Article Optical and Geometrical Properties of Cirrus Clouds over the Tibetan Plateau Measured by LiDAR and Radiosonde Sounding during the Summertime in 2014
Guangyao Dai 1 , Songhua Wu 1,2,* , Xiaoquan Song 1,2,* and Liping Liu 3 1 Ocean Remote Sensing Institute, Ocean University of China, Qingdao 266100, China; [email protected] 2 Laboratory for Regional Oceanography and Numerical Modeling, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, China 3 Laboratory of Severe Weather, Chinese Academy of Meteorological Science, Beijing 100081, China; [email protected] * Correspondence: [email protected] (S.W.); [email protected] (X.S.); Tel.: +86-0532-6678-2573 (S.W.)
Received: 27 December 2018; Accepted: 31 January 2019; Published: 2 February 2019 Abstract: Optical and geometrical characteristics of the cirrus clouds over Naqu (4508 m a.s.l., 31.48◦ N, 92.06◦ E), in the Tibetan Plateau were determined from LiDAR and radiosonde measurements performed during the third TIbetan Plateau EXperiment of atmospheric sciences (TIPEX III) campaign from July to August 2014. For the analysis of the temperature dependence, the simultaneous observations with LiDAR and radiosonde were conducted. Cirrus clouds were generally observed ranging from 5.2 km to 12 km above ground level (AGL) (i.e., 9.7 km to 16.5 km a.s.l.), with the midcloud temperatures ranging from −79.7 to −26.0 ◦C. The cloud thickness generally differed from 0.12 to 2.55 km with a mean thickness of 1.22 ± 0.70 km, and 85.7% of the measurement cases had thickness smaller than 1.5 km. The retrievals of linear particle depolarization ratio, extinction coefficient, and optical depth of cirrus clouds were provided. Moreover, the multiple scattering effect inside of cirrus clouds was corrected. The linear particle depolarization ratio of the cirrus clouds varied from 0.36 to 0.52, with a mean value of 0.44 ± 0.04. The optical depth of the cirrus clouds was between 0.01 and 3 following the scheme of Fernald-Klett method. Sub-visual, thin, and opaque cirrus clouds were observed at 4.76%, 61.90% and 33.34% of the measured cases, respectively. The temperature and thickness dependencies of the optical properties were studied in detail. A maximum cirrus thickness of around 2 km was found at temperatures between −60 and −50 ◦C. This study shows that the mean extinction coefficient of the cirrus clouds increases with the increase of temperature. Conversely, the measurements indicate that the linear particle depolarization ratio decreases with the increasing temperature. The relationships between the existence of cirrus clouds and the temperature anomaly (temperature difference from the mean value of the temperature during July and August 2014 over Naqu) and deep convective activity are also discussed. The formation of cirrus clouds is investigated and also its apparent relationship with the South Asia High Pressure, the dynamic processes of Rossby wave, and deep convective activity over the Tibetan Plateau. The outgoing longwave radiation of cirrus clouds is calculated with the Fu-Liou model and is shown to increases monotonously with the increase of optical depth.
Keywords: LiDAR; cirrus cloud; optical properties; the Tibetan Plateau
1. Introduction Cirrus clouds play a significant role in the energy budget and the hydrological cycle of the Earth’s atmosphere system [1]. Several studies show that cirrus cover on average 30% of the Earth’s surface [2].
Remote Sens. 2019, 11, 302; doi:10.3390/rs11030302 www.mdpi.com/journal/remotesensing Remote Sens. 2019, 11, 302 2 of 17
The classification of cirrus clouds has been defined on the basis of visible observations from the surface. Cirrus clouds are made predominantly or wholly of ice [3]. Only rarely do they contain supercooled liquid cloud droplets at their lower altitudes [4]. Cirrus affects the climate of the whole Earth via two opposite effects; an infrared greenhouse effect and a solar albedo effect [5]. The contribution of each effect depends strongly on cirrus optical properties [6]. Thin cirrus clouds usually cause a positive radiative effect at the top of the atmosphere, while thick cirrus clouds (e.g., opaque cirrus) may produce a cooling effect [7–9]. From the reports of the IPCC, significant uncertainties remain regarding the radiative and climate effect of cirrus clouds and cirrus clouds continue to contribute large uncertainty to estimates and interpretations of the Earth’s changing energy budget [10]. Because of the essential role that the cirrus clouds play, a clear understanding of their optical and geometrical properties is highly essential for climate modeling studies. Moreover, they are critical to understanding feedback processes that regulate or modulate the climate response to forcing. As one of the best techniques for remotely studying the characteristics and properties of cirrus clouds, LiDAR will contribute much to solve the uncertainties of cirrus. Polarization and Raman LiDARs have been employed to detect geometrical and optical properties of cirrus clouds by applying the methods that have been earlier demonstrated by previous works [11–13]. Several key optical parameters such as the extinction coefficient, LiDAR ratio and depolarization ratio as well as the mid-altitude and the corresponding midcloud temperature of cirrus clouds were measured [14]. The depolarization ratios are considered to be of special importance since they are related to microphysics properties of the ice crystals contained on cirrus clouds, while the optical depth, mid-altitude, and mid-temperature play an important role in determining cloud radiative properties. The optical depth of clouds is a key parameter in radiative transfer computations and therefore considerable effort has been put in its retrieval. The evaluation of the optical depth of cirrus clouds has been accomplished through the application of different methods which have been described in the literature [5,11,13,15,16]. Most of these methods depend on the solution of the standard LiDAR equation [11,15,16]. A different technique called “transmittance method” to determine the optical depth of cirrus clouds from elastic LiDAR signal has also been developed. It is based on the comparison of the backscattering signals below and above the cloud assuming that the LiDAR signals correctly represented the scattering medium [5,13,15]. Hence, a clear understanding of the optical properties such as the extinction coefficient, the optical depth, and the depolarization ratio of cirrus is of high priority, especially over the high-altitude area and polar regions. The Tibetan Plateau is a vast elevated plateau in the middle of the Eurasian continent with averaged elevation 4.5 km a.s.l., and it has an important role in the global and regional climate system [17]. The Tibetan Plateau lies at a critical and sensitive junction of four climatic systems: the Westerlies, the East Asian monsoon, the Siberian cold polar airflow and the Indian monsoon [18]. Especially given the elevated heat source in summer, the low pressure over the Tibetan Plateau induces a supply of moist, warm air from the Indian Ocean to the continent significantly affecting the general circulation and the Asian Summer monsoon circulation [18,19]. Considering the significant thermal effect of the Tibetan Plateau, the statistics of the cirrus clouds over the Tibetan Plateau is crucial. However, the high altitude, lack of oxygen, and extreme climate conditions make it difficult to perform long-term atmosphere research by ground-based LiDAR. We developed a high-power polarization Raman LiDAR and deployed it, for the first time to our knowledge, to the Tibetan Plateau to obtain the optical properties of cirrus in the Tibetan Plateau during July and August 2014. In this paper, we briefly describe the LiDAR system, including the transmitter, receiver, and data acquisition. The third TIbetan Plateau EXperiment of atmospheric sciences (TIPEX III) is introduced as well. Furthermore, the methodologies of the determination of the linear particle depolarization ratio, extinction coefficient, optical depth, cloud base/top height and outgoing longwave radiation (OLR) are presented. The correction of multiple scattering inside of cirrus clouds is interpreted. One measurement case of the cirrus clouds optical and geometrical properties observed in Naqu during the TIPEX III are provided. Additionally, combining with the temperature profiles obtained Remote Sens. 2019, 11, 302 3 of 17 from simultaneous observations with radiosonde, the temperature dependences of such properties are investigated. Finally, the longwave fluxes and formation of cirrus clouds are discussed briefly.
