Impacts of oasis on the atmospheric hydrological cycle over the nearby desert

doi:10.4236/ns.2010.27084

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Vol.2, No.7, 681-687 (2010) Natural Science

http://dx.doi.org/10.4236/ns.2010.27084

Impacts of oasis on the atmospheric hydrological cycle

over the nearby desert

Qiang Zhang1,2,3*, Yuhe Nan2, Sheng Wang1,3

1Key Laboratory of Arid Climatic Changes and Disaster Reduction of Gansu Province, Institute of Arid Meteorology, China

Meteorological Administration, Lan Zhou, China; *Corresponding Author: [email protected]

2College of Atmospheric Science, Lan Zhou University, Lan Zhou, China

3Meteorological Bureau of Gansu Province, Lan Zhou, China

Received 22 February 2010; revised 23 April 2010; accepted 28 April 2010.

ABSTRACT

Using the data of “A field experiment on land-

atmosphere interaction over arid region in

Northwest China” carried out in Dunhuang of

Gansu Province from May to June 2000; Char-

acteristics of the atmospheric humidity over

desert and Gobi near oasis in the Northwest

China Arid Region are analyzed. According to

the difference of the characteristics in different

wind directions, the impacts of oasis on at-

mospheric hydrological cycle over desert and

Gobi near it are revealed. The relation of at-

mosphere inverse humidity and negative water

vapor flux to wind direction and atmospheric

stability is studied. It shows that distribution of

the atmosphere inverse humidity is inconsistent

with that of the negative water vapor flux; some-

times 1-hour-average value demonstrates the

characteristic of counter-gradient transfer. And

the diurnal variation of distribution of the coun-

ter-gradient transfer and the effect of atmos-

pheric stability on the counter-gradient transfer

are also given.

Keywords: Desert or Gobi; Hydrologic Cycle;

Atmospheric Inverse Humidity; Negative Water

Vapor Flux; Counter-Gradient Transfer

1. INTRODUCTION

Oasis is one of the underlying heterogeneous factors. It

directly affects the pattern of the atmospheric energy and

hydrology transfer over grid points in the region in a

model [1-3]. The interaction of the oasis system and the

desert system is relative to the maintenance and degen-

eration processes of oasis ecology system. The special

microclimate characteristics and the hydrological cycle

mechanism formed under the condition of the interaction

are two of the important natural factors (except the fac-

tor of water resource) which maintain the oasis system

[4-6].

Some special interaction processes between oasis and

desert have been found in some experiments and studies

carried out in the Northwest China Arid Region [4-12].

However, how the oasis impacts on the water vapor and

energy transfer over desert or Gobi near it is still un-

known. Most of the former field experiments were car-

ried out in the arid climate region where annual precipi-

tation is about 150 mm, but experiments in arider cli-

mate regions where annual precipitation is blow 50 mm,

such as Dunhuang, are scarce.

Through analysis and study the data of “The Dun-

huang Experiment”, the special physical rule of the land-

surface under the interaction between oasis and desert

near it as well as the ecological maintenance mechanism

of oasis will be discussed in the following passages.

2. OBSERVATION DATA AND

ANALYTICAL METHOD

The data analyzed in this paper is from a 20-day inten-

sive observation experiment in Dunhuang Gobi micro-

meteorological central station from May 25 to June 17,

2000 (briefly, “The Dunhuang Experiment”), which is a

part of “Land-atmosphere Interactive Field Experiment

over Arid Region of Northwest China”. The station is

located at 40°10′ N, 94°31′ E. Its surface elevation is

1150 meters above sea level. It lies in the flat Shuang-

dunzi Gobi west to Dunhuang oasis, about 7 km to the

edge of the oasis. Annual precipitation is about 40 mm

and annual potential evaporation is about 3400 mm. Its

main wind direction is east, generally occurring with the

frequency of more than 50 percent, so the oasis strongly

affects the atmosphere over the observation field. The

data include the gradients of wind, temperature and hu-

Q. Zhang et al. / Natural Science 2 (2010) 681-687

682

midity on a tower, soil heat fluxes, the fluctuations of

wind, temperature, humidity and wind observed by

using a tethered balloon within hundreds of meters high.

