Modeling Insights into Precipitation Deuterium Excess as an Indicator of Raindrop Evaporation in Lanzhou, China
Higher of Geography and Environmental Science, Northwest Normal Academy, Lanzhou 730070, People’s republic of china
Laboratory of Atmospheric Physics, Department of Physics, University of Patras, GR-265 00 Patras, Greece
Author to whom correspondence should be addressed.
Received: 26 Nov 2020
Revised: 31 December 2020
Accepted: 13 January 2021
Published: fifteen January 2021
(This article belongs to the Section Hydrology)
The deuterium excess in precipitation is an effective indicator to appraise the being of sub-cloud evaporation of raindrops. Based on the synchronous measurements of stable isotopes of hydrogen and oxygen (δ
xviiiO) in precipitation for several sites in Lanzhou, western China, spanning for approximately four years, the variations of deuterium excess between the ground and the cloud base of operations are evaluated past using a 1-box Stewart model. The deuterium excess difference below the cloud base during summer (−17.82‰ in Anning, −xi.76‰ in Yuzhong, −21.18‰ in Gaolan and −12.41‰ in Yongdeng) is greater than that in other seasons, and difference in winter is weak due to the low temperature. The variations of deuterium backlog in precipitation due to beneath-cloud evaporation are examined for each sampling site and year. The results are useful to understand the modification of raindrop isotope composition beneath the cloud base at a urban center scale, and the quantitative methods provide a case study for a semi-barren region at the monsoon margin.
The sub-deject evaporation refers to the evaporation of the raindrops during their trip through the unsaturated atmosphere from the cloud base to the land. The influence of below-deject evaporation on the isotopic signatures of pelting depends on the meteorological surround (such as air temperature and relative humidity) in the atmosphere, on the original drop radius in the cloud as well equally on the stable isotopes of the surrounding moisture where the drop travels [1,2]. For the isotopic ratios when the raindrop forms at the early phase, the raindrops commonly tend to reach an equilibrium status with the ambient moisture [three]. Low-cal isotopes are depleted in advance while heavy isotopes in raindrops are enriched [4,5,half-dozen], showing that at that place has been a decrease in the deuterium excess or
twoH − 8 ×
18O . The evaporation and condensation are impacted by the different climate parameters [8,9], and the deuterium excess in meteoric water is besides sensitive to these climate weather [10,11,12], causing that the deuterium excess can exist a useful indicator to trace the source of water vapor and assess the potential existence of the sub-cloud evaporation [thirteen,xiv,fifteen,16].
Regarding the assessment of below-cloud evaporation associate to deuterium backlog, the Stewart model has been widely used. Applying the droplet evaporation model nether laboratory conditions, Stewart [i] investigated the relationship between isotope fractionation and proportions of evaporation at varied backgrounds. Froehlich et al.  modified the Stewart model and and so presented a simple frame to assess the change in deuterium excess due to evaporation. However, a few parameters used in the model  are non well-defined, and clearer parameterization is nevertheless needed [xviii]. For case, Wang et al.  used a sphere-based method to guess the raindrop remaining fraction, which shows a wide range of raindrop remaining fraction in the arid climate setting and may be more suitable for arid and semi-arid regions. These modified Stewart models are practical to other different regions [20,21,22].
At a small spatial scale, the influence of deuterium excess on precipitation greatly depends on the topography as well as the meteorological atmospheric condition with the region, so a detailed understanding of the below-cloud evaporation needs synchronous observations for surrounding sites. Hither nosotros focused on a urban center-calibration measurement in Lanzhou at the margin of the Asian monsoon. This region has been examined for the precipitation isotopes in contempo years [23,24,25,26]. However, a Stewart model-based quantitative assessment well-nigh the isotopic modification beneath cloud base is still limited in Lanzhou, and the details about inter-annual variability and spatial incoherence of raindrop evaporation are not clear for this region. The objective of this study is (1) to appraise the deuterium excess variation in atmospheric precipitation from the cloud base to the country surface; (two) to discuss the influence of main parameters on the deuterium backlog difference; and (3) to clarify the linkage between the remaining fraction of raindrop and variation of deuterium excess.
