Cirrus Optical Properties Observed with Lidar, Radiosonde, and Satellite

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, D17205, doi:10.1029/2006JD008352, 2007

Cirrus optical properties observed with lidar, radiosonde, and satellite over the tropical Indian Ocean during the aerosol-polluted northeast and clean maritime southwest monsoon P. Seifert,1 A. Ansmann,1 D. Mu¨ller,1 U. Wandinger,1 D. Althausen,1 A. J. Heymsfield,2 S. T. Massie,2 and C. Schmitt2 Received 14 December 2006; revised 4 April 2007; accepted 31 May 2007; published 14 September 2007.

[1] Cirrus formation and geometrical and optical properties of tropical cirrus as a function of height and temperature are studied on the basis of INDOEX (Indian Ocean Experiment) lidar and radiosonde measurements and satellite observations of deep convection causing the generation of anvil cirrus. Lidar and radiosonde measurements were conducted at Hulule (4.1°N, 73.3°E), Maldives, during four field campaigns carried out in February– March 1999 and March 2000 (northeast (NE) monsoon season, characterized by increased concentrations of anthropogenic aerosols over the Indian Ocean) and in July and October 1999 (southwest (SW) monsoon season, characterized by clean maritime conditions). As a result of a stronger impact of deep convection on cirrus formation during the SW monsoon season, cirrus clouds covered the sky over the lidar site in only 35% (NE), but 64% (SW) of the measurement time. Subvisible cirrus (optical depth 0.03), thin (optical depth from 0.03 to 0.3), and opaque cirrus (optical depth 0.3) were observed in 18%, 48%, and 34% (NE) and in 8%, 52%, and 40% (SW) out of all cirrus cases, respectively. Mean midcloud heights were rather similar with values of 12.9 ± 1.5 km (NE) and 12.7 ± 1.3 km (SW). In 25% of the cases the cirrus top height was found close to the tropopause. Mean values of the multiple-scattering-corrected cirrus optical depth, cirrus layer mean extinction coefficient, and extinction-to-backscatter ratio were 0.25 ± 0.26 (NE) and 0.34 ± 0.29 (SW), 0.12 ± 0.09 km1 (NE) and 0.12 ± 0.10 km1 (SW), and 33 ± 9 sr (NE) and 29 ± 11 sr (SW), respectively. A functional dependency of the extinction coefficient of the tropical cirrus on temperature is presented. All findings are compared with several other cirrus lidar observations in the tropics, subtropics, and at midlatitudes. By contrasting the cirrus optical properties of the different seasons, a potential impact of anthropogenic particles on anvil cirrus optical properties was examined. Differences in the cirrus extinction-to-backscatter ratio suggest that NE monsoon anvil cirrus originating from deep-convection cumulus clouds had more irregularly shaped and thus slightly larger ice crystals than respective SW monsoon anvil cirrus. Because the meteorological conditions were found to vary significantly between the seasons, an unambiguous identification of the influence of Asian haze on cirrus optical properties is not possible. Citation: Seifert, P., A. Ansmann, D. Mu¨ller, U. Wandinger, D. Althausen, A. J. Heymsfield, S. T. Massie, and C. Schmitt (2007), Cirrus optical properties observed with lidar, radiosonde, and satellite over the tropical Indian Ocean during the aerosol-polluted northeast and clean maritime southwest monsoon, J. Geophys. Res., 112, D17205, doi:10.1029/2006JD008352.

1. Introduction almost 50% [Wylie et al., 1994]. Tropical cirrus clouds thus have a significant impact on local meteorological processes [2] Cirrus regularly covers 20% to 35% of the globe as well as on the regional to large-scale heat budget of the [Liou, 1986; Wylie et al., 1994]. In the area of the inter- atmosphere. The radiative impact of ice clouds is complex tropical convergence zone (ITCZ) this value increases to and is determined by the greenhouse-versus-albedo effect [Liou, 1986; Wylie et al., 1994]. Whereas the scattering of 1Leibniz Institute for Tropospheric Research, Leipzig, Germany. solar radiation causes a net cooling effect for the Earth’s 2National Center for Atmospheric Research, Boulder, Colorado, USA. surface, the absorption and emission of infrared radiation leads to a trapping of energy in the Earth-atmosphere Copyright 2007 by the American Geophysical Union. system. The way in which these two effects interact and 0148-0227/07/2006JD008352

D17205 1of14 D17205 SEIFERT ET AL.: INDOEX CIRRUS OBSERVATIONS D17205 thus influence the radiation balance of the atmosphere are discussed and geometrical and optical properties of the strongly depends on the optical properties, height, thick- observed cirrus layers are presented and compared with ness, and temperature of the cirrus layers. How cirrus clouds other tropical as well as subtropical and midlatitude cirrus interact with other clouds (i.e., convective clouds), pollu- studies. In section 4, the potential impact of anthropogenic tion, and large-scale processes, such as changes in the pollution on cirrus cloud optical properties is discussed. vertical distribution of radiative heating [Stephens, 2002] Section 5 provides a short summary. or sea surface temperature [Ramanathan and Collins, 1991], is not yet completely understood. 2. Experiments [3] Several field campaigns have been carried out in 2.1. Lidar various regions of the globe to improve the knowledge of cirrus cloud occurrence, formation processes, and the cli- [7] A six-wavelength lidar [Althausen et al., 2000] was mate impact of cirrus clouds. However, especially in the employed to measure aerosol and cloud distributions over tropics vertically and temporally resolved measurements of the Maldives in the framework of INDOEX. The field site cirrus cloud properties are still scarce. Recent studies have was at the Maldives International Airport at Hulule island focused on the western Pacific warm pool [Platt et al., (4.1°N, 73.3°E). The lidar was optimized for measurements 1998, 2002; Heymsfield et al., 1998; Sassen et al., 2000; of Asian haze in the height range from 500–5000 m. As a Comstock et al., 2002]. Other lidar statistics of cirrus cloud consequence, the Raman channels for particle extinction properties in the tropics are available only for the Seychelles profiling at 355 and 532 nm could not be used to measure [Pace et al., 2003] and eastern India [Sunilkumar and light scattering coefficients in clouds from 10–17 km Parameswaran, 2005]. height. The Raman signals from these heights were too [4] This contribution presents the most comprehensive weak, too noisy. Nevertheless, the 532-nm elastic-backscatter lidar- and radiosonde-based cirrus cloud statistics for the channel was powerful enough to cover the range from region of the tropical Indian Ocean. In the scope of the 500–25000 m and thus to profile cirrus and Rayleigh Indian Ocean Experiment (INDOEX), our six-wavelength scattering up to stratospheric heights. The lidar was operated aerosol lidar [Althausen et al., 2000] performed more than at a fixed off-zenith angle of typically 5° or, in several cases, 500 hours of aerosol and cloud observations. About of 30° to avoid the unwanted impact of specular reflection by 250 radiosondes were launched. The primary goal of falling, horizontally oriented ice crystals. INDOEX was the characterization of anthropogenic aerosol [8] The geometrical properties of ice cloud layers were pollution (Asian haze) advected from south and Southeast directly determined from the range-corrected signals, clearly Asia in the lower troposphere during the northeast monsoon indicating the ice cloud layers as shown in Figure 1. Note, season [Franke et al., 2003; Mu¨ller et al., 2003]. Four field in addition, the aerosol pollution layers in the lower campaigns were carried out in February–March 1999, July troposphere during the NE monsoon season (Figures 1a 1999, October 1999, and March 2000 so that the underlying and 1c). In cases of inhomogeneous cloud structures (see data set covers two seasons of the dry northeast (NE) Figure 1b), we calculated cloud top and base heights for monsoon (February–March 1999, March 2000) as well as 10-min periods and used the respective mean cloud top one season of the rainy southwest (SW) monsoon (July and and base heights (for the entire observational period) to October 1999) [Mu¨ller et al., 2001]. The cirrus analysis calculate the mean cirrus layer depth and midcloud tem- presented here is based on the elastic-backscatter signal perature from the respective radiosonde data for this profiles measured at 532-nm wavelength. individual observation. We interpreted cirrus layers as [5] In a recent publication [Chylek et al., 2006] the different ice clouds if they were separated by 800 m. possible influence of lower tropospheric pollution on the In the statistics presented in the next section, only clouds optical properties of cirrus clouds over the Indian Ocean above 9-km height are considered to avoid a possible was discussed. Seas adjacent to the Indian subcontinent are impact of water clouds on our statistics. ideal target areas to study the anthropogenic impact on [9] For each cirrus observation lasting from a few atmospheric processes because the monsoon climate in this minutes to several hours we selected a representative period region permits contrasting polluted (NE monsoon, January of typically 5–30 min to determine the optical properties to April) and clean, maritime conditions (SW monsoon, such as the optical depth and column extinction-to-back- May to October). Chylek et al. [2006] note that MODIS scatter ratio of the cirrus layer. The column extinction-to- data indicate larger ice crystals in January (2001–2005) backscatter or column lidar ratio is the ratio of the cirrus and smaller ice crystals in September (2002–2004), optical depth to the backscatter coefficient (180° scattering corresponding to times of increased pollution and cleaner coefficient) integrated over the cirrus layer and can be conditions, respectively. Massie et al. [2007] calculated interpreted as the cirrus layer mean lidar ratio. Before cirrus reflectivity values determined from the analyses of computing the optical properties we smoothed the tempo- MODIS radiances during January–March of 2003–2005. rally averaged lidar signal profiles recorded with 15–60 m No indication of an aerosol influence, e.g., an increase of vertical resolution with a 120-m vertical smoothing window the cirrus reflectivity with increasing aerosol optical depth, length to reduce the influence of signal noise. was found. In section 4, this aspect is further discussed on [10] The so-called forward integration method of the the basis of our lidar-derived values of the cirrus layer mean Fernald method [Fernald, 1984] is applied to retrieve the extinction coefficient and extinction-to-backscatter ratio. cirrus optical depth and the cirrus lidar ratio. The method is [6] The paper is organized as follows. In section 2, the well known in the lidar community and well tested by experimental setup and data analysis methods are described. comparing the results obtained with the Fernald method and Section 3 presents the s. Cirrus formation processes from simultaneously conducted Raman lidar observations of