2. Instruments and Campaign
2.1. Water Vapor, Cloud and Aerosol LiDAR (WACAL) A three-wavelength combined elastic-backscatter Raman LiDAR, Water vapor, cloud and aerosol LiDAR (WACAL) is employed to perform continuous observations of suspended aerosol particles and cirrus clouds. The system is based on the second and third harmonic frequency of a compact, pulsed Nd:YAG laser, which emits pulses of 400, 120 and 710 mJ output energy at wavelengths of 355, 532, and 1064 nm, respectively, at a 30 Hz repetition rate. The optical receiver consists of four 308 mm diameter Newtonian telescopes. The telescope array helps to better collect the signal from both near field and far field. It also takes the collection efficiency of the strong elastic-backscatter light and the weak Raman backscatter light into account. This design also makes the system convenient to be transported and suitable for field experiments. Five Hamamatsu 10721P-110 photomultipliers and one Hamamatsu G8931-20 APD are used to detect the LiDAR signals at the wavelengths of 355, 387, 407, 532 (parallel-polarized), 532 (cross-polarized) and 1064 nm. The acquisition system employs a six-channels LICEL transient recorder including analog and photon counting modes. The vertical resolution of the signal is equal to 3.75 m and the temporal resolution is 16 s. Several data pre-processing methods are applied to the raw LiDAR return signal before retrieving the optical properties. The WACAL was built successfully in 2013 by the LiDAR group at Ocean University of China. The details of WACAL were described by Wu et al. (2015) in a separate paper [20].
2.2. Radiosonde Sounding The electronic radiosondes of GTS1 type (Nanjing Bridge Machinery Co., Ltd., China) [21] are used in our research to provide vertical profiles of temperature, pressure, and relative humidity. The radiosondes can provide temperature with accuracy of ±0.2 ◦C, relative humidity with an absolute uncertainty of ±5% and pressure with accuracy of ±1 hPa. The radiosondes were launched regularly at 00:00 UTC and 12:00 UTC during this campaign.
2.3. TIPEX III Campaign The TIPEX III campaign was conducted by a collaboration between Ocean University of China and Chinese Academy of Meteorology Science. This experiment was conducted during July and August 2014 in Naqu. As an important approach to collect the atmospheric information, the WACAL system was deployed at Naqu Bureau of Meteorology (31.48◦ N, 92.06◦ E) from 10 July to 16 August 2014. The measurement site is shown in the elevation map of Figure1 (The elevation data were downloaded from http://www.temis.nl/data/gmted2010/). The red star denotes the LiDAR site during TIPEX III while the white triangle represents the city of Qingdao. With the help of the WACAL, the optical and geometrical properties of cirrus clouds including backscatter coefficient, linear particle depolarization ratio, cloud base height, and cloud top height can be obtained. To investigate the temperature dependences of the optical and geometrical properties of cirrus, simultaneous observations by WACAL and radiosonde are obtained. Referring to the results measured by the co-located ceilometer of VAISALA CL31, almost all the high clouds including cirrus occurred at nighttime and before dawn (the period between 18:00 UTC, i.e., 02:00 LT and 22:00 UTC, i.e., 06:00 LT) [22]. Such a temporal distribution (free of sunlight) of the cirrus clouds is conducive to the joint observations by the WACAL and radiosonde sounding. Consequently, only the concurrent measurements by WACAL and radiosonde at nighttime were taken into account. Remote Sens. 2019, 11, 302 4 of 17 Remote Sens. 2018, 10, x FOR PEER REVIEW 4 of 17
Figure 1. The altitude map with the red star denoting the LiDAR site during TIPEX III while the white Figure 1. The altitude map with the red star denoting the LiDAR site during TIPEX III while the white triangle shows the location of Qingdao. triangle shows the location of Qingdao. 3. Methodology 3. Methodology In this section, the methods for retrieving the optical properties of the cirrus clouds are presented. TheIn this SNR section, of the signal the methods is calculated for fromretrieving the optical properties of the cirrus clouds are presented. The SNR of the signal is calculated fromP m(z) − P − P = noise bg Pz()SNR−− P P p , (1) = m noise bg Pm(z) SNR , (1) Pzm () where Pm(z) is the measured signal at the height of z recorded by PMT, Pnoise and Pbg denote the noise where Pzm () is the measured signal at the height of z recorded by PMT, Pnoise and Pbg denote of the PMT and the background of measured signal, respectively. In this study, the Pnoise is determined bythemeasuring noise of the the PMT signal and ofthe the background covered PMT of meas (soured called signal, “zero respectively. signal recording”). In this study, The backgroundthe Pnoise is signaldeterminedPbg (e.g., by daytimemeasuring solar the background) signal of the is obtainedcovered PMT by averaging (so called the “zero raw signalsignal at recording”). far ranges from The
18.75background km to 20.55 signal km. P Forbg (e.g., the quality daytime assurance solar background) and quality is control, obtained only by the averaging signal of the SNR raw > 10 signal is used at forfar retrieval.ranges from It should 18.75 km be emphasized to 20.55 km. that For inthe this quality study, assurance since the altitudeand quality of the control, LiDAR only measurement the signal siteof SNR>10 is 4508 mis a.s.l.,used for the retrieval. SNR of the It extremelyshould be high-altitudeemphasized that cirrus in cloudsthis study, signal since is still the commendable. altitude of the LiDARIn thismeasurement study, to avoidsite is 4508 a possible m a.s.l., impact the SNR of liquidof the cloud,extremely only hi cloudsgh-altitude with cirrus the linear clouds particle signal depolarizationis still commendable. ratios larger than 0.35 and with the optical depth smaller than 3 are taken into account. SinceIn the this linear study, particle to avoid depolarization a possible ratioimpact values of liquid of the cloud, dust and only volcanic clouds ashwith layers the linear lofted particle in high altitudedepolarization are smaller ratios thanlarger 0.35 than [23 0.35], they and canwith be the filtered optical out depth with smaller these than thresholds 3 are taken above. into Actually, account. duringSince the the linear TIPEX particle III campaign, depolarization the dust ratio and values volcanic of ashthe atdust high and altitude volcanic were ash not layers measured. lofted in high altitudeAnother are smaller main aspectthan 0.35 is the [23], accurate they can determination be filtered out of with the cirrusthese cloudthresholds base above. and top Actually, height. Theduring aspect the isTIPEX of major III campaign, importance the for dust the and precise volc calculationanic