Precipitation patterns of central Elburz

Arch. Met. Geoph. Biokl., Set. B, 21, 215--232 (1973) �9 by Springer-Verlag 1973

551. 577.21 : 551. 577.3 (55)

University of Teheran, Irrigation and Reclamation Department, Laboratory of Meteorology and Climatology, Karadj, Iran

Precipitation Patterns of Central Elburz All Khalili

With 7 Figures

Received February 7, 1973

Summary

This study is based on the 10-years homogeneous precipitation datas of 19 stations during 1960--1969.

The geographical distribution of annual precipitation shows a general decrease from west to east and from north to south, with a zone of more than 1600 mm centrad at Bandar Pahlavi.

The rainfall regime of Caspian costal plains, comprises a principal maximum in autumn, a second maximum in March and a minimum in May or July. The regime of southern central Elburz is semi Mediterranean type. The mean annual precipitation increases with altitude in southern slopes of Elburz and decreases with altitude in the northern parts of the region.

The precipitation variability was studied by the coefficient of variation. In the coastal plains of the Caspian Sea, the coefficient of variation has a constant value of about 18 and is independent of mean annual precipitation. In the southern region of Elburz, this coefficient is much larger (varies from 27 to 83) and decreases with the increasing mean of annual precipitation. The correlation coefficients between monthly rainfall of any two stations of each sides of Elburz are generally high and significant, but they are not significant between one station from northern and the other from southern slope. This demonstrates that the precipitation in both sides of central Elburz are from different origins.

The mechanism of the intense rainfalls in south west of the Caspian Sea (Bandar Pahlavi) probably consists of a advection of Siberian cold air mass through the Caspian Sea, because of which the stability of air decreases and its specific humidity increases. Caspian maritime air mass, from one side, and the Continen- tal air of central Iran, from the other side, contact at the Mouth of the Sefid Rood valley (Bandar Pahlavi region)and cause the intensive rains.

216 A. Khalili

Zusammenfassung Die Niederschlagsverh~iltnisse von Zentral-Elburz

Der Untersuchung liegen 10j~ihrige homogene Niederschlagsreihen von 19 Statio- nen aus 1960--1969 zugrunde. Die Jahresmengen des Niederschlages nehmen im allgemeinen vom Westen gegen Osten und yon Norden gegen Siiden ab und erreichen ein Maximum tiber 1600 mm im Gebiet von Bandar Pahlavi. Der Jahresgang der Niederschlagsmengen weist in den Ktistenebenen am Kaspi- schen Meet ein Hauptmaximum im Herbst, ein sekund~ires Maximum im M~irz und ein Minimum im Mai oder Juli auf, im sfidlichen zentralen Elburz zeigt er seinen semimediterranen Typ. Die Jahresniederschlagsmengen nehmen auf den sfidlichen H~ingen des Elburz mit der H6he zu, in den n6rdlichen Teilen des Gebietes aber mit der H6he ab. Die Ver~inderlichkeit des Niederschlages wird dutch den Variationskoeffizienten charakterisiert. Dieser ist in den Ebenen am Kaspischen Meer mit ungefiihr 18 fiberall nahezu gleich und unabh~ingig yon der Jahresniederschlagsmenge. Im stidlichen Teil des Elburz ist er bedeutend grSf~er (27 bis 83) und nimmt mit zunehmender Niederschlagsmenge ab. Die Korrelationskoeffizienten zwischen den Monatsniederschlagsmengen zweier Stationen auf derselben Seite des Elburz sind im allgemeinen grot~ und signifikant, sie sind abet zwischen einer Station im Norden und einer am Stidhang des Elburz nicht signifikant. Dies zeigt, daf~ die Niederschl/ige auf beiden Seiten des Elburz yon verschiedenem Ursprung sind. Die intensiven Regenf~ille sfidwestlich vom Kaspischen Meer bei Bandar Pahlavi sind wahrscheinlich auf eine Advektion sibirischer Kaltluft fiber das Kaspische Meer zurfickzuffihren, wobei die Stabilit~t der Luft abnimmt und die spezifische Feuchte zunimmt. Maritime Luft yore Kaspischen Meer und kontinentale Luft aus dem zentralen Iran treffen sich im Mfindungsgebiet des Sefid-Rood-Tales (Gebiet yon Bandar Pahlavi) und verursachen dort die intensiven Niederschl~ige.