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[11] In the Fernald retrieval, the column lidar ratio of the cirrus layer is an input parameter. By applying the forward integration method, pure Rayleigh backscattering below the cirrus is assumed. The most appropriate profile of the cirrus backscatter coefficient is obtained by varying the cirrus lidar ratio until the particle backscatter coefficient above the cirrus layer approaches zero again and varies around zero in the stratosphere because of signal noise. We checked the optimum solution by applying the backward integration method in addition. In this retrieval the computation is started from above the cirrus. In the case of the most appropriate cirrus lidar ratio, the forward and backward integration solutions coincide as illustrated by Ansmann et al. [1992]. This technique is simple and straightforward, and allowed us to accurately determine the apparent (laser- light-attenuation-related) cirrus optical depth, the cirrus layer mean extinction coefficient, i.e., the ratio of the optical depth to the geometrical depth, and of the cirrus lidar ratio. The uncertainty in the retrieval products caused by the assumption of particle free air below and above the cirrus is <10%. [12] The technique could, however, only be applied in cases with apparent cirrus optical depths from about 0.03– 0.05 to 0.6. The apparent optical depth is the directly retrieved value without applying any multiple-scattering correction. In cases with almost subvisible cirrus the Fernald retrieval was insensitive to the lidar ratio estimate (25% out of all cirrus cases). All trial values for the cirrus lidar ratio from 1–60 sr or even higher values produced almost the same backscatter coefficient profile. In these cases we used an apparent cirrus lidar ratio of 20 sr. This value of 20 sr represents the mean of all retrieved INDOEX cirrus lidar ratios for thin cirrus with optical depth <0.1. An uncertainty of the optical depth of 30%–50% remains when keeping in mind that the apparent cirrus lidar ratio can easily vary between 10 and 30 sr as a function of particle shape and size [Reichardt et al., 2002]. [13] In cases with thick cirrus with high optical depth >0.6 (6% out of all cases), Rayleigh backscattering above the ice cloud was hard to detect. The signal profiles became very noisy. Supported by several INDOEX Raman lidar observations in optically thick cirrus below 12 km height, it was found that the most appropriate cirrus lidar ratio is close to the one for which the backscatter coefficient solution becomes unstable [Seifert, 2006]. In this study we thus selected that apparent lidar ratio as the most appropriate one that produced the first stable solution when decreasing the apparent lidar ratio stepwise by 1 sr, starting with 25 sr that Figure 1. (a–c) Three cases of cirrus observations during always produced unstable solutions. This lidar ratio was INDOEX. Range-corrected 532-nm signals are shown with then applied to the obtained backscatter coefficient profile 30-s and 60-m resolution. Ht and Hb denote mean cloud top to estimate the apparent cirrus optical depth. Note, that in all and base height, respectively. In cases with strongly varying of the analyzed optically thick cirrus cases the first stable cloud base (center) and/or cloud top heights, 10-min solution showed already backscatter values around zero at averages are computed and the respective mean value was the top of the cirrus and above the cloud layer. The taken as cloud base/top height. uncertainty in the retrieval products (apparent column lidar ratio, optical depth, mean extinction coefficient) of optically thick ice clouds is estimated to be 25% as the Raman lidar ice clouds [Ansmann et al., 1992]. The Fernald method was comparisons indicate. also applied in other midlatitude, subtropical, and tropical [14] The receiver field of view of the INDOEX lidar is cirrus studies [Ansmann et al., 1993; Sassen and Comstock, 0.8 mrad and thus the results are considerably influenced by 2001; Chen et al., 2002; Pace et al., 2003]. multiple scattering. This effect is caused by strong forward scattering of laser light in the cirrus and leads to a signif-

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icant underestimation of the cirrus optical depth by 20%– scattering factor hms is a simple function of the cirrus optical 50%. The apparent values of the optical depth, layer mean depth t [hms exp(t)] and thus approaches 1 for extinction coefficient, and lidar ratio are a factor of 1.2–2 subvisible cirrus with t ! 0. lower than the desired, single-scattering-related values. The [17] The uncertainty in the optical depth and column lidar strength of this underestimation depends on cloud height, ratio introduced by the multiple scattering correction is cloud depth, strength of light scattering (scattering coeffi- estimated to be 10% because of the unknown effective size cient), size of the scatterers, laser beam divergence, and the of the ice crystals in each of the observed clouds. This value receiver field of view of the lidar [Platt, 1981; Wandinger, is estimated from the simulations with different effective 1998; Comstock and Sassen, 2001]. We applied the model particle sizes. Considering all uncertainties, the determina- provided by Hogan [2006] to determine the multiple scat- tion of the cirrus optical properties as shown in the next tering effect for each of the observed INDOEX cirrus cases. section was possible with a relative error of about 30%– The model was explicitly developed for cirrus observations 50%, 15%–20%, and 30% for cases with an apparent cirrus with ground-based and spaceborne lidars. Input parameters optical depth of <0.04, 0.04–0.6, and >0.6, respectively. are the observed cloud height, depth, and the profile of the cloud extinction coefficient that corresponds to the 2.2. Radiosonde Observations retrieved backscatter coefficient profile multiplied by a [18] During the four field campaigns 250 radiosondes of single-scattering-related column lidar ratio. Information the type Vaisala RS-80 were launched to provide vertical about the effective, i.e., the cross-section-weighted mean, profiles of pressure, temperature, and humidity up to 20– radius of the hexagonal ice particles needed in the model 25 km height. The radiosondes were released at the lidar calculations was taken from Wang and Sassen [2002] who field site at Hulule. A sonde observation was made during used lidar and radar measurements to determine the ice each of the one to six-hour lidar measurement sessions. particle size as a function of temperature. The effective 2.3. Satellite Observations radius decreased from 70 mmat30°Cto35mmat70°C. Temperature profile information for each cirrus case was [19] NOAA satellites provide an outgoing longwave determined from radiosonde data. Scattering phase func- radiation (OLR) product for the top of the atmosphere. tions for hexagonal ice crystals are implemented in the The OLR data are calculated on a daily basis by the Climate model. The selection of the appropriate scattering phase Diagnostic Center (CDC), a division of NOAA [Liebmann function depends on the chosen effective radius. and Smith, 1996]. The horizontal resolution is 2.5° 2.5°. Missing values are computed by applying spatial and [15] On the basis of the calculated profiles of the totally backscattered laser photons detected with the lidar and the temporal interpolation. OLR emitted by high, cold, deep corresponding single-scattering-related lidar signal profiles, convective clouds is much lower than the OLR emitted by warmer low clouds or the surface. Usually, values of less we corrected the multiple scattering effect following the 2 scheme of Platt [1981]. The correction procedure is de- than 170 W/m indicate deep convection. Deep convection, scribed in detail by Wandinger [1998]. in turn, indicates regions with extensive lifting of air that may play a role as source regions for cirrus clouds. [16] For a given field of view of the receiver telescope the strength of multiple scattering primarily depends on the cloud effective particle size, cloud geometrical depth and 3. Results the penetration depth of the laser beam into the cloud. When [20] During the four measurement campaigns in February the laser beam hits the cirrus cloud base, almost 50% of the and March 1999, July 1999, October 1999, and March 2000 laser light are scattered in the forward direction and remain regular observations were made each morning, noon, and in the field of view of the receiver telescope. With increas- evening on 93 days. These routine measurements lasted ing penetration depth into the cloud this forward scattered typically one to two hours. Additionally, 57 nighttime light is scattered out of the receiver field of view. Thus the measurements were made. Most observations were per- multiple scattering factor hms increases with penetration formed in February and March 1999 during the intensive depth into the cloud, almost independent of its optical field phase of INDOEX. Together with the data of depth. hms is defined as the ratio of the observed, apparent March 2000, 106 cirrus cases were counted during the NE optical depth to the single-scattering-related optical depth. monsoon season. 73 cirrus cases were sampled during The values of hms accumulated around 0.55–0.6 for geo- the SW monsoon season. The sky over the lidar site was metrical depths 1 km and mostly ranged from 0.6–0.8 for covered with cirrus during 43% of the observational time larger geometrical depths. The impact of multiple scattering (35% during the NE monsoon, 64% during the SW mon- is thus largest for geometrically thin, subvisible cirrus as soon). Several observational periods (about 30% of the total expected [Wandinger, 1998]. The optical depth of subvisible record) with thick altocumulus that prevented cirrus obser- cirrus is underestimated by almost a factor of two. This is in vations in October 1999 were excluded from this statistics. contradiction to other multiple scattering parameterizations. During the other three field campaigns, screening by lower Sassen and Comstock [2001] assumed hms =0.9for tropospheric and midtropospheric cloud layers was a neg- subvisible and optically thin cirrus, 0.8 for relatively thick ligible issue. cirrus, and 0.6–0.7 for optically thick cirrus. With 1 mrad the receiver field of view was similar to the receiver field of 3.1. Cirrus Formation Processes view of the INDOEX lidar of 0.8 mrad. Chen et al. [2002] [21] According to the current understanding, the primary used hms of 0.98–1 for subvisible cirrus, 0.86–0.98 for thin formation mechanisms of cirrus clouds in the tropics are cirrus (optical depth from 0.03–0.3), and 0.58–0.86 for outflow from deep convection, large-scale uplift of humid opaque cirrus (optical from 0.3–1). Their multiple