2. Data and Method
2.1. Sample Collecting
The Lanzhou Urban center, uppercase of the Gansu Province of China, is located at the western part of the Chinese Loess Plateau (Figure i). To empathize the isotope hydrology at the monsoon marginal region, the Northwest Normal University (NWNU) set up in 2011 an observation network of isotopes in precipitation . Inside this framework, four sampling stations embrace the key urban area (Anning) and also iii surrounding counties (i.due east., Yuzhong, Gaolan and Yongdeng). The liquid samples were collected in fourth dimension subsequently the end of each pelting or snow consequence; some large corporeality events were nerveless repeatedly considering stock-still time intervals. The samples were placed in HDPE bottles with seals and stored in a refrigerator. Solid samples (such equally snow or hail) were not filled until they were melted at the room temperature within the LDPE bags. The water samples were isotopically analyzed using a liquid water isotope analyzer (DLT-100, Los Gatos Research, Inc., Mountain View, CA, USA) in the College of Geography and Environmental Science, NWNU, with a precision of ±0.two‰ and ±0.half dozen‰, for
2H (delta note relative to VSMOW), respectively . In this study, the menstruum from April 2011 to October 2014 previously analyzed  was reexamined; run into Chen et al.  for the basic characteristics of atmospheric precipitation isotopes in Lanzhou during this menses.
According to previous studies [1,17], the difference of deuterium excess during the falling duration, i.due east., Δd, is caused by evaporation according to the formula:
the kinetic fraction factors [27,28] expressed equally:
is the condensation temperature (Grand), and
is the remaining fraction of mass. The values of
[i] are defined as:
the relative humidity.
D′ equal to 1.024 and 1.0289, respectively [1,29], and
The raindrop is treated as a spheroid, and the remaining fraction can be expressed as in Wang et al. :
the mass touching the land surface and evaporated raindrop mass, respectively;
is expressed as product of the evaporation intensity and the time required to drop to embrace the altitude from the deject base. The fall time is estimated using the cloud base top and the falling velocity when a constant motion is assumed. The terminal velocity of a raindrop with drop bore and cloud base tiptop can be calculated based on All-time [xxx]. The altitude of cloud base of operations is calculated using lifting condensation level (LCL) [19,31,32].
Evaporation intensity of a raindrop can be expressed to be a product using
(in cm) and
(in g cm−1
due south−1); here
is a part of the temperature and raindrop size, and
is of the temperature and relative humidity . Based on the experimental data by Kinzer and Gunn , the values of
at actual conditions are caused using a bilinear interpolation as suggested by Wang et al. . The median diameter of the raindrop is estimated using atmospheric precipitation intensity . Here we calculated that the median diameters of raindrops at Anning range from 0.4 to 1.7 mm, at Yuzhong from 0.4 to 1.9 mm, at Gaolan from 0.four to ane.6 mm and at Yongdeng from 0.4 to 2.0 mm, respectively.
A raindrop is usually assumed to be a sphere, then
tin can be expressed as:
the radius of the raindrop touching the country surface and
and the density of water.
3. Results and Give-and-take
3.one. Raindrop Bore, Velocity and Evaporation Intensity
three.1.1. Raindrop Diameter
Figure two shows the frequency of atmospheric precipitation events for different raindrop diameters in Lanzhou from April 2011 to Oct 2014, when the temperature above zilch. Raindrops are mainly distributed in the range from 0.4 to 1.ii mm. The arithmetic mean and the median equal both 1.0 mm; events with a raindrop bore less than one mm account for the 51.6% of the total number of atmospheric precipitation events. At the iv sites, raindrop diameters range between 0.4 and 1.7 mm at Anning, between 0.4 and one.9 mm at Yuzhong, between 0.4 and 1.six mm at Gaolan and between 0.4 to two.0 mm at Yongdeng, respectively.