1. Introduction

The Elburz range is in fact a climatic frontier between the coastal plains of the Caspian regions and the great central p la teau of Iran. This frontier is one of the mos t interesting and rare natura l climatical borders of the wor ld , par t icular ly f rom rainfall and humid i ty poin t of view; the Elburz range has p roduced two different distinct regimes, we have tried in this article to show and investigate the effect of this m oun t a i nous range on the rainfall pat terns of the regions.

The principles of p r imary research works has been based on the statistical analysis of the precipi ta t ion data of 19 selected stations in the climatic per iod f rom 1960 to 1969. It seems here tha t the homogen i ty of per iod for all stations will ove rcome the shortness of period. The names of the selected stations are ment ioned on the Fig. 4 and Tab. 1. Fur thermore , for s tudying the variat ions of precip-

d7

Precipitation Patterns of Central Elburz 217

itation in regard to altitude, stations with lesser period data were also necessary.

All the above mentioned data belong to the Archive of Iranian meteorological department and hydrometeorological network of Ministry of Power and Electricity.

2. Orography of Central Elburz

The Elburz range is a part of great wrinkled mountainous area which extends in the west to the Ararat mountainous range, in the border of Soviet Armenia, and in the east extends to Hendukesh range in Afghanistan. The central part

49 dO 57 #2 #3

OROORAPHY OF CENTRAl EZBURZ

I

CASPIAN SEA

!i

J

M

8tatzbns w#h lOyea:s dsta 0 s/a/ions w/lh less than 7Oyear~ data

Uncle:sea/eve/ o-: :0o 7500-3d50 3dYO-qYO0 0~e:~508

~A b~r C)Meh~b~d 0 Kobotor Dor reh

75 0 15 dO ~5 80 75/:m Scale. 7.7,500000

Fig. I

of this great range forms an arc in the southern parts of the Caspian Sea and separates the coastal plains from the central plateau of Iran. Fig. 1 shows the orography of central Elburz and the location of stations cited on the text.

218 A. [(halili

The highest peak of the Elburz range is in the north-eastern part of Teheran (Damavand, with an altitude of about 5671 m, which is the highest peak of Iran). The average altitude of Elburz is about 3000--3500 m and there are many valleys in the Elburz range with the minimum 2000 m altitudes. In the South-Western part of the Caspian region this range is cut by the wide valley of the Sefid-Rood. The mouth of this valley to the north is very wide and its altitude is about 300 m in a very vast area. The northern and southern air masses are in contact through this valley.

The slope of the Elburz toward the Caspian Sea is very acute, while the southern regions of Elburz have a mederate slope. So, in the synoptic scales, the ridge line of the Elburz mountains can be considered as the border of the two southern and northern regions.

3. Distribution of Annual Precipitation

Fig. 2 shows the geographical distribution of annual precipitation on central Elburz region. The most important characters of isohyetal

Geographical distr ibution of mean annual precipitat ion on

central Elburz

i I

,%

i " \ x . ".

i " I

Fig. 2

C A $ P I

S E A

A N

u.s.s.,_ i

300 "--~ J / /

, [ "~j.5 / ,~o"

lines of this plate can be summarized as below: a) Existence of a high center of precipitation, centred on Bandar Pahlavi, in south- west of Caspian Sea, with a 1600 mm rainfall, b) Existence of a


secondary center, with 1000mm rainfall centred approximately near the higher parts of Elburz. c) Decreasing of the annual precipitation from north to south (7 ram/kin) is quite distinct and this decrease is also seen with a lower gradient from west to east (1 mm/km).

The study of this map brings up a question. "Why the principal maximum rainfall is not corresponded to the highest parts of Elburz, but is corresponded to the mouth of Sefid Rood which is a region with low altitude." This problem will be discussed later.

4. Rainfall Regimes

M. H. Gandji [5], A. H. Adle [1], and Ch. Djawadi [4] have classified the climatic zones of Iran in the Koppen's system. H. Sabeti (1958) has also attempted to classify the climate of this country in the Emberger's and Gaussin's systems. E. Khodayari [11] has also classified the meteorological stations of Iran according to E. De Martonne's "concept of aridity".

All these works indicate that the coastal plains of the Caspian Sea have a humid or subhumid climate and the southern parts of central Elburz are grouped as a dry or semi dry zone.

