Advances in Space Research 36 (2005) 807–812
www.elsevier.com/locate/asr
Climatology of the subvisual cirrus clouds as seen by OSIRIS on Odin
A.E. Bourassa *, D.A. Degenstein, E.J. Llewellyn
Physics and Engineering Physics, University of Saskatchewan, 116 Science Plaza, Saskatoon, SK, Canada S7N 5E2
Received 29 November 2004; received in revised form 30 November 2004; accepted 2 May 2005
Abstract
The InfraRed Imager (IRI) subsection of the OSIRIS instrument onboard the Odin spacecraft collects limb images of the 1.53 lm
sunlight that is scattered from the upper troposphere and lower stratosphere. Due to the enhancement provided by limb geometry,
the IRI is capable of detecting cloud layers with vertical equivalent optical depths as low as 105. These measurements can be used to
map subvisual cirrus clouds present in the upper troposphere. The dusk–dawn orbit of the Odin spacecraft provides sunlit coverage
of the equatorial region throughout the year. This work presents the subvisual cirrus cloud climatology that has been measured by
the InfraRed Imager. The global seasonal distribution of sub-visible clouds detected by OSIRIS is similar to that measured previously by other instruments, but is obtained on a shorter timescale due to the sampling characteristics of the limb scatter data.
Ó 2005 COSPAR. Published by Elsevier Ltd. All rights reserved.
Keywords: Subvisual; Cirrus; OSIRIS; UTLS
1. Introduction
2. OSIRIS cloud detection
High altitude thin clouds have been the object of
much recent study and have been shown to play an
important role in the EarthÕs atmosphere, both hydrologically (Jensen et al., 1996) and radiatively (Hartmann
et al., 2001). Significant efforts have been made through
in situ and remote sensing campaigns to further the
understanding of cloud occurrence and processes. Previous satellite measurements of clouds near the tropopause have been made with the solar occultation
observations of SAGE II (Wang et al., 1996) and
through observation of the nadir radiance, such as with
the 1.36 lm channel of MODIS on Terra (Dessler and
Yang, 2003). The purpose of the current work is to develop an occurrence climatology of the high altitude
clouds using vertical images of limb scattered sunlight
measured by the Optical Spectrograph and InfraRed
Imaging System (OSIRIS) on the Odin spacecraft.
The OSIRIS instrument on the Odin spacecraft has a
channel that images the solar scattering and emission
brightness of the EarthÕs atmosphere in the limb at
1.53 lm. An on-track forward looking vertical profile
of the limb radiance, at a vertical resolution of 1 km,
is captured in each OSIRIS image. The instrument 2°
field-of-view covers approximately 100 vertical kilometers of the lower atmosphere. Images are collected every
2 s, or 15 km, along the satellite track. As the other
instruments on Odin require the satellite to make a vertical scan, the OSIRIS imager only observes the Upper
Troposphere, Lower Stratosphere (UTLS) altitudes in
approximately 60% of the collected images.
The Odin spacecraft was launched on February 20th,
2001, into a sun-synchronous polar orbit inclined of 97°
with an ascending node local time of 1800 h. As all of
the OSIRIS lines of sight are within the orbit plane,
the solar scattering angle is near 90° for all mid to low
latitudes throughout the year.
Clouds that are both vertically and optically thin are
detected in the imager observations as an enhancement
*
Corresponding author. Tel.: +1 306 966 6456.
E-mail address: [email protected] (A.E. Bourassa).
0273-1177/$30 Ó 2005 COSPAR. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.asr.2005.05.045
808
A.E. Bourassa et al. / Advances in Space Research 36 (2005) 807–812
in the line-of-sight radiance profiles. A typical example
of a tangent height radiance profile with an enhanced
scattering region from a cloud occurring at 17 km is
shown in Fig. 1. The shape of this enhancement below
17 km altitude is characteristic of a vertically thin layer,
of large horizontal extent, imaged in the limb. Path
lengths of the lines-of-sight that intersect the cloud layer
vary with tangent height. The pixels with the highest
lines-of-sight that sample the cloud observe the cloud
at the tangent point. These pixels have a large path
length through the cloud and measure a correspondingly
large enhancement above the background. All pixels
measuring at tangent heights lower than the cloud
height observe the cloud only on the near and far sides
of the tangent point with successively smaller path
lengths and, therefore, measure a smaller enhancement
beyond the background.
The background Rayleigh scattering can be removed
by interpolating the trend above and below the region of
enhancement from the cloud as shown by the dotted line
in Fig. 1. This allows the optical depth of the cloud
along the line-of-sight to be estimated. The observed
radiance, I, is the line integral of the volume scattering
rate, V, along the entire line-of-sight, s.
Z
I ¼ V ds.
