Estimation of the Aerosol Radiative Effect over the Tibetan Plateau Based on the Latest CALIPSO Product

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  • Corresponding author: Yuzhi LIU, liuyzh@lzu.edu.cn
  • Funds:

    Supported by the Strategic Priority Research Program of Chinese Academy of Sciences (XDA2006010301), National Natural Science Foundation of China (91737101, 41475095, and 41405010), Fundamental Research Funds for Central Universities (lzujbky-2017-63), and China 111 Project (B13045)

  • doi: 10.1007/s13351-018-8060-3

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  • Based on the CALIPSO (Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation) Version 4.10 products released on 8 November 2016, the Level 2 (L2) aerosol product over the Tibetan Plateau (TP) is evaluated and the aerosol radiative effect is also estimated in this study. As there are still some missing aerosol data points in the daytime CALIPSO Version 4.10 L2 product, this study re-calculated the aerosol extinction coefficient to explore the aerosol radiative effect over the TP based on the CALIPSO Level 1 (L1) and CloudSat 2B-CLDCLASS-LIDAR products. The energy budget estimation obtained by using the AODs (aerosol optical depths) from calculated aerosol extinction coefficient as an input to a radiative transfer model shows better agreement with the Earth’s Radiant Energy System (CERES) and CloudSat 2B-FLXHR-LIDAR observations than that with the input of AODs from aerosol extinction coefficient from CALIPSO Version 4.10 L2 product. The radiative effect and heating rate of aerosols over the TP are further simulated by using the calculated aerosol extinction coefficient. The dust aerosols may heat the atmosphere by retaining the energy in the layer. The instantaneous heating rate can be as high as 5.5 K day–1 depending on the density of the dust layers. Overall, the dust aerosols significantly affect the radiative energy budget and thermodynamic structure of the air over the TP, mainly by altering the shortwave radiation budget. The significant influence of dust aerosols over the TP on the radiation budget may have important implications for investigating the atmospheric circulation and future regional and global climate.
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  • Fig. 1.  The altitude–orbit cross-sections of (a–d) total attenuated backscattering intensity at 532 nm and (e–h) depolarization ratio over the vicinity of the TP (27.5°–45.N, 80°–100°E) from 6 to 10 August 2008 along the scanning orbit paths of CALIPSO satellite. The last column shows the orbits of the satellite (red lines).

    Fig. 2.  The red–green–blue (RGB) color composite image over the vicinity of the TP on 7 August 2008 sounded by MODIS instruments aboard (a) Terra and (b) Aqua. The red line shows the trajectory of the CALIPSO satellite over the TP on 7 August 2008. The blue line denotes the region where the CALIPSO satellite sounds the aerosols as clouds.

    Fig. 3.  The feature of atmosphere classifications for the dusty event on 7 August 2008 from CALIPSO (a) V3 and (b) V4 products. (c) The distribution of clouds along the scanning orbit path of CloudSat satellite.

    Fig. 4.  Frequencies of dusty events (black histograms) in the vicinity of the TP (27.5°–45°N, 80°–100°E) in summer during 2007–10. The gray histograms indicate the frequencies of all the sections detected by CALIPSO in day (light) and night (dark) times, respectively. The slanting shadow indicates the missing frequency of the aerosol data in the CALIPSO V4.10 L2 product due to the misjudgment.

    Fig. 5.  (a) Flow chart for calculating the aerosol extinction coefficient from the CALIPSO L1 data and CloudSat products, and (b) structure schematic of the atmospheric layers.

    Fig. 6.  The altitude–orbit cross-sections of aerosol extinction coefficient from CALIPSO (a) V3.30 and (b) V4.10 L2 products along the satellite orbit path on 7 August 2008. (c) The aerosol extinction coefficient calculated based on the method shown in Fig. 5.

    Fig. 7.  Variations in SW and LW (infrared) radiation at (a, b) the TOA and (c, d) the surface on 7 August 2008 along the satellite orbit. The observations from CERES and CloudSat are indicated in green and orange, respectively. The red dots and blue curves represent model simulations performed by using aerosol extinction coefficient from the CALIPSO L2 product (red) and the calculated one (blue) as inputs, respectively.

    Fig. 8.  The altitude–orbit cross-sections of (a) direct shortwave, (b) infrared, and (c) net radiative forcing (W m–2) along the CALIPSO orbit path over the TP on 7 August 2008.

    Fig. 9.  The altitude–orbit cross-sections of (a) shortwave, (b) infrared, and (c) net heating rates (K day–1) due to the aerosols along the CALIPSO orbit path over the TP on 7 August 2008.

