Lidar observations of Cirrus clouds at Palau island (7◦33′ N, 134◦48′ E)

A polarization diversity elastic backscatter lidar has been deployed in the equatorial island of Palau in February and March 2016, in the framework of the EU Stratoclim project. The system operated unattended in the Atmospheric Observatory of Palau Island from 15 February to 25 March 2016, during nighttime. Each lidar profile extends from the ground to 30 km height. Here the dataset is presented and discussed in terms of the temperature structure of the UTLS, obtained from co-located radiosondings. During the campaign, several high altitude clouds were observed, peaking approximately 3 km below the Cold 5 Point Tropopause (CPT) located above 17 km. Their occurrence was associated with cold anomalies in the Upper Troposphere (UT). Conversely, when warm UT anomalies occurred, the presence of cirrus was restricted to a 5 km thick layer centered 5 km below the CPT. Thin and subvisible cirrus were frequently detected close to the CPT. The particle depolarization ratios of these cirrus was generally lower than the values detected in the UT clouds. CPT cirrus occurrence showed a correlation with cold anomalies likely triggered by stratospheric wave activity penetrating the UT. The back trajectories study revealed a thermal and 10 convective history compatible with the convective outflow formation for most of the cirrus clouds, suggesting that the majority of air masses related to the clouds had encountered convection in the past and had reached the minimum temperature during its transport in less than 48 hours before the observation. A subset of SVC, with low depolarization and high lidar ratio and with no sign of significative recent uplifting, may be originated in situ.


Introduction
Cirrus are clouds composed of ice particles which form in the upper troposphere, covering about 20-25 % of the Earth (Rossow and Schiffer, 1999). They play an important role in climate: cirrus control the amount of solar radiative energy reaching the variability in depolarization and greater optical thickness at lower temperatures in tropical cirrus clouds; in later measurements from Kavieng (2 • 50 S, 152 • 7 E) Platt et al. (1998) reported a decrease of the lidar integrated attenuated depolarization ratio 55 with temperature, from 0.42 at -70 • C to 0.18 at -10 • C, which actually contradicts some later findings. For instance, Sassen and Benson (2001) show for midlatitude cirrus a quite regular increase for decreasing temperatures, from values around 0.3 at 240 K to 0.45 at 195 K; depolarization measurements from Mahe (4.4 • S, 55.3 • E) from Pace et al. (2003) do not show a regular behavior in temperature, while in observations from Gadanki (13.5 • N, 79.2 • E) Sunilkumar and Parameswaran (2005) observed an increase with decreasing temperature, albeit in both cases the depolarization dropped to its lowest values at the 60 lowest temperatures (i.e. highest altitudes) observed. These apparently contradictory findings suggest that there may not be an univocal relationship between temperature and depolarization, as the latter may be influenced by the history rather than the instantaneous value of the air mass temperature, with high depolarization produced by fresh particles in cirrus clouds originating from the outflow of convective cells and intermediate to low depolarization associated with aged outflows and in situ formed cirrus, the latter often in the form of SVC, dwelling at or slightly below the Tropopause, as suggested by Pace et al. 65 (2003). Cirrus at the tropical tropopause are of particular interest as they represent the last thermodynamic process of water vapor before it enters the Stratosphere. Nee et al. (1998) recorded the presence of high tropospheric SVC, often showing an optical thickness of less than 0.01, at Chung-Li (25 • N, 121 • E), for almost 50% of the observational period between May and September 1993-95. Boehm and Verlinde (2000) related the occurrences of high and persistent cirrus clouds in Nauru (0.5 • S, 170 • E) to the temperature perturbation induced by equatorial Kelvin waves. The role of Kelvin as well as gravity-and Rossby 70 waves and their link to extensive, persistent laminar cirrus has then been addressed extensively (Pfister et al., 2001;Garrett et al., 2004). Wang et al. (2019) shows from 10 years of lidar satellite observations that these optically thin laminar cirrus occurs frequently in the west/central tropical Pacific, equatorial western Africa, and northern South America, thus preferably in the tropical large-scale ascending zones. Similarly, Wang and Dessler (2012) have shown with satellite observations that the convective fractions of cirrus increase with height until the cold point tropopause is reached, peaking in their geographical 75 occurrence over equatorial Africa, the tropical western Pacific, and South America. They have reported at least ∼30 % of cirrus in the TTL are of convective origin. Some studies have focused on the western Pacific tropical warm pool (TWP) (Platt et al., 2002;Heymsfield et al., 1998;Sassen et al., 2000;Comstock et al., 2002). They have reported that there thin cirrus are particularly frequent and can cover more than 50 % of the area (Prabhakara et al., 1993;Dessler and Yang, 2003). This area, which spans the western waters of the equatorial Pacific, is characterised by a mean sea surface temperature (SST) 80 exceeding 28 • C, weak trade winds, deep convection extending above 15 km, and holds the warmest seawaters in the world. It has a large effect on surrounding monsoon regions and on climate so as to be called the "heat engine of the world" influencing remote regions and large scale weather variability (De Deckker, 2016). It is also a region of primary importance for the troposphere to stratosphere transport: with a Lagrangian trajectory analysis of tropospheric airmasses entering the stratosphere. Kremser et al. (2009)  March 2016, collecting atmospheric profiles of aerosol and clouds from the ground to 30 km. Cirrus clouds were observed on several occasions. We present here an analysis of the data obtained during that campaign. The instrument and the data processing are described and the measurements are discussed, aiming at characterizing cirrus morphology, optical properties 100 and connection with TTL temperature. A backtrajectory analysis is also presented, trying to connect the characteristics of the cirrus with their origin and mechanism of formation.
2 Instrument and data processing