The seasonal distribution of rainfall has a fundamental r6te in the identification of these climatical areas. The monthly values of mean annual precipitations of these stations are written in Tab. 1.

By plotting the monthly average variation of the precipitation for all the 19 selected stations, we can group them from the point of view of their characteristics in the following three categories:

a) The rainfall regime of the Caspian coastal plains, comprises a principal maximum in autumn, and a second maximum in winter. Between these two, there is a relative minimum rainfall in spring. There is no true dry season in this region and dryest month of the year receives more than 20 mm of rainfall. In the eastern side of this region the secondary maximum has the same importance as the principal maximum. The zonal percentage mean of seasonal rainfall for all the seven stations of this region is as below:

Winter Spring Summer Autumn

24 ~ 14 ~ 22 ~ 40 ~

b) The rainfall regime of southern central Elburz is semi mediterranean type. This regime is characterized by a dry period of about

2 2 0 A . K h a l i l i

u

0

0

on

0

0

H

~ d d d ~ d

I I I t TM

0 ~ 0 0 ~ 0 ~ 0 ~ O 0

~ c r t ~

kO

~ ~ = ~ . . . . . . . . . . . o~ ~ " : : : : : : E Z ~ ",~ -= . . . . . . . . . . ~ , = . . . . ~ - "n ~

~ o ~ ~ ~ ~ ~ E =~ ~ = ~'> ~p ~ ~ ~


3- -6 months, which coincides mostly with the warm season of the year. The winter is the most rainy season, and the precipitation occurs mostly in the cold period of the year. The zonal percentage mean of seasonal rainfall for all 12 selected stations of the region are as below:


42 ~ 31 ~ 2 ~ 25 ~

c) The rainfall regime of Elburz higher altitudes in both two sides, are similar to above mentioned regime. For example we have given on the Tab. 1 the pluvial regime of the two typical stations at 2750 m and 2900 m altitude; "North Kandavan" at the northern slope, and "South Kandavan" at the southern slope of Elburz respectively (in Tehran-Chalus road). The zonal percentage mean of seasonal rainfall of 2 selected stations of the region is as below:


50 ~ 22 ~ 3 ~ 25 ~

5. Annual Rainfall Variability

In order to study the year to year variations of annual precipitation the coefficient of variation (CV) is computed for each station.

This coefficient has the following form:

CV= 100 -~=a 100 [X(PI-F)2]I/2 P F (N- 1)1/2

(1)

where Pi is annual precipitation of one year i, o is standard deviation of P, P is mean annual precipitation.

F. Hashemi [6] has used this formula for monthly precipitation of Tehran. Our calculation shows two different results as below:

a) In the southern region of central Elburz the coefficient of variation is relatively large and varies between 27 and 82 in different stations.

The variation of CV as a function of P is plotted on Fig. 3. It can be seen that, generally the coefficient of variation (CV) decreases while the mean annual precipitation increases. In order to study the degree of this decrease the coefficient of linear regression be-

Arch. Met. Geoph. BiokL B. Bd. 21, H. 2-3 15

222 A. Khalili

tween Pi and CVi is calculated for all the southern stations of central Elburz from following formula:

27 (CVt - CV) (Pi -~-) R = (N-l) ~ecrcv ' (2)

where ~ is mean annual rainfall of one station i (in mm), CVi

coefficient of variation of one station i, CV is the mean of CV~, P is

the zonal mean of Pi.

R is significant (1 percent) and equal to - -0 .55; the equation of linear regression is:

C V = - 8 . 5 �9 10 -a P + 56.58. (3)

The scatter diagram in Fig. 3 suggests that a linear fonction does

not explaine the relationship between CV and P. Therefore the following decreasing and non linear equations were used. The similar model has been suggested by Brooks and Carruthers [3]:

Yc = A + - - B

X B

Yc = A + x~ (4)

B Yc = A + - -

Yc = A e B X,

where Yc is the calculated coefficient of variation (CV) in o/% X is

mean annual precipitation (P) in ram, e =2.71828. All of these equations are transformed into linear form and the values of A and B are estimated by the least squares method.

The goodness of fit is measured by magnitude of correlation coefficient ~ , between the observed values of Y, and the corresponding values Yc, computed from the formula of the fitted curve.