    Table 1.  Wave bands in Fu–Liou radiation transfer model

    Spectral band
    Longwave (cm–1)2200–1900, 1900–1700, 1700–1400, 1400–1250, 1250–1100, 1100–980, 980–800, 800–670, 670–540, 540–400, 400–280, 280–0
    Shortwave (μm)0.175–0.7, 0.7–1.3, 1.3–1.8, 1.8–2.5, 2.5–3.5, 3.5–4
    Visible band
    (0.175–0.700 μm)
    0.1754–0.2247, 0.2247–0.2439, 0.2439–0.2857, 0.2857–0.2985, 0.2985–0.3225, 0.3225–0.3575, 0.3575–0.4375, 0.4375–0.4975, 0.4975–0.5950, 0.5950–0.6896
    Download: Download as CSV

    Table 2.  SSA and ASY of 18 aerosol types at a relative humidity of 50% used in Fu–Liou radiation transfer model

    Aerosol typeSSAASY
    Marine1.00.8113
    Continental0.95650.6293
    Urban0.93270.6270
    0.5-micron dust0.96530.6622
    1.0-micron dust0.92550.6931
    2.0-micron dust0.87150.7725
    4.0-micron dust0.79491.0
    8.0-micron dust0.69960.8795
    Insoluble0.74860.8163
    Water soluble0.97300.6570
    Soot0.17890.3079
    Accumulated sea salt1.00.7724
    Coarse sea salt1.00.8464
    Nucleating mineral dust0.97400.6521
    Accumulated mineral dust0.90930.7162
    Coarse mineral dust0.71180.8741
    Transported mineral dust0.87630.7532
    Sulfate droplets1.00.7634
    Download: Download as CSV
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Estimation of the Aerosol Radiative Effect over the Tibetan Plateau Based on the Latest CALIPSO Product

    Corresponding author: Yuzhi LIU, liuyzh@lzu.edu.cn
  • Key Laboratory for Semi-Arid Climate Change of the Ministry of Education, College of Atmospheric Sciences, Lanzhou University, Lanzhou 730000
Funds: Supported by the Strategic Priority Research Program of Chinese Academy of Sciences (XDA2006010301), National Natural Science Foundation of China (91737101, 41475095, and 41405010), Fundamental Research Funds for Central Universities (lzujbky-2017-63), and China 111 Project (B13045)

Abstract: Based on the CALIPSO (Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation) Version 4.10 products released on 8 November 2016, the Level 2 (L2) aerosol product over the Tibetan Plateau (TP) is evaluated and the aerosol radiative effect is also estimated in this study. As there are still some missing aerosol data points in the daytime CALIPSO Version 4.10 L2 product, this study re-calculated the aerosol extinction coefficient to explore the aerosol radiative effect over the TP based on the CALIPSO Level 1 (L1) and CloudSat 2B-CLDCLASS-LIDAR products. The energy budget estimation obtained by using the AODs (aerosol optical depths) from calculated aerosol extinction coefficient as an input to a radiative transfer model shows better agreement with the Earth’s Radiant Energy System (CERES) and CloudSat 2B-FLXHR-LIDAR observations than that with the input of AODs from aerosol extinction coefficient from CALIPSO Version 4.10 L2 product. The radiative effect and heating rate of aerosols over the TP are further simulated by using the calculated aerosol extinction coefficient. The dust aerosols may heat the atmosphere by retaining the energy in the layer. The instantaneous heating rate can be as high as 5.5 K day–1 depending on the density of the dust layers. Overall, the dust aerosols significantly affect the radiative energy budget and thermodynamic structure of the air over the TP, mainly by altering the shortwave radiation budget. The significant influence of dust aerosols over the TP on the radiation budget may have important implications for investigating the atmospheric circulation and future regional and global climate.

1.   Introduction
  • Aerosols exert a profound impact on the radiation balance of the earth–atmosphere system and both global and regional climate. Aerosols not only affect the radiation balance by directly scattering and absorbing the incoming solar and thermal radiation (Charlson et al., 1992; Sekiguchi et al., 2003; Kim et al., 2005; Takamura et al., 2007), but also influence cloud microphysical properties and precipitation rate by acting as condensation nuclei for cloud formation (Twomey, 1977; Albrecht, 1989; Ackerman et al., 2000; Su et al., 2008; Huang el al., 2009). In addition, aerosols can accelerate the evaporation of low-level cloud droplets and further reduce the cloud water path through a semi-direct effect (Rosenfeld et al., 2014; Seiki and Nakajima, 2014). Any change in the amount or composition of aerosols can affect the radiation budget and climate in various ways (Nakajima et al., 2007; Mukai et al., 2008; Su et al., 2008; Zhang et al., 2010; Choi et al., 2014; Rosenfeld et al., 2014). Given that aerosols are continuously increasing over the developing countries surrounding the Tibetan Plateau (TP) such as China (Uno et al., 2001; Guo et al., 2011; Li et al., 2017) and India (Müller et al., 2003; Toth et al., 2016), their related radiative effects must be better understood and quantified.