ash at ofhigh the alti opticaltude andwere geometrical not measured. properties of cirrusAnother clouds. main aspect is the accurate determination of the cirrus cloud base and top height. The aspect is of major importance for the precise calculation of the optical and geometrical properties of 3.1. Calibration of the Linear Particle Depolarization Ratio and Error Analysis cirrus clouds. WACAL provides the linear particle depolarization ratio at 532 nm. For the precise measurement of3.1. the Calibration linear particle of the Linear depolarization Particle Depolarization ratio, an Ratio effective and Error and accurateAnalysis calibration method called ± ◦ “ 45WACAL-calibration” provides is used the linear in this particle paper [depolarizati24]. In this method,on ratio at the 532 influences nm. For the of precise the gain measurement ratio, offset angleof the oflinear the receiver particle moduledepolarization and the ratio, properties an effective of the polarizing and accurate beam calibration splitter (PBS) method are calibratedcalled “±45°- by usingcalibration” half wave is used plate. in Afterthis paper the calibration, [24]. In this the method, linear particle the influences depolarization of the gain ratio ratio, can beoffset represented angle of asthe a receiver function module of component and the variablesproperties including of the polarizing measured beam ratio splitterm(r), (PBS) gain ratioare calibratedG, offset by angle usingϕ, thehalf reflectivities wave plate. and After the the transmittances calibration, the (TP ,linearTS, R Pparticleand RS depolarization. The subscripts ratio “P” indicatescan be represented the reflectivity as a andfunction transmittance of component for the variables parallel-polarized including measured light while ratio the subscriptmr(), gain “ Sratio” represents G , offset the angle reflectivity ϕ , the and transmittance for the cross-polarized light.) of PBS by reflectivities and the transmittances ( TP , TS , RP and RS . The subscripts ” P ” indicates the reflectivity and transmittance for the parallel-polarized light while the subscript “ S ” represents the reflectivity and transmittance for the cross-polarized light.) of PBS by
Remote Sens. 2019, 11, 302 5 of 17
m(r)T − GR + (m(r)T − GR ) tan2 ϕ v( ) = P P S S δ r 2 and (2) GRS − m(r)TS + (GRP − m(r)TP) tan ϕ (1 + δm)δv(r)R − (1 + δv(r))δm δp(r) = , (3) (1 + δm)R − (1 + δv(r)) with δv is the volume depolarization ratio, δp is the linear particle depolarization ratio, δm is the depolarization ratio of molecule which is set as 0.0036 for 532 nm and R is the backscatter ratio. The backscatter ratio is defined as R = (βmol + βaer)/βmol, where βaer is the aerosol backscatter coefficient and βmol denotes the molecule backscatter coefficient. In this study, the gain ratio of the perpendicular channel to the parallel channel G is determined from the measurements with half wave plate at angles of 0◦ and 45◦. It is calculated by
v 2 2 [TPδ (r) + TS + TP tan ϕ]TP G = m ◦ (r) m ◦ (r) 45 0 v 2 . (4) [RSδ (r) + RP + RS tan ϕ]RS
The error of the gain ratio is calculated by
2 2 2 2 2 2 ∆G ∆G ∆G ∆m0◦ ∆m45◦ ∆ϕ ( ) = ( ) + ( ) = FG[( ) + ( ) ] + FG( ) . (5) m ◦ ◦ ϕ G total G systematic G random m0 m45 ϕ
f (x) The propagation factor Fx is a factor by which the relative error in f (x) is magnified with respect to the relative error in the component variables x. It is defined as
2 f (x) x ∂ f (x) F = ( ) . (6) x f (x) ∂x
The relative uncertainty of the offset angle is around 5% and the error of the measured ratio is calculated via ∆m 2 ∆P ∆P 2 1 1 ( ) = ( T )2 + R ) = + 2 2 . (7) m PT PR SNRPT SNRPR The total error of the volume depolarization ratio is determined with
2 2 2 ∆δv ∆δv ∆δv ( v ) = ( v ) + ( v ) δ total δ systematic δ random 2 2 ∆δv δv ∆G 2 δv ∆ϕ ( v ) = F ( ) + Fϕ ( ) . (8) δ systematic G G ϕ 2 ∆δv δv ∆m 2 ( v ) = Fm ( ) δ random m
Once the volume depolarization ratio was calibrated, the particle depolarization ratio can be determined via Equation (3). The error of the particle depolarization ratio can be calculated by
p 2 2 v 2 ∆δ (r) δp ∆R(r) δp ∆δ (r) ( ) = F ( ) + F v ( ) . (9) δp(r) R R(r) δ δv(r)
δp δp The propagation factors FR and Fδv are determined by Equation (6). The detailed error analysis of volume and linear particle depolarization ratio was described in a separate paper [25].
3.2. Determination of the Extinction Coefficient Based on the elastic-backscatter signal at wavelength of 532 nm, the particle extinction coefficient p α (z ) profile can be calculated by using Fernald method [11,26]. In this paper, the particle LiDAR λ0 0 Remote Sens. 2019, 11, 302 6 of 17
p ratio S (z) is determined by means of the updated method of Comstock and Sassen (2001) [27,28]. λ0 p The S (z) of cirrus clouds over Naqu calculated by this method is 28.4 ± 5.2 sr. λ0 The method for determination of the reference height z0 is known as “Minimum value method”, which is also described in Wu et al. (2015) [20]. In this method, the vertical profile of P (z)z2/βmol(z) λ0 λ0 is calculated and the minimum value is found at a certain height of z. This height can act as reference height z0. Once the reference height is fixed, the Fernald method is applied to determine the aerosol p p extinction coefficient α (z). Beforehand, the particle extinction coefficient at reference height α (z ) λ0 λ0 0 is assigned an initial value. In this study, the initial value is 4 × 10−6 km−1. An iteration process is p used to determine the final α (z ). From the derived extinction coefficient result, the mean value λ0 0 p p α (z ) is computed. In each iteration step, the input value of α (z ) is increased by 10% than last λ0,mea 0 λ0 0 p p value until the relative difference between α (z ) and α (z ) is less than 5% [29]. λ0 0 λ0,mea 0
3.3. Multiple Scattering Correction The multiple scattering (MS) returns of elastic signal from cirrus clouds should be considered. The amount of the MS returns relies on the forward diffraction peak in the phase function [28]. The MS factor is the function of laser penetration, cloud range, FOV and mean effective radius for ice particles [5,30–32]. However, there is no obvious dependence of the forward diffraction peak on ice crystal shape, which was reported by Macke et al. (1996) [33]. In this study, to accurately correct the MS influence, we perform a full treatment of multiple scattering following the model of Hogan et al. (2006) [34]. The model is also introduced and applied by Seifert et al. (2007) [35], Kienast-Sjögren et al. (2016) [36] and Gouveia et al. (2017) [37]. In this model, the typically operated FOV of 1.3 mrad and the full divergence angle of 0.5 mrad in WACAL are used. Moreover, the particle size of cirrus clouds which is modeled according to the method in Krämer et al. (2016) [38] is applied into the MS model to correct the effect in cirrus clouds as well.