ð1Þ
If the magnitude of the solar flux density, Fx, is constant along the entire line-of-sight through the cloud,
the volume scattering rate can be approximated as
V ¼ stot F pðHÞ
;
4p
ð2Þ
where p(H) is the scattering phase function of the cloud
particles. Combining Eqs. (1) and (2) yields an estimate
Tangent Height (km)
30
20
10
0
0
2
4
6
photons
Limb Radiance
cm 2 s sterad
Fig. 1. A typical OSIRIS limb radiance image showing an enhancement from a thin cloud occurring at 17 km altitude.
of the optical depth of the cloud along the line-of-sight
in terms of the measured radiance, I
stot ¼
4pI
.
pðHÞF ð3Þ
This expression assumes that the cloud is sufficiently
thin that multiple scattering events can be ignored and
~ is unity as ice does
that the single scatter albedo, x,
not absorb strongly in the near infrared.
If further assumptions are made about the physical
extent of the cloud, the total optical depth can be converted to an equivalent vertical optical depth such as
that measured by a lidar in the zenith or by a nadirlooking satellite instrument. If the vertical extent of
the cloud, Dz, is estimated based on the shape of the
enhancement in the radiance profile, the path length of
the line-of-sight through the cloud at the tangent point,
Ds, can be calculated. The ratio of Dz to Ds is the conversion factor from total optical depth, stot, to vertical
optical depth, s
s ¼ stot
Dz
.
Ds
ð4Þ
Using a Mie phase function for log-normally distributed
ice particles with a 10 lm mode radius, the typical optical depths of OSIRIS cloud detections are near
s = 0.003. This is an order of magnitude below the accepted threshold for ‘‘subvisual’’ clouds, s < 0.03, derived by Sassen et al. (1989). Many clouds that are
detected in the OSIRIS profiles are sufficiently optically
thick that they saturate the detector and the optical
depth cannot be calculated. A radiance that just saturates the detector corresponds to an optical depth of
s = 0.005. Therefore, this is the maximum optical depth
that the OSIRIS imager can resolve. The only statement
that can be made about clouds that saturate the imager
is that the optical depth is greater than s = 0.005. With
respect to the OSIRIS cloud detections and this climatology study, ‘‘thin’’ clouds and ‘‘thick’’ clouds are separated by this optical depth threshold. The thinnest
clouds that have been detected in the OSIRIS data set
to date have an optical depth near 105.
A multi-layer perceptron (MLP) network, often referred to as a ‘‘neural net’’ is used to identify images
in the data set that contain the signature of a cloud.
The MPL is trained with data set of a large number of
manually chosen images that are known to contain
cloud signatures. Due to the extreme sensitivity of the
imager for tropospheric brightness levels (the exposure
times are tuned for strat/meso science) the imager data
set cannot be used with the MLP detection scheme for
detecting clouds below 12 km altitude. All cloud detections made by OSIRIS are between 12 and 25 km altitude. For each image taken by OSIRIS, the MLP
makes a decision whether or not the image contains
the cloud signature. The number of cloud detections
A.E. Bourassa et al. / Advances in Space Research 36 (2005) 807–812
and the total number of samples are logged for a geographic bin size of 5° latitude and 20° longitude. This
bin size reflects the longitudinal separation between
sampled locations in successive orbits at the equator
and the altitude scan rate of Odin that dictates the frequency of UTLS samples in latitude. A frequency of
detection can then be calculated for each bin, for a given
period of time, as the ratio of the total number of detections to the total number of samples.
3. Cloud climatological distributions
The geographic distribution of OSIRIS imager cloud
detections between 12 and 25 km altitude for the first
three years of the mission (July, 2001, to July, 2004) is
shown in Fig. 2. The left panel plots the frequency of
detection for all clouds whereas the right panel shows
only the ‘‘thin’’ cloud detections. This climatology
shows good agreement with the 6-year thin cloud climatology measured by SAGE II (Wang et al., 1996) both in
terms of geographic location and frequency of detection.
The highest frequency of cloud occurrence (approximately 75%) occurs over Indonesia, west-central Africa
and the northern part of South America. These are regions that are often associated with strong convective
activity and biomass burning. A similar pattern in the
distribution is observed in the ‘‘thin’’ cloud detections,
although the maximum frequency of detection is much
lower, near 10%.
The seasonal average of all cloud detections is shown
in Fig. 3. As cloud detection measurements cannot be
made in the absence of sunlight locations that are not
illuminated at the Odin local time (0600/1800 h) are coloured black on the maps. It is interesting to note that the
regions of high frequency of cloud occurrence shown in
the 3-year average in Fig. 2 are most fully developed in
809
the boreal winter. During the boreal summer, a very different global distribution has formed consisting of a single highly developed maximum of cloud occurrence over
south Asia. Distributions of cloud occurrence during
spring and autumn are similar in nature to the boreal
winter but, on average, the detection frequency is lower
and the maximums are not as highly developed.
