Simultaneous occurrence of two bands of inter tropical convergence zone (ITCZ) on either side of the equator, generally
known as double-ITCZ (DITCZ), over the Equatorial Indian Ocean (EIO) is investigated using the cloud characteristics
derived from NOAA-14/16-AVHRR data (1996-2003), the monthly mean cloud characteristics obtained from the
International Satellite Cloud Climatology Project (ISCCP-D2) (1984-2004) and the monthly mean outgoing long wave
radiation (OLR: 1974-2004) data obtained from NOAA. A well discernible signature of DITCZ could be observed over
the EIO in terms of total as well as high cloud amount and OLR. The doubling of ITCZ occurs mainly in the western part
of EIO between 50°E and 80°E. The frequency of occurrence of DITCZ over the Indian Ocean is largest in November
(percentage of occurrence ~85%) and December (~62%), which is significantly larger than that reported from earlier
studies. The most preferred latitude for the northern and southern bands of DITCZ is ~5°N and ~7.5°S respectively in
November and ~5°N and ~10°S in December. The amplitude of the DITCZ, defined as the difference between the total
cloud fraction in the equatorial region of minimum cloudiness and that in the respective bands of the DITCZ, is 0.05 to
0.25 for the southern branch and 0.05 to 0.15 for the northern branch. The corresponding amplitude in terms of OLR is
10Wm-2 to 15Wm-2 in the southern band and 5Wm-2 to 10Wm-2 in the northern band.
Quantitative estimates of the spatio-temporal variations in deep convective events over the Indian subcontinent, Arabian
Sea, Bay of Bengal, and tropical Indian Ocean are carried out using the data obtained from Advanced Very High
Resolution Radiometer (AVHRR) onboard NOAA-14 and NOAA-16 during the period 1996-2003. Pixels having
thermal IR brightness temperature (BT) less than 245K are considered as high altitude clouds and those having BT<220
K are considered as very high altitude clouds. Very deep convective clouds are observed over north Bay of Bengal
during the Asian summer monsoon season when the mean cloud top temperature reaches as low as 190K. Over the Head
Bay of Bengal (HBoB) from June to September, more than 50% of the observed clouds are deep convective type and
more than half of these deep convective clouds are very deep convective clouds. Histogram analysis of the cloud top
temperatures during this period shows that over HBoB the most prominent cloud top temperature of the deep convective
clouds is ~205K over the HBoB while that over southeast Arabian Sea (SEAS) is ~220K. This indicates that most
probably the cloud top altitude over HBoB is ~2 km larger than that over SEAS during the Asian summer monsoon
period. Another remarkable feature observed during the Asian summer monsoon period is the significantly low values of
deep convective clouds observed over the south Bay of Bengal close to Srilanka, which appears as a large pool of
reduced cloud amount surrounded by regions of large-scale deep convection. Over both SEAS and HBoB, the total, deep
convective and very deep convective cloud amounts as well as their corresponding cloud top temperatures (or the
altitude of the cloud top) undergo large seasonal variations, while such variations are less prominent over the eastern
equatorial Indian Ocean.
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