    As the tallest plateau on the earth, the TP has substantial implications for atmospheric circulation, the hydrological cycle, and regional and global climate change because of its special geographical and configurational features and the strong solar heating and low air density over this plateau (Qian et al., 2004, 2011; Zhu et al., 2008; Zhou et al., 2013). Moreover, the TP locates at the juncture of several important natural and anthropogenic aerosol sources, and these aerosols can be transported into the atmosphere over the TP via the airflow. In summer, the TP acts as an elevated heat source in the middle troposphere (Yang et al., 2009; Wonsick et al., 2014), which favors the vertical transport of dust aerosols from neighboring sources through triggering strong convergence (Yang et al., 1992; Lau et al., 2006). Recently, many studies have revealed that dust and anthropogenic aerosols accumulate over the TP (Huang et al., 2007; Liu et al., 2008a, 2015; Xia et al., 2008), especially in summer and autumn (Chen et al., 2013). The transported aerosols could significantly influence the melting of snow and the vertical thermal structure by affecting the radiation balance and thus changing the plateau heat pump effect of the TP (Wu et al., 2002, 2006; Lau et al., 2010; Yasunari et al., 2010; Lau, 2016). Simultaneously, the variation in the air temperature caused by aerosols could further induce a horizontal temperature gradient, which may modify the wind field (Wang et al., 2013; Huang et al., 2014), alter the atmospheric stability and convection, and eventually affect the atmospheric dynamics (Kuhlmann and Quaas, 2010). Moreover, some studies have noted that the aerosols accumulated against the northern and southern slopes of the TP may advance the rainy periods and subsequently intensify the summer monsoon (Lau et al., 2006; Lau and Kim, 2010). For suspended aerosols over the TP, the effects of this special topography and high surface albedo feedback may be quite different, suggesting that distinct effects arise at different atmospheric levels (Huang et al., 2009, 2014).

    Satellite observations provide us an unprecedented opportunity overcoming the lack of long-term and systemic observations, especially over the ocean and plateaus. The Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite, which was launched in 2006, provides a wealth of actively remote sensed vertical structures of aerosols at regional and global scales, even over bright surfaces and beneath thin cloud as well as under clear sky conditions (Huang et al., 2007, 2008; Liu et al., 2008a, b). However, when the aerosol extinction data from the CALIPSO Level 2 (L2) product were used to estimate the radiative effect of aerosols over the TP, many values were frequently found to be absent due to misidentification of cloud and aerosol. The CALIPSO Version 3 CAD (cloud aerosol distinction) algorithm tends to classify elevated aerosol as cloud, particularly at mid–high latitudes. Fortunately, in the latest version of CALIPSO lidar data product (Version 4.10), dust detection is much improved. So far, there are few studies to assess the improvement of new products and calculate the aerosol radiative effects with the latest CALIPSO aerosol product over the TP.

    In this study, aerosol extinction profiles from the CALIPSO L2 Version 4 and Version 3 products over the TP were evaluated and re-calculated based on a typical dusty case on 7 August 2008. The aerosol optical depths (AODs) obtained from aerosol extinction profiles from CALIPSO L2 Version 4 product and recalculation were further used as the inputs to a radiative transfer model to estimate the aerosol radiative effects. To evaluate the recalculated aerosol extinction profiles, the simulated shortwave (SW) and infrared radiation budgets at the top of the atmosphere (TOA) and surface (SFC) were compared with those from the Earth’s Radiant Energy System (CERES) and CloudSat observations. Finally, the radiative and heating effects of dust aerosols over the TP were simulated and analyzed.

2.   Datasets, model description, and methodology
  • The CALIPSO onboard provides new insight into the vertical structure and properties of clouds and aerosols. On 8 November 2016, the CALIPSO project began the release of Version 4.10 (V4.10 or V4 for short) of the CALIPSO Level 1B (L1) data and the attendant L2 products. The CALIPSO L1 product provides three calibrated and geolocated lidar profiles of total attenuated backscatter at 532 and 1064 nm, and the perpendicular polarization component of the backscatter at 532 nm. The features of atmosphere classification from the CALIPSO VFM (vertical feature mask) files are also analyzed to explore cloud and aerosol locations and types. The CAD algorithm of V3 (Versions 3.01, 3.02, and 3.03 for different periods) tends to classify the elevated dust as cloud, especially at mid–high latitudes, which is reduced in V4 through introducing definition of elevated layers and fringes (Omar et al., 2016), leading to a better classification of aerosol and cloud at high latitude and high altitude. Through adding new aerosol type and implementation of the Subtype Coalescence Algorithm for AeRosol Fringes, aerosol detection is much improved in V4 data (Omar et al., 2016). The CALIPSO L2 aerosol profile product also provides the vertical distribution of the aerosol extinction coefficient, which is a key parameter that is required to calculate the radiative effect (Satheesh et al., 2009; Kuang et al., 2015). However, despite some improvements, a substantial proportion of the aerosol extinction coefficient data obtained directly from the CALIPSO V4 L2 product are missing over the TP; thus, it brings challenges to estimate the aerosol radiative effect.

  • As a part of the A-Train constellation of satellites, CloudSat is an experimental satellite that utilizes radar to observe clouds and precipitation from space. The product of CloudSat, remaining in particularly close proximity to CALIPSO by providing near-simultaneous and collocated observations, could advance the understanding of clouds abundance, distribution, structure, and radiative properties. In this study, the heights of cloud top and base and the cloud classification data from CloudSat 2B-CLDCLASS-LIDAR product are used to evaluate the CALIPSO L2 product and calculate the aerosol extinction coefficient. 2B-CLDCLASS-LIDAR products from CloudSat and CALIPSO measurements are used to describe the cloud types and distributions (Henderson et al., 2013). The 2B-CLDCLASS-LIDAR not only improves cloud detection but also provides more reliable information for cloud type characterization by taking advantages of more complete cloud vertical structure from lidar and radar measurements. The algorithm of 2B-FLXHR-LIDAR product derives estimation of broadband fluxes and heating rates consistent with liquid and ice water content estimation from the CPR (CloudSat Profiling Radar), MODIS (Moderate Resolution Imaging Spectroradio-meter), and CALIPSO datasets. It performs independent flux calculations over 12 longwave (LW) bands (4.55 µm–∞) and 6 solar bands (0.2–4.0 µm) (L’Ecuyer et al., 2008). Ultimately, these bands are combined into the two broadband flux estimates (LW and SW). The aforementioned mean radiation budgets at the TOA and SFC from CloudSat 2B-FLXHR-LIDAR data are used to evaluate the calculated aerosol extinction coefficient through comparing with the model simulations.