4of14 D17205 SEIFERT ET AL.: INDOEX CIRRUS OBSERVATIONS D17205 layers, and cooling due to wave activity in the upper (cold-point tropopause) of each radiosonde ascent. The troposphere [Jensen et al., 1996]. Convective cumulonim- black dots at an altitude of 6 km show when the lidar was bus clouds are the primary source for outflow cirrus which operated. White vertical lines correspond to observed cirrus forms when upper tropospheric winds blow ice particles clouds. The red areas in Figure 2b correspond to a positive away from their convective cores. This cirrus is usually deviation of 5 K and the dark areas to a negative deviation denoted anvil cirrus. Also, the anvil clouds that remain in of 5 K from the average temperature. Between Julian day 70 the troposphere after the deep convective cloud dissipated and 85 a cold perturbation can be found at an altitude of are counted as outflow cirrus. Such cirrus clouds appear as about 15 km. During this time period the highest frequency rather opaque, textured clouds with irregular boundaries. of cirrus clouds was observed. During warmer periods, as They generally persist for 0.5 to 3 days. Their top heights between Julian day 64 and 70, almost no cirrus were are restricted to the maximum altitude deep convection can detected, despite the high number of lidar observations. reach. Except for a small fraction of less than 1.8%, the The downward propagating Kelvin waves can best be upper boundary for deep convection is 14 km [Liu and recognized at altitudes above 17 km. They show a wave- Zipser, 2005]. length of about 12 days with an amplitude of more than 5 K. [22] Large-scale dynamic lifting of humid layers supports [25] The analysis of the three additional field campaigns the formation of cirrus at higher altitudes. The lifting leads in July and October 1999 and in March 2000 shows the to a cooling of the layer until the air is supersaturated with same wave characteristics in the lower stratosphere. Be- water vapor and ice nucleation occurs. Jensen et al. [1996] cause of the shorter duration of these campaigns the graphs named different processes which can cause dynamic lifting: are less representative and are not presented in this study. flow over continental-scale bulges in the tropopause, flow [26] As shown in Figure 2c the occurrence of cirrus was, over large-scale convective systems, and lifting above however, also strongly correlated with the distance to deep stratiform regions of mesoscale convective systems. Clouds convection. The data for the bar chart in Figure 2c were induced by these processes usually appear as rather calculated from the CDC OLR data set (see section 2.3). For tenuous, laminar layers of low optical depth spreading each Julian day, the bars show the geographical distance homogeneously over large areas. They have a geometrical between Hulule (Maldives), where the lidar was located, depth of about 1 km and are located at altitudes above and the nearest location with deep convection. It is obvious 14 km, often close to the tropopause or strong temperature from the graph that especially between Julian day 70 and 85 inversions. there was strong convective activity close to Hulule. On [23] Deep convective clouds are also capable of causing day 75, corresponding to 16 March 1999, deep convection considerable radiative cooling above their anvils. Thin and was observed directly at the lidar site. subvisible cirrus can form above deep convective clouds. [27] Also cirrus statistics based on satellite-borne lidar These layers are comparable to pileus clouds but more measurements, as with the ICESat/Geoscience Laser Altimeter persistent [Hartmann et al., 2001; Garrett et al., 2004, System (GLAS), reveal an increase of the occurrence fre- 2006]. After the deep convective clouds dissolved, the quency of cirrus with decreasing OLR [Dessler et al., 2006]. ‘‘trapped’’ cirrus remains in the upper troposphere as a However, an investigation on the connection between the layer located above the remnant of the dissipating anvil occurrence of cirrus and stratospheric waves, as done here, cloud and the outflow cirrus. was not performed. [24] Recent studies reveal a strong link between cirrus [28] A clear separation of the processes that control cirrus clouds and atmospheric waves which cause in situ cooling formation is not possible from our INDOEX data. Figure 2 on a synoptic scale. Massie et al. [2002] note that subvisible suggests that cirrus occurrence in general increases when cirrus are associated 50% of the time with regions of the strong cold perturbations in the upper troposphere occur, tropics, near the tropopause, for which backward trajectories and that, during such perturbation events, convective activ- are not associated with deep convection. These observations ity also increases, possibly linked to the cold perturbation. are consistent with the formation of cirrus that is due to in Seasonal means and mean distance of areas of deep con- situ cooling of humid layers. Boehm and Verlinde [2000] vection to the lidar site at Hulule are given in Table 1. found a strong correlation between descending stratospheric 3.2. Cirrus Geometrical Properties Kelvin waves and the occurrence of cirrus. According to their study, negative temperature deviations from the aver- [29] Table 2 and Figures 3 and 4 summarize the obser- age temperature in the upper troposphere are directly related vational results concerning the geometrical and thermal to the occurrence of cirrus. Information about the mecha- properties of cirrus. To avoid a possible, unwanted inclusion nism of the stratospheric Kelvin waves is given by Holton of altocumulus, i.e., of water clouds, we restricted the [1992]. A similar behavior is found in our INDOEX analysis to clouds with cloud base heights 9 km. As can radiosonde/lidar data set. Figure 2a shows the radiosonde be seen from Table 2, rather similar geometrical properties temperature field and Figure 2b the deviation from the were found during the NE and SW monsoon seasons. average temperature of all 54 measurement days in spring [30] Figure 3 shows seasonal and total frequency distri- 1999. The red line in both figures indicates the lapse rate butions of cirrus cloud base height, cloud top height, and tropopause. The lapse rate tropopause, also referred to as the cloud depth. In 50% out of all cases, the top height was WMO tropopause, is defined as the lowest level at which found above 14 km. Base heights show a broad distribution the lapse rate decreases to 2 K/km or less, and the average peaking at 12–13 km and 11–12 km height during the NE lapse rate from this level to any level within the next higher and SW monsoon season, respectively. Cloud geometrical 2 km does not exceed 2 K/km [Krishna Murthy et al., depth was in about 70% and 50% out of all cases 2km 1986]. The white line served minimum temperature during the NE and SW monsoon season, respectively.

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Figure 2. (a) Temperature field created from 54 days of radiosonde data in spring 1999. (b) Temperature deviation from the average temperature of all radiosondes for the same time period. Descending stratospheric waves are visible. Red line indicates WMO tropopause, white line indicates cold-point tropopause, white vertical lines indicate observed cirrus, and black dots at 6 km indicate periods of lidar operation. (c) Distance between Hulule (Maldives) and nearest location with deep convection (OLR < 170 W/m2). 1° 110 km. The correlation between distance to deep convection and cirrus occurrences is noteworthy.

[31] Because of the permanent presence of deep convection around the Maldives during the SW monsoon season Table 2. Mean Values and Standard Deviations (in Parentheses) (see Table 1, July, October) the detected clouds were most of Geometrical and Thermal Cloud Propertiesa likely often fresh remnants of cumulonimbus anvils. This may explain the higher number of cirrus layers with depths All Cases NE Monsoon SW Monsoon >2 km during the SW monsoon season. Analyzed cases 179 106 73 Cloud base height, km 11.9 (1.6) 12.1 (1.6) 11.6 (1.6) Cloud top height, km 13.7 (1.4) 13.7 (1.4) 13.7 (1.3) Cloud depth, km 1.8 (1.0) 1.6 (0.9) 2.1 (1.2) Table 1. Seasonal Average and Range of Distance of Deep Midcloud height, km 12.8 (1.4) 12.9 (1.5) 12.7 (1.3) 2 Convection (OLR < 170 Wm ) to Hulule (Maldives) in Cloud base temperature, °C 50 (13) 51 (13) 48 (13) Geographical Degrees (1° 100 km) Cloud top temperature, °C 65 (11) 65 (11) 64 (10) Midcloud temperature, °C 58 (11) 58 (11) 57 (11) Mean Distance Range (Min–Max) Tropopause height, km 16.3 (1.0) 16.7 (0.9) 15.5 (0.4) Feb/Mar 1999 13° 0–30° Tropopause temperature, °C 81 (4.0) 83 (3) 77 (2) Mar 2000 13° 8–25° aShown are also height and temperature of the lapse rate tropopause. The July 1999 3° 0–9° values are given for all observed cirrus cases as well as for the northeast Oct 1999 2° 0–4° monsoon and the southwest monsoon.

