Characteristics of cirrus clouds from ICESat/GLAS observations

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L09810, doi: /2007gl029529, 2007 Characteristics of cirrus clouds from ICESat/GLAS observations Nawo Eguchi, 1 Tatsuya Yokota, 1 and Gen Inoue 2 Received 30 January 2007; revised 31 January 2007; accepted 6 April 2007; published 9 May [1] Cloud observations from the Geoscience Laser Altimeter System (GLAS) revealed characteristics of cirrus clouds in boreal autumn The vertical distribution of the central altitude of cirrus peaks 2 km below the climatological tropopause, which is 14.5 km in the tropics and 9.5 km in the northern midlatitudes. The mean location of the peak in deep convection is north of the Equator (7.5 N) but the top of zonally averaged cirrus is almost constant at 14.5 km in the tropics. This suggests that the height of tropical cirrus is closely linked to anvil cirrus from deep convection and lower temperatures in the tropopause symmetric with respect to the Equator. Cirrus clouds in the midlatitudes have a greater optical depth than those at other latitudes. The zonally averaged thickness of cirrus is about 1.6 km regardless of latitude. Citation: Eguchi, N., T. Yokota, and G. Inoue (2007), Characteristics of cirrus clouds from ICESat/GLAS observations, Geophys. Res. Lett., 34, L09810, doi: / 2007GL Introduction [2] Cirrus clouds in the upper troposphere affect both the terrestrial radiative balance and the water content and chemical balance of the stratosphere [e.g., Ramanathan and Collins, 1991; Gao et al., 2004]. Despite their importance, a lack of observational data has precluded a quantification of their effects in the atmosphere. Consequently, although cirrus clouds affect the remote sensing of other atmospheric parameters, such effects are not considered in most data analysis algorithms, leading to large data uncertainties. [3] The Greenhouse gases Observing SATellite (GOSAT), scheduled to launch at the end of 2008, will measure CO 2 column values [Yokota et al., 2004]. Cirrus clouds comprise a major source of error that may cause inaccurate estimates of CO 2 column because reflectance from cirrus contaminates the reflectance from lower layers used to derive the retrieved CO 2 column. The GOSAT retrieval algorithm therefore uses a priori information on cirrus parameters such as cloud top altitude and optical depth (t) to reduce errors. [4] Previous in situ measurements (i.e., aircraft- and ground-based observations) have clarified several aspects of cirrus parameters [Dowling and Radke, 1990]. However, such data have limited availability in time and space, and global observations are needed to fully understand cirrus 1 Center for Global Environmental Research, National Institute for Environmental Studies, Tsukuba, Japan. 2 Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan. Copyright 2007 by the American Geophysical Union /07/2007GL features. The Geoscience Laser Altimeter System (GLAS) on the Ice, Cloud, and land Elevation Satellite (ICESat) can measure the vertical distribution of cirrus clouds on a global scale, with t down to 0.01 [Spinhirne et al., 2005]. Many past studies have investigated cloud parameters such as cloud height and t, and validated data quality by using GLAS cloud data and comparing it to data from other in situ and satellite measurements and reanalysis data sets [e.g., Palm et al., 2005; Dessler et al., 2006]. However, the vertical distribution of cirrus parameters on the global scale has had little discussion. The present study applied GLAS data to investigate the height, geometric thickness, and optical depth (t) of cirrus at global scales. 2. Analysis Data [5] The GLAS has two laser wavelength channels: one is at 1064 nm and the other is at 532 nm. The 1064-nm channel detects mainly the surface and the height of relatively thick clouds. The 532-nm channel obtains the heights and optical properties of thinner clouds and aerosol layers [Palm et al., 2002]. The laser footprint is about 60 m, and data are recorded at 40 Hz with a vertical and horizontal resolutions of 76.8 and 172 m, respectively [Spinhirne et al., 2005]. The present report considers top and bottom heights of the cloud layer derived from the GLA09 product (Release-26) and optical depth (t) derived from the GLA11 product (Release-26). Data were obtained between 1 October and 18 November 2003, and were limited to this period because data from the first 7 days of the laser 2A period might include substantial errors (J. Spinhirne, personal communication, 2006). The average of the top and bottom heights of the cloud layer is the cloud center. [6] Cirrus clouds are defined in this paper as having a geometric thickness of 8 km or less and t of 3 or less. In addition, the bottom altitude must exceed 8 km in the tropics (15 S 15 N) or 5 km in the extratropics. This bottom threshold is defined by temperature. Ice