  • CERES is one of the highest-priority scientific satellite instruments developed for the earth observing system (EOS) and provides the first long-term global estimates of the radiative fluxes in the earth’s atmosphere. CERES products include both solar-reflected and earth-emitted radiation from TOA to the earth’s surface. The Single Scanner Footprint (SSF) L2 product provides the observed radiation at the TOA associated with the cloud and aerosol properties at the instantaneous footprint level from MODIS observations. The radiation budgets of the SW (0–5 μm), LW (5–100 μm), and window (WN; 8–12 μm) channels in the CERES product are derived from empirical angular distribution models.

    Here, the radiation data from CERES and CloudSat observations are used to evaluate the accuracy and validity of the re-calculated extinction coefficient and to assess the model simulation. For the sake of comparison, the CALIPSO data are interpolated to the CloudSat data with a lower resolution. As for the CERES observations, the error induced from resolutions is smaller than that from different satellite scanning times.

  • In this study, a radiative transfer model that was originally developed by Fu and Liou (1992, 1993) and modified by Rose and Charlock (2002) is used to estimate the radiative effect and heating rate of aerosols over the TP. A delta-four stream scheme is adopted in this model with 15 SW spectral bands ranging from 0.175 to 4.000 µm and 12 LW spectral bands ranging from 2200 to 0 cm–1 (Table 1). The correlated k-distribution method is used to treat the non-gray gaseous absorption resulting from H2O, CO2, N2O, O3, and CH4 (Fu and Liou, 1992). The observed AOD at multiple wave-lengths is allowed as an input to the model. The aerosol wavelength-dependent single-scattering albedo (SSA) and asymmetry parame-ter (ASY) are determined by choosing an assumed aerosol type or a mixture containing several constituents. The aerosol type and properties are obtained from the Optical Properties of Aerosols and Clouds (OPAC) software package (Hess et al., 1998) and the studies of Tegen et al. (1996) and D’Almeida et al. (2005). SSA and ASY of 18 aerosol types at a relative humidity of 50% are given in Table 2.

    Spectral band
    Longwave (cm–1)2200–1900, 1900–1700, 1700–1400, 1400–1250, 1250–1100, 1100–980, 980–800, 800–670, 670–540, 540–400, 400–280, 280–0
    Shortwave (μm)0.175–0.7, 0.7–1.3, 1.3–1.8, 1.8–2.5, 2.5–3.5, 3.5–4
    Visible band
    (0.175–0.700 μm)
    0.1754–0.2247, 0.2247–0.2439, 0.2439–0.2857, 0.2857–0.2985, 0.2985–0.3225, 0.3225–0.3575, 0.3575–0.4375, 0.4375–0.4975, 0.4975–0.5950, 0.5950–0.6896

    Table 1.  Wave bands in Fu–Liou radiation transfer model

    Aerosol typeSSAASY
    Marine1.00.8113
    Continental0.95650.6293
    Urban0.93270.6270
    0.5-micron dust0.96530.6622
    1.0-micron dust0.92550.6931
    2.0-micron dust0.87150.7725
    4.0-micron dust0.79491.0
    8.0-micron dust0.69960.8795
    Insoluble0.74860.8163
    Water soluble0.97300.6570
    Soot0.17890.3079
    Accumulated sea salt1.00.7724
    Coarse sea salt1.00.8464
    Nucleating mineral dust0.97400.6521
    Accumulated mineral dust0.90930.7162
    Coarse mineral dust0.71180.8741
    Transported mineral dust0.87630.7532
    Sulfate droplets1.00.7634

    Table 2.  SSA and ASY of 18 aerosol types at a relative humidity of 50% used in Fu–Liou radiation transfer model

    As the inputs to the radiative transfer model, the profiles of the pressure, temperature, and concentrations of atmospheric components are derived from CALIPSO data, and the surface albedo data are from the CloudSat 2B-FLXHR product. The aerosol extinction profiles, including those directly obtained from CALIPSO L2 data and recalculation, are used to obtain AODs serving as input fields for the radiative transfer model to simulate the radiation effects of aerosols over the TP.

  • According to the analysis in this study, there are substantial absent values over the TP in the aerosol extinction coefficients derived from the CALIPSO L2 product. Here, based on the attenuated backscatter coefficient ($\beta' $) data from the CALIPSO L1 product and cloud classifications observed by CloudSat, the extinction coefficient profiles of aerosol (σaer) are calculated approximatively (hereinafter, the aerosol extinction coefficient calculated from the CALIPSO L1 and CloudSat products is referred to as the “calculated aerosol extinction coefficient”).