The lidar system
The lidar deployed in Palau is a version of a small, portable, home-made instrument already described in Cairo et al. (2012).

105
Similar systems are designed to work unattended in remote sites and have been used in previous campaigns in Africa (Cavalieri et al., 2010(Cavalieri et al., , 2011 and Europe Rosati et al., 2016;Bucci et al., 2018). We will briefly describe the setup used in this work. The system is contained in a 30 × 40 × 50 cm aluminum box, electronically shielded and thermally insulated with polyurethane. An inclined quartz window on its top allows the transmission of the laser beam and the collection of the backscattered signal. The temperature in the aluminum box is controlled by four cooler-heater Peltier cells, 20 W each; 110 an additional 200 W Peltier cell conditioner has been added to improve the temperature control in equatorial conditions. The laser source (Bright Solutions, Wedge) is an air cooled, diode pumped Nd-YAG, with second-harmonic generation and active Q switching. The laser pulse duration is 1 ns and the emission is at 532 nm (green) with energies of 1 mJ/pulse The pulse repetition rate is 1 kHz. The laser beam divergence is reduced to 0.4 mrad by a beam expander and the laser is aligned to the telescope Field of View (FOV) by a steerable dielectric mirror, placed before the beam expander. The telescope is Newtonian 115 with a diameter of 20 cm, f /1.5, with a FOV of 0.75 mrad. The overlap of the laser beam with the FOV begins at 40 m from the instrument and is completed at 600 m. An overlap correction function O(r) is used to reconstruct the backscatter signal over that region (Biavati et al., 2011). Narrow band interference filters, with 5 nm bandwidth, high transmission and negligible temperature dependence, select the backscattered light from the sky background. A cube polarizer is used to further divide the radiation at 532 nm in the components parallel and perpendicular to the polarization of the emitted light. Additional polarizers 120 are placed in front of the detectors, which are miniature photomultiplier modules (Hamamatsu 6780-20) with low thermal noise (less than 10 counts/s at 25 • C). A characterization cross-talk between the channels has been performed following the method outlined in Snels et al. (2009), resulting to be negligible. The photomultiplier signals are amplified with a bandwidth of 250 MHz then recorded both in current and in photocounting mode by an acquisition card (Embedded Devices, APC-80250DSP).
The two records can be merged during data processing. In current mode, the photomultiplier signal is filtered through a 15 125 MHz low pass to avoid aliasing and then digitized into an 8 bits waveform, at adjustable sampling rates. The sample duration can be set to 12.5, 25, 50 or 100 ns and the waveform is reconstructed from 1024 samples, providing a spatial resolution from 1.875 to 15 m and a spatial range from 1.875 to 15 km. The system has been carefully checked for linearity throughout the measurement dynamical range. An absolute calibration of the channels gain ratio was also performed before the deployment.
In photon counting mode the photo impulses which are above an adjustable threshold are counted in 1024 consecutive time 130 bins, whose length may be set from 25 to 1000 ns in 25 ns increments, providing a spatial resolution from 3.75m to 150 m and a spatial range from 3.75 to 150 km. The dead time estimated from the maximum photon counting rate result to be 6 ns, and its effects are taken into account following Donovan et al. (1993). In both modes the first 24 bins are collected before the laser shot and used for measuring background light. The acquisition card provides the sum of the signals integrated over a user defined time interval that can range from 1 s to possibly several tens of hours. Data are stored in the memory board of the 135 system. An external computer is used to access the system via USB or TCP/IP connection. A visualization of the measurements in real time is possible for system checking or for alignment. A synopsis of the system specifications is reported in table 1. In the Palau campaign configuration, the system was set to operate every night for 10 hrs, providing 5 minutes averaged vertical profiles of atmospheric elastic backscattering with a vertical resolution of 30 m, extending from the ground up to 30 km.