3.4. Extraction of the Cloud Base and Top Heights To determine the cloud base and top heights, an algorithm that combines “differential zero-crossing algorithm” [39] and “threshold algorithm” [40] is used when the clouds can be penetrated by laser. In most of cases, cloud heights can be defined directly from these zero crossings of the first derivative of backscatter intensity dP(z)/dz. However, to exclude interference of spurious zero crossings, the dP(z)/dz is calculated via least-squares fit by using a multipoint window that slides through successive points from z0 to the end of the LiDAR profile. Additionally, based on the noise in the recorded LiDAR signal, a series of range-dependent thresholds are defined. Only the signal excursion above the corresponding threshold value is identified as a cloud. By this algorithm, cloud layers apparent in the return signal are identified. However, if the lower clouds were too thick to be penetrated, the higher clouds cannot be identified [20,22]. During this campaign, the cloud base heights determined with WACAL and that with the co-located VAISALA CL31 ceilometer were compared and the results were reported by Song et al. (2017) [22]. A good agreement is found. In this study, to ensure that the LiDAR can detect the cloud top, the LiDAR signal above the cirrus is checked. When the signal still tends to be attenuated, the cirrus top is ensured. Moreover, since the LiDAR is restricted when the optical depth is bigger than 3, only cirrus clouds with the whole path optical depth smaller than 3 are taken into account.
3.5. Cirrus OLR Cirrus OLR is a parameter that has been used extensively to study cloud-radiation interactions. The Fu-Liou radiative transfer model is used here to estimate OLR. The model is available freely online (https://www-cave.larc.nasa.gov/cgi-bin/fuliou/runfl.cgi). It is a delta-four steam radiative transfer scheme with 15 spectral bands from 0.175 to 4.0 µm in shortwave (SW) and 12 longwave (LW) spectral bands between 2850 and 0 cm−1. The corrected k-distribution method is used to parameterize the Remote Sens. 2019, 11, 302 7 of 17
non-gray gaseous absorption by H2O, CO2, O3, N2O, and CH4 [8]. The Fu-Liou model inputs here are visible optical depth, phase, cloud top and base pressure, and particle size. The optical depth can be obtained from above cirrus optical properties retrieval. Cloud phase is an input as either WATER or ICE phase, which affects the input of cloud particle size. Here the cirrus clouds are chosen as ICE phase. The cloud top and base pressures can be obtained from radiosonde data.
4. Discussion During the TIPEX III, 21 measurement cases of cirrus clouds were measured, with 17 of them observed at nighttime while the rest detected before dawn. In this section, the optical and geometrical properties of these cirrus clouds are presented. With the temperatures measured by radiosondes that are launched at Naqu meteorological bureau twice per day, at 0000 UTC and 1200 UTC (LST – 8 h), the temperature dependence of optical and geometrical properties is investigated and discussed. Furthermore, the relationship of the appearance of cirrus clouds and the temperature anomaly, as well as the cirrus OLR are investigated. According to the measurement results from CL31 and WACAL in Song et al. (2017), the diurnal variations of CBH distribution proportion and CBH frequency have a similar variation trend with a distinct “U” shape in the time dimension, which analyze the cloud structure variation at spatial and temporal aspects, respectively [22]. Therefore, almost all the high clouds including cirrus occurred at nighttime (mainly the period between 18:00 UTC (02:00 LT) and 22:00 UTC (06:00 LT)). In our study, for the analysis of the temperature dependence of cirrus optical and geometrical properties, the simultaneous observations with LiDAR and radiosonde were conducted. The time difference between cirrus observations with LiDAR and radiosonde is less than 2 h. Therefore, the frequency is a relative value. It should be noted that in our database, almost all the cirrus meets the requirement (time difference less than 2 h). Thus, the sampling has only a tiny impact on the results.
4.1. Measument Case and Short Summary For detailed study of the optical and geometrical properties of cirrus clouds, we present one measurement case on 4 August 2014 in Figure2. During this measurement period, two cirrus clouds layers were observed. According to Figure2a,c, the developments of linear particle depolarization ratio and extinction coefficient inside of the cirrus clouds are expressed in detail with a temporal resolution of 80 s. For instance, one depolarization ratio profile and one extinction coefficient measured at 21:00 LST are presented in Figure2b,d. It should be emphasized that only the depolarization ratios inside of cirrus clouds are plotted and the extinction coefficients are shown with and without MS correction. Due to the inherent inhomogeneity inside of the cirrus clouds, the linear particle depolarization ratio and extinction coefficient vary as the cirrus evolves and ice crystal habit changes. The value of linear particle depolarization ratio under 7 km above ground level (AGL) is generally smaller than that of cirrus higher. Conversely, the value of extinction coefficient under 7 km AGL is larger than that of cirrus higher. The results may be explained by the temperature dependence changes of the crystal shape, shape ratios, number concentration, and size distribution of ice crystals in cirrus clouds. With the increase of the height, the temperatures gradually decrease. According to the previous work [5,14,27,35], temperature has great impact on linear particle depolarization ratio and extinction coefficient by dominating the crystal shape, number concentration, and other microphysics properties of cirrus. Furthermore, the mean cloud base height and the mean top height of the first cirrus layer are 6.02 ± 0.60 km and 8.56 ± 0.69 km, respectively. The mean linear particle depolarization ratio and mean extinction coefficient are 0.39 ± 0.02 and 0.17 ± 0.08 km−1, respectively. The mean temperature at the layer top is −39.8 ± 4.0 ◦C. The mean cloud base height and mean top height of the second cirrus layer are 9.70 ± 0.39 km and 11.77 ± 0.76 km, respectively. The corresponding mean linear particle depolarization ratio and mean extinction coefficient are 0.40 ± 0.02 and 0.07 ± 0.04 km−1, respectively. Remote Sens. 2019, 11, 302 8 of 17
In addition, the mean temperature at the layer top is −67.0 ± 4.4 ◦C. In this study, the geometrical and opticalRemote Sens. properties 2018, 10, x of FOR all PEER the 21 REVIEW cirrus clouds are retrieved. 8 of 17