    The attenuated backscatter profile ($\beta' $) is defined as the product of the backscatter coefficient (β) and two-way attenuation of the atmosphere (T2):

    $$ \beta _i' = {\beta _i}T_i^2, $$ (1)

    where $T_i^2 = {e^{ - 2\eta {\tau _{i - 1}}}}$, ${\tau _i} = \mathop \smallint \limits_z^\infty {\sigma _i}{\rm{d}}z$ (Omar et al., 2009; Young and Vaughan, 2009), η is a range-dependent multiple-scattering function dependent on the scattering phase function of the particles and the sensing geometry of the lidar, and a constant value of 0.6 is used for simplifying the algorithm (Young and Vaughan, 2009), same with that in the CALIPSO V3 retrievals (Garnier et al., 2015). The parameter τi denotes the optical depth of the atmosphere, which is derived from extinction coefficient (σi), and σi is defined as the product of the backscatter coefficient (βi) and lidar ratio (S), i.e., σi = i (Kovalev et al., 2007). According to the Eq. (1), the backscatter coefficient can be derived from ${\beta _i} = \beta _i'T_i^{ - 2}$. Then, for a certain radar ratio, the extinction coefficient can be calculated. The footstep of i means the i-th layer of the atmosphere, depending on the vertical resolution of CALIPSO.

    The total attenuated backscatter and depolarization ratio from CALIPSO L1 product are combined to identify atmospheric molecules, aerosols, and clouds. Aerosol can usually be differentiated from clouds based on the backscatter intensity because the backscatter signal of cloud is much larger than that of aerosol (Winker et al., 2009). The depolarization ratio, which is defined as the ratio of perpendicular to parallel components of the attenuated backscatter at 532 nm, is useful for identifying spherical particles (Sassen, 1991; Liu et al., 2008a; Chen et al., 2010, 2014; Li et al., 2013). The thresholds of the total attenuated backscatter and depolarization ratio for identifying aerosols are taken as 0.000,8–0.048 km–1 sr–1 and 0.06–0.40, respectively (Liu et al., 2008a; Shen et al., 2010; Li et al., 2013; Jia et al., 2015). Here, the particle with a total attenuated backscatter less than 0.000,8 km–1 sr–1 is considered to be atmospheric molecule. If the total attenuated backscatter is larger than 0.048 km–1 sr–1 or the depolarization ratio is larger than 0.4, it can be regarded as cloud. However, when aerosols are very dense, especially for the elevated aerosols, it may result in misclassification of dense aerosol layers as clouds because of the similar backscattering intensities (Liu et al., 2008a; Huang et al., 2009), leading to substantial amounts of missing values in the relevant aerosol data. The cloud classifications from CloudSat together with the height of cloud top and base are used to revise the presence of clouds and aerosols.

    As the optical properties of air molecules are very stable and the scale is much smaller than 532 nm, the radar ratio (S) can be strictly obtained of 8π/3 according to Rayleigh scattering theory (Omar et al., 2009). While the optical properties of aerosol or cloud particles are very complex, the accurate laser radar ratios depend on their optical properties. For aerosols, the revised tropospheric lidar ratios in the CALIPSO_L2_V4 algorithms are used (Omar et al., 2016). According to the following analysis and discussion, the aerosols in the studied area are dust (S = 44). To simplify the algorithm implementation, the radar ratio for all the cloud is approximately considered to be 28.

    The reference altitude z0 is chosen where the aerosol scattering is negligible (Omar et al., 2009). In this study, it is postulated that the aerosol and cloud are absent in the atmosphere above 20 km. Simultaneously, in order to simplify the algorithm, the effect of atmospheric molecules above 20 km on the radiation transfer is also neglected. Under this assumption, the optical depth of the first layer of the atmosphere (20 km–∞), where the downward calculations begin, is equal to zero, i.e., τ0 = 0. Then, after several iterations until reaching the surface, the extinction coefficient profiles of aerosol (σaer) are derived.

    The AOD for a given layer (z) is retrieved in terms of the aerosol extinction coefficient σaer, i.e., σaer(z), as follows:

    $$ \tau \left( z \right) = \mathop \smallint \nolimits_z^\infty {\sigma ^{\rm aer}} {\rm d}z, $$ (2)

    where τ(z) is the AOD at layer z, and dz is the vertical resolution (the interval between the layer base and top).

    Aerosol radiative effect is defined as the difference between the net radiation under actual sky conditions and that in the absence of aerosols. The net SW and LW radiations are defined as:

    $$ \begin{array}{l} {T_{{\rm{SW}}}} = F_{{\rm{SW}}}^{{\rm{down}}} - F_{{\rm{SW}}}^{{\rm{up}}},\\ {T_{{\rm{LW}}}} = F_{{\rm{LW}}}^{{\rm{down}}} - F_{{\rm{LW}}}^{{\rm{up}}}. \end{array} $$ (3)

    The aerosol radiative effect is given as:

    $$ \begin{array}{l} {C_{{\rm{SW}}}} = {T_{{\rm{SW}}}} - T_{{\rm{SW}}}^{{\rm{clear}}},\\ {C_{{\rm{LW}}}} = {T_{{\rm{LW}}}} - T_{{\rm{LW}}}^{{\rm{clear}}},\\ {C_{{\rm{net}}}} = {C_{{\rm{SW}}}} + {C_{{\rm{LW}}}}, \end{array} $$ (4)

    where $F_{{\rm{SW}}}^{{\rm{down}}}\left( {F_{{\rm{SW}}}^{{\rm{up}}}} \right)$ and $F_{{\rm{LW}}}^{{\rm{down}}}\left( {F_{{\rm{LW}}}^{{\rm{up}}}} \right)$ are the downward (upward) SW and LW radiation flux intensities, respectively; hereafter, this variable will be abbreviated as radiation. The superscript “clear” denotes the same meteorological condition as the actual sky except that aerosols are absent.