Data processing 140
In elastic lidars, the two physical quantities of interest, the particle backscattering β a (r) and extinction α a (r) coefficients, must be determined from the elastic backscatter equation: where P (r) is the power of the backscattered radiation received by the lidar telescope from range r, E is the transmitted laser-pulse energy, C is the lidar constant including its optical and detection characteristics, O(r) is the overlap function and 145 β m (r) and α m (r) are the molecular backscatter and extinction coefficient respectively, that can be derived by meteorological data of air density and molecular scattering theory. To determine the two unknowns, the particle backscattering β a (r) and extinction α a (r), a relationship must be assumed between them, often in the form of a constant extinction-to backscatter ratio, the so-called Lidar Ratio (LR). This parameter is an intensive aerosol property, strongly depending on its size, shape and composition.

150
The backscatter ratio (BR), defined from the particle (a) and molecular (m) backscattering coefficients as: is then derived with the Klett inversion method (Klett, 1985) with LR assuming piecewise constant values in regions where clouds or different typologies of aerosols were present. To identify such regions, the values of BR, altitude and volume depolarization ratio δ are iteratively inspected during processing. The volume depolarization ratio δ is defined as: where par and cross refers to the backscattered light with polarization respectively parallel and orthogonal to that of the laser.
These values are iteratively inspected during the data processing to recursively adjust the LR accordingly. For instance, when thin liquid or ice clouds are identified, LR there is set to values known from literature respectively to 19 sr (Chen et al., 2002) and 29 sr (O'Connor et al., 2004). The LR for aerosol may easily range from 20 sr in the case of marine aerosol 160 (Papagiannopoulos et al., 2016) to 80 sr for biomass burning aerosol (Weinzierl et al., 2011). We used a constant aerosol lidar ratio set to 26 sr where no clouds were present (Dawson et al., 2015). The values of β m were retrieved from temperature and pressure profiles (Collis and Russell, 1976) by co-located radiosoundings which were launched twice a day by the Weather Service Office of Palau, accessible through http://weather.uwyo.edu/upperair/sounding.html.
The calibration altitude for BR was chosen typically between 8 and 12 km when free of aerosol and clouds. We set there 165 the calibration value of BR tot 1.02, as suggested in Kar et al. (2018). The uncertainty associated with the data from the lidar used in this study is extensively discussed in Cairo et al. (2012) and Rosati et al. (2016). At a given altitude, it depends on the attenuation of the signal from the ground, which is affected by low level clouds and aerosol. For the present purposes, a typical upper limit to the absolute random errors on BR and δ in the tropospheric range may be quantified to be 0.1 and 5% respectively.