Figure 2. Time serials of (a) depolarization ratio, (b) one depolarization ratio profile, (c) cloud base/top Figure 2. Time serials of (a) depolarization ratio, (b) one depolarization ratio profile, (c) cloud base/top heights on 04 August 2014. In this figure, (d) extinction coefficient and (e) one extinction coefficient heights on 04 August 2014. In this figure, (d) extinction coefficient and (e) one extinction coefficient profile measured at 21:00 LST are provided, as well as the (f) relative humidity/temperature. Please profile measured at 21:00 LST are provided, as well as the (f) relative humidity/temperature. Please note that only the depolarization ratios inside of the cirrus are provided. In the Figure (d), the extinction note that only the depolarization ratios inside of the cirrus are provided. In the Figure (d), the coefficient with multiple scattering correction and without the correction are plotted and the results are extinction coefficient with multiple scattering correction and without the correction are plotted and denoted by dark green solid line and blue dashed line, respectively. the results are denoted by dark green solid line and blue dashed line, respectively. 4.2. Geometrical Characteristics and Temperature Dependence 4.2. Geometrical Characteristics and Temperature Dependence Cloud thickness and structure are important factors in determining the cirrus cloud properties. BasedCloud on the thickness measurements and structure of WACAL, are important the structures factors of in cirrus determining clouds were the cirrus provided. cloud In properties. Figure3, weBased present on the the measurements cirrus structure of WACAL, and the vertical the structures distribution of cirrus of theclouds occurrence were provided. frequency In ofFigure cloud 3, base/topwe present heights the cirrus measured structure during and the the TIPEX vertical III. The distribution abscissa axis of coordinatesthe occurrence in style frequency of “MMDD-No.” of cloud indicatebase/top the heights measurement measured date during (MMDD) the TIPEX and the III. serial The abscissa number axis (No.) coordinates of cirrus on in that style day. of Figure“MMDD-3a showsNo.” indicate the vertical the measurement extent of cirrus date clouds (MMDD) measured and bythe WACAL serial number over Naqu. (No.) Theof cirrus variation on that of cloud day. base/topFigure 3(a) heights shows were the vertical also presented. extent of Figure cirrus3 clbouds presents measured the occurrence by WACAL frequency over Naqu. of cloud The basevariation and topof cloud heights base/top at 1 km heights altitude were interval. also presented. The maximum Figure frequencies 3(b) presents of the the cirrus occurrence cloud base frequency and top of height cloud werebase and 28.57% top atheights height at of 1 6–7km altitude km AGL interval. and 33.34% The maximum at height of frequencies 8–9 km AGL, of the respectively. cirrus cloud The base cirrus and basetop height height were ranged 28.57% from at 5 height to 12km of 6-7 AGL, kmwith AGL a and mean 33.34% value at of height 7.46 ± of1.79 8-9 km km AGL, AGL. respectively. Meanwhile, ± theThe cirrus cirrus top base height height ranged ranged from from 5 to 135 to km 12 AGL, km AGL, with awith mean a valuemean ofvalue 8.41 ±of 1.887.46 km1.79 AGL. km AGL. ± Meanwhile,Histograms the cirrus of cirrus top cloud height middle ranged height from (geometric 5 to 13 km center AGL, height)with a andmean temperature value of 8.41 at cirrus1.88 topkm areAGL. also presented in Figure3. The middle heights of the cirrus clouds are generally observed in the TibetanHistograms Plateau region of cirrus from cloud 5 to 12 middle km AGL height and 61.91%(geome oftric them center are height) located and between temperature 6 and 9 kmat cirrus AGL. Accordingtop are also to presented Figure3d, in the Figure maximum 3. The frequency middle heig of thehts temperaturesof the cirrus clouds at midcloud are generally is 33.33% observed between in the Tibetan Plateau region from 5 to 12 km AGL and 61.91% of them are located between 6 and 9 km AGL. According to Figure 3(d), the maximum frequency of the temperatures at midcloud is 33.33% between −50 °C and −40 °C and the midcloud temperatures range from −79.7 to −26.0 °C with a mean value of −44.0 ± 15.0 °C .
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◦ ◦ ◦ −Remote50 C Sens. and 2018−40, 10,C x FOR and PEER the midcloud REVIEW temperatures range from −79.7 to −26.0 C with a mean9 value of 17 of −44.0 ± 15.0 ◦C. FromFrom Figure Figure3 e,3(e), it can it can be be found found that that the the cirrus cirrus thickness thickness distribution distributionhas has aa skewedskewed distribution. 85.7%85.7% of of the the case case studies studies have have thickness thickness smallersmaller thanthan 1.51.5 kmkm and the cloud thickness ranges ranges from from 0.12 0.12 toto 2.55 2.55 km km with with a a mean mean thickness thickness of of 1.22 1.22±± 0.70 km.
FigureFigure 3. 3.( a(a)) Cirrus Cirrus structure, structure, ( b(b)) the the histogram histogram of of cloud cloud base/top base/top heights, (c) (c) cirrus middle height height occurrenceoccurrence frequency, frequency, ( d(d)) temperature temperature occurrenceoccurrence frequencyfrequency andand ((e)e) cloudcloud thickness measured measured by by WACALWACAL over over Naqu Naqu during during TIPEX TIPEX III. III. 4.3. Optical Properties and Temperature Dependence 4.3. Optical Properties and Temperature Dependence In this study, the extinction coefficient, the linear particle depolarization ratio and optical depth In this study, the extinction coefficient, the linear particle depolarization ratio and optical depth are calculated. We applied the methods which were briefly presented in the methodology section are calculated. We applied the methods which were briefly presented in the methodology section to to all the case studies of cirrus clouds during the TIPEX III over Naqu. In this study, the linear all the case studies of cirrus clouds during the TIPEX III over Naqu. In this study, the linear particle particle depolarization ratio and cirrus mean extinction coefficient and the corresponding statistical depolarization ratio and cirrus mean extinction coefficient and the corresponding statistical uncertainties are calculated. The depolarization ratio ranges from 0.36 to 0.52, with the cirrus mean extinction coefficient differs from 0.05 km-1 to 0.40 km-1. For further investigation of the extinction coefficient and optical depth, we present the distribution and histogram of the cirrus clouds optical depth in Figure 4. The optical depth of the
Remote Sens. 2019, 11, 302 10 of 17 uncertainties are calculated. The depolarization ratio ranges from 0.36 to 0.52, with the cirrus mean extinction coefficient differs from 0.05 km−1 to 0.40 km−1. RemoteFor Sens. further 2018, 10 investigation, x FOR PEER REVIEW of the extinction coefficient and optical depth, we present the distribution 10 of 17 and histogram of the cirrus clouds optical depth in Figure4. The optical depth of the cirrus clouds iscirrus between clouds 0.01 is andbetween 3. We 0.01 found and that3. We 4.76% found of that cirrus 4.76% clouds of cirrus are sub-visual clouds are (optical sub-visual depth<0.03), (optical 61.90%depth<0.03), of cirrus 61.90% clouds of arecirrus optical clouds thin are (0.03
Figure 4. (a)(a) Distribution of optical depth and the classification, classification, (b) (b) Occurrence frequency of different types of cirrus clouds.clouds.