The sensors of wind, temperature and humidity on the

tower are installed at 18, 8, 4 and 2 m high respectively.

The sensor of wind direction is installed at 10 m high.

The sensors of supersonic fluctuation instrument were

installed at 2.9 m with a data processing system that

can give the hourly-average momentum flux, sensible

heat flux, and latent heat fluxes averaged. The details

about the data and the station in Dunhuang experiment

have been described in the paper [5,9]. The accuracy of

the instruments refers to the correlated references [13-

14].

During the Dunhuang experiment, there are 445 valid

sets of data which are hourly averaged and 10-day’s data

to show full diurnal variation. So the amount of samples

of hourly data is 445 and that of diurnal variation is 10.

Among the ten days, one day is affected evidently by the

precipitation. In order to make the results universal, the

data which is evidently affected by the precipitation is

picked out. The turbulent sensible and latent heat flux is

computed by the supersonic fluctuation data.

3. ATMOSPHERIC INVERSE HUMIDITY

STRUCTURE AND WATER VAPOR

FLUX

In Figure 1，the characteristics of 9-day average daily

variation of atmospheric specific humidity at four levels

(a) and latent heat flux at 2.9 m high (b) are given in

May-June over Dunhuang Gobi. As seen in Figure 1,

because of the influence of the oasis, the atmospheric

specific humidity over Gobi is inverse humidity from

0:00 to 6:00. In the daytime, although the specific hu-

midity mainly decreases with height, the inverse humid-

ity appears at 2-8 m high after 14:00 in the afternoon.

Figure 1(b) shows the negative water vapor flux domi-

nates at night and the water vapor transfers up in the

daytime. From its overall characteristics, the diurnal

variation of the specific humidity can be divided into

four stages that are respectively called the wet stage, the

losing-water stage, the dry stage and the attainting-water

stage [15].

The daily integration of latent heat flux is 0.117

MJ/m2 and that of sensible heat flux is 8.692 MJ/m2.

Difference between them is in two orders of magnitude.

The climatic Bowen ratio reachs 74.5 which is an ex-

treme arid climate index.

In fact, the impact of the oasis on desert near it is dy-

namic, which is quite different under different types of

synoptic situation. Especially, the wind direction is es-

sential to the impact. The representative data on two full

days are selected to analyze. One is 16 June on which

(a)

2.5

3.5

4.5

5.5

0 24 6 810121416182022

Local time/hour

atmosphere

humidity /g.kg-1

1m 2m

8m 18

(a)

(b)

Figure 1. Daily variation of atmospheric specific humidity at

four levels. (a) and latent heat flux at 2.9 m; (b) average data

from May 27 to June 15 over Dunhuang Gobi.

wind is from the oasis (named oasis wind), another is 14

June on which wind is from desert (named desert wind)

expect 4:00 and 5:00 in which wind direction can not be

clearly distinguished into oasis wind and desert wind.

Figure 2 shows the four stages of the wet stage (a), the

losing-water stage (b), the dry stage (c) and the attaint-

ing-water stage (d) in daily variation of the specific hu-

midity profile in the wind from oasis. It indicates that the

specific humidity profile is mainly inverse humidity all

day. In the wet stage, inverse humidity appears below 18

m high. In the losing-water stage Inverse humidity ap-

pears at about 2 m high and in 8-18 m and the humidity

diminishes obviously with time. In the dry stage, inverse

humidity appears only below 2 m, and inverse humidity

disappears in few stronger heating hours. In the water-

gaining stage, inverse humidity strengthens gradually

and moves up, at last, all is inverse humidity below 18m

and humidity increases obviously.

(b)

-10

-5

0246810121416182022

local time/hour

surface latent

heat flux/w.m-2

m-2

Q. Zhang et al. / Natural Science 2 (2010) 681-687

683

(a)

5 5.5 66.5 7 7.5

atmosphere humidity/g.kg

-1

height/m

(a) (b)

(c)

4.2 4.44.6 4.855.2

atmosphere humidity/g.kg

-1

height/m

12h

13h

14h

15h

16h

17h

18h

(d)

45678

atmosphere humidity/g.kg

-1

height/m

19h

20h

21h

22h

23h

Figure 2. The four stages of the wet stage. (a) the losing-water stage; (b) and the dry stage; (c) the attainting-water stage; (d) in

daily variation of the specific humidity profile in the wind from oasis.

