a. dody
nuclear research center, negev, Beer sheva, Israel
Abstract
rainfall events were sampled in high resolution for stable isotope analyses during four rainy seasons in the central negev of Israel. each sample is equivalent to 1–2 mm of rain.
High variability in the isotopic composition was found in fractions of rain during storms. two modes of isotopic distribution were found. The first is a wave shaped distribution, where iso -topic compositions showed enriched to depleted graded changes and vice versa. the second mode is a step function where each rain cell displayed a constant δ18o value, but varied greatly from the other rain cells. new interpretation suggests that during the transport of the air parcel system three processes can occur. The first process is a complete blending among the rain cells.
the second is a partial isotopic mixing between the rain cells. finally the third case is when each rain cell maintains its own isotopic values separate from the other rain cells. the third case of no mixing showed unexpected results due to the high air turbulence, vertically and horizontally. there was no evidence of complete mixing among the rain cells of identical air parcel systems. the processes in the air parcel trajectory itself suggested here is put forward as a new way to explain the changes in the isotopic composition during the rain.
1. IntroductIon
In principle, stable isotopes in atmospheric water are used as tracers in order to improve our understanding of the hydrological cycle. there are a large number of observations of stable isotopes in precipitation with monthly time resolution all over the globe [1]. It is well known that temperature, altitude, latitude and continental at-tributes affect isotopic composition [2]. others, like refs [3, 4] suggest additional parameters such as evaporation, relative humidity (rH) and sea surface temperature (sst). Based on ref. [5] the isotopic composition is relatively uniform over a wide area on any particular day, but differs appreciably from storm to storm. later on, refs [6] and [7] showed in their study in the same geographical area that isotopic compo-sition differs dramatically even in a single rain cell. ref. [8] discussed the affect of the trajectory of stable isotopes in water vapour in the eastern Mediterranean. they calculated the air parcel trajectory based on the rH as measured in the last point.
ref. [9] also mentioned generally the effects of the synoptic trajectory on isotopic
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composition. ref. [10], which was based on the isotope data of ref. [6] mentions the local parameters such as rain intensity which influence the isotopic composition in a single rain spell. It is obvious today that in order to improve our understanding of the fractionation process under atmospheric conditions, sampling in high resolution is needed. only few measurements are available such as in refs [4, 11]. this study presents high resolution sampling of isotopes in precipitation under desert conditions.
unfortunately, at that time of the research (early 1990s), the sampling of precipita-tion for isotopes was done with no regard to the synoptic trajectory and its processes.
later, ref. [10] analysed part of the data from ref. [6] with respect to the trajectory of the water vapours. the average annual precipitation of 86 mm varied from 20 to 191 mm, and was distributed during 20 rainy days per season on average [12].
the research ran during four rainy seasons.
all of the rain samples were analysed at the International atomic energy agen-cy (Iaea). samples of three rain storms were analysed at both the Iaea labora-tory in Vienna and at the niedersächsisches landesamt for Bodenforschung (nlfB) Laboratory in Hannover, Germany. A standard deviation of ± 1‰ for δ2H and ± 0.1‰
for δ18O are acceptable accuracies for δ2H and δ18o analyses, respectively.
the aim of this work is to present a new interpretation explaining how isotopic composition varies during a single storm.
2. MetHodology
a mechanical sequential rain sampling with high resolution in time was used to collect approximately 250 to 500 ml of water equivalent to 1–2 mm of rain, respec-tively [13]. the novel approach to this method is that once an assigned volume of water is collected, the self-potential energy of the loaded containers is used to operate a mechanical seal for isolating the rain sample from the atmosphere. the sampler has 20 rain bags allowing the collection of rain water from a storm in amounts of 20 to 40 mm. a detailed description of the sampler was presented in ref. [13].
3. results
eight storms were monitored and sampled during four rainy seasons from 1990 to 1993. all data are shown in detail in ref. [9]. the isotopic composition of rainfall (rf) varied dramatically from enriched to depleted and vice versa in each storm. two modes of δ18o distribution during the rain were characterized:
• Wave shape distribution (Fig. 1). The δ18o is changed gently from enriched to depleted values and vice versa.
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• Step function, where each rain cell has constant δ18O values but is signifi-cantly different between them (fig. 2). changes can also be seen in the tri-tium values between the rain cells.
4. tHe proBleM
In a given area and rain storm, the variability in the changes in isotopic compo-sition during a single storm as shown in figs 1 and 2 could not be explained by simple changes in the air temperature, rH and sst. there must be other processes involved.
5. dIscussIon
Based on ref. [8], a change in 12 ºc in the sst reveals a change of less than 1‰ for δ18o (fig. 3b). In our data (fig. 2), the difference in the isotope values be-tween two rain cells is around 7‰ for δ18o, which is equivalent to a change in 84 ºc of the sst while assuming a linear correlation. this is, of course, not acceptable.
FIG. 1. Wave shape distribution of δ18O during the rain.
FIG. 2. Constant δ18O values in each rain cell. Different tritium values between the rain cells.
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Moreover, the change in sst in the Mediterranean sea between winter and summer is 18ºc, which means that in average the change per day is around 0.1º. therefore in a given single storm, the effect of sst on the isotopes’ values is negligible. thus, we should look for a better explanation.
Significant differences in the isotopic composition due to variations in the syn-optic conditions (both in terms of the source of the water vapours and the trajectory of the rain cell) were reported previously by refs [14–16] and [8].the new interpre-tation describes the processes occurring in the air parcel system along the trajectory.
each of the individual rain cells may represent a small amount of rainfall with a spe-cific isotopic composition. The rate at which the rain storm progresses in a spespe-cific location induces the isotopic composition of precipitation at the sampling point, as well as the rate of temporal change in the isotopic variations. ref. [10], which partly analysed the data this article is based on, claimed that the isotopic composition of a single rain spell is related primarily to the origin and trajectory of the air parcel system. this state supports the second mode of no mixing between the sequential rain spells (Fig. 2). The first mode wave shape distribution (Fig. 1) is explained by ref. [10] as being due to local parameters, especially rain intensity. the present paper suggests that the waveshape distribution is a result of mixing between two or three sequential rain spells. It must be stated here that based on ref. [6] no correlation was found between rain intensity versus isotopic composition, as explained by ref. [10].
FIG. 3. Measured values of δ2H and δ18O plotted against average SST at humidity uptake (after Ref. [8]).
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6. conclusIons
air parcel systems combine different rain cells from different origins. the iso-topic composition of each rain cell is determined by meteorological conditions when it joins the synoptic system. a new interpretation suggests that the isotopic composi-tion in precipitacomposi-tion is also controlled by the processes in the synoptic system, such as mixing or not, between sequential rain cells. the isotopic composition of each rain cell can be saved along the trajectory with no mixing although air turbulence may be high both vertically and horizontally.
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FIG. 4. Air parcel system contains rain cells with different isotopic compositions showing: (a) partial mixing between the rain cells; (b) no mixing between the rain cells.
a. partial mixing between the cells
Origin of the air parcel system
b. no mixing between the cells
Legend
enriched water vapours depleted water vapours
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