The optical properties and longwave radiative ... - Wiley Online Library

PUBLICATIONS Geophysical Research Letters RESEARCH LETTER 10.1002/2014GL059432 Key Points: • The characteristics of cirrus clouds in their lateral boundary layer were revealed • These clouds provide significant radiative forcing on the atmosphere

Supporting Information: • Readme • Text S1 • Figure S1 • Figure S2 • Figure S3 Correspondence to: Y. Fu, [email protected]

Citation: Li, R., H. Cai, Y. Fu, Y. Wang, Q. Min, J. Guo, and X. Dong (2014), The optical properties and longwave radiative forcing in the lateral boundary of cirrus cloud, Geophys. Res. Lett., 41, 3666–3675, doi:10.1002/ 2014GL059432. Received 27 JAN 2014 Accepted 2 APR 2014 Accepted article online 4 APR 2014 Published online 28 MAY 2014

The optical properties and longwave radiative forcing in the lateral boundary of cirrus cloud Rui Li1,2, Hongke Cai1,3, Yunfei Fu1, Yu Wang1, Qilong Min2,4, Jingchao Guo1, and Xue Dong1 1

Key Laboratory of the Atmospheric Composition and Optical Radiation, CAS, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, China, 2Atmospheric Science Research Center, State University of New York at Albany, Albany, New York, USA, 3Department of Atmospheric Science, Chengdu University of Information Technology, Chengdu, China, 4The State Key Laboratory of Information Engineering in Surveying, Mapping and Remote Sensing, Wuhan University, Wuhan, China

Abstract

Through observations from the Cloud-Aerosol Lidar with Orthogonal Polarization onboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation, we detected a common feature of narrow and subvisible lateral boundary layer in cirrus cloud. In this layer the lidar backscatter, the depolarization ratio, the ice water content, the effective radius of ice particles, and the cloud optical depth all decrease sharply toward the cloud edge. In general, the width of this layer (6.4 ± 3.1 km over land) decreases with increasing ambient temperature. The estimated longwave radiative forcing associated with the layer is about 10 W/m2. Due to its extremely small optical depth (less than 0.3), such lateral boundary layer may be missed by conventional satellite passive optical sensors. As a consequence, the mentioned radiative forcing has not been credited with its deserved share in the Earth’s radiative energy budget.

1. Introduction Cirrus clouds composed of nonspherical ice particles cover about 20–30% of the globe, providing significant radiative forcing (RF) on the Earth climate system and playing an important role in regulating the water vapor concentration in the upper troposphere and lower stratosphere [Liou, 1986; Stephens et al., 1990; Rossow and Schiffer, 1999; Rosenfield et al., 1998]. In spite of its unquestionable importance in global water and energy cycles, our understanding of the spatial and temporal distributions of cirrus cloud still has large uncertainties due to the difficulties of observation. Satellite passive optical sensors such as the Moderate Resolution Imaging Spectroradiometer (MODIS) have the effective capability of detecting thin cirrus cloud [Gao et al., 2002, 2003], but they may misclassify up to 40% of optical thin cirrus clouds with optical depth less than 0.3 as cloud-free region[Lee et al., 2009]. The spaceborne W-band (94 GHz) cloud profiling radar (CPR) on the CloudSat satellite can provide better detection of ice clouds at lower altitude but still misses many of the thin cirrus cloud at high altitudes[Stein et al., 2011]. The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) [Winker et al., 2003] provides a significant improvement in detecting high-level thin cirrus clouds [Nazaryan et al., 2008; Stein et al., 2011]. The cloud-clear-sky interface, which was hard to identify precisely before the launch of CALIPSO, is an ideal test bed to study the formation and the dissipation processes of clouds and the possible cloud-aerosol interactions. Several recent studies using lidar observations [Redemann et al., 2009; Su et al., 2008; Tackett and Di Girolamo, 2009; Varnai and Marshak, 2011] have found a transition zone in the clear sky surrounding lowlevel cloud in which aerosol optical properties undergo sharp changes. Most of these studies focus on warm clouds and the aerosol properties near the cloud edge. To our best knowledge, few satellite observational studies have been conducted to examine the optical properties and the consequent RF of cirrus cloud in the near-clear-sky environment. In directly contact with the unsaturated clear-sky environment, the cirrus cloud in the lateral boundary layer is expected to undergo a sublimation process and this can affect the water vapor concentration in the upper troposphere and lower stratosphere [Jensen et al., 1996a, 1996b; Corti et al., 2006]. The ambient temperature, humidity, and other large-scale dynamic and thermodynamic parameters may play roles in controlling the optical properties of clouds in this layer [Joos et al., 2008; Reverdy et al., 2012]. In addition, the aerosol, either from the lower atmosphere levels or aviation emission, may also affect the optical properties of ice cloud [DeMott et al., 2003; Min and Li, 2010].