Compared with Figure 2, the daily variation of the

specific humidity profile over desert near oasis in the

four stages in the wind from desert (given in Figure 3) is

quite different. It shows the specific humidity decreases

progressively with height all day. In the wet stage, weak

inverse humidity appears mainly in 2 m in general, but

in all layer in transient wind direction. In the losing-

water stage, inverse humidity below 2 m diminishes gra-

dually and disappears or moves up to 2-8 m. In the dry

stage, inverse humidity in 2-8 m and air becomes moist

obviously. In the attainting-water stage, the profiles of

specific humidity decreases up and changes gradually

into constant humidity with time. Except 4:00 and 5:00,

there is no water vapor advection from oasis to Gobi, but

the weak inverse humidity still maintains in long time in

dry stage. The reasons are worthy to be discussed.

Figure 4 is the daily variation of the latent heat flux at

2.9 m over Gobi near the oasis in the wind from the oa-

sis (a) and from Gobi or desert (b). In the wind from the

oasis, positive latent heat flux is much smaller than that

in the wind from the desert ,and further, negative water

vapor flux appears mainly in the evening, which are own

to the effect of atmospheric inverse humidity. But in the

wind from the desert, not only is the latent heat flux

bigger, but also the water vapor still transfers generally

up in the evening.

The daily integration of the latent heat flux is –0.051

MJ/m2 in the wind from oasis and is 0.443 MJ/m2 in the

wind from desert. The contributions of the latent heat

flux to the surface heat balance in different kind of wind

(b)

5 5.56 6.57 7.5

atmosphere humidity/g. kg-1

height/m

10h

atmosphere humidity/g·kg-1

Q. Zhang et al. / Natural Science 2 (2010) 681-687

684

(a)

55.566.577.5

atmosphere humidity/g.kg

-1

height/m

(b)

5 5.5 6 6.577.5

atmosphere humidity/g. kg-1

height/m

10h

11h

atmos phere humidity/g·kg

-1

(a) (b)

(d)

45678

atmosphere humidity/g.kg

-1

height/m

19h

20h

21h

22h

23h

Figure 3. The four stages of the wet stage. (a) the losing-water stage; (b) the dry stage; (c) the attainting-water stage; (d) in daily

variation of the specific humidity profile in the wind from desert.

directions are completely different. When the latent heat

flux is converted to the water vapor flux, its daily inte-

gration is –0.0155 mm in the wind from the oasis and is

0.1355 mm in the wind from desert. In the common

synoptic background annual evaporation over desert and

Gobi is bigger than the climatic mean value without the

infect of the oasis. This result means that it is the effect

of the oasis makes the climatic state and the vegtation

ecological type better in the desert region near the oasis

than that in dersert or Gobi. It can be called the clmatic

effect of inverse humidity. Dunhuang Oasis is smaller,

so maybe its climatic effect isn’t more evidient than that

of bigger or more prosperous oasis. The point isn’t found

in former relative studies on arid region.

-15

-10

-5

0246810 12 14 16 18 20 22

local time/hour

surface latent

flux/w.m

-2

desert wind

oasis wind

Figure 4. (a) Characteristics of daily variation of the latent

heat flux at 2.9 m over Gobi in the wind from oasis; (b) and

from Gobi or desert.

(c)

2 2.5 3 3.5

atmosphere humidity/g.kg-1

height/m

12h

13h

14h

15h

16h

17h

18h

atmosphere humidity/g·kg-1

Q. Zhang et al. / Natural Science 2 (2010) 681-687

685

4. DISTRIBUTION OF ATMOSPHERIC

INVERSE HUMIDITY AND NEGATIVE

WATER VAPOR FLUX AS WELL AS

COUNTER-GRADIENT TRANSFER

Figure 5 is a comparison of distribution of the wind

from the oasis and inverse humidity as well as negative

water vapor within 1-2 m over Gobi in Dunhuang. It

indicates that frequency of oasis wind keeps at about 50

percent all day, frequency of the atmosphere inverse

humidity is bigger at night than that in the daytime. Fre-

quency of inverse humidity is bigger than that of oasis

wind at night, but smaller at noon and in the afternoon.