170
An independent evaluation of the cirrus extinction, and hence of the cirrus LR, has been implemented following the approach of Young (1995), obtaining the value of the cloud transmittance determined from the elastically scattered lidar signals from clear regions below and above the cloud. When this approach does not produce results, due to optical thickness too small or noise of the profile below and/or above the cloud, a fixed value of LR=29 sr was assumed (Chen et al., 2002).
Multiple scattering should be considered when cirrus clouds are analyzed. The effect depends on the lidar FOV and on the 175 optical thickness of the clouds, increasing when the two get larger and becoming not negligible when τ approaches unity.
It tends to produce observed extinctions and depolarization respectively smaller and greater than the real (effective) ones; the effect on the backscattering coefficient tends to be less important (Bissonnette, 2005). Different correction algorithms have been proposed although there is still no consensus on a univocal and rigorous correction method. We have followed the procedure suggested in Chen et al. (2002), multiplying our τ and the retrieved LR by a multiple scattering factor η given by: In our analysis, this correction ranges from close to 1 in very thin clouds to 0.7 for the thickest ones. No corrections were made to the backscattering and depolarization coefficients. The inversion of the lidar data delivers profiles of Backscatter Ratio BR and hence of particle backscatter coefficient β a , and volume depolarization δ. From the latter, a value of the particle depolarization δ a can be obtained following Cairo et al. (1999). The uncertainty associated to δ a very much depends on the associated BR and in our data, can be as large as 100% for a single measurement on very thin clouds. However this uncertainty is the result of random errors that do not produce a bias in the mean values of the measurements, so that the averages presented in our work may not be precise but are accurate.

Data analysis
Threshold values of 1.15 for BR were used to identify the cloud base z bottom and top z top as limits for the calculation of the 190 cloud integrated characteristics; clouds were identified as such in an altitude interval in which condition R > 1.15 was continuously met for at least 150 m. For them, the vertical extension of the cloud ∆z and optical thickness τ have been computed.
The mid cloud optical altitude z o , average temperature, average particle depolarization, average potential temperature have been computed as weighted averages over the cloud vertical extension, with the weight given by the cloud particle backscatter coefficient β a . The mid cloud geometrical altitude z g is defined as the average of the cloud top and base altitude. analysis at 3hr resolution, using diabatic vertical motion. The convective influence is individuated from the 3-hourly brightness temperature (BT) images at 11 µm from the climate quality Gridded Satellite dataset GRIDSAT-B1 (GRIDSAT78 BI) (Knapp 205 et al., 2011). A convective source is therefore individuated when, in a specific geographical bin, a trajectory is traveling below (has a temperature warmer than) a convective cloud top level, selected from the satellite measurement with BT < 230 K, as similarly done in Tzella and Legras (2011) and Tissier and Legras (2016). More details on the trajectory-convective clouds coupling methods can be found in Bucci et al. (2020) and Legras and Bucci (2019). The age of theair masses is computed as the time intercurred between the time of release of the cluster, i.e. at the time of the lidar observation of the airmass, and the 210 convective cloud crossing.

Meteorological context
Palau is located on the north western edge of the TWP. Its average daily temperature is 28 • C throughout the year with very small changes from season to season. Its weather is influenced by the meandering of the Intertropical Convergence Zone, season is from May to October, affected by West Pacific Monsoon that brings heavy rainfall, while the driest season is from February to April (Kubota et al., 2005). Winds come from the north-east from December to March, then revert to westerly between May and July, to September and December.
The  Point Tropopause (CPT) temperature is 192 K at 17400 m at 382 K potential temperature on average; that altitude is nearly coincident with the Lapse Rate Tropopause (LRT) , defined as the level at which the lapse rate becomes less than 2 K/km 235 and the average lapse rate is less than 2 K/km for 2 km above this level, The Level of Neutral Buoyancy (LNB), which we consider to be the one at which the potential temperature equals the equivalent potential temperature at the ground, is at 11600 m at 348K potential temperature on average. In our case it can be shown that this level coincides with the Level of Minimum Stability (LMS), where the vertical gradient of the potential temperature attains a minimum, and thus can be taken as the lower boundary of the TTL (Sunilkumar et al., 2017).