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Figure 5. Frequency distributions of cloud depth for clouds with 10 km < Hb < 14 km (dashed line), and Hb > 14 km (solid line). Data of both monsoon seasons are included.

would dissolve quickly [Ackerman et al., 1988; Jensen et al., 1996]. [33] The location of the tropopause level which on Figure 3. Frequency distributions of (a–c) cloud base average varied by more than 1 km in height and 6°Cin height, (d–f) cloud top height, and (g–i) cloud depth for all temperature between both monsoon seasons (see Table 2) observed cirrus cases, the NE monsoon season, and the SW also played a role in the distribution of the clouds, espe- monsoon season. cially regarding the top heights. Because of the lower tropopause during the SW monsoon season, no cirrus was observed above 16 km at temperatures below 80°C during [32] The following effects must also be kept in mind this season. when discussing possible differences between SW and NE [34] Figure 4 shows that the cirrus center heights most monsoon cirrus. The, on average, very close distance to an frequently were between 11 and 14 km height (in both area of convective activity during the SW monsoon season seasons, see Table 2). Corresponding midcloud temper- suggests that the dissolution process had often not started atures ranged most frequently from 50 to 70°C. The when clouds were detected at the upper edge of deep dependence of cirrus depth on height or temperature was convective cells during this season [Liu and Zipser, weak with a trend to geometrically thin clouds with de- 2005]. In contrast, NE monsoon clouds usually had their creasing temperature (see Figure 4c). origin in greater distance to the Maldives. Therefore they [35] The general opinion that the occurrence of upper had already aged before they were detected with the lidar. tropospheric cirrus is decoupled from lower tropospheric Entrainment and detrainment of air at the cloud boundaries processes such as deep convection is corroborated by during the life cycle of the cirrus layer cause a decrease of Figures 5 and 6. Upper tropospheric cirrus formed by the cloud thickness until the cloud decays [Platt et al., 1989; large-scale uplift or wave activity is usually physically thin Sassen et al., 1989; Sivakumar et al., 2003]. The mainte- and has a laminar structure [Pfister et al., 2001]. A sharp nance of anvil cirrus for several hours to more than a day depth distribution peaking at 400–800 m is found for also requires a slow rise of the clouds by radiative heating. clouds with cloud base above 14 km. Anvil cirrus appears If the cirrus clouds would remain at their initial level, they more structured, has variable cloud boundaries, and thus is characterized by a broad distribution of geometrical depths. [36] Figure 6 shows the distribution of the distance of the cirrus cloud top to the lapse rate tropopause for the different cirrus types (laminar cirrus above 14 km, anvil cirrus). The graph contains the data of 140 cirrus cases of both monsoon seasons. The radiosonde ascent that was closest to the lidar observation was used in Figure 6. Two distinct maxima are found. One maximum of occurrence is located close to the tropopause level. The lower maximum is obviously related Figure 4. (a) Frequency of occurrence of midcloud height. to anvil cirrus from cumulonimbus clouds. (b) Frequency of occurrence of midcloud temperature. [37] The 4-year geometrical cirrus cloud statistics of the (c) Mean cloud depth per midcloud temperature interval and tropical Indian station Gadanki (13.5°N, 79.2°E) pre- standard deviations. In all graphs the cirrus cases of both sented by Sivakumar et al. [2003] and Sunilkumar and monsoon seasons are i d. Parameswaran [2005] agree well with the Maldives data

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contrast to the findings for the Maldives region, besides the 25% of the cirrus clouds at Gadanki that were close to, but below the tropopause, another 25% of all clouds were reported to have appeared in altitudes up to 3 km above the tropopause. This is possibly caused by the fact that Sivakumar et al. [2003] used radiosonde data from a station located 120 km away from Gadanki where radiosondes were launched at fixed times. [41] We also compared our tropical data set with lidar measurements at midlatitudes [Ansmann et al., 1993; Sassen and Campbell, 2001; Sassen and Comstock, 2001; Wang and Sassen, 2002; Reichardt, 1999] in order to identify similarities and differences in the cirrus properties for different climate regimes (latitudinal bands). Ansmann et al. [1993] present 4-week cirrus observations performed at Norderney island (54°N) in northern Germany during the Figure 6. Frequency distribution of the distance between International Cirrus Experiment (ICE) in the autumn of cloud top and the respective lapse rate tropopause. The 1989, Reichardt [1999] summarizes observations conducted underlying data set contains the data of both monsoon at Geesthacht (53°N) in northern Germany from May 1994 seasons. to March 1996, Sassen and Campbell [2001] present data recorded at Salt Lake City (40°N), Utah, from 1986–1996, set. A very similar occurrence frequency distribution of Sassen and Comstock [2001] analyzed data observed at Salt cloud depths as shown in Figure 3g was observed over Lake City, from 1992–1999. Wang and Sassen [2002] used Gadanki. The midcloud height distribution was similar to Raman lidar observations at the Atmospheric Radiation that presented in Figure 4a. The distribution of the cloud Measurement (ARM) site in Oklahoma (37°N) taken during depth per midcloud temperature interval as shown in the period from November 1996 to November 2000. Figure 4c is in good agreement with the observations [42] Most of the midlatitude cirrus clouds begin to form performed at Gadanki as well as at Nauru and the Seychelles. in the uppermost troposphere close to the tropopause. The Comstock et al. [2002] analyzed lidar data collected at top heights were found most frequently from 11–12 km Nauru (0°, 167°E) in the western Pacific region which is height over northern Germany as well as over Utah. 70% of known to have the highest altitude of occurrence of thin the observed cirrus top heights coincided with the tropo- cirrus clouds [Wang et al., 1996] and Pace et al. [2003] pause height over northern Germany [Reichardt, 1999]. characterized clouds at the Seychelles (4.4°S, 55.3°E) by Mean midcloud height was at 11 km ± 2 km over Utah. means of lidar. A broad frequency-of-occurrence distribution for the cloud thickness from a few hundred meters to 7 km was found [38] The comparison of the Nauru [Comstock et al., 2002] and the Maldives data sets further reveals that the cloud top over Utah with a mean value of 1.8 km [Sassen and in the western Pacific region was most frequently around Campbell, 2001]. The mean cirrus depth was 2–2.5 km 16 km at Nauru and around 14–15 km at Hulule. The cloud over Oklahoma [Wang and Sassen, 2002]. These values are base was typically found about 2 km below cloud top at very similar to the mean cloud depths presented in Table 2. both tropical sites. A higher mean tropopause height of Most ice clouds had a thickness of 0.5–2.5 km (75% out of 16.9 km over the western Pacific warm pool [Sassen et al., all cases) over the more northern midlatitude sites in 2000] in contrast to the 16.3 km height over the Maldives northern Germany. (see Table 2) implies differences in the meteorological 3.3. Cirrus Optical Properties conditions between both regions. High sea surface temper- [43] The cirrus data set for the retrieval of cirrus optical atures in the western Pacific region are also supposed to properties was considerably smaller (96 cases) than the total enhance deep convection [Ramanathan and Collins, 1991; data set of 179 cirrus observations. In many cases low-level Liu and Zipser, 2005]. Similar characteristics of cloud depth clouds temporarily obstructed the cirrus layers so that the distributions as shown in Figure 5 were also found by time periods left for the optical properties retrieval were Comstock et al. [2002] and Pace et al. [2003]. often too short resulting in a too low signal-to-noise ratio. In [39] Cloud top distributions based on a 5-week data set several cases with strong variations of cloud structures and obtained with the spaceborne ICESat/Geoscience Laser optical properties with height and time, a reliable retrieval Altimeter System (GLAS) confirm these findings [Dessler of the optical depth based on the averaged signal profile was et al., 2006]. The GLAS measurements show that the not possible [Ansmann et al., 1992]. highest frequency and highest cloud heights of cirrus occur [44] An overview of the optical properties of the analyzed over the western Pacific warm pool. Lower cloud frequen- clouds is given in Tables 3 and 4. The classification of the cies and lower heights are reported for the tropical Indian cloud types in Table 3 follows the scheme of Sassen and Ocean over the Maldives. Cho [1992]. They defined cirrus clouds with t < 0.03 as [40] Sivakumar et al. [2003] also investigated the distan- subvisible cirrus, clouds with 0.03 < t < 0.3 as thin cirrus, ces from cloud top to the tropopause (see Figure 6) and and clouds with t > 0.3 as opaque cirrus. According to found one maximum right below the tropopause and another Table 3, 15% of all analyzed cirrus cases were subvisible one about 3 to 4 km below the tropopause. However, in cirrus, about 50% thin cirrus, and 36% opaque cirrus. More