There are two main reasons for this feature. On the one

hand, the turbulent mixing is very strong and demolishes

the inverse humidity structure in the daytime. Thus the

inverse humidity structure is hard to maintain. On the

other hand, surface evaporation in the daytime can partly

counteract the effect of the water vapor advection.

Therefore, if the water vapor advection isn’t strong

enough the inverse humidity will not appear in the day-

time. But at night the turbulent mixing is weaker,

evaporation is less and atmosphere is of generalized con-

servation (It means atmospheric motion and exchange is

inactive), and inertia of the atmosphere inverse humidity

is strong. Thus, the inverse humidity can still keep for a

long time, even if there isn’t the water vapor advection

from the oasis. These conclusions haven’t been drawn

from the observations in desert near Zhangye-Linze

prosperous oasis in Heihe region [2]. There are two main

reasons about it. On the one hand, the wind direction is

more irregular in the observation of Heihe region which

is affected by Qilian Mountain. On the other hand, per-

haps the water vapor transfer of Dunhuang oasis isn’t as

strong as Zhangye-Linze oasis’s.

Generally the direction of the water vapor flux is con-

trolled by the specific humidity gradient. For example, if

the specific humidity decreases with height the water

vapor flux will be positive, namely upwards, vice versa.

100

0 2 4 6 810121416182022

local time/h

probablity

inverse humidity

wind from oasis

ative moist flux

Figure 5. A comparison of distribution of the wind from the

oasis, inverse humidity and negative water vapor within 1-2 m

over Gobi in Dunhuang.

Figure 5 shows that, the distribution of the negative wa-

ter vapor flux is consistent with that of the inverse spe-

cific humidity at night, but is quite inconsistent with it in

the daytime. Frequency of the negative water vapor flux

is under 20 percent and less than that of the inverse spe-

cific humidity in the daytime. It means that, the latent

heat bulk transfer coefficients must be positive, namely

counter-gradient transfer appears most times. It also ap-

pears over desert near oasis in Heihe region, but is omit-

ted as error in the former studies. The counter-gradient

transfer is partly related to the observational error be-

cause both the humidity and latent heat flux are too

small. But its distribution shows such good rule which

makes us to look for other reasons. Some studies show

that the counter-gradient transfer can result from the

horizontal heterogeneity which is caused by the motion

of convective eddy, complicated underlying and cumulus

convection [6,7]. The strong horizontal heterogeneity of

water vapor caused by distribution of oases in arid re-

gion is a typical mesoscale process. Based on Taylor

hypothesis, temporal averaged values can respond to the

effect of the spatial heterogeneity, and it results in oppo-

site symbol between the hourly averaged gradient and

the turbulent flux.

Figure 6 is a comparison of the characteristics of

daily variation of distribution of counter-gradient trans-

fer (a) and the bulk Richardson number (b). Figure 6(a)

indicates that the frequency of the counter-gradient

transfer over desert and Gobi near oasis in the daytime is

bigger than that at night. Its maximum is over 60 percent

at 9:00 o’ and the minimum is about 20 percent at 18:00.

It basically maintains about 40 percent in a longer time.

The daily variation of the counter-gradient transfer

may be related to the atmospheric stability. Comparing

Figure 6(a) with Figure 6(b), the daily variation of the

distribution of counter-gradient transfers and is mainly

correlative to that of the Richardson number. The bigger

the frequency of counter-gradient transfer is, the stronger

the atmospheric instability is, vice versa.