To investigateinvestigate thethe temperaturetemperature dependencedependence of linearlinear particleparticle depolarizationdepolarization ratio and mean extinction coefficientcoefficient distribution,distribution, we plotted the scatter diagramsdiagrams of linearlinear particleparticle depolarizationdepolarization ratio, mean extinction coefficient,coefficient, andand midcloudmidcloud temperature,temperature, cirruscirrus middlemiddle height.height. In Figure Figure 55,, the linear linear particle particle depolarization depolarization ratios ratios dependences dependences on onmidcloud midcloud temperature temperature and andcirrus cirrus middle middle height height are arepresented. presented. The The bars bars show show the the statistical statistical uncertainties uncertainties of of linear particle depolarization ratio while the black lines are thethe fittingfitting curves.curves. From Figure 55,, thethe linearlinear particleparticle depolarization ratioratio ofof cirruscirrus varies varies from from 0.36 0.36 to to 0.52. 0.52. With With the the increase increase of of the th temperature,e temperature, the the values values of theof the linear linear particle particle depolarization depolarization ratio ratio are graduallyare gradua reduced,lly reduced, which which agrees agrees with previouswith previouspapers papers [5,14]. Depolarization[5,14]. Depolarization ratio isratio sensitive is sensitive to crystal to crystal shape shape and and shape shape ratios ratios [44 [44].]. With With the the extremelyextremely low temperature, the formation of crystals with small mean sizes and more complex form with irregular structures maymay initiate. initiate. According According to to the the measurement measurement results results from from Heymsfield Heymsfield and and Iaquinta Iaquinta (2000) (2000) [45] and[45] Miloshevichand Miloshevich and Heymsfield and Heymsfield (1997) [46(1997)], we [4 propose6], we thatpropose with thethat decrease with the of thedecrease temperature of the andtemperature the increase and ofthe cirrus increase middle of cirrus height, middle the irregular height, typethe irregular ices with type smaller ices size with and smaller greater size aspect and ratiogreater are aspect formed, ratio which are resultformed, in thewhich increase result of in linear the increase particle of depolarization linear particle ratio depolarization of the cirrus ratio clouds. of Accordingthe cirrus toclouds. Figure According5, the linear to particle Figure depolarization5, the linear particle ratio with depolarization higher temperature ratio with and lowerhigher altitudetemperature is slightly and lower more sensitivealtitude tois theslightly ambient more temperature. sensitive to Under the ambient the condition temperature. of low temperature, Under the itcondition is proposed of low that thetemperature, existence ofit theis proposed stable cirrus that particles the existence with smaller of the radius stable makes cirrus the particles temperature with sensitivitysmaller radius gradually makes weaker the temperature with the decreasingsensitivity temperature.gradually weaker with the decreasing temperature. Figure 6 shows the mean extinction coefficient dependences on midcloud temperature and cirrus middle height. The bar presents the standard deviation of extinction coefficient of a certain cirrus layer while the black lines are the fitting curves. From Figure 6, as expected, the mean extinction coefficient decreases with decreasing temperature for the measured cirrus clouds over the Tibetan Plateau, which agrees with previous papers [27,35]. The particle extinction coefficient is essentially
Remote Sens. 2019, 11, 302 11 of 17
Figure6 shows the mean extinction coefficient dependences on midcloud temperature and cirrus middle height. The bar presents the standard deviation of extinction coefficient of a certain cirrus layer whileRemote theSens. black2018, 10 lines, x FOR are PEER the fittingREVIEW curves. From Figure6, as expected, the mean extinction coefficient 11 of 17 Remotedecreases Sens. with2018, 10 decreasing, x FOR PEER temperature REVIEW for the measured cirrus clouds over the Tibetan Plateau, 11 which of 17 agreesinfluenced with by previous the number papers concentration [27,35]. The particle and size extinction distribution coefficient of ice is crystals essentially in cirrus influenced clouds. by influenced by the number concentration and size distribution of ice crystals in cirrus clouds. theAlthough number the concentration cirrus radiative and properties size distribution depend on of icecloud crystals microphysics, in cirrus it clouds. is the ambient Although temperature the cirrus Although the cirrus radiative properties depend on cloud microphysics, it is the ambient temperature radiativethat governs properties the radiative depend transfer. on cloud It microphysics, dominates the it is microphysics the ambient temperatureproperties such that governsas number the that governs the radiative transfer. It dominates the microphysics properties such as number radiativeconcentration transfer. and size It dominates distribution. the microphysicsPlatt et al. (1988) properties reported such that the as number relationship concentration between the and cirrus size concentration and size distribution. Platt et al. (1988) reported that the relationship between the2 cirrus distribution.extinction coefficient Platt et al. and (1988) atmospheric reported thattemp theerature relationship yields the between follow the equation cirrus extinction of α=aT coefficient + bT + c extinction coefficient and atmospheric temperature yields the follow equation of α=aT2 + bT + c and[30]. atmosphericWang and Sassen temperature (2002) described yields the the follow temperat equationure ofdependenceα = aT2 + ofbT cirrus+ c [30 extinction]. Wang and coefficient Sassen [30]. Wang and Sassen (2002) described the temperature dependence of cirrus extinction coefficient (2002)by using described the LiDAR the and temperature radar data dependence with a three-orde of cirrusr polynomial extinction function coefficient [47]. by Here using we theapplied LiDAR the by using the LiDAR and radar data with a three-order polynomial function [47]. Here we applied the andsecond-order radar data functional with a three-orderforms in representing polynomial the function temperature [47]. dependence Here we applied of cirrus the mean second-order extinction second-order functional forms inα= representing4.58 × 10−−52 T the + 7.4 temperature × 10 3 T + 0.39 dependence (km − 1 ) of cirrus mean extinction functionalcoefficient and forms the in formula representing is the temperature−−52 dependence 3 of cirrus − 1. Moreover, mean extinction we also coefficient provided coefficient and the formula is α=−4.582 × 10 T + 7.4− × 10 T + 0.39− (km ) . Moreover, we also provided andthe relationship the formula between is α = 4.58 the× cirru10 5sT mean+ 7.4 extinction× 10 3T + coefficient0.39 (km and1). Moreover,the height we AGL also and provided the fitting the therelationshipcurve relationship is presented between between in theFigure cirrusthe 6(b).cirru mean Thes mean extinction temperature extinction coefficient se coefficientnsitivity and of the and cirrus height the mean height AGL extinction and AGL the and fitting coefficient the curve fitting is curvepresentedgradually is presented inweakened Figure in6b. Figure with The temperature the6(b). decreasing The temperature sensitivity temperature. se ofnsitivity cirrus Consequently, mean of cirrus extinction mean temperature extinction coefficient coefficient ishas gradually a more is graduallyweakenedprominent withweakened impact the on decreasing extinctionwith the temperature. decreasingcoefficient for Consequently,temperature. warmer and temperatureConsequently, lower cirrus. has temperature a more prominent has a impactmore prominenton extinction impact coefficient on extinction for warmer coefficient and lower for warmer cirrus. and lower cirrus.
Figure 5. (a) Scatter diagram and correlation of depolarization ratio and midcloud temperature, (b) Figure 5. (a) Scatter diagram and correlation of depolarization ratio and midcloud temperature, FigureScatter 5.diagram (a) Scatter and diagram correlation and ofcorrelation depolarization of depola ratiorization and cirrus ratio andmiddle midcloud height. temperature, The black lines (b) (b) Scatter diagram and correlation of depolarization ratio and cirrus middle height. The black lines Scatterindicate diagram the fitting and curves. correlation of depolarization ratio and cirrus middle height. The black lines indicateindicate the the fitting fitting curves. curves.