240
On the left panel of the figure the time series of temperature anomaly profiles observed during the campaign are reported.
The altitudes of the CPT (blue dots) are also displayed. The pronounced wave-fronts in the stratosphere that are seen to descend from the stratosphere and sometimes penetrating below the CPT are the result of large scale tropical wave activity. We can see here how the structure and variability of the TTL are greatly affected by these fluctuations induced by such gravity waves or Kelvin waves, that influence tropopause height, temperature, cloud occurrence, (Fueglistaler et al., 2009).

245
Prevalent winds were from East, veering South with altitude and becoming southerly close to the Tropopause. altitude clouds, which are the ones we will focus our attention on, appear from 10 km upward, with two peaks of maximum occurrence respectively at 12 km and at 15 km; then their occurrence tapers off until the tropopause is reached. linear increase of τ with ∆z is apparent, with two different slopes respectively for the SVC and the thin cirrus classes. Figure   260 6 reports the relative difference between the geometrical and optical mid cloud altitude (z g -z o )/z g , vs altitude, colour coded with τ . Positive values indicate that the backscattering is more intense in the upper part of the cloud, which is then radiatively more effective than the lower part. This parameter may vary depending on the distribution of Ice Water Content (IWC) inside the cloud, in turn affected by the microphysics of particle formation and redistribution by sedimentation and evaporation. In our case, the lower and upper parts of the cirrus appear to produce the same scattering effect for small to medium values of τ ,

265
indicative of an even distribution of backscatter inside, which we may take as a proxy for the distribution of IWC. Conversely, the thickest clouds tend to have lower backscatter in their bottom part with respect to the top part, with few exceptions for the highest ones; this is arguably due to mass redistribution by sedimentation and subsequent evaporation.

Clouds optical properties
The trend of depolarization with the altitude, i.e. with decreasing temperature, shows a compact linear relationship with a 270 progressive increase towards higher altitudes, as shown in figure 7. This linearity is apparent from 10 km to slightly below the tropopause. It is interesting to note that approaching the tropopause, a different behavious sets in, so that between 15 and 17 km an entire range of depolarization values is also observed. It is possible to show that the data associated with the compact linear increase of depolarization toward higher altitude are associated with particle backscatter coefficients β a covering the entire range of their variability (in our data, between 10 −8 and 10 −4 m −1 sr −1 ). Conversely, those depolarization data at high altitude 275 which are almost evenly distributed between 10 and 60%, are associated only with medium to low values of β a . The fact that different optical typologies of clouds coexist in the upper part of the TTL can be drawn also from the inspection of the relation between average particle depolarization δ a and optical thickness τ , shown in figure 8. There, two branches can be discerned: for medium to high values of τ associated to thin cirrus, those arguably topping at 15 km in figure 4, there is a decreasing trend of δ a vs τ , while for small to medium values of τ associated to SVC, the particle depolarization is more spread through 280 its variability range. The analysis of the LR adds another piece of information, as it is a parameter linked to the average size dimension of the particles: the lower the LR, the larger the particles. In figure 9 we report the occurrence of LR vs τ .  figure 11 where LR is reported in relation to the cloud temperature.
So in the upper part of the TTL, two classes of clouds are present, namely thin cirrus with high depolarization and low LR, and SVC with medium to low depolarization and larger LR. The optical parameters that we used to discern these two cirrus classes are indicative of differences in the average shape and dimension of the particles that compose them.