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These uncertainties in satellite observations coexist in current understandings of high cloud feedbacks to climate change and the associated RF (Intergovernmental Panel on Climate Change (IPCC), AR4, and AR5 reports) [i.e., Forster et al., 2007; Boucher et al., 2013]. In this study, we aim to understand these uncertainties by using the CALIOP measurements with advanced detecting effectiveness on subvisible cirrus cloud.

2. Data and Method As a part of the effort to understand the role of the Asia monsoon in global weather and climate system, we focused on three study areas in southeastern China and the surrounding region in June, July, and August from 2006 to 2011. The selected study areas include one over land in eastern China (Land, 105°E ~ 125°E, 20°N ~ 40°N, 1635 cirrus cloud cases) and two over oceans to the east (SeaE, 120°E ~ 130°E, 20°N ~ 40°N, 785 cirrus cloud cases) and to the south (SeaS, 110°E ~ 120°E, 5°N ~ 25°N, 1225 cirrus cloud cases) of China. The well-validated CALIPSO-CALIOP backscatter profiles (L1B) and vertical feature masks (L2-VFM) data [Winker et al., 2007, 2009; McGill et al., 2007; Liu et al., 2009; Rogers et al., 2011] were used to identify cirrus cloud and its interface with the clear sky and to derive the associated optical properties in the lateral boundaries. The L1B product provides dual-wavelength (532 and 1064 nm) attenuated backscatter profiles at the horizontal resolution of 1 km and the vertical resolution of 60 m at altitudes of 8.3–20.2 km. Only nighttime data were used to avoid the low signal-to-noise ratio (SNR) caused by strong solar radiation in the daytime [Kim et al., 2008]. Because of the two-way attenuation, attenuated backscatter in different environments cannot be compared directly, so backscatter profiles (β) calculated from the attenuated backscatter profiles ( β′) were used. Depolarization ratio δ532 ( β′532;⊥ =β′532;== , ratios of the perpendicular and parallel components of the 532 nm backscatter) and the color ratio χ (β1064 /β532, ratios of total backscatter coefficients at 1064 to that at 532 nm) were also investigated to study the particle shape and size information. In L2-VFM data, the scene classification algorithm [Liu et al., 2009] defines the type of detected layer based on color ratio information mainly and also defines the cloud thermodynamic phase based on the depolarization information. Its vertical resolution is consistent with that of L1B. To mitigate the impact of lower SNR, horizontal averaging up to 80 km is used in L2-VFM data. However, in this study, we only used samples with horizontal averaging less than 1 km to ensure the precise positioning of the interface between cirrus cloud and clear sky. Other constraints were applied to select the proper cases in this study: (1) cloudy and clear-sky areas should be wide enough (at least 45 km ) to ensure one interface only, (2) L2-VFM data should indicate clearsky and ice-phase cirrus in cloudy areas with high reliability, (3) the cloud optical depth should be limited to less than 3 to ensure cirrus rather than deep convective clouds [Sassen and Cho, 1992], (4) the 532 nm backscatter in the cloud should be significantly stronger (> the monthly mean value plus two standard deviations under 3σ quality control) than that in clear sky, and (5) the static stability of atmosphere computed from reanalysis data should be positive (∂θ ∂z > 0) to ensure the selected cirrus cloud formed in stable stratification with weak vertical movement. National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) data (with an hourly time resolution and a horizontal resolution of 0.5° latitude × 0.5° longitude) [Saha et al., 2010] at 18:00 UTC (~2:00 A.M. local time close to CALIPSO’s overpasses) were used to analyze the possible dependence of cirrus cloud optical properties on environmental parameters and largescale dynamics. As a first-order approximation, we used the methods of Heymsfield et al. [2005] to estimate the ice water content, the effective radius (re), and the optical depth of the cirrus cloud based on the measured β532 (refer to the supporting information). The Fu-Liou radiative transfer model [Fu and Liou, 1992, 1993] was used to calculate the longwave radiative forcing (wave number of 0–2200 cm 1) with assumptions of midlatitude (for the Land and SeaE) and subtropical (for the SeaS) standard atmosphere profiles. The emissivities of background continent and oceanic surface were based on Capelle et al. [2012] and Hanafin and Minnett [2005]. Since the differences between clear and cloudy sky (i.e., the cloud radiative forcing)

