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. 810 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) 807–812 811 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 812 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. References Dessler, A., Yang, P. The distribution of tropical thin cirrus clouds inferred from Terra MODIS data. J. Climate 16, 1241–1247, 2003. Hartmann, D.L., Holton, J.R., Fu, Q. The heat balance of the tropical tropopause, cirrus, and stratospheric dehydration. Geophys. Res. Lett. 28, 1969–1972, 2001. Jensen, E.J., Toon, O.B., Pfister, L., Selkirk, H.B. Dehydration of the upper troposphere and lower stratosphere by subvisible cirrus clouds near the tropical tropopause. Geophys. Res. Lett. 23, 825– 828, 1996. Sassen, K., Griffin, M.K., Dodd, G.C. Optical scattering and microphysical properties of subvisual cirrus clouds and climatic implications. J. Appl. Meteorol. 28, 91–98, 1989. Wang, P.H., McCormick, M.P., Minnis, P., et al. A 6-year climatology of cloud occurrence frequency from SAGE II observations (1985–1990). J. Geophys. Res. 101, 29407–29429, 1996.
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