~d = y 1 Z (Ycl _y~)2 x(y~-~)~ (5)

Y being the mean value of the observed Y's. The corresponding values of ~ for the Eqs. (4) are respectively: (0.42), (0.29), (0.47), (0.34) and (0.51). The best result is obtained by the least model and the equation of fitting curve is:

CV =57.339 e -~176176 (6)


The heavy line in Fig. 3 shows the graphical representation of this equation.

| Garmso.r

6C

sc u.

~c

3C

| DQmghm

| Abbaso

80

,~ Bonkuh

~ r z h r a n |

Varam ~ -~---------------------------~h S bad Takcstan '~.~. .

j | Karadi

-O.902P CV = 57,339 e

-2 - - . - - CV=-8,57.10 D +56,58

~meh

- ' ~ - - - . ~ ~ ~oo, " ~ ' ~ ..~ . ~

~ " ~ , ~

s o IoO ISO 20Q 2 5 0 n o 3 s o ~oo

Fig. 3. Variation coefficient CV as function of precipitation P in the southern region of central Elburz

b) In the coastal plains of Caspian Sea, the coefficient of variation is smaller and has approximately a constant value of about 15.6 + 2.5, and is independent of mean annual rainfall (Fig. 4). This

22

20 r ,'v "'. 18 uJ

o o

16

-~ 1/, >~

12

| R a m s a r

n a n d a r P u h i a v i G o r g a n | o w e :babr 5~

A s t o r a Baboisur

Rasht

Mean Qnr uot precip tation ram. 600 800 tO00 1200 a&OO l iO0 a8OO 2000

Fig. 4. Variation coefficient CV as function of precipitationPin the coastal plains of Caspian Sea

15"

224 A. Khalili

difference shows that the origin of those factors which cause rainfall in both sides, is different and the rainfall mechanism is much regular in the north in comparison with the south.

6. Annual Rainfall Variation in Regard to Altitude

Generally, the amount of precipitation increases with altitude and this increase appears to continue only up to a certain altitude, called the zone of maximum precipitation, above which there is a de- cline.

For verification of this law, concerning our interested regions, two groups of data were used and analysed separately. Both groupes of data were punched on cards and processed by computer.

a) The first group of data, included a certain annual mean of precipitations established for period (1960--1969).

According to J. Lomas [12] the following three models were used:

Y=ax2+bx+c

Y=a+bx +clogx (7)

Y=a+bx,

where Yc is calculated annual mean of precipitation in mm, x is altitude of station in meters. The best results in the present study were obtained by using the 3rd model.

o =

<

~- 300

ac L

P = 0,17h -44 ,67

- - i L( P o l o o r ' ~

/ i @ G ~azvin / I

@ Dr , .a ,~om'~

i ~ ~ - - " | Takest zn Varamin ~ .

' Abbasaic ad

1 ~ Damghal J

iooo 121)o ILOO IGI)O leoo 20oo 2leo i~oo 21~oo

Fig. 5. Scatter diagram and regression line of annual precipitation and altitude (1960--1969)

Precipi tat ion Pat terns of Centra l Elburz 225

For the southern slopes of Elburz the correlation coefficient is equal to +0.77 for a small sample (with n = 12) and is significant at 1 percent. Equation of simple linear regression for the Elburz southern slope is as below:

P = 0.17 h - 44.67. (8)

Fig. 5 shows the relationship between annual rainfall and altitude in the southern part of Elburz. The correlation coefficient for the northern slope was not significant and its sample was 'very small.

b) In the second group are included a network of rainfall stations, which are located across the two main roads cutting the Elburz mountains. One road connects Tehran to Amole, and the other

Tab le 2. Correlation between Mean Annual Rainfall (p) and Altitude (h) in the Northern Slope of Elburz (p in ram, h in meter)

Cases Period No. of stations Correlation coefficient Equation of regression

B 1969--70 8 - 0,62 P = - 0,22 h + 985,92 A 1966--68 6 -0,93:":" P= - 0 , 6 8 h + 1155,38 C 1 9 6 5 - 6 6 8 - 0 , 5 9 P = - 0 , 2 4 h + 874,01

connects Tehran to Chalus. These stations are selected in three groups, according to the their duration of records. The simple linear regression between annual rainfall and altitude of these stations have given the best results (model 3). The results of this calculation are summarized in Tab. 2 and 3.