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Figure 1. The height-distance cross section of CALIPSO/CALIOP observed. (a) β1064: the backscatter at 1064 nm. (b) β532: the backscatter at 532 nm. (c) χ: the color ratio. (d) δ532: the depolarization ratio at 532 nm related to a cirrus cloud case on 25 June 2008 over the selected Land area (105°E ~ 125°E, 20°N ~ 40°N). Distance zero stands for the cloud edge.

rather than the radiation fluxes themselves are our main interests, the uncertainties associated with the assumptions of atmosphere profile and surface emissivities should not bias our conclusions significantly.

3. Results 3.1. The Optical Properties at the Cloud Boundary Considering the case on 25 June 2008 over Land (Figure 1), a clear lateral boundary layer surrounding the main cloud body is identified, in which the β1064 and β532 dramatically drop from ~0.01 to 0.03 km 1 sr 1 to less than 10 3 km 1 sr 1 toward the cloud edge in just a few kilometers. Comparing to the sharp change in the boundary layer, relatively uniform β1064 and β532 are observed in the main cloud body and the clear sky. In addition to the backscatter, the depolarization ratio δ532 (Figure 1d) also shows a decrease trend in the lateral boundary layer but not as clear as that shown in the backscatter. The color ratio (Figure 1c) does not show a reliable trend there. In the cloud-free region, the color ratio is close to zero, indicating no aerosol particles. Although the low-level aerosol particles may be transported vertically to higher atmosphere by deep convections [Yin et al., 2005; Cui and Carslaw, 2006] and show up in the residue of cirrus cloud, in this case their concentration maybe too low to be detected by CALIOP. The backscatter at 532 nm in cloud-free region (Figure 1b) is stronger than that at 1064 nm (Figure 1a) because the molecular Rayleigh scattering is proportional to 1/λ4. The SNR for backscatter at 1064 nm in cloud-free region is low, and this leads to a large uncertainty in the color ratio as well; therefore, we will only use β532 and δ532 in the following discussions. The above features of cirrus cloud in its lateral boundary layer are not unique to this single case but are common (>90%) in multiple cases. Based on the 1635 cases selected over Land, the median and 75% and