clouds exist at temperatures cooler than 40 C, and the altitude corresponding to this temperature is 8 km in the tropics and 5 km in the extratropics. The geometric thickness threshold comes from in situ measurements described by Dowling and Radke [1990]. The threshold value of t is from the ISCCP cloud type definition ( nasa.gov/cloudtypes.html) and is a threshold sufficient to detect opaque cirrus [Lynch et al., 2002]. If the ground was not detected in the profile, then data from the lowest cloud layer were not used in the analysis. Most deleted data were from below 8 km (5 km) in the tropics (northern midlatitudes). [7] Cirrus clouds detected using the above criteria in the tropics represent about 40% of the clouds during the analysis period. Specifically, 17% of the total GLAS data L of6

2 Figure 1. Vertical distributions of the fraction of cirrus cloud data to total data observed with ICESat/GLAS data in (a) the tropics (15 S 15 N) and (b) the northern midlatitudes (30 N 60 N). The solid, dotted, and dashed-dotted lines represent vertical profiles of the center, top, and bottom, respectively, of cirrus clouds. The dashed line indicates the geometric thickness of cirrus clouds. (c) Vertical distributions of cirrus cloud geometric thickness in the tropics (15 S 15 N) (solid line) and the northern midlatitudes (30 N 60 N) (dashed line). The vertical axis represents the altitude of cirrus tops. (d) As in Figure 1c but for the cirrus cloud optical depth. included only cirrus (both single and multilayered cirrus) and 23% included cirrus and other clouds types. 3. Vertical Distribution of Cirrus Clouds [8] Figures 1a and 1b show vertical distributions of cirrus frequencies in the tropics and in northern midlatitudes (30 N 60 N) as calculated from GLAS data. Cirrus frequency is defined as the ratio of the number of cirrus cloud events to the total number of soundings during the analysis period. Cirrus frequency is calculated for each 1 km altitude step. The cirrus frequency at 8.5 km, for example, represents the averaged cirrus frequency for altitudes between 8 and 9 km. Data from the tropics and midlatitudes were used to investigate high cirrus frequency in the tropics and to compare the differences in cirrus in the tropics and midlatitudes. Vertical frequency distributions of cirrus top, center, and bottom altitudes have a Gaussian distribution regardless of latitude. The cirrus center frequency peaks at 14.5 (9.5) km with the value of 10 (10)% in the tropics (northern midlatitudes). The most common cirrus thickness is less than 2 km for both latitudes. [9] The maximum frequencies of the cirrus cloud top (16.5 km) and bottom (14.5 km) in the tropics (Figure 1a) are 13 and 8%, respectively. The climatological tropopause during autumn is 16.5 km [Seidel et al., 2001]. Integration of cirrus frequencies at altitudes above 16 km show that cirrus tops (centers) located above 16 km account for 18 (9)% of all data points. Furthermore, about 40% of the cirrus clouds are less than 2 km thick. In northern midlatitudes (Figure 1b), frequencies of cirrus top and bottom altitudes peak at 10.5 and 8.5 km, respectively. The frequency of cirrus decreases both above the climatological tropopause (11 km) and below 6 km. Top (center) cirrus levels above the climatological tropopause account for 18 (10)% of all cirrus, and the most prevalent cirrus thickness, which accounts for 31% of all cirrus, is 2 km or less. [10] Figure 1c shows the vertical distributions of the cirrus cloud thickness in the tropics (solid line) and northern midlatitudes (dashed line). The vertical axis represents the altitude of the top of the cirrus. Thus, Figure 1c shows the averaged thickness of cirrus as a function of the altitude of the top of the cirrus. Similar to Figure 1a (Figure 1b), most 2of6

3 Geometric thickness in northern midlatitudes decreases with altitude below 13 km. Generally, the thickness increases with the altitude up to just below the tropopause. However, the presence of a thick cloud (e.g., a cloud having t larger than 4) in the upper layer increases the possibility that GLAS will not detect any lower layer. [11] Figure 1d shows vertical distributions of t. In the tropics, t is nearly constant (0.27) below 14 km; t above 14 km decreases with increasing altitude such that t at 18 km is 0.1. In northern midlatitudes, t below the tropopause (11 km) has a peak value, 0.4, at 10.5 km. In general, when the cirrus top is below the tropopause, t is larger in the midlatitudes. Figure 2. (a) Altitude of the center of cirrus [km], (b) geometric thickness [km] and (c) optical depth. Contours indicate Outgoing Longwave Radiation (OLR) values of 240 and 210 [W/m 2 ]. The white region shows where cirrus clouds were not detected. (d) Latitudinal distributions of cirrus top height [km] (blue), bottom height [km] (red), geometric thickness [km] (purple), optical depth 10 (orange), temperature (green), and OLR (black). cirrus data occur between 12 and 16 km (7.5 and 11.5 km) in the tropics (northern midlatitudes). Geometric thickness in the upper and lower tropical troposphere is less than 1 km; cirrus thickness at intermediate altitudes exceeds 1.5 km. 4. Horizontal Distribution of Cirrus Clouds [12] Figures 2a, 2b, and 2c show horizontal maps of the altitude of the center of the cirrus cloud, the geometric thickness and t, respectively. Contours depict the average Outgoing Longwave Radiation (OLR) obtained from the National Oceanic and Atmospheric Administration (NOAA) operational satellites during the same time period. Cirrus clouds were not detected in some areas (white region in Figure 2). [13] The height of the center of the cirrus clouds rises and t decreases as latitude decreases. In contrast, geometric thickness is nearly constant (1.6 km). Zonal structures are more uniform in mid- and high latitudes than in the tropics. The centers of cirrus clouds are relatively high in tropical convective regions over the maritime continents and the Pacific and Atlantic oceans. In particular, extreme values for the altitude of the center of the cirrus cloud and the cirrus cloud thickness occur over the tropical eastern Pacific. There, the center altitude is almost 1 km higher and the geometric thickness is about 0.6 km smaller than in other tropical regions. [14] Figure 2d shows that the top and bottom altitudes of cirrus clouds decrease with latitude. In the tropics, where the active convection (that is, the local minimum in OLR) is north (7.5 N) of the Equator, the average top (bottom) altitude of cirrus is nearly constant around 14.5 (13) km. Cirrus levels are closely linked to the altitude at which anvil cirrus forms in the outflow from convection. The level distributions reflect the distribution of low temperatures in the upper troposphere, which are symmetric about the Equator. Figure 2d also shows that t for cirrus clouds in both northern and southern midlatitudes is greater than t at other latitudes. The average geometric thickness is about 1.6 km regardless of latitude. [15] Figures 3a and 3b show cirrus frequencies derived from GLAS and from MODerate resolution Imaging Spectrometer (MODIS) on board the Terra spacecraft, respectively. MODIS cirrus fraction data were obtained from the monthly mean cloud product (MOD08M3, Ver. 4). MODIS cirrus fraction data are inferred from the cirrus bidirectional reflectance data retrieved from the 0.66 and 1.38 mm channels [Gao et al., 2002]. Water vapor absorbs almost all of the radiance at 1.38 mm before it reaches the satellite, and most water vapor (90%) is below 10 (7) km in the tropics (extratropics). Thus, if cirrus exists above those altitudes, it can be detected by apparent reflectance in the 1.38 mm band. 3of6

4 Figure 3. (a) Cirrus frequency [%] from ICESat/GLAS averaged between 1 October and 18 November 2003 and (b) cirrus fraction [%] from MODIS/Terra averaged data from October and November The white region in Figure 3a is where cirrus clouds were not detected. Contours in Figure 3a show OLR values of 240 and 210 [W/m 2 ]. (c) Latitudinal distribution of cirrus cloud frequencies from GLAS (red) and MODIS (blue), and OLR (black). ON in the title of Figure 3b indicates October and November. [16] In Figure 3a, cirrus occurs frequently over regions of tropical convection (more than 70%). Cirrus is less frequent (less than 30%) south of the Equator over the eastern Pacific, the Atlantic, and over the subtropics. Figure 3b shows high cirrus frequencies over the Antarctic, the Tibetan plateau, and the west coast of Chile. Chen and Liu [2005] investigated seasonal cirrus variations over Tibet. [17] A comparison of data from GLAS and MODIS (Figure 3c) shows relatively good agreement in the tropics. However, differences over mid- and high latitudes in both hemispheres are particularly noticeable. In the northern polar regions, the frequency of cirrus is higher in GLAS than in MODIS data. It is difficult to observe northern polar regions with the passive MODIS sensor during the polar night starting at the end of September. [18] Two cases occur in which it is difficult to identify cirrus clouds correctly. The first is that of a dry upper and middle troposphere (e.g., a region of subsidence) in which case lower clouds can be misidentified as cirrus clouds. The second occurs over high terrain (e.g., mountains). In that 4of6