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Table 3. Seasonal Frequency, Mean Optical Depth, Mean Extinction Coefficient, and Mean Lidar Ratio of Subvisible Cirrus, Thin Cirrus, and Opaque Cirrusa Subvisible Cirrus, Thin Cirrus, Opaque Cirrus, t < 0.03 0.03 < t <0.3 t >0.3 All seasons (96 cases) Frequency, % 15 49 36 Optical depth 0.015 (0.005) 0.13 (0.07) 0.59 (0.27) Ext. cf., km1 0.017 (0.007) 0.08 (0.04) 0.22 (0.09) Lidar ratio, sr 34 (12) 29 (7) NE monsoon (61 cases) Frequency, % 18 48 34 Optical depth 0.015 (0.005) 0.13 (0.07) 0.55 (0.24) Ext. cf., km1 0.016 (0.006) 0.09 (0.05) 0.23 (0.08) Lidar ratio, sr 36 (11) 30 (5) SW monsoon (35 cases) Frequency, % 8 52 40 Optical depth 0.015 (0.007) 0.14 (0.07) 0.67 (0.29) Ext. cf., km1 0.02 (0.009) 0.07 (0.04) 0.2 (0.1) Lidar ratio, sr 29 (12) 28 (9) aStandard deviations of the mean values are given in parentheses. The cloud types are classified after Sassen and Cho [1992]. Ext. cf., extinction coefficient.

subvisible cirrus (18%) was observed during the NE mon- Figure 7. Frequency distributions of (a–c) mean optical soon than during the SW monsoon season (8%). The depth, (d–f) mean extinction coefficient, and (g–i) lidar seasonal mean values of the optical depth, the cirrus layer ratio with respect to midcloud temperature intervals of 10 K mean extinction coefficient (ratio of optical depth to geo- for all cases, NE monsoon, and SW monsoon. metrical depth), and cirrus lidar ratio indicate almost the same optical properties during both SW and NE monsoon seasons. The Nauru observations (0°S, 166°W, April – perature intervals are presented in Figure 7. As expected, November 1999, also SW monsoon season) reveal more the optical depth and mean extinction coefficient decreased subvisible cirrus (20–25%) and more thin cirrus (60–70%). with decreasing temperature for typical cirrus temperatures [45] Table 4 provides an overview of the mean values of of 40 °C. The impact of increased convective activity cirrus optical properties considering all cases and, separately, during the SW monsoon in comparison to the NE monsoon anvil (14 km) and remaining laminar or nonanvil cirrus is indicated by larger values of optical depth and extinction clouds. The higher frequency of subvisible cirrus during the coefficient at higher temperatures. NE monsoon season led to the lower mean cirrus optical [47] Sunilkumar and Parameswaran [2005] present a depth when compared with the SW monsoon mean optical relatively strong, almost exponential increase of the optical depth. The average values of the cirrus layer mean extinction depth with increasing temperature for the Gadanki data set. coefficient are remarkably similar for the different seasons. Their optical depth values are lower than those determined Differences in the lidar ratio mean values for the different from the Maldives data, with, e.g., a mean optical depth of seasons are also found to be small (Table 4). Only for thin about 0.15 for temperatures between 40 and 50°C cirrus a considerable difference is found (Table 3). The lidar compared to 0.35 over the Maldives. ratio statistics is discussed in more detail below. [48] The analysis of the data set from the Seychelles [46] Mean values of cirrus optical depth, cirrus layer [Pace et al., 2003] revealed a rather constant average optical mean extinction coefficient, and lidar ratio for 10 K tem- depth of about 0.7 at temperatures >60°C. However, at temperatures between 60 and 90°C the optical depth decreased rapidly from about 0.5 to 0.003. Reasons for the Table 4. Optical Cirrus Properties of All Clouds, Clouds With observed differences between the three data sets of Gadanki, Top Below 14 km, and Clouds With Top Above 14 kma the Seychelles, and the Maldives are unclear. They might All Cases NE Monsoon SW Monsoon result from different data analysis methods including the Analyzed cases 96 61 35 approach to correct for multiple scattering effects. Another Optical depth (all) 0.28 (0.29) 0.25 (0.26) 0.34 (0.29) reason may be differences in the definition of a single cirrus Optical depth (14 km) 0.32 (0.28) 0.31 (0.28) 0.35 (0.29) cloud layer. Pace et al. [2003] defined cloud layers sepa- Optical depth (>14 km) 0.25 (0.28) 0.21 (0.23) 0.34 (0.30) rated by more than 400 m as different clouds. In the present Ext. cf. (all), km1 0.12 (0.09) 0.12 (0.09) 0.12 (0.10) 1 study layers were evaluated separately when more than Ext. cf. (14 km), km 0.15 (0.10) 0.16 (0.11) 0.14 (0.11) Ext. cf. (>14 km), km1 0.09 (0.08) 0.10 (0.08) 0.09 (0.07) 800 m of clear air appeared between them. Thinner clouds Lidar ratio (all), sr 32 (10) 33 (9) 29 (11) would also lead to lower optical depths. For the analysis of Lidar ratio (14 km), sr 33 (11) 34 (10) 32 (12) the Gadanki data set, Sunilkumar and Parameswaran Lidar ratio (>14 km), sr 30 (9) 32 (8) 27 (8) [2005] applied a constant lidar ratio of S =20srtoall aValues are given for all cirrus cases and separately for each season. cirrus cases. This leads to an underestimation of the optical Standard deviations are given theses. Ext. cf., extinction coefficient.

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4.15 105 T 2. For comparison additional fit functions are presented for the tropical region of Gadanki (dashed-dotted line [Sunilkumar and Parameswaran, 2005]) and for the mid latitudinal region of Oklahoma, USA (dashed line [Wang and Sassen, 2002]). Retrieved cirrus layer mean extinction coefficients from the ICE data set (northern Germany, open squares [Ansmann et al., 1993]) are shown in addition. Good agreement is found for the Maldives and Oklahoma data sets when taking the standard deviations (INDOEX, solid circles) into account. Almost the same decrease of the extinction coefficient with decreasing temperature is found. The respective polynomial function for the Gadanki data deviates strongly from the INDOEX polynomial function, obviously the result of differences in the lidar data analysis and the retrieval assumptions. Good agreement is also found with ICE extinction-versus- temperature behavior. [51] The distribution of the INDOEX cirrus lidar ratios versus temperature is shown in Figures 7g–7i and 9. Although the mean values of the cirrus lidar ratio for the different seasons, different optical depth classes, and cloud Figure 8. Mean extinction coefficient versus temperature types (anvil, nonanvil) are very similar with values from (10-K intervals) determined from the entire Maldives data 27–36 sr (Tables 3 and 4), Figures 9 and Figure 7 indicate set (solid circles). The solid curve shows the polynomial fit some differences between the seasons. 63% of the NE function of second-order parameterization of the mean monsoon lidar ratios in Figure 9 are >30 sr and are well cloud extinction coefficient for the Maldives data set (see distributed over the 27–57 sr range (80% out of all NE text). The dashed-dotted curve is the polynomial fit function monsoon cases), whereas 68% of the SW monsoon lidar of the respective Gadanki distribution [Sunilkumar and ratios are <30 sr and accumulate from 16–31 sr (80% out of Parameswaran, 2005], and the dashed curve shows the all SW monsoon cases). A clear dependence on temperature polynomial fit function of midlatitude cirrus clouds is not obvious. observed over Oklahoma [Wang and Sassen, 2002]. In [52] According to model calculations [Reichardt et al., addition, mean values of the cirrus mean extinction 2002] of optical properties of ice crystals with weak shape coefficient for 10-K temperature intervals (shifted by 2 K distortion (slight deviations from the regular hexagonal from the interval center for better comparison, open structure and a rough surface) column-like crystals mostly squares) found during ICE [Ansmann et al., 1993] are produce lidar ratios of the order of 5–20 sr, whereas plate- shown. like crystals lead to lidar ratios in the range predominately from 15–35 sr. The model study further shows that the lidar ratio is rather sensitive to the crystal shape and especially to depth of clouds that actually had a higher lidar ratio. On deviations from the ideal hexagonal structure (pristine average, the cirrus lidar ratio was 30–35 sr at Hulule (see particles). With increasing distortion (irregularity) the lidar Tables 3 and 4 and Figure 7). ratios increase. It can be concluded that the lidar ratios for [49] The comparison of the optical properties of our larger, more complex crystals that are assumed to show a tropical data set with respective midlatitudes observations reveals that in midlatitudes subvisible, thin, and opaque cirrus occurred in about 10%, 60–65%, and 25–30% out of all cases over central Europe [Reichardt, 1999] and Okla- homa [Wang and Sassen, 2002], and roughly 10%, 30%, and 60% over Utah [Sassen and Comstock, 2001]. Mean cirrus optical depth was 0.58 over Oklahoma and 0.75 over Utah [Sassen and Comstock, 2001], and about 0.25–0.3 above northern Germany [Ansmann et al., 1993; Reichardt, 1999] as at Hulule (Table 4). [50] In order to accurately determine the radiative properties of clouds, improved knowledge on the cirrus extinction coefficient a as a function of temperature T is a basic requirement [Heymsfield and McFarquhar, 2002]. Parame- terization functions determined from distributions of the extinction coefficient dependent on temperature are useful Figure 9. Column lidar ratio versus temperature for NE to improve cirrus modeling. The parameterization function (triangles) and SW monsoon (crosses) seasons. 63% of the derived from the INDOEX data set is shown in Figure 8 NE monsoon values are >30 sr and 68% of the SW (solid line). The equation is a polynomial function of monsoon values are <30 sr. The lidar ratio of 30 sr is second-order of the for ) = 0.5072 + 8.91 103 T + indicated as a thin dashed line.

