Lidar-derived CODs are discussed in Section 4 in connection with those derived from visible fluxes measured simultaneously by passive radiometers onboard the ARAT.
Liquid water content derived from the lidar data analysis in convective cells is used to Ž
. derive an estimate of droplet concentration and effective radius at cloud top Section 5 .
The cloud parameters retrieved from the lidar data will be further compared to other estimates from in situ and remote sensing instruments in the conclusion paper of the
Ž .
series Pawlowska et al., 2000b .
2. Airborne measurements
A high pressure system was centered over England bringing cold subsiding air from Ž
northern Europe over the north-western part of France see Section 4 and Fig. 2 of .
Brenguier and Fouquart, 2000 . The structure of cloud radiance observed by AVHRR at Ž
. 08:30 UTC in the visible channel is given in Fig. 1 of Fouilloux et al.,
2000 .
Ž . Ž .
Fig. 1. a Vertical potential temperature and b water vapour mixing ratio profiles measured during the ascent and descent of the ARAT near points A and M.
Stratocumulus, characterized by higher radiance values are seen to extend over the Ž
. English Channel, eastern England and over western Brittany France . Flights were
Ž .
performed by the ARAT on a leg AM about 100 km long, northwest of Brittany above Ž
. this cloud deck see Fig. 1 in Brenguier and Fouquart, 2000 . AVHRR radiances
revealed that the stratocumulus deck was breaking west of point A and did not extend very far beyond this point. The track was flown eight times between these two points.
Ž .
Once between 9:15 and 9:40 UTC, at a level of 300 m further referred to as AM1 to Ž
. document cloud properties from below cloud base height and extinction , and seven
times at a level of 4500 m between 9:58 and 12:18 UTC to retrieve cloud top height and Ž
extinction coefficient as would be obtained from a space-borne system Table 1 in .
Brenguier and Fouquart, 2000 . In this study, we focus on the second AM and MA legs Ž
. flown by the ARAT further referred to as AM2 and MA2 between 10:34 and 10:54
Ž .
UTC, and 10:57 and 11:17 UTC Table 1 in Brenguier and Fouquart, 2000 . The vertical potential temperature and humidity profiles measured during ARAT
ascent and descent near points M and A, at 9:40 and 12:10 UTC, respectively, are reported on Fig. 1. They show a significant evolution in the vertical structure along the
leg defined by these two points, and during the mission, as the temperature inversion height decreases from about 1000 to 800 m going from M to A. The values of water
vapor mixing ratio in the upper mixed boundary layer are comparable at both points, about 4.1 and 4.3 g kg
y1
in A and M, respectively. A 1.2-K increase in temperature is Ž
. observed between A and M. The lifting condensation level LCL calculated from these
values corresponds to an altitude of about 600 m. These observations are in agreement Ž
with those obtained by the Merlin IV see Pawlowska et al., 2000a, which is further .
Ž referred to as PBB . The visible image obtained from AVHRR see Fouilloux et al.,
. 2000 showed that the radiance values were much larger near M, emphasizing the
increased reflectance and optical depth of the cloud deck closer to the coast.
3. Lidar data analysis