Clouds close to the tropopause 295
It is worthwhile zooming in on what happens close to the Tropopause, therefore we move to a reference system centered at the altitude of the CPT, and we replot the statistics of the particle depolarization data with respect to the distance from the CPT in figure 12; again we are able to see the presence of two modes in the depolarization, paricularly evident from 2 to 1 km below the tropopause, and around the tropopause, one with high values and one with medium to low values of the depolarization.
It is worth noting that clouds, especially those with medium to low depolarization values, do not extend significantly above 300 the CPT. In figure 13 for the same subset of observations depicted in figure 12, we report the statistics of the simultaneous occurrence of particle depolarization and local temperature anomalies, these latter calculated as deviations of the temperature profile measured by the concomitant radiosounding with respect to the average temperature profile during the campaign, as reported in the right panel of figure 2. It is evident that cloud occurrence is associated with cold temperature anomalies. It is noteworthy that clouds with high depolarization are present throughout the whole variability range of cold anomalies, while 305 those with medium to low depolarization occur only when the temperature drops 2 to 3 K below the average. It can be shown that such correlation between cloud occurrence and negative temperature anomaly is not present for clouds at lower levels.

Trajectory analysis
The trajectory analysis allows possible linking of the optical characteristics of the clouds with the thermal and convective history of the airmass. Clusters of 1000 trajectories were launched backward from the altitude and time of the clouds obser-310 vations, and the averaged trajectory over each cluster, together with its dispersion, was used for further analysis. Along each of the averaged backtrajectories, the minimum temperature encountered T min and the time elapsed since that, t(T min ), was computed, together with the derivatives of temperature dT /dt, potential temperature dθ/dt and pressure dp/dt at the time of lidar measurements, computed as averages over the past 18 hours. Moreover, the following quantities were also computed: the time during which the air mass remained below the temperature attained at the lidar observation t(T (t) < T 0 ); the minimum https://doi.org/10.5194/acp-2020-1057 Preprint. Discussion started: 19 October 2020 c Author(s) 2020. CC BY 4.0 License. potential temperature θ min and the maximum pressure p max and the relative times elapsed from the lidar observation t(θ min ) and t(p min ).
For each averaged backtrajectory, the 'convective fraction' -defined as the percentage of the trajectories in the cluster that had met convection -was also computed. The time elapsed since the most probable convection encounter was defined as the time from the maximum increase of the convective fraction in the cluster.

320
This information obtained from the analysis of the retro-trajectory was connected to the measured optical parameters: depolarization, optical thickness and LR. We give here a brief resume of the main results; a more extensive analysis and supporting figures are available in the supplementary material.
Most of the clouds in the upper part of the TTL are stationary or experience a slight warming, while some of those in the lower part, especially among the thickest ones, are cooling (figures S4, S5 and S6 in Supplementary material).

325
Most of the cloud observations had max temperature differences along the backtrajectories which are below 5 K, with few exceptions for clouds in the mid TTL between 12 and 15 km, with medium to high optical thickness and medium to low depolarization, which experienced temperature differences as large as a few tens of Kelvin (figures S1, S2 and S3 in Supplementary material).
Most of the clouds encountered their minimum temperature T min less than 48 hrs before observations therefore relatively 330 recently, with few exceptions of clouds below 14 km, with high optical thickness and medium to low depolarization for which the temporal distance from the minimum of temperature along the backtrajectory was as large as 100-200 hours (figures S13, S14 and S15 in Supplementary material).
Generally, the clouds spent less than 48 hrs below the temperature T 0 , so indicative of relatively recent origin, with the exception of some clouds present both in the upper part of the TTL, and below 14 km, which spent several days below T 0 335 (figures S16, S17 and S18 in Supplementary material).
A more interesting picture came from the analysis of the maximum potential temperature difference along the trajectory, indicative of the altitude jump: while for clouds below 16 km this difference is smaller than 10 K, above that level θ max −θ min is consistently greater, ranging from 15 to 25 K (figure S19, S20 and S21 in Supplementary material). In fact, above that altitude the potential temperature gradient begins to assume stratospheric characteristics. Moreover, the clouds in the TTL and 340 particularly in its upper part had met θ min several days before the observation,while on the contrary, in the backtrajectories from clouds at lower levels θ min was temporally significantly closer to the observations, going back to some hours up to one or two days before (figures S22, S23 and S24 in Supplementary material).
The convective influence on the cloud airmass spans between 20 and 60 % , evenly spread across altitude, somewhat larger for the highest and optically thinnest clouds (figures S28, S29 and S30 in Supplementary material); the time elapsed from the 345 most likely convective encounter is very often below two days, except for very few cases, scattered throughout the altitude range, when it is greater (figure S31, S32 and S33 in Supplementary material).
It is worthwhile noting that, for the set of high level SVC with low optical thickness and depolarization and high LR which may be depicted on the left side of figures 9 and 10, they have the highest potential temperature difference along the backtrajectory (figures S19, S20 and S21 in Supplementary material), and the longest time interval from θ min (figures S22, S23 and S24 in Supplementary material) and below T 0 (figures S16, S17 and S18 in Supplementary material), possibly suggesting an origin other than the recent convection one.