10 of 14 D17205 SEIFERT ET AL.: INDOEX CIRRUS OBSERVATIONS D17205 higher degree of distortion than small crystals, are consid- visible extinction coefficient for the cirrus layer is signifi- erably larger than 30 sr [Reichardt et al., 2002]. cantly overestimated by using the LIRAD method [Sassen [53] According to these simulations one may conclude and Comstock, 2001]. This overestimation appears to in- that the crystals were, on average, smaller and of more crease with increasing height (decreasing temperature) of regular shape during the SW monsoon season than during the cirrus layer. A similar discrepancy between Raman lidar the NE monsoon season. One reason for this finding may be and LIRAD observations of midlatitude cirrus lidar ratios that the majority of SW monsoon lidar ratios was deter- was already found earlier [Ansmann et al., 1992]. mined in anvil cirrus (Figure 9). On the other hand, if we only consider the lidar ratios for the anvil-cirrus temperature 4. Anthropogenic Aerosol Effects on Cirrus range (midcloud temperatures of about 60 ± 5°Cin Clouds Figure 9) the NE-to-SW differences remain. Different meteorological conditions and different aerosol conditions [59] Chylek et al. [2006] investigated the effect of aero- (polluted northern hemispheric air versus pristine southern sols on the size distribution of cloud droplets and ice hemispheric air) may have caused this difference (see crystals based on observations with the spaceborne MODIS discussion in section 4). (Moderate-resolution Imaging Spectroradiometer). They [54] It is interesting to note that a similar behavior found a larger effective radius of ice crystals in cirrus (influence of convection) of the lidar ratio was found by clouds over the Indian Ocean adjacent to the Indian sub- Whiteman et al. [2004] at the Andros Islands (25°N, continent during episodes of increased pollution (Januaries August–September 1998), Bahamas. When they excluded of 2001–2005) than during the more clean SW monsoon hurricane-influenced observations, they found a steady season (Septembers of 2000–2004). They proposed a increase from about 15–17 ± 10 sr at 30°C over 22 ± combination of natural seasonal variability of meteorological 8srat50°C to 28–33 ± 12 sr at temperatures around conditions and an ‘‘inverse aerosol indirect effect’’ (i.e., 70°C. When considering the hurricane-influenced cases a decrease in ice crystal numbers accompanied by growth (deep convection up to 16.5 km or 80°C) the mean lidar into larger sizes) caused by heterogeneous ice nucleation as a ratios decreased to values of roughly 15–22 sr for the possible explanation of the observed ice crystal growth. temperature range from 60°Cto80°C. The impact of They argue that this ‘‘inversion’’ may occur when pollution strong convection seems to lower the lidar ratio. Stronger aerosol supply heterogeneous ice nuclei (soot particles, dust convection may initiate the formation of a larger number of particles) that initiate ice nucleation at supersaturations crystals that then show a smaller mean size and more simple lower than those needed for homogeneous nucleation. This forms with regular structures (hexagonal columns and causes ice to form and grow on these heterogeneous ice plates). The lidar ratios were mostly in the range from nuclei first, thereby reducing the ambient water concentra- 10–40 sr (mean values of 20 ± 8 sr) over the Andros tions and supersaturation so that many of homogeneous ice Islands. Multiple scattering was estimated to be of minor nuclei are not activated, thereby decreasing the total number importance (5% impact) and thus not corrected. Some of ice crystal concentration. lowering of the lidar ratios due to specular reflection (strong [60] As observed with our lidar, aerosol layers reached backscatter coefficients in the case of a vertically pointing into heights of up to 4 km during the NE monsoon season lidar) cannot be excluded in this statistics. [Franke et al., 2003] (note, in addition, the aerosol pollution [55] Similar lidar ratios as observed during INDOEX were layers in Figures 1a and 1b). The particle optical depth was also derived from lidar observations in Taiwan (25°N, found to be a factor of three higher during the NE monsoon August 1999 to July 2000) presented by Chen et al. [2002]. season over the Maldives compared to conditions during the About 90% of their measured, multiple-scattering-corrected clean SW monsoon season. Above 4 km the atmosphere values were 50 sr. In the height ranges from 12–15 km was always rather clean according to the lidar observations. (50 to 70°C) they found single-scattering-related lidar Under such conditions the following effect may occur: Deep ratios of about 35 ± 15 sr. For heights between 15–16 km convection that is initiated in such pollution layers below (73 ± 3°C), the mean lidar ratio was 20 ± 8 sr. 4-km height may be considerably affected by the enhanced [56] Seven-year observations of the cirrus lidar ratio over concentration of aerosol particles. These particles may act as Utah revealed mean values of about 35, 27, 22, 22, and 21 sr additional cloud condensation nuclei (CCN [Twomey, for optical depths around 0.12, 0.25, 0.5, 1, 1.2, and 2.5, 1974]). For a constant liquid water content, enhanced respectively. Most retrieved values ranged from 15–60 sr particle concentrations may thus increase the cloud droplet [Sassen and Comstock, 2001]. number concentration, and decrease the mean cloud droplet [57] As mentioned by Whiteman et al. [2004], cirrus lidar diameter. Heymsfield and McFarquhar [2001] reported ratios from about 10–50 sr are in strong contradiction to droplet number concentrations in maritime convective values published by Platt et al. [2002] (Melville Island, in clouds north of the ITCZ that were up to five times higher northern Australia, 11°S, November–December 1995) for than in convective clouds south of the ITCZ. The decrease tropical cirrus. They found mean lidar ratios of 104 ± 20 sr of the effective droplet radius was also reported by Chylek (75°C), 90 ± 25 sr (65°C), 47 ± 20 sr (55°C), 48 ± et al. [2006]. 20 sr (45°C), and 39 ± 18 sr (35°C), respectively, by [61] In the case of smaller droplets an important ice applying the so-called LIRAD (lidar radiometer) technique. production process in tropical clouds, the Hallett-Mossop [58] In the LIRAD method the backscatter coefficient is process [Hallett and Mossop, 1974; Mossop and Hallett, measured with the lidar, whereas the visible extinction 1974; Pruppacher and Klett, 1997], might be suppressed coefficient is estimated from the column IR absorption [Rosenfeld and Woodley, 2000]. The Hallett-Mossop pro- coefficient measured radiometer. Obviously, the cess describes the important role ice particle splintering