Tab le 3. Correlation between Annual Rainfall (p) and Altitude (h) in the Southern Slopes of Elburz (p in ram, h in meter)

Cases Period No. of stations Correlation coefficient Equation of regression

B 1969--70 13 + 0,75** P = 0,24 h - 29,46 A 1966--68 14 + 0,84*': P = 0,26 h - 112,29 C 1965--66 18 +0,68** P=0 ,28 h - 87,28

* Significant at 5 percent

** Significant at 1 percent

The regression calculations for southern and northern slopes were made and are presented separately.

In the Figs. 6 and 7 scatter diagram and the regression line of the 3 cases are plotted. This calculation shows also, that in the southern

226 A. Khalili

slopes of Elburz, the rule of increasing precipitation with altitude is justified. Its gradient of increasing varies between 24- -28 mm per

800

60 (

40(3

20[

60(

20C

0

1000

800

600

400

200

P= 0,26h-112,29b

A : Poriod {1966-BB) | G ~ch sor

| ~ o d o k

?Oor+ lorJo ,z~tso I1,1) o rood I~,oo

I Ran&wan S,

;Lar Pc, Ic Dr

ALTITUDE m 2ADO 2200 2&OB 2600 2SO0 )OOO

I i j P=o.v.,-29.46 ,~Lo~ioo / i

@ , I I 4 ~ Soda, bal | Polo ~r

! i | JZarrin D, tsh t i

/ K, botar I I ' Dq rroh

| Garm: ar I I

i ooo t2oo tODD toDD 18oo zooo z2oo ZSOD Z~O0 2800 3ODD

P : 0 , 2 8 h - 87,28

C : Period ( 1965- 66]

L

Kabotar Darrch

I i

i

| I adok

F . J | E~Qchsar

..... ~ I | I

! | ~ . . . ~ Lar Poh ; A f j ~ h **.~,~*~ **'- ' | 162

: ") Hamand i e) ~ - , : | D c r n ~ v r a n d i

, bmcc an i | Ma~abad I

i : !

I

o J Lcfloon

2000 2200 2&O0 2600 ZS00 30DO

Fig. 6. Correlation between mean annual precipitation and altitude in the southern slopes of central Elburz

100 meters. With regard to scatter diagrams the zone of maximum precipitations would be at about 2000 to 2400 meters altitude.

120C

80C

e GOC

z

o

- / *OC

L 120G

u

100(

80[

60C <

= 40C

z 120C

~c

I00( =

<

80C x

60(

/*OC

20C


ElfIN U w ~ I1 u,I I

~ hol on Talclr

K ~ r ~ ~ Sh'rgah

i |

l

i

1 ', I o 2 0 0 &eo

L e l N o w b h a r

i | Ghor 'm Tatar

l

I

i pqal

| S h a h i

i J

200

"~i r~uw onw

l G h o r ( : |

i

i l

i

~ Katar At: ad l l i i

o Zoo

6 0 0 8oe w o o

( n U l l ~ O ( Cl =

n Tatar

| Shirgal"

| tar Uasht

a z o o a&oo

| Kal~ x Dasht

IOOO 1 2 0 0

| Kale Dasht

18OO ,

P =-0,68h +1155~38

A: Period (1966-68)

|

1800 Z 0 0 I

I I P =-0,22h +985,92 B ; Period (1969-?0)

I

[3al ~and Sat |

~LTITU~E m.

2 2 0 0 2&O0

LT~UOE In.

tO00 2000 2200 2&O0

P=-0,2Lh + 87/*,08 C: Period (1965-66)

&O0 600 880 tOOO t200 l&O0 1688 aSo0

| Dar )and ~LTITIInI~ m

2 0 0 8 22O0 2&O0

Fig. 7. C o r r e l a t i o n b e t w e e n m e a n a n n u a l p r e c i p i t a t i o n a n d a l t i t u d e i n t h e n o r t h e r n s l opes of c e n t r a l E l b u r z

228 A. Khalili

In the northern slopes of Elburz, the result is completely converse, all the correlation coefficient here are negative, and the annual precipitation decreases with altitude, and maximum rainfall occurs in the coastal plain and the gradient of decreasing rainfall varies between 22--68 mm of rain per 100 meters.