3.   Evaluation of aerosol extinction properties for CALIPSO L2 product
  • The TP is a sensitive area in response to the climate change. It is surrounded by the earth’s highest mountains (e.g., the Himalayas and Pamir and Kunlun Mountains) and several important aerosol sources, such as the Taklimakan Desert to the north, the Great Indian Desert to the southwest, and the Gobi Desert to the northeast, as shown in Fig. 1 from Jia et al. (2015). Recently, many studies have indicated that substantial dust and anthropogenic aerosols are transported to the TP during the summer because of the plateau heat pump effect (Ge et al., 2014; Jia et al., 2015; Liu et al., 2015). In this study, based on the latest CALIPSO products and a radiative transfer model, a heavy dust event over the TP from 6 to 10 August 2008 is selected and investigated in detail.

  • Figure 1 presents the altitude–orbit cross-section of the total attenuated backscatter coefficient at 532 nm and the depolarization ratio from 6 to 10 August 2008. The light gray section indicates the topography. Generally, the CALIPSO product indicates aerosol and clouds using green–yellow–orange and white–gray color schemes, respectively. As indicated by the factors of altitude and aerosol distribution continuity, on 6 August 2008, aerosol emissions may have occurred over the Taklimakan Desert, bordering the northern slope of the TP. The gray parts over the Taklimakan Desert should be dense aerosols, not clouds. In the following days, the aerosols entrained into the air were uplifted and transported to the TP. The aerosols still remained spread over the TP on 10 August.

    Figure 1.  The altitude–orbit cross-sections of (a–d) total attenuated backscattering intensity at 532 nm and (e–h) depolarization ratio over the vicinity of the TP (27.5°–45.N, 80°–100°E) from 6 to 10 August 2008 along the scanning orbit paths of CALIPSO satellite. The last column shows the orbits of the satellite (red lines).

    To detect the detailed properties of the aerosols during this event, the analysis on 7 August 2008 is performed further for that it contains a half-orbit in daytime. As shown in Figs. 1c, d, the aerosol starts to be lifted upwards and transported to the TP on 7 August 2008. Figure 2 presents the red–green–blue (RGB) color composite cloud images on 7 August in the vicinity of the TP from MODIS instruments aboard (a) Terra and (b) Aqua. In addition to the topography, Fig. 2 depicts a complicated scene comprising cloud, snow cover, and dust aerosols around the TP. As indicated in Fig. 2, the clouds mainly exist in southern and eastern TP, which is shown in white like cotton without distinct boundaries. The snow cover, which appears white with clear peripheries and not moves with time in Fig. 2, spreads along the terrain at high altitude over the Tianshan Mountains and the TP. The special surface feature of the Taklimakan Desert makes it easy to distinguish cloud and aerosol from the earth’s surface in the satellite cloud image. Combining the strong backscattering signal from CALIPSO (Fig. 1) and the MODIS observed cloud images, the grayish objects suspended over the Taklimakan Desert (as shown in Fig. 2) are decided to be the dust plumes. Strong dust storms almost pervade the whole Tarim Basin, including the location of the CALIPSO trajectory as the blue thick line shown in Fig. 2.

    Figure 2.  The red–green–blue (RGB) color composite image over the vicinity of the TP on 7 August 2008 sounded by MODIS instruments aboard (a) Terra and (b) Aqua. The red line shows the trajectory of the CALIPSO satellite over the TP on 7 August 2008. The blue line denotes the region where the CALIPSO satellite sounds the aerosols as clouds.

    Figures 3a and 3b show the feature of atmosphere classification on 7 August 2008 from CALIPSO V3 and V4 VFM data, respectively. Figure 3c shows the vertical distribution of cloud classifications along the scanning orbit path of CloudSat satellite. Unexpectedly, the CALIPSO L2 VFM product identifies the plumes visible at heights ranging from 3 to 5 km over this region (39°–42°N, 84.5°–83.4°E, indicated by the thick blue lines shown in Fig. 2) as clouds where both CloudSat and MODIS observed no cloud. Because these satellites (CALIPSO, Aqua, and CloudSat) fly in formation and are separated by just a few minutes, they provide approximately collocated observations of cloud and aerosol properties almost simultaneously. The results are comparable. Clearly, some improvements can be seen in CALIPSO V4 product compared with CALIPSO V3 (Figs. 3a, b) through re-assigning to the most frequent subtype of the overlying adjacent aerosol layers by adding the definition of fringes and Subtype Coalescence Algorithm for AeRosol Fringes. However, substantial absent data points from the aerosol extinction coefficient profiles in the CALIPSO V4.10 L2 product are found and there still are some incorrect determinations of cloud and aerosol. As shown in Fig. 3, dense aerosols clearly extend over the northern slope of the TP; furthermore, some aerosol particles also can be found over the southern slope and immediately above the TP. Figure 3 also shows that dust is the main component of the aerosols over the TP.

