  In order to study the relation between the components of atmospheric
circulation and summer precipitation over the Iranian plateau, daily rainfall
data of synoptic stations of Iran were obtained from IRIMO for the period 1970
to 2003. Composite maps of different components such as pressure levels, vector
wind, U and V winds, vertical velocity, Specific humidity, OLR, streamlines,
vorticity, and the temperature was drawn and analyzed. The results showed that
the summer circulation of the atmosphere over South West Asia after a sudden
change from late May to early June was established in its normal position.
Different new pressure systems such as Zagros Trough were identified over the
surface and the Turkmenistan Anticyclone, the quasi-stationary trough of east
Turkey at 700 hPa level. During the establishment of Subtropical High Pressure
in the middle and upper troposphere over Iran, the summer circulation of
southwest Asia reached its highest intensity and expansion and continued to
mid-August. On the other hand, the establishment of the Turkmenistan
Anticyclone and Pakistan Low caused the 120 days winds of Sistan, and the
development of High pressure over the Arabian Peninsula and Zagros Trough
resulted in the Shomal wind. The main region of summer precipitation was
identified as a triangular area to the east of 58.30 E and south of 28.30 N.
Synoptic patterns responsible for the summer precipitation of the area are
grouped into four categories as Iran low pressure, Monsoon Depression, Pakistan
low pressure, and local convective cells. Iran low pressure was responsible for
more than 65 percent of summer rainfall, while the Monsoon and Pakistan lows
produced only 15 percent of the rain. Most of the moisture of rain generating
systems came from the west of Oman sea and the northwest Arabian sea via the
southwesterly flow below 850hPa level.