This result shows also that the origin of the major parts of the precipitation at both sides of Elburz mountains is different.

7. Local Inter Correlation

The result of the statistical analysis presented in items 5 and 6 suggests that there is a rather high correlation between rainfalls of the most stations on each sides of Elburz mountains. It was also interesting to check how closely the various stations were correlated. This calculation will permit us to find an idea on the origin of precipitation in the two sides of central Elburz mountains. For this purpose the methods of linear correlation were used, and the datas were divided in four groups, corresponding to four seasons of the year.

In one station nominated with No. i, the amount of X~.j represents the monthly rainfall of different months of ea& season (for example, January, February and March) and years (from 1960 to 1969). Since j varies from 1 to 30, and since the number of selected stations are 19, the number of i varies from 1 to 19. So we obtain a pseudo- matrix table (19 X 30) for each season of the year.

o

r 4.a

Various Stations

XI, 1 . . . X K , 1 . . . Xi, 1 . . . Xm, 1 . . . X19, t

X1,] . . . X K , J . . . Xf , . i . . . X m , j . . . X 1 9 , j

X 1 , a o �9 �9 �9 X K , 3 o �9 �9 �9 X i , 3 o �9 �9 �9 X m , a o . �9 �9 X 1 9 , a o

The coefficient of correlation R (K, m) between the elements of each two columns (m and k) is calculated by the following formula:

3 0

(x~, j - x k) (xm, j - x%) R (Xk, X,~) = i<

2 9 a x k " a x m (9)

where X a and X~ are mean, %a, and Ox,~ are standard deviation of mensual precipitation for stations No. k and m.


Therefore, we have obtained 117 correlation coefficients for each season. In our calculation circumstances, when R ~ 0.36 this coefficient is significant at 5 o/% and when R ~ 0.46, it is significant at 1%. For giving an idea in Tab. 4 we have offered the zonal mean of correlation coefficient between monthly precipitation of Tehran (called center of correlation) and all other stations.

Table 4. Zonal Mean o[ Correlation Coefficient between the Monthly Precipitation

Winter Spring Summer Autumn No. of stations

Center of Northern correlation of Elburz 0,51 0,55 0,68 0,55 7 Now Shahr Southern

of Elburz 0,31 0,05 0,07 0,10 12

Center of Northern correlation of Elburz 0,24 0,19 0,16 0,18 7 Tehran Southern

of Elburz 0,53 0,78 0,49 0,71 12

Also, the zonal mean of correlation coefficient between Now Shahr (called center of correlation) and all other stations is given in the same table.

The two regions are treated separately. The results obtained from different values of R, are of great importance, and their most important results are as below:

a) The stations can be divided to northern and southern stations from the importance, rigidity and significance of their (R), point of view.

b) When the center of correlation is Now Shahr, there is a positive, large, and significant correlation between monthly precipitation of all stations in general, while (R) is significant for all the southern stations. The average zonal correlation coefficient, in the north region, is larger in summer (Tab. 4). This correlation exists to a 304 km distance, straightly from center of correlation to Astara, and also extends to a 260 km distance in the east to Gorgan. In other words the correlation extends parallel to Elburz range and coastal plains.

Conversely, when the center of correlation is selected as Tehran, the correlation coefficient between Tehran and all other southern stations is significant (except some special cases). The distance of two stations with significant correlation is on an average 120 km, and in one case this distance increases to 262 kin.

230 A. Khalili

For the northern stations the value of R is insignificant. In both cases, by crossing the ridge line of Elburz, the value of R becomes insignificant even for very near stations. Therefore, the origin of the precipitation in both sides of Elburz is quite different except in winter time, where the origin of the precipitation can be considered the same.

c) The coefficient of correlation between monthly precipitation of Now Shahr and Lar Plour (the only high station of Elburz in this category) is equal to 0.58 in winter and is insignificant in the rest of the year.

Conversely the correlation coefficient between Tehran and Lar Plour is significant in all the times, except in winter. This problem could be solved by accepting that the higher altitudes of Elburz are affected by the north westerly flow of polar air masses.

The above mentioned results are in accordance with research works of Khalili [9, 10] and Khodayari [11].

The mediterranean systems also, affect the whole country during the cold seasons, specially winter, and cause: equally rainfall at both sides of Elburz mountains. That is why the zonal mean correlation coefficient concerning this season is larger than in other season (Tab. 4).