    Figure 3.  The feature of atmosphere classifications for the dusty event on 7 August 2008 from CALIPSO (a) V3 and (b) V4 products. (c) The distribution of clouds along the scanning orbit path of CloudSat satellite.

    According to above analysis, there still is misidentification of aerosols and clouds in CALIPSO V4.10 product (as shown in Fig. 3). In this view, the dusty events over the TP and its vicinity for summers during 2007–10 are investigated deeply (Fig. 4). The occurrence frequencies of dust events in the vicinity of the TP are calculated by combining the total attenuated backscatter and depolarization ratio from CALIPSO L1 product and atmosphere classifications from CALIPSO VFM product, which identifies the presence of clear air, cloud and aerosol types at each horizontal and vertical grids (Adams et al., 2012; Guo et al., 2016). All the sections of atmosphere classifications and aerosol extinction coefficient detected by CALIPSO in the day and night time are analyzed in detail through comparing with the CloudSat cloud profiles and the frequencies of misidentifying cloud and aerosol are obtained. The MODIS satellite images are used to verify the misjudgment of cloud–aerosol. As given in Fig. 4, the detection of dust events in nighttime are well improved in V4.10 data. However, there is high frequency of misidentifying the cloud and aerosol in the daytime shown as absent values in CALIPSO V4.10 L2 APro product.

    Figure 4.  Frequencies of dusty events (black histograms) in the vicinity of the TP (27.5°–45°N, 80°–100°E) in summer during 2007–10. The gray histograms indicate the frequencies of all the sections detected by CALIPSO in day (light) and night (dark) times, respectively. The slanting shadow indicates the missing frequency of the aerosol data in the CALIPSO V4.10 L2 product due to the misjudgment.

  • According to the dust detection using the CALIPSO L1 product, as shown in Figs. 1, 3, substantial dust aerosols should exist at exactly the points that are missing from the CALIPSO L2 data. Figures 6a and 6b show the altitude–orbit cross-section of aerosol extinction coefficient from CALIPSO V3.01 and V4.10, respectively. Clearly, because the CALIPSO L2 product contains so many absent values of aerosol extinction coefficient, this product cannot be relied upon when seeking to obtain a better understanding of the radiative effect of aerosols over the TP. Here, the aerosol extinction coefficient is calculated (Fig. 6c) based on the CALIPSO L1 product and cloud products from CloudSat according to the method described in Section 2.3 and Fig. 5. The calculated extinction coefficient is in agreement with that obtained from the CALIPSO L2 product (Figs. 6a, b). Correspondingly, AOD is derived from the calculated aerosol extinction coefficient as an input for the model.

    Figure 5.  (a) Flow chart for calculating the aerosol extinction coefficient from the CALIPSO L1 data and CloudSat products, and (b) structure schematic of the atmospheric layers.

    Figure 6.  The altitude–orbit cross-sections of aerosol extinction coefficient from CALIPSO (a) V3.30 and (b) V4.10 L2 products along the satellite orbit path on 7 August 2008. (c) The aerosol extinction coefficient calculated based on the method shown in Fig. 5.

    To evaluate the estimation method, we calculate the SW and infrared radiation at the TOA and surface along the satellite orbit path on 7 August 2008 and compare the results with those obtained from the CERES/CloudSat observations, as shown in Fig. 7. Thereinto, the dust aerosol type is determined by comparing the simulated reflected SW radiation at the TOA with the observation from CERES and CloudSat to confirm the SSA and ASY (figure omitted). The result shows that the aerosol type that fits best is transported mineral dust. In Fig. 7, the observations from CERES and CloudSat are presented by green and orange plots, respectively. The red dots and blue plot indicate model simulations using the CALIPSO L2 product and retrieval as inputs, respectively. Here, the CloudSat observation has the advantage of almost contemporaneous scanning times with CALIPSO, overcoming the defect of CERES. As shown in Fig. 7, the estimates of simulations using the CALIPSO L2 product and the calculated one as inputs are in good agreement among the available data except for several points. Figure 7 also shows that the estimations of the simulated radiation budget with the calculated extinction coefficient as inputs are in better agreement with the observations from CERES and CloudSat than those with CALIPSO L2 product. Thus, the calculation of aerosol extinction coefficient shown in Fig. 5 is reasonable. The disagreement in the SW radiation between the CERES observations and the simulation may be attributable to the large difference in the satellite scanning time. Thus, the calculated aerosol extinction coefficient reasonably describes the vertical distributions of dust aerosols over the TP, and the model estimation is sufficiently reliable to obtain information about radiative effect and the heating effect of the dust aerosols.

    Figure 7.  Variations in SW and LW (infrared) radiation at (a, b) the TOA and (c, d) the surface on 7 August 2008 along the satellite orbit. The observations from CERES and CloudSat are indicated in green and orange, respectively. The red dots and blue curves represent model simulations performed by using aerosol extinction coefficient from the CALIPSO L2 product (red) and the calculated one (blue) as inputs, respectively.

4.   Characteristics of the heating effect of aerosols over the TP
  • Based on the calculated extinction coefficient profiles, the radiative and heating effects of aerosols over the TP are further investigated. The distribution of the radiation budget simulated with the calculated aerosol extinction coefficient is analyzed by subtracting that estimated without aerosols to examine the effect of aerosols on the radiation balance in detail.