11 of 14 D17205 SEIFERT ET AL.: INDOEX CIRRUS OBSERVATIONS D17205 plays during the riming process in clouds. The resulting ice haze on tropical cirrus clouds cannot be drawn from our splinters act again as ice nuclei, supporting the Bergeron- data. Findeisen ice production process. In order to be effective, the Hallett-Mossop process requires relatively high number 5. Summary concentrations of large droplets. Since droplets are comparably small in polluted clouds, ice production at relatively [66] We analyzed the INDOEX lidar-radiosonde measure- high temperatures might be less effective than it would be in ments with respect to the geometrical and multiple-scattering- clean maritime clouds. Hence more droplets would be lifted corrected optical properties of 179 different ice clouds with up to altitudes with temperatures below 37.5°C. At such base above 9-km height as a function of height and temperatures the droplets finally freeze homogeneously to temperature. Satellite observations of convective activity plate- and column-like ice crystals [Hallett et al., 2002]. (OLR) together with the temperature profiles of the According to this theory, cirrus clouds (anvil cirrus) that are 250 INDOEX radiosondes were used to study the impact influenced by anthropogenic pollution should have more of deep convection and stratospheric waves on cirrus and smaller ice particles than pristine cirrus clouds formation over the lidar site in the tropical Indian Ocean. [Ramanathan et al., 2001]. In comparison to cirrus not We found that the occurrence of cirrus clouds correlates to affected by pollution, the comparably high concentration both, the distance to deep convection and the phase of of ice crystals should lead to higher values of the cloud stratospheric waves. Statistics of geometrical and optical mean extinction coefficients and possibly lower lidar cirrus properties were presented for the entire data set as ratios (caused by smaller and more regular ice crystals) well as separately for the dry, aerosol-polluted NE monsoon during the NE monsoon season. and the rainy SW monsoon season. Average values of the [62] Our INDOEX observations reveal rather similar cirrus properties varied significantly between the two sea- cirrus layer mean extinction coefficients for the different sons. They were most likely caused by contrasting meteo- NE and SW monsoon seasons. As shown in Table 4, the rological conditions. Cirrus clouds covered the sky over the mean values of the extinction coefficient were 0.16 ± lidar site in only 35% of the measurement time during NE 11 (NE) and 0.14 ± 0.11 (SW) for anvil cirrus. On the other monsoon, but 64% during SW monsoon. With 2.1 ± 1.2 km hand, the column lidar ratios of anvil cirrus show conside- cloud geometrical depth during the SW monsoon was shifted rable differences between the seasons. Higher values were to larger values compared to the NE monsoon (1.6 ± 0.9 km). found during the NE monsoon season supporting the Mean midcloud heights were rather similar with values of finding of Chylek et al. [2006]. Here, we assume that higher 12.9 ± 1.5 km during NE and 12.7 ± 1.3 km during SW lidar ratios indicate a higher degree of irregularity of the ice monsoon. 25% of all observed clouds reached the tropopause crystal shapes, that in turn is linked to the presence of more level but less than 4% were observed above it. complex and thus larger ice crystals. When considering only [67] According to their optical depth the cirrus clouds the anvil cirrus clouds in Figure 9, about 74% of the NE were separated into subvisible (optical depth 0.03), thin monsoon cirrus lidar ratios are larger than 27 sr, whereas the (optical depth from 0.03 to 0.3), and opaque cirrus (optical SW monsoon anvil cirrus lidar ratios still accumulate depth 0.3). Thin cirrus was the dominating cirrus type between 16–31 sr (80% out of all anvil cirrus cases). during both seasons (48% during NE and 52% during SW [63]AsWhiteman et al. [2004] mentioned, deep- monsoon). The lower impact of deep convection during the convection-related cirrus show lower lidar ratios than NE monsoon is expressed in higher frequencies of subvis- laminar, nonanvil cirrus. It cannot be excluded that many ible cirrus (18% compared to 8% during SW monsoon) but ice clouds with top heights below 14 km during the NE less opaque cirrus (34% compared to 40% during SW monsoon season were still nonanvil cirrus. As was dis- monsoon). The average optical depth of all observed cirrus cussed above, during the NE monsoon season, aged cirrus clouds also showed significant seasonal differences, differ- was frequently observed that may have descended from ing between 0.25 ± 0.26 during NE and 0.34 ± 0.29 during above 14 km, while during the SW monsoon season, freshly SW monsoon. The average extinction-to-backscatter ratio formed anvil cirrus dominated. was lower during SW monsoon (29 ± 11 sr) than during NE [64] It also cannot be excluded that the lidar ratio monsoon (33 ± 9 sr). The average of the cirrus layer mean increases (indicating an increasing irregularity effect of extinction coefficient equaled for both seasons (0.12 ± 1 the crystals) with increasing lifetime of the ice cloud, 0.1 km ). although it appears unlikely that the degree of crystal shape [68] Comparisons of the results of the INDOEX measure- irregularity increases with time in a slowly dissolving and ments with other tropical, subtropical, and midlatitude cirrus evaporating cirrus [Reichardt et al., 2002]. studies were presented. Similar optical properties were [65] The difference in the extinction-to-backscatter char- mainly found for the different regions. For better compar- acteristics in ice clouds below 14 km height is consistent ison parameterizations of the temperature dependence of the with the finding of Chylek et al. [2006] stating that effective cirrus extinction coefficient were presented for 4 different cirrus particle size increases with increasing aerosol optical cirrus studies. Observed differences in the geometrical depth (increasing impact of aerosol pollution). However, properties can mostly be explained by differing meteoro- because significant differences in the meteorological con- logical conditions. Over the INDOEX region cirrus was ditions, especially below 14 km height, was also observed detected less frequent and at lower heights in comparison to during the NE and SW monsoon seasons, the differences in studies based on data sets from the western Pacific region the lidar ratios can also be simply explained by natural and the southwestern Indian Ocean. A cirrus study based on effects without the need of an anthropogenic effect. Thus an measurements of the spaceborne ICESat/Geoscience Laser unambiguous conclusio arding the influence of Asian Altimeter System (GLAS) found a similar distribution of

12 of 14 D17205 SEIFERT ET AL.: INDOEX CIRRUS OBSERVATIONS D17205 cirrus clouds over the Indian Ocean as reflected in the field Hallett, J., W. P. Arnott, M. P. Bailey, and J. T. Hallett (2002), Ice crystals in cirrus, in Cirrus, edited by D. K. Lynch et al., pp. 41–77, Oxford Univ. studies conducted in this region. Press, New York. [69] Cirrus optical properties observed during the polluted Hartmann, D. L., J. Holton, and Q. Fu (2001), The heat balance of the NE monsoon season and the clean, maritime SW monsoon tropical tropopause, cirrus and stratospheric dehydration, Geophys. Res. Lett., 28, 1969–1972. season were compared to identify a possible impact of man- Heymsfield, A. J., and G. M. McFarquhar (2001), Microphysics of made aerosols on ice formation and the optical properties of INDOEX clean and polluted trade cumulus clouds, J. Geophys. Res., ice clouds. Cirrus layer mean extinction coefficients were 106, 26,653–28,676. rather similar during both seasons. Lidar ratios, on the other Heymsfield, A. J., and G. M. McFarquhar (2002), Mid-latitude and tropical cirrus, in Cirrus, edited by D. K. Lynch et al., pp. 78–101, Oxford Univ. hand, were found to differ between NE and SW monsoon Press, New York. season suggesting more irregularly shaped and thus larger Heymsfield, A. J., G. M. McFarquhar, W. D. Collins, J. A. Goldstein, F. P. J. ice crystals during the NE monsoons season than during the Valero, J. Spinhirne, W. Hart, and P. Pilewskie (1998), Cloud properties leading to highly reflective tropical cirrus: Interpretations from CEPEX, SW monsoon season. Large ice crystals during the NE TOGA COARE, and Kwajalein, Marshall Islands, J. Geophys. Res., 103, monsoon season were also derived from satellite data 8805–8812. [Chylek et al., 2006]. A clear conclusion concerning a Hogan, R. J. (2006), Fast approximate calculation of multiply scattered lidar returns, Appl. Opt., 45, 5984–5992. possible impact of anthropogenic haze on cirrus properties Holton, J. R. (1992), Middle atmosphere dynamics, in An Introduction to is not possible because of significantly different meteoro- Dynamic Meteorology, 3rd ed., pp. 402–432, Elsevier, New York. logical conditions during the different seasons that can also Jensen,E.J.,O.B.Toon,H.B.Selkirk,J.D.Spinhirne,andM.R. be responsible for the found differences. Schoeberl (1996), On the formation and persistence of subvisible cirrus clouds near the tropical tropopause, J. Geophys. Res., 101,21,361– 21,376. [70] Acknowledgments. This research was supported in part by NASA Krishna Murthy, B. V., K. Parameswaran, and K. O. Rose (1986), Temporal grant NNG04GF31G, which is overseen by Hal Maring. We would like to variations of the tropical tropopause characteristics, J. Atmos. Sci., 43, thank the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, for providing 914–922. the interpolated OLR data on their web site http://www.cdc.noaa.gov/. Liebmann, B., and C. A. Smith (1996), Description of a complete (interpolated) outgoing longwave radiation dataset, Bull. Am. Meteorol. Soc., References 77, 1275–1277. Liou, K. N. (1986), Influence of cirrus clouds on weather and climate Ackerman, T. P., K.-N. Liou, F. P. Valero, and L. Pfister (1988), Heating processes: A global perspective, Mon. Weather Rev., 114, 1167–1199. rates in tropical anvils, J. Atmos. Sci., 45, 1606–1623. Liu, C., and E. J. Zipser (2005), Global distribution of convection penetrat- Althausen, D., D. Mu¨ller, A. Ansmann, U. Wandinger, H. Hube, E. Clauder, ing the tropical tropopause, J. Geophys. Res., 110, D23104, doi:10.1029/ and S. Zo¨rner (2000), Scanning 6-wavelength 11-channel aerosol lidar, 2005JD006063. J. Atmos. Oceanic Technol., 17, 1469–1482. Massie, S., A. Gettelman, W. Randel, and D. Baumgardner (2002), Dis- Ansmann, A., U. Wandinger, M. Riebesell, C. Weitkamp, and W. Michaelis tribution of tropical cirrus in relation to convection, J. Geophys. Res., (1992), Independent measurement of extinction and backscatter profiles 107(D21), 4591, doi:10.1029/2001JD001293. in cirrus clouds by using a combined Raman elastic-backscatter lidar, Massie, S. T., A. J. Heymsfield, C. Schmitt, D. Mu¨ller, and P. Seifert Appl. Opt., 31, 7113–7131. (2007), Aerosol indirect effects as a function of cloud top pressure, Ansmann, A., et al. (1993), Lidar network observations of cirrus morpho- J. Geophys. Res., 112, D06202, doi:10.1029/2006JD007383. logical and scattering properties during the International Cirrus Experi- Mossop, S. C., and J. Hallett (1974), Ice crystal concentration in cumulus ment 1989: The 18 October 1989 case study and statistical analysis, clouds: Influence of the drop spectrum, Science, 186, 632–634. J. Appl. Meteorol., 32, 1608–1622. Mu¨ller, D., K. Franke, F. Wagner, D. Althausen, A. Ansmann, and Boehm, M. T., and J. Verlinde (2000), Stratospheric influence on upper J. Heintzenberg (2001), Vertical profiling of optical and physical particle tropospheric tropical cirrus, Geophys. Res. Lett., 27, 3209–3212. properties over the tropical Indian Ocean with six-wavelength lidar: Chen, W.-N., C.-W. Chiang, and J.-B. Nee (2002), Lidar ratio and depolar- 1. Seasonal cycle, J. Geophys. Res., 106, 28,567–28,575. ization ratio for cirrus clouds, Appl. Opt., 41, 6470–6476. Mu¨ller, D., K. Franke, A. Ansmann, D. Althausen, and F. Wagner (2003), Chylek, P., M. K. Dubey, U. Lohmann, V. Ramanathan, Y. J. Kaufman, Indo-Asian pollution during INDOEX: Microphysical particle properties G. Lesins, J. Hudson, G. Altmann, and S. Olsen (2006), Aerosol in- and single-scattering albedo inferred from multiwavelength lidar observa- direct effect over the Indian Ocean, Geophys. Res. Lett., 33, L06806, tions, J. Geophys. Res., 108(D19), 4600, doi:10.1029/2003JD003538. doi:10.1029/2005GL025397. Pace, G., M. Cacciani, A. di Sarra, G. Fiocco, and D. Fua`(2003), Lidar Comstock, J. M., and K. Sassen (2001), Retrieval of cirrus cloud radiative observations of equatorial cirrus clouds at Mahe´Seychelles, J. Geophys. and backscattering properties using combined lidar and infrared radio- Res., 108(D8), 4236, doi:10.1029/2002JD002710. meter (LIRAD) measurements, J. Atmos. Oceanic Technol., 18, 1658– Pfister, L., et al. (2001), Aircraft observations of thin cirrus clouds near the 1673. tropical tropopause, J. Geophys. Res., 106, 9765–9786. Comstock, J. M., T. P. Ackerman, and G. G. Mace (2002), Ground-based Platt, C. M. R. (1981), Remote sounding of high clouds. III: Monte Carlo lidar and radar remote sensing of tropical cirrus clouds at Nauru Island: calculations of multiple-scattered lidar returns, J. Atmos. Sci., 38,156–167. Cloud statistics and radiative impacts, J. Geophys. Res., 107(D23), 4714, Platt, C. M. R., J. D. Spinhirne, and W. D. Hart (1989), Optical and doi:10.1029/2002JD002203. microphysical properties of a cold cirrus cloud: Evidence for regions of Dessler, A. E., S. P. Palm, W. D. Hart, and J. D. Spinhirne (2006), Tropo- small ice particles, J. Geophys. Res., 94, 11,151–11,164. pause-level thin cirrus coverage revealed by ICESat/Geoscience Laser Platt, C. M. R., S. A. Young, P. J. Manson, G. R. Patterson, S. C. Marsden, Altimeter System, J. Geophys. Res., 111, D08203, doi:10.1029/ R. T. Austin, and J. H. Churnside (1998), The optical properties of 2005JD006586. equatorial cirrus from observations in the ARM pilot radiation observa- Fernald, F. G. (1984), Analysis of atmospheric lidar observations: Some tion experiment, J. Atmos. Sci., 55, 1977–1996, doi:10.1175/1520- comments, Appl. Opt., 23, 652–653. 0469(1998)055. Franke, K., A. Ansmann, D. Mu¨ller, D. Althausen, C. Venkataraman, M. S. Platt, C. M. R., S. A. Young, R. T. Austin, G. R. Patterson, D. L. Mitchell, Reddy, F. Wagner, and R. Scheele (2003), Optical properties of the Indo- and S. D. Miller (2002), LIRAD observations of tropical cirrus clouds in Asian haze layer over the tropical Indian Ocean, J. Geophys. Res., MCTEX. Part I: Optical properties and detection of small particles in cold 108(D2), 4059, doi:10.1029/2002JD002473. cirrus, J. Atmos. Sci., 59, 3145–3162. Garrett,T.J.,A.J.Heymsfield,M.J.McGill,B.A.Ridley,D.G. Pruppacher, H. R., and J. D. Klett (1997), Microphysics of Clouds and Baumgardner, T. P. Bui, and C. R. Webster (2004), Convective generation Precipitation, 954 pp., Springer, New York. of cirrus near the tropopause, J. Geophys. Res., 109, D21203, doi:10.1029/ Ramanathan, V., and W. Collins (1991), Thermodynamic regulation of 2004JD004952. ocean warming by cirrus clouds deduced from observations of the Garrett, T. J., et al. (2006), Convective formation of pileus cloud near the 1987 El Nin˜o, Nature, 351, 27–32. tropopause, Atmos. Chem. Phys., 6, 1185–1200. Ramanathan, V., P. Crutzen, J. T. Kiehl, and D. Rosenfeld (2001), Aerosols, Hallett, J., and S. C. Mossop (1974), Production of secondary ice particles climate, and the hydrological cycle, Science, 294, 2119–2124. during the riming process, Nature, 249, 26–28.