A zonal average of the 3-year seasonally separated
OSIRIS cloud detections is shown as a function of altitude in Fig. 4. As the cloud detection algorithm is only
sensitive above 12 km altitude the apparent lack of
clouds between 10 and 12 km is due to this deficiency,
and not a physical phenomenon. A seasonal shift in
the latitudes that contain the highest frequency of cloud
occurrence is also evident in this plot. The distribution
of the cloud detections is centered about the equator
in both the spring and autumn. The clouds tend to favour formation in the summer hemisphere as the distribution is centered south of the equator in the boreal
winter and moves north of the equator in the austral
winter. It should also be noted that the maximum magnitude of detection frequency and latitudinal extent decreases in the austral winter.
The longitudinal dependence of the cloud detection
frequency for the entire mission is shown in Fig. 5(a).
In this plot, all latitudes are counted equally so that
the frequency of detection is correspondingly much lower than for a single bin near the equator. The major
maxima in detection frequency noted previously are also
evident in this plot. Panels (b) and (c) are plots of the
longitude/altitude dependence as a difference in cloud
detection frequency at the two Odin local times. During
the ascending track of the satellite (South Pole to North
Pole) the local time is near 1800 h, whereas for the
descending track, the local time is 0600 h. Thus OSIRIS
provides both a morning view and an evening view of
the equatorial region. Panel (b) plots the difference
Fig. 2. Full mission OSIRIS cloud detection climatology for clouds between 12 and 25 km altitude. All cloud detections are shown in the left panel
and only ‘‘thin’’ cloud detections are shown in the right panel.
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A.E. Bourassa et al. / Advances in Space Research 36 (2005) 807–812
Fig. 3. OSIRIS 3-year climatology of all cloud detections separated by season.
detections. There is a large difference between the morning and evening cloud occurrences in panel (b) that inverts near 85° longitude. In the morning, the cloud
detection frequency is high to the west of 85°, and in
the evening, it is high to the east of 85° longitude. An
interesting feature can also be seen in panel (c) where
the maximum in cloud detection frequency varies between morning and evening four times at nearly equal
spacing across longitudes.
4. The OSIRIS thin cloud detection data product
Fig. 4. Zonal average of seasonal cloud detection distribution.
between the morning view and the evening view of cloud
detection frequency for all clouds detections in the mission. Panel (c) plots the same for only ‘‘thin’’ cloud
A unique and appealing feature of the OSIRIS thin
cloud detections is that a relatively large number of samples can be made globally in a few days. Fig. 6 shows
geographic distributions of OSIRIS cloud detection calculated on a monthly basis over the course of one year.
Again, the black areas of the map correspond to darkness at the satellite local time. The seasonal trends discussed in the previous section are certainly observable
in the monthly maps; however, there are distinct
monthly differences within the same season. A large
number of high altitude thin clouds were detected near
the south pole during September. The Antarctic UTLS
region was relatively cloud-free again by October.
A.E. Bourassa et al. / Advances in Space Research 36 (2005) 807–812
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Fig. 5. (a) An average of the full mission cloud detection distribution over all latitudes. (b) Longitudinal distribution of difference in cloud detections
between local time samples (0600–1800 h). (c) Same as (b) for ‘‘thin’’ clouds only.
Fig. 6. The OSIRIS cloud detection data product: global one-month averages.
We speculate that these clouds are the well known Polar
Stratospheric Clouds (PSCÕs); however, no work has
been done to extend the analysis of this data set.
The OSIRIS subvisual cirrus data product is composed of global geographic distributions calculated on
a two week basis similar to the monthly distributions
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A.E. Bourassa et al. / Advances in Space Research 36 (2005) 807–812
shown in Fig. 6. These are available for the entire 3-year
mission. Each individual cloud detection is also available on an image-by-image basis; this includes cloud
height, thickness and radiance, which is a bi-product
of the MLP algorithm. These detections have not yet
been validated with other satellite cirrus products.
5. Summary and conclusions
The OSIRIS imager is able to detect optically and
vertically thin high altitude clouds as an enhancement
in the line-of-sight radiance profiles collected at
1.53 lm. The global distribution of these cloud detections over the 3-year mission (up-to-date) shows a correlation with the 6-year climatology measured by SAGE
II. A maximum frequency of occurrence, approximately
60%, is observed near equatorial regions.
The limb scatter technique allows a large number of
global samples to be made over a relatively short time
period. A thin cloud detection data product in the form
of biweekly distributions of cloud detection frequency
for the entire mission from the University of Saskatchewan OSIRIS web-site.
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