Ž .
Fig. 6. Vertical distribution of the cloud top extinction coefficient at 532 nm for legs AM2 and MA2. The Ž
. dotted line represents Eq. 11 . The solid line marks the altitude of the highest cloud observed.
backscatter coefficient. Extinction and altitude simultaneous increases and decreases are indicative of convective cells, already identified in Fig. 3. The correlation coefficient
between these two parameters is 0.92. Small-scale fluctuations appear to be more important on the extinction coefficient than on the cloud top altitude. This results in the
vertical distribution of the cloud top extinction shown in Fig. 6. Data from legs AM2 and MA2 have been included to emphasize vertical fluctuations of the cloud optical
properties caused by the entrainment process at top of the cloud deck. In Fig. 6, 99.9 of the values of the cloud-top height are observed between 1120 and 850 m. The largest
value of cloud-top extinction is about 0.17 m
y1
. Precision in the filtered values of the extinction coefficient is expected to be of the order of 10. This figure is further
discussed in Sections 5 and 6.
4. COD
The COD has then been estimated from the retrieved extinction coefficient at cloud Ž
. top using cloud geometrical thickness retrieved from lidar Fig. 3 . It was assumed that
for each measurement point, the extinction coefficient was linearly increasing from cloud base up to the value derived at cloud top. This hypothesis is used as an
approximation of the ratio of the vertical distribution of the liquid water content and Ž
. effective radius defining the extinction coefficient as discussed in Section 5 in both
Ž .
upward and downward motions see in situ measurements in PBB . The estimated optical depth t is thus given by
1 t s
a z , 4
Ž .
t t
2 where a and z
are the extinction coefficient at the top of the cloud and the cloud
t t
geometrical thickness, respectively. Ž
Results from legs AM2 and MA2 are reported in Fig. 7a and b, respectively values .
Ž .
are filtered over 5 points . The convective structures near 48.68N previously identified in Fig. 5 are also clearly evidenced here. CODs as large as 40 are observed in the most
Ž .
energetic updrafts, and a large variability within a factor 3 is observed between updraft and downdraft.
Simultaneously to lidar measurements, the upward and downward shortwave fluxes were measured onboard the ARAT using the broadband visible Eppley pyranometers. A
calibration flight performed with the Merlin IV of Meteo–France and the Falcon 20 of
´ ´
DLR was designed to assess the consistency of the visible flux measurements made by the three aircraft. On average, they agreed to within 0.2 as the three aircraft were
Ž .
flying at an altitude of 1.5 km Sauvage et al., 1999 . The plane albedo A of the
a
surface–cloud–atmosphere system was deduced from the ratio of the upward and downward shortwave flux F
measured by the upward and downward looking
SW
pyranometers as the ARAT flew over the stratocumulus layer F
≠
SW
A s .
5
Ž .
A
F x
SW
As data were taken on a constant level leg near noon, no attitude nor solar zenith Ž
. elevation corrections were made on the measured solar fluxes Saunders et al., 1992 .
Ž .
Albedo values increase from about 0.2 near A to 0.7 near point M not shown for leg MA2. Using a plane-parallel cloud model and assuming no absorption, two-stream
approximations in radiative transfer calculations show that the COD, t , can be deduced Ž
. from the total plane albedo A as Meador and Weaver, 1980
A
A
A
t s ,
6
Ž .
g 1 y A
Ž .
A
Ž where g is a parameter depending on the radiation model used Meador and Weaver,
. Ž
. 1980 . In the case of the Eddington scheme, the value of g is 3r4 1 y g , where g is
the asymmetry factor of the cloud droplet distribution. As g s 0.85 for water spheres, we obtain g s 8.8. Fig. 8 shows a comparison between the COD derived from the
albedo analysis and the COD deduced after fitting a fifth order polynomial function on the lidar-retrieved optical depth given in Fig. 7b. At the mesoscale, the tendencies are
Ž .
Ž Fig. 7. Estimated cloud optical depth at 532 nm vs. latitude as derived from lidar data filtered extinction
. Ž .
Ž . coefficient and cloud top altitudes along leg a AM2 and b MA2.
Ž .
Fig. 8. Comparison of the optical depth retrieved from lidar measurements at 532 nm and from the cloud albedo analysis using ARAT flux measurements for leg MA2.
comparable between the two. In the optically denser part of the cloud, values are in good Ž
. agreement about 20 or better . On the other hand, CODs retrieved from passive
measurements are larger in the other parts of the cloud. Most likely, this discrepancy is due to a combination of effects: cloud inhomogeneity and the fact that the lidar and the
pyranometers have different fields of view. It could also be related to the simplified
Ž Ž ..
COD calculation used i.e. Eq. 6 . This point will be further investigated.
5. Retrieval of cloud microphysics
