there is always a tenuous stratospheric aerosol layer between the gondola and the highests clouds observed by the lidar BeCOOL . The variability of this layer has been investigated during the first Stratéole2 scientific campaign, using CALIOP nighttime overpasses, averaged between 50 to 230° E and -10 to 5° N, which are the closest to the flight tracks. Figure  illustrates the variability of the attenuated scattering ratio several kilometers below the flight level. The closest the ratio to 1, the better the approximation that Rayleigh scattering is prevailing. This is the case from 1 to 2 km below the flight level, the region we chose to estimate the value of K and compute B(r).

We calculated the value of K for each profile to account for the variations of power emitted by the laser diode and other overall transmission of the optical system. The left panel of Fig.  shows that K is driven by the laser diode temperature. During the night, given the limited insulation of the system, the stratosphere is cooling the system, whereas operating the laser diode during a cycle is warming it. The higher the laser diode temperature, the lower the value of K. The right panel of this Fig.  also shows this strong correlation, except between 2 and 4 h (local time). This is due to the presence of stratospheric aerosols.

Figure  shows a cross section along the flight track of a stratospheric balloon of the attenuated backscatter B(r) for twelve consecutive nights. The atmosphere is cloud free above an altitude of 17.5 km. Several cloudy structures are observed below. The lidar is able to resolve them when they are optically thin enough. In these cases, BeCOOL is also observing the marine boundary layer and the ocean surface. When the laser beam encounters an optically thick structure, such as a large convective cloud, the signal is fully attenuated at some point within this cloud and the ocean surface is not observable. The white arrow on this figure is pointing towards the profile B(z)=β(z)[T(z)]2 we are going to use to illustrate the inversion procedure to compute β(z) and the ability of BeCOOL to measure the optical depth τ of thin cirrus (left panel of Fig. ). From now on, we will use the altitude z instead of the range r as the vertical coordinate.

We first assume that there is no multiple scattering (η=1). We choose an a priori value of S*. The integration of the lidar Eq. () is done backward, which is numerically more stable . A first profile of β(z) is computed, making it possible to estimate the apparent transmission of the cloud T*=e-τ*. This integration is done several time, the apparent lidar ratio S* being iteratively adjusted until the value of T* matches the observed one (see the middle panel of Fig. ). In other words, starting with the initial condition B(zb)=Br(zb), the apparent lidar ratio is adjusted until B(zt)=Br(zt).

In a second step, we estimate the multiple scattering correction factor η, comparing BeCOOL's distribution of apparent lidar ratios S* to CALIOP's distribution of lidar ratio S for clouds above 10 km, with an optical depth in the 0.2 to 1.5 range, over the tropical area explored by the BeCOOL flights (10° S to 5° N) between October 2021 and January 2022. Extensive work has been done to parameterize CALIOP's multiple scattering correction factor η using combined lidar and infrared radiometer observations . We thus considered the values reported in CALIOP dataset Level 2, V4-21,, already corrected for multiple scattering, as the best estimate of the lidar ratio distribution.

Setting η=0.88 is reconciling the mode of these two distributions (see Fig. ). It is also optimizing the overlap between the two distributions. This value is close to the one (0.80 ± 0.05) derived from dense low level water cloud observations by with a similar instrument. Several adjustments of the value of η have been tested. We estimate that its uncertainty is about 0.1. The value of 0.88 has been used for η to compute the cloud optical depth τ and the backscatter profile (right panel of Fig. ).

The distribution of the cloud optical depth computed with this method is shown on Fig.  (green line). Constrained retrieval can be used for an optical depth between 0.02 and 2. For larger values, BeCOOL is not powerful enough to go through the cloud. For smaller value, it is not possible to constrain the retrieval.

When a cloud is optically thin, it is not possible to use the cloud free regions above and below this cloud to constrain its optical depth. The backward retrieval is only done once with an a priori lidar ratio (case 2 of Fig. ). This value is derived from CALIOP’s successfully constrained lidar ratios over the time period and region covered by BeCOOL observations. These lidar ratios were vertically binned at 1 km resolution, and the most frequent value (mode) was retained for each altitude bin, producing a vertical profile of the most likely lidar ratios. The a priori lidar ratio used for BeCOOL's unconstrained retrieval is interpolated from this profile, and no further correction of the multiple scattering is needed.

The distribution of the cloud optical depth computed this way is shown on Fig.  (red line). Unconstrained retrieval is used for an optical depth between 2×10-5 and 0.2. The highest values (τ≳0.1) correspond to a limited number of cases of mid-level clouds for which the uncertainty of Br is too large to estimate the apparent transmission. Comparing the constrained and unconstrained distribution, we consider an optical depth of 3×10-2 as the usual threshold value to switch from one method to the other.

When a cloud, such as a convective cloud, is optically so thick that the laser beam does not go through it or when we want to investigate cloud or aerosol layers close to the surface, it is also not possible to constrain the retrieval. Moreover, as there is no reference available below the cloud, we cannot use backward integration anymore. Switching to forward unconstrained retrieval, it is only possible to estimate a lower bound of the cloud optical depth (case 3 on Fig. ). We will not develop this point here, as BeCOOL was first and foremost developed to investigate cirrus in the upper troposphere.