3.one.ii. Raindrop Velocity
The size distribution of the to a higher place-mentioned raindrop diameters defines the value of the terminal velocity. Figure 3 shows the frequency of pelting events for different terminal velocity values in Lanzhou from April 2011 to October 2014. The terminal velocity of all rainfall events varies from 2.i to 6.9 m/s; the most frequent rainfall events have terminal velocities betwixt 3 and v thou/s. The number of rainfall events with terminal velocity values between 2.v and 4.5m/s is 123 at Anning (69.9% of the total), 123 (65.4% of the full) at Yuzhong, 174 (73.iv% of the total) at Gaolan and 189 (70.8% of the total) at Yongdeng.
iii.1.3. Raindrop Evaporation Intensity
Figure 4 and Figure 5 illustrate the regional and monthly variation of raindrop evaporation intensity in the study region. The range of evaporation intensity in precipitation is large; the minimum is 0.026 ng/s and the maximum iv.ix ng/south. The evaporation rate changes evidently with season, but its spatial difference is smaller, and the median is less than one.0 ng/due south amid the four sites. On a monthly basis, as air temperature increases in summer, evaporation rate presents a rising tendency; it is relatively high from May to September and starts to decrease after Oct. Equally shown in Effigy five, the alter of evaporation rate during the different years is generally similar.
3.ii. Deuterium Excess Deviation for Each Site and Year
As shown in Effigy 6 and Figure 7, Δd
varies depending on the spatial and seasonal patterns, similarly to the variation of
f. The deuterium excess difference differs depending on the site and on the season. According to the values of the median, the variation of Δd
in precipitation is relatively smaller (more than 20‰) for the study region. During the winter months, in that location is no variation of deuterium backlog for some months. During the summer months, variation of deuterium backlog is much greater in the recent four years. Figure 7 shows that the variations of Δd
during the four years at the iv sites are consistent with the full changes shown in Effigy six. The variations of Δd
in precipitation for each site and year reflect the climate background at dissimilar spatial and temporal scales.
Table 1 exhibits the values for each sampling site in this study. The annual weighted hateful Δd
value varies from −17.84‰ (Gaolan) to −10.55‰ (Yuzhong); the four-year arithmetics mean is −13.81‰; notwithstanding, the everyman Δd
values were observed at Anning in 2013 (−24.39‰), and the highest Δd
value observed at Anning is −8.53‰ in 2014. On a seasonal basis (Table 2), the annual weighted mean Δd
ranges from −17.50‰ to −x.30‰ during spring, between −21.18‰ and −xi.76‰ during summer and between −12.48‰ and -8.50‰ during autumn, respectively. Overall, the deuterium backlog difference below the cloud base during summer (−17.82‰ in Anning, −eleven.76‰ in Yuzhong, −21.18‰ in Gaolan and −12.41‰ in Yongdeng) is greater than that in spring and autumn; in addition, the amplitude of variation of Δd
betwixt the country surface and the deject base during summertime is greater than in spring and autumn. The results in Table 2 are consistent with seasonal variations shown in Effigy six and Figure seven.
3.3. Climate Parameters and Deuterium Excess Difference
In Effigy 8a, when the air temperature is very low, there is no liquid raindrop and the sub-cloud evaporation tin be neglected for the cold environment. In Figure 8b, small atmospheric precipitation tin be widely seen, although larger precipitation corresponds the low evaporation. For the relative humidity in Effigy 8c, high humidity unremarkably means low below-deject evaporation. In Figure 8d, very large raindrop may result in depression evaporation. Although the amplitude of the variation of deuterium excess is close to −200‰ during individual precipitation events, these correspond to the minimal rainfall intensity and corporeality. If precipitation is weighted, this miracle may not announced. The results near the climate parameters are of assistance to empathize the raindrop evaporation and deuterium excess departure especially in the semi-barren regions.
3.4. Raindrop Evaporation and Deuterium Excess Difference
The remaining fraction varies depending on the specific site and the time menstruum, and it ranges from 2% to 100% (Effigy ix). Considering the very weak below-cloud evaporation in cold period, the winter evaporation remaining fraction are usually close to 100%. The medians of
at the four sites all exceed 70%. Figure ten illustrates the intra-annual variation of
at the iv sites in Lanzhou, indicating that the basic patterns are consistent to Figure 9.
In many instance studies, the Δd
have a good correlation [17,18,xix], which is generally consistent with this study (Effigy 11 and Figure 12), i.due east., a 1% increase of evaporation may cause deuterium excess in raindrop to reduce by approximately 1‰ (1.1‰ per 1% in this study). Under conditions of depression remaining fraction, the relationship is usually weak . In Anning, Yuzhong, Gaolan and Yongdeng, the regression coefficients are 0.87‰, 1.09‰, 1.eighteen‰ and 1.10‰ per i%, respectively (Figure eleven). In 2011, 2012, 2013 and 2014, the regression coefficients are ane.03‰, 1.15‰, 1.xviii‰ and ane.09‰ per ane%, respectively (Figure 12). The results in the study region are generally consistent with the previous findings, and the inter-annual variations practice exist during the sampling period. In addition, the spatial diversity of this regression slopes can also be seen in such a small expanse.
