Geoscience Reference
In-Depth Information
could explain in part the difference in the clear sky occurrence between the two types
of measurements (12%). Also, note that ISCCP assumes that if a cloud is present,
there is a single cloud structure in the atmospheric column. The inference of CTH for
optically thin cirrus is problematic and such a cloud may be assessed as being a lower-
level cloud. The assumption of a single cloud structure in a satellite imager pixel can
lead to biases in CTH when multilayered cloud structures exist. It is interesting to note
that the multilayered cloud classes (H+M), (H+L+M), and (H+L) retrieved from LITE
profi les are redistributed in the middle and low cloud classes of the ISCCP climato-
logy. This could explain the great percentage of middle and low cloud structures in the
ISCCP products. Moreover, Evan et al. (2007) demonstrated that the long term global
trends in the cloudiness from the ISCCP record are infl uenced by artifacts associated
with satellite viewing geometry. This study underlines a non-physical decrease of the
total cloud amount as given by ISCCP of about 6% between 1987 and 2000. This can
also explain partially the differences observed in the proportion of clear sky observations.
The lack of optically thin cirrus clouds in the ISCCP products is also described in
Jin et al. (1996). They assessed the latitudinal spread of the thin cirrus cloud fraction,
in a comparison of the ISCCP and high resolution infrared radiation sounder (HIRS)
high cloud products. In all the cases and considering all the latitude range, ISCCP
seems to underestimate the fraction of cirrus cloud type. In particular, we note a larger
difference in the ITCZ region.
Comparison to Cloud Statistic from MODIS Measurements
Cloud products are available from the MODerate-resolution Imaging Spectrometer
(MODIS; Platnick et al., 2003) and provide global CTH. The approach assumes only a
single-layer cloud in the entire atmospheric column, and effort is underway to include
multispectral approaches for the detection of multilayered clouds, specifically for the
case of cirrus overlying a lower-level water cloud.
High-level and mid-level clouds are analyzed with the CO
2
-slicing method deve-
loped by Menzel et al. (2002); this approach infers cloud top pressure and effective
cloud amount (emittance multiplied by cloud fraction) for clouds at pressures lower
than approximately 700 hPa. The MODIS results are similar to those presented in
this study for the spaceborne lidar systems. In particular, the ratio of semitransparent
clouds to the total cover of high clouds shown in Table 2 is closer to the results de-
duced from LITE dataset.
Comparison to Clouds Statistics from TOVS and Others
Other cloud products derived from passive sensors, such as TOVS Path-B (Stubenrauch
et al., 2006a, b), or from atmospheric infrared sounder (AIRS), identify more cir-
rus clouds than ISCCP. From analysis of TOVS Path-B products from 1987 to 1995,
the percentage was ~4% in the middle latitudes (30º-60ºN and 30º-60ºS), and up to
20% in the Tropics (15ºN-15ºS). For the AIRS L2 data analyses, the ITCZ high cloud
amount is retrieved as being 10% larger than that from ISCCP (Stubenrauch et al.,
2006a). As for MODIS, the ratio of semitransparent clouds to the total cover of high
clouds shown in Table 2 indicates that a higher proportion of optically thick clouds
than that derived from LITE measurements for all the latitudes.