The atmospheric stability indirectly affects the cou-

nter-gradient transfer through its influence on the oasis

effect. Generally, the stronger the heterogeneity is, the

counter-gradient transfer caused by the temporal aver-

aging [16] is more obvious. At night the atmosphere over

oasis is stable, inactive and its impact on desert near it is

smaller, so the frequency of the counter-gradient transfer

is less. But in the daytime the great instability makes the

atmosphere over oasis active, and the influence of at-

mosphere over the oasis on the atmosphere over desert

near oasis is greater, so the frequency of the counter-

gradient transfer is bigger. After the surface atmosphere

exchanges strongly between oasis and desert during a

period in the daytime, horizontal distribution of atmos

Q. Zhang et al. / Natural Science 2 (2010) 681-687

686

(a)

100

1357911 13 1517192123

local time/hour

distribution of counter-

gradient moisture transfer/%

observed curve

Polynomial (observed curve)

(a)

(b)

-0.8

-0.4

0.4

0.8

1.2

0 246810121416182022

local time/hour

Riichardson number

(b)

Figure 6. (a) A comparison of the characteristics of daily varia-

tion of distribution of counter-gradient transfer; (b) and the

bulk Richard-son number.

pheric humidity tends to even and the impact of the oasis

on desert become weaker. So the counter-gradient trans-

fer of water vapor appears at 9:00, not at noon or in the

afternoon.

5. CONCLUSIONS AND DISCUSSIONS

The daily variation of the specific humidity over desert

near oasis can be divided into four stages which are the

wet stage, the losing-water stage, the dry stage and the

attaining-water stage. This is similar to the general Gobi

or desert. Differences caused by the oasis wind are given

in the following content. Firstly, the atmosphere specific

humidity over Gobi near oasis is basically in inverse

humidity and the water vapor flux is negative at night.

Secondly, the specific humidity decreases progressively

with height and the water vapor flux is positive in the

daytime. Thirdly, the difference between the daily inte-

grated values of the latent and sensible heat flux are in

two orders of magnitude.

The structure of humidity profiles and the water vapor

transfer in desert wind are very different from those in

oasis wind. In the wind from desert, the specific humid-

ity decreases with height all day and the water vapor

basically transfers up. However, in the wind from oasis,

the inverse humidity is dominant and the water vapor

flux is negative all day except few hours. The daily inte-

grated value of water vapor flux in the wind from oasis

is significantly different from that in the wind from de-

sert. Such effect of the oasis indicates that less annual

precipitation can support the climatic state or the vegta-

tion ecological type with greater annual evaporation over

desert region near the oasis. Namly, the effect of the oa-

sis makes the climatic state and the vegtation ecological

type better in the dersert region near the oasis than that

in the desert or Gobi.

The atmosphere over oasis is conservative at night and

active in the daytime. So the frequence of inverse hu-

midity is bigger than that of the oasis wind at night,

while is smaller in the daytime. Because of the influence

of horizontal spatial heterogeneity of water vapor caused

by distribution of oasis, sometimes the direction of water

vapor transfer is inconsistent with specific humidity gra-

dient, namely the counter-gradient transfer of water va-

por appears in a longer time.

The frequency of the counter-gradient transfer in the

daytime is bigger than that at night and it is much rela-

tive to the atmospheric stability. These features are rela-

tive to the counteraction of the turbulent mixing on the

horizontal heterogeneity of water vapor over oasis.

The effects of thermal circulation between oasis and

desert near it, which were found in the former studies

[12] on the Northwest China Arid Region, doesn’t ap-

pear in the Dunhuang experiment. The flowing are the

possible reasons. Firstly, Dunhuang oasis is smaller. Sec-

ondly, the thermal circulation is weaker. At last, its in-

fluence on atmospheic humidity over gobi or desert may

be offset by the strong turbulent mixing resulting from

extreme arid environment.

6. ACKNOWLEDGEMENTS

We thank Mr. Zeyong Hu, Xuhong Hou, Ping Hou and Yanjiang Nei et al.

for their assistance in the observation and data procession. We also

thank for the comments of the reviewer. This work was supported

by the National Natural Science Foundation of China (Grant No.

40830957) and the National Key Program for Basic Sciences – Re-

search on Formation Mechanism and Prediction Theory of Heavy

Climatic Disasters in China (Grant No.G1999040904-2).

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