8. Discussion on the Origin of the Caspian Coastal Precipitations

After Birot and Dresh [2], the origin of the Caspian and Black Sea coastal precipitation are known to be due to contact of Indian Ocean Monsoons and polar air masses. Khalili [7] in the case of the rainfall of the coastal precipitations of the Black Sea, and Khodayari [11] in the case of the Caspian Sea precipitations, have shown that this precipitations are due to the advection of the cold polar air masses from the relative warm beds and surface of the sea.

Khalili [8] has shown that besides the above mentioned phenomenon, these precipitations are due to asendance of air masses on local front surface. This local front is formed between the maritime air mass in the north and continental air mass of the south. These two air masses are in contact in the mouth of Sefid Rood. This front works as a high mountain, against the northern air flow.

By compiling the above mentioned different ideas, we could offer the below hypothesis, for explaining, the rainfall of this region and


specially for explaining the annual m a x i m u m precipitation. From the beginning of au tumn the Siberian air mass becomes too cold, and the high pressure center (anticyclone) is formed. This anticyclone persists for 6 months of the year, and one of the branches of the air f low of this center crosses the Caspian Sea f rom NE to SW. Due to relative warmness of the Caspian Sea water, in this period of the year the advection of this air mass results in two phenomena: a) Rapid evaporat ion of water and increasing the specific humidity, b) Rapid decrease of air stability.

This two phenomena are intensified and aggravated if the difference between the air and water temperature increases. This phenomenon happens in au tumn where the precipitation is maximum. This humid and instable air mass, when reaches to the southern parts of Caspian Sea, ascends on the mountains or on the local front surface, and causes intensive rainfall. Since the path of air mass to SW of the Caspian Sea is larger, there is more precipitation in that region (e. g. Bandar Pahlavi). This mechanism causes rainy low-clouds, which could not reach to the higher elevations. Tha t is why, decreasing precipitation with altitude is marked at pa ragraph 6.

References

1. Adle, A.H.: Climats de l'Iran. Univ. of Teheran, No. 584, pp. 98--100, 121-- 131 (1960).

2. Birot et Dresch: La Mediterran~e et la Moyen Orient. Tome II, pp. 160--163. Presse Univ. Paris, 1956.

3. Brooks, C. E.P., and N. Carruthers: Handbook of Statistical Methods in Meteorology. Pp. 286, 303. London: Her Majesty's Stationery Office, 1953.

4. Djawadi, Ch.: Distribution climatiques en Iran. Monographie M~teorol. Nat. Paris, 1966.

5. Gandji, H.: Distribution climatique en Iran. Rev. de fac. des lettres, Univ. de T~h&an, No. 1, 3, 3--7 (1954).

6. Hashemi, F.: A Statistical Analysis of the Annual Monthly and Daily Pre- cipitation of Teheran. Iranian Meteorol. Dept., pp. 5--6, Teheran, 1969.

7. Khalili, A.: Contribution ~t l'etude statistique et dynamique des pr~cipitations atmosph&iques au sud est de la Mer Noire. Pp. 143--146. Thesis presented to Fac. of Sci. Univ. of Paris, 1965.

8. Khalili, A.: On the Origine of Southern Caspian Sea Rainfall. Pub. of Iranian Met. Dept., March 1971.

9. Khalili, A.: Rainfall Regimes of Central Elburz. Irrigation and Drainage Seminar, Ministry of Water and Electricity, Teheran 1970.

10. Khalili, A.: Elburz, grande fronti~re climatique de l'Iran, Journal de Tdhdran, No. 10519, 1970.

232 A. Khalili: Precipitation Patterns of Central Elburz

11. Khodayari, E.: Contribution fi l'~tude des precipitations atmosph~riques au sud-ouest de la met Caspienne. Thesis presented to Fac. of Sci. Univ. of Paris. Pp. 82--92 (1970).

12. Lomas, J.: Pr~vision du rendement des r~coltes de bld en Iran ~ partir de donn~es pluviom~triques. Bulletin de I'OMM 21, 6--11 (1972).

Author's address: Dr. All Khalili, Faculty of Agriculture, Department of Irrigation and Reclamation, Laboratory of Meteorology and Climatology, Karadj, Iran.

Documents

Precipitation patterns of central Elburz