Discussion
The analysis shows how clouds occurrence extends throughout the whole TTL, with two peaks, one at 10-13 km and the other at or slightly below the CPT; clouds do not extend significantly above it. It is noteworthy how only those upper TTL clouds are 355 largely affected by local atmospheric temperature anomalies.
Depolarization increases with height and generally decrease with temperature. In the upper part of the TTL, depolarization may vary over a wide range, with some of the highest and thinnest SVC clouds attaining medium to low depolarization and LR greater than those found in other SVC at the same altitude, and in thicker clouds below. In situ measurements, reviewed in Some of the observed SVC may have had rather long life times, and are associated with negligible lifting in the hours preceding the observation. The study of the convective influence is not conclusive, so we may speculate that this class of SVC may have originated by in situ formation, with nucleation processes that induce morphologies, sizes and concentration of ice 370 crystals that differs from those that we can observe at lower altitudes (11-14 km), where the depolarization and the optical thickness is higher and the LR is lower. Such in situ formation may be triggered by the activity of tropical waves that may induce temperature anomalies as large as 2-3 K close to the CPT, while the clouds in the lower part of the TTL, more likely originated by convective events, are dependent on local temperature conditions only for their subsistence and not for their formation. of recent significative uplifting in the histories of those airmass suggests a mechanism of formation by in situ condensation, triggered by temperature fluctuation. The fact that the presence of small particles in cirrus particle size distribution increase in percentage with decreasing temperature i.e increasing altitude, and with increasing time since convective influence, has been reported by Woods et al. (2018).

385
Conversely, the majority of cirrus in the TTL have thermal and convective histories compatible with formation from convective outflows, bringing up moisture and ice. In the thickest of them, the negative difference between the optical and geometrical mid cloud altitude gives a hint that larger ice crystals are being removed from the TTL by sedimentation, leaving smaller particles on optically thinner cirrus that can survive for several hours. Compared to observations in other climatically important regions, observations of equatorial cirrus in the TWP are scarse so this work, albeit limited to lidar observations during a 390 relatively short time frame, contributes to enriching observations. The work would have benefitted from the simultaneous use of balloon hygrometers to better explore the effect of cirrus clouds on the distribution of water vapor in the TTL. An observational campaign with balloon hygrometers and the deployment of the more performing lidar COMCAL by the Alfred Wegener Institute will help to better define the morphology and microphysical processes in TTL in this region.
Data availability. Lidar data are temporarily available at https://stratoclim.icg.kfa-juelich.de/AfcMain/CampaignDataBase/DataMicro, they will migrate to a definitive database in the future. Contact the first author for further information.
Author contributions. FC, MDM, MS, LDL, set up and extensively tested the system and software; FC, MDM deployed and maintained the system in campaign; FC and AS performed the data analysis; SB, BL, AK, provided bactrajectories and convective analysis; SG did a preliminary study on backtrajectory during his master thesis; FC wrote the manuscript which was reviewed by everyone; the software for analysis was produced by FC, MS, LDL, AS, SB.
Ruhe, Impres GmbH, Bremen, which very much assisted us remotely and on the field; finally Maurizio Viterbini, ISAC-CNR now retired without whose invaluable technical support this work would not have been possible.