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25% percentile β532 and δ532 (as functions of distance to the cloud edge) all decrease steeply within a narrow lateral boundary layer (Figures 2a and 2b). Accompanied with this sharp change in cloud boundary are the much smoother variations in the main cloud body and in the adjacent clear sky. We used quadratic fitting of median values of ln(β1064) and ln(β532) in the cloud main body and linear fitting of them in the cloud-free region. It is found that the continuous 3 km horizontal deviation of fitted values in cloud area sharply changed at the distance of about 10 km from the cloud edge. The median values of ln(β1064) and ln(β532) in this lateral boundary layer fit are best fitted with a straight line (red dashed lines in Figures 2a and 2b). The intersection of this fitted line with those in cloud main body and cloud-free region determined the exact width of the lateral boundary layer, which is about 6.4 ± 3.1 km over the Land area. The derived ice water content, the effective radius, and the optical depth based on the median β532 (Figures 2c, 2d, and 2e) also decrease proportionally with the distance to the cloud edge. It should be noted that the mean cloud optical depth in the lateral boundary layer is smaller than 0.3, which is the detection threshold of MODIS [Lee et al., 2009] and CloudSat CPR [Haladay and Stephens, 2009]. As a result, those cloud boundaries would not be detected by these sensors. The lateral boundary of cirrus cloud is in contact with unsaturated ambient environment and thus undergoes sublimation processes. This may lead to less ice water content, smaller particle size, and consequently weaker lidar backscatter compared to those in the main cloud body. However, the physical understanding of the decrease of depolarization ratio at the lateral boundary is not clear and warrants further study. The formation of cirrus cloud largely depends on the environmental conditions and large-scale dynamics. In order to reveal the potential controlling factors at the lateral boundary, we studied the correlations between the width and the depolarization ratio to parameters of ambient temperature (T), relative humidity (RH), vertical velocity, horizontal divergence, and convective available potential energy using the temporal and spatial nearest NCEP data in accordance with the altitude of the cirrus cloud. Statistically significant correlations are found among the width, the depolarization ratio, and the T as shown in Figures 3a and 3c. The width of the lateral boundary layer decreases with increasing ambient temperature. The correlation coefficient is 0.11, above the 95% statistical confidence level. In other words, cirrus cloud formed at warmer temperatures tends to have narrower lateral boundaries compared to those formed at colder temperatures. This is consistent with Lin 1983 (equation (31)), in which the sublimation rate of ice clouds increases with ambient temperature and decreases with supersaturation ratio. We speculate that the width of the lateral boundary can be used as an indicator of the sublimation rate and the observed width-T dependence can be explained by the fact that the sublimation rate of ice particles is faster at warmer temperature [Lin et al., 1983]. On the other hand, there is only insignificantly weak positive correlation between the width and the largescale RH (Figure 3b). The moister environment close to the lateral boundary is expected to slow down the sublimation process and result in a broader lateral boundary of cirrus cloud. However, the large-scale RH from NCEP is substantially different from that near the actual cloud edge. The above dependence would have been stronger if using in situ water vapor. The depolarization ratio also decreases with temperature (Figure 3c), similar to effects reported by previous studies [Noel et al., 2002; Wang et al., 2008; Sassen and Zhu, 2009]. The possible contamination from spherical supercooled water particles and the changes of shape may explain this decrease at relatively warmer temperatures. The relationships between the width and the depolarization ratio to other dynamic parameters are weak (see supporting information Figures S1 and S2 ), indicating either the NCEP CFSR reanalysis data are not suitable for this kind of study (due to the spatial and temporal mismatch with satellite observation) or those parameters have little impacts on the lateral boundary of cirrus clouds. 3.2. The Radiative Forcing at the Cloud Boundary The cirrus clouds can trap outgoing longwave radiation from the Earth-atmosphere medium and contribute a positive radiative forcing (RF) effect on the Earth climate system. Based on the above findings, the optical properties of cirrus cloud in the lateral boundary layer substantially differ from those in its main body. Due to its extremely small optical depth, however, the associated RF might be missed by conventional satellite

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Figure 2

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Figure 3. The scatterplots of the width of the lateral boundary of cirrus cloud to (a) ambient temperature and (b) ambient relative humidity (RH). Overlapped are the associated mean curves with standard derivation. (c, d) Same as Figures 3a and 3b but for the depolarization ratio at 532 nm.

optical sensors. Current satellite sensors measuring the radiation flux such as the Clouds and the Earth’s Radiant Energy System have too coarse spatial resolution (~20 km) to catch this subtle feature (with a scale less than ~10 km). In this study, we employed the retrieved ice water content and effective radius (in Figures 2d and 2e) as input to the radiative transfer model of Fu and Liou [1992, 1993] to calculate the cloud radiative forcing (cloud-clear) of the cirrus cloud in its lateral boundary layer. That model has been successfully applied to study the possible impacts of dust on longwave radiation forcing from ice clouds in Min and Li [2010]. The RF results in the selected Land area in China are shown in Figures 2f and 2g. As expected, the outgoing longwave radiation at the top of atmosphere (TOA) over cirrus covered area is ~30–40 W/m2 smaller than that in the clear sky (warming effect) because cirrus cloud trapped more longwave radiation. The trapped energy is mainly used to heat the atmosphere beneath, and only a slight increase (~0.5 W/m2) of downward longwave radiation is received at the Earth surface (Figure 2g). We will be focusing on the RF at the TOA in the following discussion. In the lateral boundary of cirrus clouds, the radiative forcing (RF) decreases with the distance to the cloud edge almost linearly. The overall mean RF in the whole lateral boundary is about 10 W/m2, which is ~20 W/m2

Figure 2. (a) The median (blue solid) and 75% and 25% (black dash) percentile backscatter at 532 nm and the associated fitting lines (red dash) in the main cloud body, the lateral boundary, and the clear-sky regions; (b) the same as Figure 2a but for depolarization ratio; (c) the mean optical depth; (d) the mean effective radius; (e) the mean ice water content; (f) the mean upward longwave radiative forcing (cloud minus clear sky) at the top of atmosphere; and (g) the mean downward longwave radiative forcing at Earth surface as functions of the distance to cloud edge (distance = 0) over the selected Land area.