In this study, the divergence of deuterium excess of raindrops from the cloud base of operations to the land surface is evaluated past using a model in order to discuss the influence of below-cloud evaporation in Lanzhou, China, at the monsoon margin. The changes in deuterium excess in summer are greater than in other seasons, and changes in winter are close to aught due to the depression temperature. Here we focus on the below-cloud evaporation effect on raindrop stable isotopes in a city scale where the sampling sites are located closely, and the variations of deuterium excess are examined for each sampling site and year. In this study, different isotopic characteristics due to below-cloud evaporation can exist seen due to the meteorological controls. The results are useful to sympathise the modification of raindrop isotope composition below the cloud base at a urban center scale, and the quantitative methods used hither provide a instance study for raindrop evaporation assessment for a semi-barren region at the monsoon margin.
Conceptualization, M.Z. and S.W.; Software, F.C.; Validation, F.C.; Formal assay, F.C.; Investigation, F.C.; Methodology, S.Westward. and A.A.A.; Resources, Q.M., X.Z., X.W. and J.C.; Data curation, F.C.; Writing—original draft training, F.C.; Writing—review and editing, F.C., M.Z. and S.Westward.; Supervision, M.Z.; Funding conquering, M.Z. and S.W. All authors have read and agreed to the published version of the manuscript.
The research is supported past the National Natural Science Foundation of Mainland china (No. 42071047 and 41771035), the Foundation for Distinguished Immature Scholars of Gansu Province (No. 20JR10RA112) and the Scientific Research Program of Higher Educational activity Institutions of Gansu Province (No. 2018C-02).
Institutional Review Board Argument
Informed Consent Argument
Data Availability Statement
The data used in this paper are bachelor from M.Z. ([email protected]) upon request.
The authors thank the meteorological bureaus for their assistance in sampling.
Conflicts of Involvement
The authors declare no conflict of involvement. The piece of work described here has not been submitted elsewhere for publication, in whole or in role, and all the authors listed have approved the manuscript that is enclosed.
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Spatial distribution of network in Lanzhou used in this study. Small-scale map shows the study area in China.
Frequency of atmospheric precipitation events for different rain drop diameters: (a) Lanzhou and (b) four sampling sites.
Frequency of precipitation events for different last velocities: (a) Lanzhou and (b) 4 sampling sites.
Regional (a) and monthly (b) variations of raindrop evaporation intensity.
Regional (a,c,due east,yard) and monthly (b,d,f,h) variations of raindrop evaporation intensity in 2011 (a,b), 2012 (c,d), 2013 (e,f) and 2014 (one thousand,h).
Regional (a) and monthly (b) variations of Δd
Regional (a,c,e,g) and monthly (b,d,f,h) variations of Δd
in 2011 (a,b), 2012 (c,d), 2013 (e,f) and 2014 (yard,h).
Relationship between meteorological parameters ((a). air temperature, (b). precipitation amount, (c). relative humidity, (d). raindrop bore) and Δd
in atmospheric precipitation.
Regional (a) and monthly (b) variations of
in atmospheric precipitation.
Regional (a,c,e,thousand) and monthly (b,d,f,h) variations of
in 2011 (a,b), 2012 (c,d), 2013 (eastward,f) and 2014 (g,h).
in precipitation for each site (a). Anning, (b). Yuzhong, (c). Gaolan, (d). Yongdeng.
in precipitation in 2011 (a), 2012 (b), 2013 (c) and 2014 (d).
Comparison of deuterium excess (d) in precipitation at land surface and cloud base of operations for each yr.
|Basis||Cloud Base||Δd||Ground||Deject Base||Δd||Ground||Deject Base||Δd||Basis||Cloud Base of operations||Δd||Ground||Cloud Base||Δd|
Comparing of deuterium excess (d) in atmospheric precipitation at land surface and cloud base of operations for each season.
|Basis||Cloud Base||Δd||Ground||Cloud Base||Δd||Ground||Deject Base||Δd||Ground||Cloud Base of operations||Δd|
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