Figure 5 displays the vertical profiles of the retrieved extinction coefficient from CALIOP level 2.01 operational cloud profile product for the measurements acquired on (Figure 5a) 16, (Figure 5b) 23, (Figure 5c) 25, and (Figure 5d) 26 May. The CALIOP extinction coefficient is obtained with 5 km and 60 m horizontal and vertical resolutions, respectively. The superimposed red lines are the Falcon flight altitudes of the cloud sequences reported in Table 1.
 We recall that the in situ extinction coefficient can be derived from both the Polar Nephelometer and the combined FSSP-300 and CPI measurements. As we will discuss below, cross-correlations performed between extinction measurements obtained from these two different techniques highlight very good results which validate the probe calibrations and the methods of data processing. Indeed, the Polar Nephelometer (PN) extinction will be used in the following. It should be noticed that the in situ observations have been averaged over the horizontal CALIOP pixel resolution (i.e., 5 km (see Figure 5) or about 25 s according to the mean Falcon airspeed). The flight trajectory was first projected onto the CALIOP vertical plane by considering the mean wind advection at the corresponding levels and the time difference between satellite and in situ measurements. This was done to reduce inherent errors in comparing quasi-instantaneous spaceborne observations and aircraft measurements carried out during a much longer duration. Then for each satellite pixel the spatial collocation was realized according to combined corrected latitude-longitude coordinates of the satellite track and the DLR F20 aircraft.
 The results of the CALIOP validation are displayed in Figure 6. Figure 6 displays the CALIOP extinction versus the PN extinction for the (Figure 6a) 16, (Figure 6b) 23, (Figure 6c) 25, and (Figure 6d) 26 May cirrus cases. The horizontal bars represent the standard deviation of the in situ extinction, which results from the horizontal cloud variability over 25 s. The examination of the results shows a very good correspondence between the two measurements for the 23 and 25 May situations despite very different situations: outflow cirrus and thin frontal cirrus, respectively. The slope parameters of the linear fits are 0.94 and 0.90, with maximum extinction values of about 1.2 and 0.6 km−1, respectively. The correlation coefficient is much better for 25 May (0.69) than for 23 May (0.36) because of a smaller number of measurements with a larger dispersion.
 As for the 16 and 26 May situations, significant differences are evidenced between the two measurements. For the 16 May comparison, systematically larger CALIOP extinctions than PN observations are evidenced (slope parameter of 2.27). Indeed, for CALIOP values in the range from 0.4 km−1 to 0.9 km−1, no agreement can be found, since the PN extinctions remain no larger than about 0.25 km−1. In contrast, the 26 May CALIOP values are systematically lower than the in situ observations with a slope parameter of 0.62 and a correlation coefficient of 0.72, and without apparent saturation on either of the signals. We now discuss the interpretation of the measurements in order to explain the differences evidenced on the 16 and 26 May cirrus cases.
4.3.1. The 16 May Cirrus Case
 We recall this situation concerns a frontal thin cirrus over ocean like the 25 May cirrus case with quite similar geometrical properties (see Figures 5a and 5c) but with lower temperatures (−56°C to −59°C against −50°C to −54°C; see Table 1). The very coherent nature of the in situ observations argues strongly against any systematic errors in the in situ measurements when comparing the extinction relationships in Figure 6. As a matter of fact, Figure 7 reports comparisons between the extinction coefficients from the combined FSSP-300 and CPI instruments and the Polar Nephelometer probe for the (Figure 7a) 16, (Figure 7b) 23, (Figure 7c) 25, and (Figure 7d) 26 May situations. Cloud data at 1 Hz frequency are shown in Figure 7. The results emphasize that the two measurements fit very well for the four cirrus cases, with quasi-identical slope parameters of the linear fits (1.06 ± 0.03) and correlation coefficients close to 0.9. Likewise, very similar particle size distributions and extinction distributions are observed for the 16 and 25 May situations, as reported in Figures 8a and 8b, respectively. Figures 8a and 8b represent the particle size and extinction distributions measured by the FSSP-300 and CPI probes and are averaged over the cloud sequences for each of the four cirrus cases. Table 2 reports the mean values of the microphysical parameters. Considering the 16 and 25 May observations, because only few ice particles larger than 100 μm in diameter were observed (0.5 and 1.5 L−1, respectively; see Table 2), with no ice crystals larger than about 350 μm, we may expect that the shattering effects are probably not very important and are not greater than the usual random uncertainties (i.e., 25% for the PN extinction [see Gayet et al., 2002b, Table 1]. However, hypothesizing shattering occurrence, the effects on measurements (on both FSSP-300+CPI and Polar Nephelometer) should be of the same order for the 16 and 25 May situations because of the similarities of the size distributions.