13 of 14 D17205 SEIFERT ET AL.: INDOEX CIRRUS OBSERVATIONS D17205

Reichardt, J. (1999), Optical and geometrical properties of northern mid- Stephens, G. L. (2002), Cirrus, climate, and global change, in Cirrus, edited latitude cirrus clouds observed with a UV Raman lidar, Phys. Chem. by D. K. Lynch et al., pp. 433–448, Oxford Univ. Press, New York. Earth, Part B, 24, 255–260. Sunilkumar, S. V., and K. Parameswaran (2005), Temperature dependence Reichardt, J., S. Reichardt, M. Hess, and T. J. McGee (2002), Correlations of tropical cirrus properties and radiative effects, J. Geophys. Res., 110, among the optical properties of cirrus-cloud particles: Microphysical in- D13205, doi:10.1029/2004JD005426. terpretation, J. Geophys. Res., 107(D21), 4562, doi:10.1029/ Twomey, S. (1974), Pollution and the planetary albedo, Atmos. Environ., 8, 2002JD002589. 1251–1256. Rosenfeld, D., and W. L. Woodley (2000), Deep convective clouds with Wandinger, U. (1998), Multiple-scattering influence on extinction- and sustained supercooled liquid water down to 37.5 degrees., Nature, 405, backscatter-coefficient measurements with Raman and high-spectral- 440–442. resolution lidars, Appl. Opt., 37, 417–427. Sassen, K., and J. R. Campbell (2001), A midlatitude cirrus cloud clima- Wang, P. H., P. Minnis, M. P. McCormick, G. S. Kent, and K. M. Skeens tology from the Facility for Atmospheric Remote Sensing. Part I: Macro- (1996), A 6-year climatology of cloud occurence frequency from Strato- physical and synoptic properties, J. Atmos. Sci., 58, 481–496. spheric Aerosol and Gas Experiment II observations (1985–1990), Sassen, K., and B. S. Cho (1992), Subvisual thin cirrus lidar data set for J. Geophys. Res., 101, 29,407–29,429. satellite verification and climatological research, J. Appl. Meteorol., 31, Wang, Z., and K. Sassen (2002), Cirrus cloud microphysical property re- 1275–1285. trieval using lidar and radar measurements. Part II: Midlatitude cirrus Sassen, K., and J. M. Comstock (2001), A midlatitude cirrus cloud clima- microphysical and radiative properties, J. Atmos. Sci., 59, 2291–2302. tology from the Facility for Atmospheric Remote Sensing. Part III: Ra- Whiteman, D. N., B. Demoz, and Z. Wang (2004), Subtropical cirrus cloud diative properties, J. Atmos. Sci., 58, 2113–2127. extinction to backscatter ratios measured by Raman Lidar during Sassen, K., M. K. Griffin, and G. C. Dodd (1989), Optical scattering and CAMEX-3, Geophys. Res. Lett., 31, L12105, doi:10.1029/2004GL020003. microphysical properties of subvisual cirrus clouds, and climatic implica- Wylie, D. P., W. P. Menzel, H. M. Woolf, and K. I. Strabala (1994), Four tions, J. Appl. Meteorol., 28, 91–98, doi:10.1175/1520-0450(1989)028. years of global cirrus cloud statistics using HIRS, J. Clim., 7, 1972– Sassen, K., R. P. Benson, and J. D. Spinhirne (2000), Tropical cirrus cloud 1986. properties from TOGA/COARE airborne polarization lidar, Geophys. Res. Lett., 27, 673–676. Seifert, P. (2006), Seasonal dependence of geometrical and optical properties D. Althausen, A. Ansmann, D. Mu¨ller, P. Seifert, and U. Wandinger, of tropical cirrus determined from lidar, radiosonde, and satellite observa- Leibniz Institute for Tropospheric Research, Permoserstr. 15, 04318 Leipzig, tions over the polluted tropical Indian Ocean (Maldives), Diploma thesis, Germany. ([email protected]; [email protected]; [email protected]; 81 pp., Univ. of Leipzig, Leipzig, Germany. [email protected]; [email protected]) Sivakumar, V., Y. Bhavanikumar, P. B. Rao, K. Mizutani, T. Aoki, A. J. Heymsfield, S. T. Massie, and C. Schmitt, National Center for M. Yasui, and T. Itabe (2003), Lidar observed characteristics of the tropical Atmospheric Research, P.O. Box 3000, Boulder, CO 80307, USA. cirrus clouds, Radio Sci., 38(6), 1094, doi:10.1029/2002RS002719. ([email protected]; [email protected]; [email protected])

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