    Figure 8 shows the estimated SW, infrared and net radiative effect of aerosols along the CALIPSO scanning orbit over the TP on 7 August 2008. The SW radiative effect is positive and could be as high as 16 W m–2 at the dust layer. Simultaneously, Fig. 8b shows that the dust aerosols can cool the air at approximately –4 W m–2 in the infrared spectral bands. Figure 8c shows the net radiative effect (i.e., the total radiative effect, including SW and infrared). Figure 8c indicates that the dust aerosols can heat the atmosphere at the dust layer. Thus, the radiation absorbed by the aerosols will be retained in the dust layer.

    Figure 8.  The altitude–orbit cross-sections of (a) direct shortwave, (b) infrared, and (c) net radiative forcing (W m–2) along the CALIPSO orbit path over the TP on 7 August 2008.

    The heating effect of the dust aerosols over the TP is also investigated (Fig. 9). As shown in Fig. 9a, the SW heating rate shows a peak corresponding to the dust aerosol layer with the maximum extinction coefficient. By absorbing the SW radiation, dust aerosols can heat the atmosphere at an instantaneous rate of up to 5.5 K day–1. As shown in Fig. 9b, the infrared heating rate is slightly negative and thus could partially compensate for the large positive SW heating effect. Although the aerosols have a relatively small effect on the infrared radiation, the cooling effect at the aerosol layers cannot be ignored.

    Figure 9.  The altitude–orbit cross-sections of (a) shortwave, (b) infrared, and (c) net heating rates (K day–1) due to the aerosols along the CALIPSO orbit path over the TP on 7 August 2008.

5.   Conclusions and discussion
  • In this study, the latest version of CALIPSO L2 product (V4.10) over the TP was evaluated. The improved Version 4 data can detect dust events well in nighttime, while there are substantial absent data in the daytime. To explore the vertical aerosol effect, aerosol extinction profiles was revised based on a typical summer dust event from 6 to 10 August 2008. During this dust event, dense aerosols occurred over the Taklimakan Desert on 6 August 2008, and the aerosols entrained in the air were uplifted and transported to the TP in the subsequent days. The uplifted aerosols spread over the TP and could still be observed on 10 August 2008. However, many values are missing from the CALIPSO L2 data over the TP because of misclassified aerosols as clouds.

    To completely understand the radiative effects of aerosols over the TP, the aerosol extinction coefficient profile is calculated based on CALIPSO L1 data and cloud product from CloudSat. The calculated extinction coefficient can reasonably describe the vertical distribution and the amount of dust aerosols over the TP. Subsequently, the radiative energy budgets at the TOA and surface over the TP along the scanning orbit of the CALIPSO satellite are calculated by using the Fu–Liou radiative transfer model. Comparing the radiation budgets based on simulations and observations reveals that the estimation produced by using the calculated aerosol extinction data as inputs are in better agreement with the CERES and CloudSat observations than with those obtained directly from the CALIPSO L2 product. Based on the calculated aerosol extinction coefficient, the radiative effect and heating rate of aerosols over the TP are further simulated. The results show that dust aerosols over the TP significantly affect the radiative energy budget and thermodynamic structure, mainly by altering the shortwave radiation. The instantaneous heating rate can be as high as 5.5 K day–1 depending on the density of the dust layers.

    Optical depth and type of aerosols are not only the determinants of its radiative effects but also the vertical distribution of aerosols and clouds. Most of the current estimates of aerosol radiation effects are largely based on model studies (Ramanathan et al., 2007) and the inputs for radiation models are derived from the measured aerosol properties observed at the surface by translating to column properties through making assumptions about vertical profiles (Satheesh et al., 1999). There is a significant uncertainty, especially for the vicinity of the TP. The vertical transport of aerosols induced by the “heat pump effect” may go against with the assumptions. Thus, calculating the aerosol extinction coefficient over the vicinity of the TP is significance in addressing the radiative impact of aerosols and realizing the implications of TP for the atmospheric circulation and future regional and global climate.

    The radiative effect and heating effect of dust over the TP were also investigated in this work. The dust aerosols significantly affect the radiative energy budget and thermodynamic structure of the atmosphere over the TP mainly by affecting the SW radiation budget. These results show that the SW radiative effect is positive and can be as high as 16 W m–2, whereas the infrared radiative effect is negative and has a value of approximately –4 W m–2. The instantaneous heating rate of the dust can be as high as 5.5 K day–1 in dense dust layers in which the aerosols will retain the radiation energy.

    Although this study provides a relatively complete understanding of the effects of dust over the TP by revising the extinction data, the analysis mainly focuses on the north slope of the TP with more missing aerosol extinction coefficient in the CALIPSO L2 APro product. Further investigations of the aerosol effect over the south part of the TP, instead of case study, are needed since the aerosol types are more complex than that over the north slope of the TP and it may involve complex interactions between clouds and aerosols.

    Acknowledgments. The CALIPSO, CloudSat, and CERES data were obtained from the NASA Langley Research Center Atmospheric Sciences Data Center, and the authors gratefully acknowledge their efforts in making these data available online. We also gratefully acknowledge Q. Fu and K. N. Liou for providing the Fu–Liou model.

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