Figure 7. Comparison between extinction coefficients from combined FSSP-300 and CPI instruments and Polar Nephelometer probe for the (a) 16, (b) 23, (c) 25, and (d) 26 May situations. The slope parameters with their uncertainties and correlation coefficients are reported. Horizontal gray bars represent the 25% uncertainties on Polar Nephelometer measurements.
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Figure 8. (a) Particle size distributions and (b) particle extinction coefficient distributions determined by the FSSP-100 and CPI probes as a function of diameter and averaged over the cloud sequences related to the 16, 23, 25, and 26 May cirrus cases.
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Table 2. Mean Values of Microphysical Parameters Obtained During Cirrus Cloud Sequencesa
|Day (in 2007)||Particle Conc. (cm−3)||Particle Conc. (D > 25 μm) (L−1)||Particle Conc. (D > 100 μm) (L−1)||IWC (mg m−3)||IWC (D > 25 μm) (mg m−3)||Extinction (km−1)||Extinction (D > 25 μm) (km−1)||Deff (μm)||Deff (D > 25 μm) (μm)|
 In conclusion, the close agreement between the in situ measurements from 16 and 25 May strongly suggests that the disparities seen in Figure 6a are not due to errors in the Polar Nephelometer data, but should instead be attributed to overestimates generated by the CALIOP retrieval. Therefore, one possible explanation may be the preferential orientation of the planar-shaped ice crystals, which can provoke a dramatically stronger lidar backscatter than would be expected for randomly oriented ice particles [Sassen, 1980; Hu et al., 2007]. A stronger extinction value will be retrieved in that case. In order to give arguments to support this hypothesis, the CPI ice particle shape classifications (represented for number, surface, and mass percentages) are displayed in Figure 9. The comparison of the results shown in Figures 9a (16 May) and 9c (25 May) clearly highlights significant differences in dominant crystal shape within the temperature (or altitude) domains in which the CALIOP comparisons have been made. Pristine-plate ice crystals dominate the ice crystal shape during the 16 May cirrus case, as clearly evidenced by CPI examples of ice crystal images in Figure 9a, whereas for the 25 May situation (Figure 9c) the main shape of the particles is irregular with some bullets and plates (see examples of ice crystals on Figure 9c). Pristine-plate ice crystals with sizes up to 300 μm could be horizontally oriented [Bréon and Dubrulle, 2004] and may therefore explain the poor extinction comparison for the 16 May data.
Figure 9. CPI classification of the ice particle shape in number, surface, and mass percentages for the (a) 16, (b) 23, (c) 25, and (d) 26 May situations. The rectangles represent the temperature domains in which the CALIOP and in situ comparisons have been made.
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 These findings nicely confirm the interpretation of the comparison results between CALIOP and LNG measurements discussed in section 4.1. A signature of oriented ice crystals is evidenced in region 2 (16 May situation; see Figure 3), particularly in the half lower part of the cirrus layer, which was sampled by the DLR F20 aircraft during the second cloud sequence (see Table 1). On the contrary, no signatures of oriented particles are found in the cirrus layer sampled by the DLR F20 during the 25 May situation, as evidenced from LNG observations.