Figure 6. ((a)a) ScatterScatter diagramdiagram ofof extinction extinction coefficient coefficient and and midcloud midcloud temperature, temperature, (b )(b) Scatter Scatter diagram diagram of Figureextinctionof extinction 6. (a) coefficient Scatter coefficient diagram and and cirrus ofcirrus middleextinction middle height. coefficient height. The black The and black lines midcloud indicatelines indicate temperature, the corresponding the corresponding (b) Scatter fitting diagram curves. fitting ofcurves. extinction coefficient and cirrus middle height. The black lines indicate the corresponding fitting curves.The dependence of cirrus mean optical and geometrical properties on midcloud temperature with an interval of 10 ◦C is also provided in Figure7. According to Figure7a,d, with the decrease of the The dependence of cirrus mean optical and geometrical properties on midcloud temperature temperatures, the developments of linear particle depolarization ratio and mean extinction coefficient withThe an intervaldependence of 10 of°C cirrus is also mean provided optical in and Figure geom 7. etricalAccording properties to Figure on midcloud7 (a) and temperature(d), with the withshow an opposite interval change of 10 ° tendencies.C is also provided Since the in number Figure concentration 7. According and to Figure size distribution 7 (a) and of(d), ice with crystals the decrease of the temperatures, the developments of linear particle depolarization ratio and mean decreaseextinction of coefficient the temperatures, show opposite the developments change tend ofencies. linear Since particle the numberdepolarization concentration ratio and and mean size extinctiondistribution coefficient of ice crystalsshow opposite in cirrus change clouds tend areencies. temperature-dependent, Since the number concentration the mean extinction and size distributioncoefficient increases of ice crystalswith increasing in cirrus temperature. clouds are In temperature-dependent,contrast, the linear particle the depolarization mean extinction ratio coefficientwhich is sensitiveincreases towith ice increasing crystal shape temperature. and shap In econtrast, ratios increasesthe linear alongparticle with depolarization the decrease ratio of whichtemperature, is sensitive which tois alsoice crystaldemonstrated shape inand Platt shap et eal .ratios (1987) increases [48]. It can along also bewith noted the from decrease Figure of 7 temperature, which is also demonstrated in Platt et al. (1987) [48]. It can also be noted from Figure 7
Remote Sens. 2019, 11, 302 12 of 17
in cirrusRemote clouds Sens. are 2018 temperature-dependent,, 10, x FOR PEER REVIEW the mean extinction coefficient increases with increasing 12 of 17 temperature. In contrast, the linear particle depolarization ratio which is sensitive to ice crystal shape and shape(b) and ratios (c) increasesthat the mean along cirrus with thickness the decrease and opti ofcal temperature, depth have the which similar is also tendencies. demonstrated Accordingly, in Platt et al. (1987)the maximum [48]. It can optical also depth be noted and fromcloud Figure thickness7b,c are that both the found mean at cirrusthe temperatures thickness andbetween optical −60 depthand ° have the−50 similarC . In tendencies. Platt et al. Accordingly,(1987) [48], the the similar maximum temperature optical dependency depth and cloudof optical thickness depth is are also both found atpresented. the temperatures between −60 and −50 ◦C. In Platt et al. (1987) [48], the similar temperature
dependency of optical depth is also presented.
Figure 7. (a) Mean extinction coefficient (b) optical depth (c) thickness and (d) depolarization ratio as a functionFigure of midcloud 7. (a) Mean temperature extinction coefficient at 10 ◦C (b) intervals. optical depth (c) thickness and (d) depolarization ratio as a function of midcloud temperature at 10 °C intervals. 4.4. Cirrus Occurrence and Temperature Anomaly 4.4. Cirrus Occurrence and Temperature Anomaly The temperature anomalies (difference from the mean value of the temperature) during July and August 2014The over temperature Naqu is anomalies provided (difference in Figure 8from. The the simultaneous mean value of the tropopause temperature) heights during and July cirrus and August 2014 over Naqu is provided in Figure 8. The simultaneous tropopause heights and cirrus base/top heights were provided as well. The tropopause heights are determined by using temperature base/top heights were provided as well. The tropopause heights are determined by using data. Accordingtemperature to data. the previous According reports to the previous (e.g., Gage reports and (e.g., Green, Gage 1982 and [Green,49]), the 1982 height [49]), ofthe tropopause height of −1 = − · -1 can be determinedtropopause can when be determined the dT/dz when2 theK kmdT/2Kkm dz and=− the ⋅ temperature and the temperature lapse rate doeslapse notrate exceeddoes 2 K · km−1 for at least two kilometers. From this figure, most of the cirrus were below tropopause not exceed 2 Kkm⋅ -1 for at least two kilometers. From this figure, most of the cirrus were below except thetropopause cases on except 10 and the 18 cases July on 2014. 10 and However, 18 July 2014. we still However, took them we still into took account them sinceinto account the cirrus since top heightsthe of these cirrus two top casesheights were of these smaller twothan cases 1.5 were km sma aboveller thethan tropopause. 1.5 km above The the warmtropopause. color correspondsThe warm to the positivecolor corresponds deviation whileto the positive the cool deviation color corresponds while the cool to negative color corresponds deviation. to The negative cirrus deviation. cloud base and topThe heights cirrus are cloud also base plotted and top in Figure heights8. are The also pink plotted star symbolin Figure denotes 8. The pink the cirrusstar symbol base heightdenotes while the the blackcirrus square base symbol height representswhile the black the cirrus square top symbol height. represents The LiDAR the observationcirrus top height. can be The separated LiDAR in six stages:observation 10 to 11 can July, be 18 separated to 22 July in six and stages: 31 July 10 toto 811 August July, 18 forto 22 the July cirrus-existence and 31 July to 8 stagesAugust and for the 12 to 17 July, 23cirrus-existence to 30 July, and stages 9 to and 16 August12 to 17 forJuly, the 23 cirrus-inexistence to 30 July, and 9 to stages.16 August During for the the cirrus-inexistence 3 cirrus-existence stages. During the 3 cirrus-existence stages, the temperatures below the cirrus clouds were higher stages, the temperatures below the cirrus clouds were higher than the average value, which indicates than the average value, which indicates that the formation of the cirrus clouds may be related to the that the formation of the cirrus clouds may be related to the local ground heating and deep convective local ground heating and deep convective activity over the Tibetan Plateau. In this period, cold activityperturbations over the Tibetan were Plateau.found at height In this of period, about 12 cold km perturbations AGL. During werethe 3 cirrus-inexistence found at height ofstages, about 12 km AGL.oppositely, During the thetemperatures 3 cirrus-inexistence below the cirrus stages, clouds oppositely, were lower the than temperatures the average. Consequently, below the cirrus it clouds werecan be lower found than that thethe average.tropospheric Consequently, temperatures it with can the be foundpresence that of thecirrus tropospheric were about 5 temperatures°C higher with thethan presence that with of no cirrus cirrus were clouds. about Based 5 ◦onC the higher appearance than that of the with colder no and cirrus warmer clouds. air masses Based at on the the appearanceheight of of the above colder 12 km and AGL warmer (16.5 km air a.s.l.), masses the atRossby the height waves ofcan above be recognized 12 km AGL at the (16.5 same km height a.s.l.), the Rossbyin the waves tropospheric can be recognizedlayer and with at the a duration same height cycle inof theabout tropospheric 10 days. The layer formation and with of the a duration cirrus cycle of about 10 days. The formation of the cirrus clouds may also be affected by South Asia High RemoteRemote Sens. Sens.2019 2018, 11,, 30210, x FOR PEER REVIEW 13 of13 17 of 17 Remote Sens. 2018, 10, x FOR PEER REVIEW 13 of 17 clouds may also be affected by South Asia High Pressure (SAHP), which has characteristics of quasi- Pressurecloudsbiweekly (SAHP), may oscillation. also which be affected In has summer, characteristicsby South there Asia are High oftwo quasi-biweekly Pressuremain heating (SAHP), centers oscillation. which on has the Incharacteristics latitude summer, band there ofnear quasi- are the two SAHP Center, one on the Tibetan Plateau and one in the middle and lower reaches of the Yangtze mainbiweekly heating oscillation. centers on In thesummer, latitude there band are neartwo main the SAHP heating Center, centers one on the on latitude the Tibetan band Plateau near the and oneSAHPRiver. in the FollowingCenter, middle one and the on lower development the Tibetan reaches Plateauof of the the heating Yangtzeand one fi eld River.in theand middle Followingthe surrounding and thelower development atmospheric reaches of the ofcirculation, theYangtze heating fieldRiver.these and two theFollowing surroundingcenters the change development atmospheric with characteristics of the circulation, heating of quasi-biweekly field these and twothe surrounding centers oscillation. change atmospheric with characteristics circulation, of quasi-biweeklythese two centers oscillation. change with characteristics of quasi-biweekly oscillation.