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Figure 4. The comparisons of (a) median β532, the backscatter at 532 nm; (b) median δ532, the depolarization ratio at 532 nm; (c) the mean upward longwave radiative forcing at the top of atmosphere; and (d) the mean downward longwave radiative forcing at Earth surface as functions of the distance to cloud edge among three selected area of Land, SeaE, and SeaS.

weaker than that in the main cloud body. After comparing with the results in the selected SeaE and SeaS regions (Figure 4c), no significant regional variations were found. As we mentioned, satellite passive optical sensors have great difficulty detecting most lateral boundaries in cirrus cloud. So far, no global atmosphere circulations models (GCM) take account of such an effect. Therefore, a lateral boundary cirrus cloud-induced bias of longwave RF is expected in both satellite observations and GCM model simulations. Considering the large coverage of cirrus cloud (~25% globally), this bias may be significant to the TOA energy balance. To give a rough quantitative estimation of such effect before doing detailed global statistics, we assume that the mean optical properties and RF (i.e., 10 W/m2) in the lateral boundary of cirrus cloud are valid globally, the mean width of the cirrus cloud lateral boundary is 6 km, and the global cirrus coverage is 25%. At the most conservative estimation, if all cirrus clouds are assembled into a big round plate without any holes embedded, the total length L0 of lateral boundary gets its minimum of 2πR0(R0 = 6370 km, the radius of Earth). Based on the above assumptions and considerations, the associated global longwave RF is at least 0.0047 W/m2 (Global RF = (L0 × Width × 10 W/m 2)/(4πR02) = (2πR0 × 6 × 103 × 10 W/m 2)/(4πR02) = 30 × 103/R0 = 0.0047 W/m 2). Given the fact that most cirrus clouds have fibrous and/or silky sheen appearances, the real value of L0 and RF should be substantially larger. As referenced from IPCC 2007 report [Forster et al., 2007], the RF for persistent line-shaped contrails is about +0.01 W/m2, and the RF for aviation-induced cloudiness is about +0.03 W/m2. Therefore, the lateral boundary cirrus clouds-induced RF (> > 0.005 W/m2) is comparable to, and even larger than, these recognized important radiative forcing agents.

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Different parameterizations of the radiative properties (single scattering albedo, phase function, and such) of cirrus cloud also have impacts on the above estimations [Stephens et al., 1990] but are out of the scope of this paper. We believe that the found RF related to lateral boundary cirrus cloud should be significant enough to overcome the potential errors and uncertainties in radiative transfer modeling. 3.3. Comparisons Among Different Areas Although the color ratio measurements suggest no aerosol in the cloud-free region adjacent to the cirrus cloud edge (Figure 1c), this does not necessarily exclude the possibility of aerosol-cloud interaction. The background aerosol loading is different among the three areas based on MODIS-observed aerosol optical depth (AOD, supporting information Figure S3). The mean AOD among Land, SeaE, and SeaS are ~0.7, ~0.4, and ~0.1 during June-July-August 2006–2011. Meanwhile, as measured by CALIOP, the mean effective temperatures of the cirrus clouds among the three areas are 42.7°C, 44.1°C, and 47.3°C, respectively. Also, the median β532 over Land area is higher than those over SeaE and SeaS. The δ532 shows very similar behavior over those areas. Warmer cirrus cloud over higher aerosol contaminated area is consistent with the hypothesis that continental dust aerosol serves as ice nuclei to enhance the heterogeneous ice nucleation process which occurs in relatively warmer temperature and lower super saturation conditions compared to those required by the homogeneous ice nucleation [DeMott et al., 2003; Sassen et al., 2003; Min and Li, 2010]. The reason for such difference can also be attributed to the different dynamic and thermodynamic conditions in the study areas. There are no significant differences of the optical properties and the width of the lateral boundary layer among the three selected areas (Figure 4). The radiative forcing at the TOA and at the surface also shows little difference. This indicates that the features of lateral boundary cirrus cloud do not have large regional variations in China.

4. Conclusion and Discussion The optical properties and the longwave radiative forcing in the lateral boundary of cirrus cloud in China were investigated by using nighttime observations from spaceborne lidar the CALIPSO/CALIOP. A narrow (6.4 ± 3.1 km), subvisible cirrus cloud lateral boundary over land can be readily identified with the CALIOP-measured backscatter at both 532 and 1064 nm. The small optical depth (