 The preferential orientation signature could also be verified from the diagram of the CALIOP layer-integrated attenuated backscatter (γ′) versus the layer-integrated depolarization ratio (δ) as proposed by Hu et al.  and Cho et al. . These diagrams are reported in Figure 10. The results in Figures 10a (16 May) and 10c (25 May) clearly show the signatures of both the low-level water clouds and the randomly oriented ice crystals in cirrus clouds [see Hu et al., 2007]. The observation of pristine-plate ice crystals at the CALIOP validation levels during the 16 May cirrus case are consistent with the location of the data points in Figure 10a at the upper left portion of the scatterplot, which corresponds to horizontally oriented ice crystals as hypothesized by Hu et al. . The number of pixels is poor due to the small CALIPSO data set available during these limited cloud sequences. Nevertheless, this feature is not observed for 25 May, and this would confirm our findings about the orientation effect of particles. The boundary stratiform clouds over the sea on the 16 and 25 May situations were detected by CALIOP because of the relatively low cirrus optical depth (∼0.5) and fractional structure of the cirrus layers during the considered flight sequences. This feature is not observed during the outflow cirrus cases shown in Figures 10b and 10c. We note in passing that no more clouds were observed between the cirrus and the stratiform cloud layers for these two case studies.
Figure 10. Diagram of γ′ − δ from CALIOP data for the (a) 16, (b) 23, (c) 25, and (d) 26 May situations. Color scale represents the frequency of occurrence, and the resolutions of each pixel are Δγ′ = 0.004 sr−1 and Δδ = 0.02.
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 The presence of horizontally oriented plates in cirrus clouds is attested to by numerous previous studies from spaceborne reflectance observations [Chepfer et al., 1999; Bréon and Dubrulle, 2004; Noel and Chepfer, 2004]. On a global scale, this feature is apparent for roughly half of the cirrus clouds observed by Polarization and Directionality of the Earth Reflectances (POLDER) [Bréon and Dubrulle, 2004], and more frequently at high latitude. As a result, larger extinction values and subsequently larger cirrus optical depths should be retrieved from CALIOP observations. In order to avoid such biases in the CALIPSO retrievals, since November 2007, the CALIOP laser beam is tilted of 3° ahead of the nadir pointing direction. On the other hand, the climatology of the oriented plate ice crystals is of great interest regarding the optical and radiative properties of cirrus clouds. Direct retrieval of optical depths using backscatter from water clouds or surface echoes [Hu et al., 2007; Josset et al., 2008] is expected to provide new insights for the analysis of these properties.
4.3.2. The 26 May Cirrus Case
 We recall that the 26 May case addresses outflow cirrus like the 23 May situation, but with a larger optical depth, since the PN extinction reaches 2.5 km−1 against 1.2 km−1 (see Figures 6d and 6b, respectively). As in the previous discussion, the coherence of the in situ observations eliminates concerns about systematic errors in the in situ measurements when comparing the extinction relationships in Figure 6d. Quasi-identical slope parameters (1.06 and 1.05) are found for the two considered situations (see Figures 7b and 7d). Likewise, quite similar ice crystal shape distributions are observed for the both outflow cirrus cases within the temperature (or altitude) domains in which the CALIOP comparisons have been made. The dominant shape (bullet-rosette) is observed about 30% and 50% of the time (in number, surface, and mass distributions) for 23 and 26 May, respectively, as exemplified by the CPI images shown in Figures 9b and 9d.