Figure 8. Temperature anomalies during July and August 2014. Figure 8. Temperature anomalies during July and August 2014. Figure 8. Temperature anomalies during July and August 2014. To research the significant impact of the radiative effect and climate effect caused by cirrus clouds,To researchTo weresearch calculated the the significant significant the longwave impact impact fluxes ofof the theof cirr radiativeradiatus cloudsive effect effect based and and on climate climate the Fu-Liou effect effect causedmodel. caused bySince bycirrus the cirrus clouds,clouds,microphysical we we calculated calculated properties the the longwave suchlongwave as mean fluxes fluxes effective ofof cirruscirr radiusus clouds for ice based based particles on on the theis unavailableFu-Liou Fu-Liou model. model. from Since WACAL Since the the microphysicalmicrophysicalsystem, we propertiesmodeled properties the such icesuch particle as as mean mean size effective effective by the method radius for fordescribed ice ice particles particles in Krämer is isunavailable unavailable et al. (2016) from from[38]. WACAL Figure WACAL 9(a) shows that the cirrus OLR F increases monotonously with the increase of optical depth. F is system,system, we we modeled modeled the the ice ice particle particlec size size by by the the method method described described in in Krämer Krämer etet al.al. (2016)(2016) [[38].38]. Figure Figurec 9a − − smaller than 6 Wm 2 for sub-visual cirrus while for thin cirrus, the values of F lies between 6 Wm 2 shows9(a) that shows the that cirrus the OLRcirrusF cOLRincreases Fc increases monotonously monotonously with thewith increase the increase of optical ofc optical depth. depth.Fc is F smallerc is − − thanand 6 Wm 25 Wm−2 for2 . As sub-visual for−2 the opaque cirrus cirrus, while the for value thin ofcirrus, F is greater the values than 25 ofWmF lies2 . Comstock between et 6 Wmal. (2002)−2−2and smaller than 6 Wm for sub-visual cirrus while for thinc cirrus, the values ofc Fc lies between 6 Wm −2 −2 25 Wmestimated. As that− for2 thea cloud opaque becomes cirrus, significant the value in oftermFc ofis greaterOLR when than the 25 cloud Wm −forcing2 . Comstock is approximately et al. (2002) and 25 Wm . As for the opaque cirrus, the value of Fc is greater than 25 Wm . Comstock et al. (2002) estimated− that2 a cloud becomes significant in term of OLR when the cloud forcing is approximately estimated10 Wm [50].that aAccording cloud becomes to Figure significant 9(a), inthe term cirrus of OLRclouds when in thisthe cloudstudy forcing with OD>0.095 is approximately have a −2 10 Wm [−502 ]. According to Figure9a, the cirrus clouds in this study with OD>0.095 have a significant 10significantWm [50]. cloud According forcing and to theFigure value 9(a), of OD the is cirrus slightly clouds bigger in than this that study reported with byOD>0.095 Comstock have et al.a cloudsignificant(2002) forcing [50]. andcloud Moreover, the forcing value the and of relationships ODthe value is slightly of betweenOD bigger is slightly cl thanoud bigger thickness, that reported than meanthat byreported extinction Comstock by Comstockcoefficient, et al. (2002) et and al. [ 50 ]. Moreover,(2002)OLR are [50]. the shown relationshipsMoreover, in Figure the between9(b) relationships and (c). cloud It isbetween thickness,found there cloud mean is thickness,no extinctionobvious mean direct coefficient, extinction dependency and coefficient, OLRamong are these and shown in FigureOLRparameters. are9b,c. shown It is foundin Figure there 9(b) is and no (c). obvious It is found direct there dependency is no obvious among direct these dependency parameters. among these parameters.
Figure 9. Relationships between optical depth, cloud thickness, extinction coefficient, and longwave flux.
Remote Sens. 2019, 11, 302 14 of 17
5. Results and Conclusions This study provides an analysis of the geometrical and temperature characteristics as well as the optical properties and OLR of cirrus clouds over the Tibetan Plateau during TIPEX III between July 10 and August 16. Our results show that cirrus clouds are occurring between 5.2 km to 12 km AGL (i.e., 9.7 km to 16.5 km a.s.l.), with midcloud temperatures ranging from −79.7 to −26.0 ◦C. The mean cirrus top temperature is −44.0 ± 15.0 ◦C. The cloud thickness ranges from 0.12 to 2.55 km with a mean thickness of 1.22 ± 0.70 km. The linear particle depolarization ratio differs from 0.36 to 0.52, with a mean value of 0.44 ± 0.04. According to the measured optical depth, the cirrus clouds are classified as sub-visual cirrus (4.76%), optical thin cirrus (61.90%) and opaque cirrus (33.34%). From the results, the fitting curves about linear particle depolarization ratio, mean extinction coefficient depending on the midcloud temperature and the cirrus middle height are plotted. The dependence of cirrus mean optical and geometrical properties on temperature with an interval of 10 ◦C is also provided. With the decrease of the temperatures, the developments of linear particle depolarization ratio and mean extinction coefficient show opposite change tendencies. The mean cirrus thickness and optical depth have the similar tendencies. The maximum optical depth and cloud thickness are both found at the midcloud temperatures between −60 and −50 ◦C. According to the temperature anomaly during the July and August in 2014, the formation of cirrus clouds has apparent relationship with the dynamic processes of Rossby wave and South Asia High over the Tibetan Plateau. The OLR of sub-visual, thin, and opaque cirrus was calculated respectively and they increase gradually along with the optical depth.
Author Contributions: Conceptualization, G.D. and S.W.; methodology, G.D.; validation, S.W. and X.S.; formal analysis, G.D.; investigation, G.D., S.W. and X.S.; resources, S.W. and L.L.; data curation, G.D.; writing—original draft preparation, G.D.; writing—review and editing, G.D. and S.W.; supervision, S.W.; project administration, S.W. and L.L.; funding acquisition, S.W. and L.L. Funding: This research was funded by National Key Research and Development Program of China, grant number 2016YFC1400904, National Natural Science Foundation of China (NSFC), grant numbers 41375016 and 91337103; and the China Special Fund for Research in the Public Interest, grant numbers GYHY201406001. Acknowledgments: The authors are very grateful to Patric Seifert from Leibniz Institute for Tropospheric Research, Germany for providing us comments about cirrus filters, quality control, and multiple scattering. We also thank Diego A. Gouveia from University of São Paulo, Brazil for discussion and providing scripts for correcting multiple scattering of cirrus. We appreciate the contribution made by Meteorological Bureau of Tibet Autonomous Region, Naqu Bureau of Meteorology. We also thank Dongxiang Wang, Xiaochun Zhai and Qichao Wang for their help with the observations and discussions. Conflicts of Interest: The authors declare no conflict of interest.
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