5 Table 1. Mean Values of the Altitude of the Top, Center, and Bottom of Cirrus Clouds, Geometric Thickness and Optical Depth (t)inthe Tropics and Subtropics in Both Hemispheres, Midlatitudes in Both Hemispheres, and Globally a Top, km Center, km Bottom, km Thickness, km t Tropics (15 S 15 N) 14.4 (2.1) 13.6 (2.1) 12.8 (2.3) Subtropics in NH (15 N 30 N) 12.8 (2.8) 12.0 (2.7) 11.2 (2.8) Subtropics in SH (15 S 30 S) 11.4 (2.7) 10.7 (2.6) 10.0 (2.7) Middle Latitude in NH (30 N 60 N) 10.1 (2.0) 9.2 (1.9) 8.4 (2.0) Middle Latitude in SH (30 S 60 S) 9.5 (2.0) 8.7 (1.8) 8.0 (1.9) Globe (86 S 86 N) 11.1 (3.0) 10.3 (3.0) 9.5 (3.0) a Numbers in parentheses indicate the 1 s (standard deviation). NH and SH indicate Northern Hemisphere and Southern Hemisphere, respectively. event, the retrieval may be affected by surface reflectance and other conditions (e.g., humidity). 5. Summary [19] Table 1 shows that cirrus clouds in the tropics have an average top and bottom altitude of 14.4 and 12.8 km, respectively. Standard deviations (STD) of top, center, and bottom altitudes are larger in the subtropics than in other latitudes because the subtropics are affected by variations in cirrus and convective clouds in the tropics and middle latitudes. Furthermore, a big change occurs with latitude in the subtropics of the tropopause and the temperature and top height of convective clouds. At midlatitudes in both hemispheres, the top and bottom altitudes of cirrus clouds are approximately 10 and 8 km, respectively. These altitudes are 4 5 km lower than corresponding values in the tropics; altitudes in the Southern Hemisphere are slightly lower than those in the Northern Hemisphere. [20] Figure 1a shows that the vertical distribution for the height of the bottom of cirrus clouds has a peak at 14.5 km. This altitude corresponds to the base of the tropical tropopause layer (TTL) [Highwood and Hoskins, 1998]. Anvil cirrus frequently forms over colder regions around the base of the TTL when upwelling stops in the convective clouds. Outflow then develops from the convection at the base of the TTL. Consequently, anvil cirrus prevails over the tropics as shown in Figures 2a and 2d. The center (bottom) of the cirrus cloud is above 14 km about 30% (22%) of the time in the tropics. This frequency is similar to that of cirrus in the TTL and to results from the SAGE II [Wang et al., 1996]. [21] The zonally averaged geometric thickness of cirrus is about 1.6 km, regardless of latitude. Present results show that cirrus clouds are optically thin in the tropics (less than 0.2) near the climatological tropopause, and t is approximately constant at 0.27 below the TTL (Figure 1d). Moreover, the mean value of t increases with latitude (Table 1 and Figure 2d). The geometric thickness is almost constant, but t varies with latitude. This suggests that physical characteristic of cirrus, including water ice content and crystal shape that depends on temperature, differ between the tropics and midlatitudes. [22] Figures 2 and 3 show that although cirrus clouds occur with lower frequency over the tropical eastern Pacific than over other tropical regions, the altitude of the center of the cirrus there is 1 km higher and the geometric thickness and t are smaller. Thin cirrus at high altitudes is often undetected by passive measurements and can have a significant impact on the retrieval of other atmospheric parameters such as CO 2 column density. In the tropics, cirrus fraction from GLAS showed good agreement with that of MODIS. However, cirrus fraction from GLAS is somewhat larger than that from MODIS. Differences between GLAS and MODIS increase with increasing latitude because MODIS can detect lower clouds more effectively and is greatly affected by surface reflectance. [23] Further investigation of cirrus parameters, especially their temporal variation, is required to validate GOSAT algorithms. CALIPSO measurements of the atmosphere started at the end of April 2006, and cloud data derived from CALIOP will be a useful tool to analyze cirrus cloud parameters. [24] Acknowledgments. We thank David Crisp (JPL), M. Patrick McCormick (Hampton Univ.), James Spinhirne (NASA) and two anonymous reviewers for valuable comments. This research is conducted for the GOSAT project of NIES, Japan. This study was partly supported by the Global Environment Research Fund from by Ministry of the Environment of Japan. References Chen, B., and X. Liu (2005), Seasonal migration of cirrus clouds over the Asian monsoon regions and the Tibetan Plateau measured from MODIS/ Terra, Geophys. Res. 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6 Wang, P. H., P. Minnis, P. McCormick, G. S. Kent, and K. M. Skeens (1996), A 6-year climatology of cloud occurrence frequency from Stratospheric Aerosol and Gas Experiment II observations ( ), J. Geophys. Res., 101, 29,407 29,429. Yokota, T., H. Oguma, I. Morino, and G. Inoue (2004), A nadir looking SWIR FTS to monitor CO 2 column density for Japanese GOSAT project, paper presented at 24th International Symposium on Space Technology and Science, Jpn. Soc. for Aeronaut. and Space Sci., Miyazaki, Japan. N. Eguchi and T. Yokota, Center for Global Environmental Research, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki, , Japan. (eguchi.nawo@nies.go.jp) G. Inoue, Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi, , Japan. 6of6