 The extinction results retrieved from the CALIOP data are strongly dependent on the lidar ratio and multiple-scattering coefficient, and any cause for possible variability of these parameters must be examined. Because the lidar ratio depends on the shape, size, and orientation of the ice particles [Chen et al., 2002], the observation of rather similarly shaped (nonplanar) ice crystals allows us to assume that any crystal-shape effect could be neglected. However, compared to the 23 May case, significantly larger ice particle sizes and extinction coefficients are evidenced for the 26 May situation (see Figures 8a and 8b and Table 2) with a consequently greater optical depth. If the CALIOP lidar ratio is changed from 25 to 40 sr, the CALIOP extinction coefficient will increase by a factor of 1.6 (40/25), and the linear fit in Figure 6d becomes very close to unity (0.62 × 1.6 = 0.992). Therefore the CALIOP data would be more consistent with the in situ observations. A lidar ratio value of 40 sr is within the upper part of the one sigma variation from the Sassen and Comstock  results at the corresponding optical depth. Nevertheless, for clouds with high optical depths the multiple-scattering effect lowers the effective lidar ratios compared with single-scattering condition [Chen et al., 2002]. Because CALIOP and PN extinctions agree well for the 23 May situation, this implies that the lidar ratio and multiple-scattering values of 25 and 0.6 are suitable for this case. Therefore with a similar ice crystal population but with a larger extinction (and subsequent optical depth) the multiple-scattering coefficient and the lidar ratio would have lower values according to the trend from Sassen and Comstock , a conclusion which is contradictory with our 26 May observations.
 As already indicated, large ice crystals (up to 800 μm) are measured during the 26 May situation (see Figure 8). Therefore, the contamination of the FSSP-300, CPI, and Polar Nephelometer measurements by the shattering of ice crystals could likely be more important than for the other cases, since the concentration of particles with diameter larger 100 μm is significant (10 L−1, see Table 2). It is conceivable that the effects of shattering depend on the design of the probe inlet [Heymsfield, 2007]. The extinction coefficients are inferred from the FSSP-300 + CPI and from the Polar Nephelometer probes, which all have very different inlet designs (for instance, inlet diameters of 40, 23, and 10 mm, respectively). The hypothesis that the shattering of large ice crystals affects the FSSP-300+CPI and PN measurements in the same way, or with a same efficiency, appears unlikely. This is supported by the consistency of comparison results between extinctions calculated from two different techniques (FSSP-300 + CPI and PN) and obtained during very different microphysical cloud properties (rather sharp and broad size distributions; see Figure 8a). This would appear unlikely if artifacts dominate the measurements. Otherwise, with regard to the very good agreement between CALIOP and PN observations when small ice crystals are evidenced (23 and 25 May), the subsequent shattering contamination of the FSSP-300 and PN measurements in presence of more numerous and larger ice crystals seems a plausible explanation for the larger PN values (38% larger than CALIOP extinction values) evidenced in Figure 10d. In conclusion, the relative importance of the effects of shattering of ice crystals on the in situ measurements (the extinction coefficient in our case) remains an open question. For example, contradictory conclusions have been drawn about the reliability of the Cloud Integrating Nephelometer (CIN) [Gerber et al., 2000] with regard to the shattering contamination. Garrett  suggested an absence of sensitivity to shattering of particles on the CIN aperture. Comparisons of lidar volume extinction from the airborne Cloud Physics Lidar (CPL) and in situ CIN extinction measurements have shown very good agreement [Noel et al., 2007].
 Another plausible explanation for the differences observed could be the weak spatial and temporal coincidence with the satellite observations due to (1) the restricted flight area over Germany by the Air Traffic Control Authority, which permitted only rather short flight legs under the satellite trace (see Figure 5d, which shows cloud sequences of 0.6° latitude long against more than 2° for the other cases); (2) the internal structure of the cirrus clouds, which varied very rapidly during the time of observations [Protat et al., 2009]; indeed the outflow cirrus sampled on 26 May topped the main convective system, whereas during the 23 May situation the sampled cirrus resulted from an advected anvil, as seen in Figure 5b; and (3) the horizontal inhomogeneities in the cloud properties: these could be estimated from the standard deviation (or variance) when averaging the 1 Hz (or ∼200 m horizontal resolution) Polar Nephelometer extinction over the CALIOP pixel resolution (5 km); the standard deviations (see the error bars in Figure 6) clearly show large values for the 26 May case (Figure 6d), with the one sigma variation overlapping the 1:1 slope.