High-resolution dual polarized micropulse lidar (MPL) observations have been used to investigate the diurnal evolution of atmospheric boundary layer (ABL) during winter (2008–2011) over Thiruvananthapuram (8.5°N, 77°E), a tropical coastal station located at southwest Peninsular India, adjoining the Arabian Sea. The lidar observations are compared with the boundary layer characteristics derived from concurrent balloon-borne radiosonde observations. This study shows that the mixed layer height over this coastal station generally increases from <300 m in the morning to ∼1500 m by the afternoon. Growth rate of the mixed layer height is rapid (∼350 m/hr) during 09–11 IST and slows down with time to <150 m/hr during 11–14 IST and <90 m/hr during 14–16 IST. Thermal internal boundary layer during the afternoon, caused by sea breeze circulation, extends up to ∼500 m altitude and is characterized by highly spherical aerosols, while a distinctly non-spherical aerosol layer appear above this altitude, in the return flow arising from the landmass.
High-resolution observations of downwelling spectral irradiance in the wavelength band of 350-2500 nm carried out using Spectroradiometer at Thiruvananthapuram (8.5°N, 77°E), a coastal station located in the southwest Indian Peninsula adjoining the Arabian Sea, have been used to investigate the instantaneous spectral aerosol direct radiative forcing efficiency (ISADRFE) and its variation with solar zenith angle. This study shows that, during the pre-monsoon season (March-May), magnitude of the ISADRFE maximizes at the wavelength of ~460 nm around noon, with mean values ranging between -270 to -330 Wm-2μm-1τ500 -1 at 15°<SZA<50°. The wavelength at which the ISADRFE peaks as well as the spectral width of the peak increases with solar zenith angle (SZA), especially at SZA>70°. About 47% of the to the total (broad band) instantaneous shortwave aerosol radiative forcing is contributed by the photosynthetically active radiation at solar zenith angle of 20°, while the corresponding percentage contribution decreases to ~40% at SZA of 75°. At solar zenith angle of 20°, ~8% of IADRFE is contributed by the wavelength bands above 1.5 μm, while at SZA of 75°, the corresponding contribution is ~17%.
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.
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.
Impact of atmospheric aerosols on the radiation budget of the Earth-atmosphere system and climate is well recognized.
The spatial distribution and the properties of atmospheric aerosols over the oceanic areas around the Indian subcontinent
during the Asian dry season (November-April period) are significantly governed by the transport of continental air from
the adjoining landmass. In the present study, we report the estimated spatial distribution of monthly mean aerosol direct
radiative forcing (ADRF) at the top-of-atmosphere (TOA), within the atmosphere, and at the earth's surface over the
Arabian Sea, Bay of Bengal and Equatorial Indian Ocean during the Asian dry season, based on the regional distribution
of aerosol optical depth (AOD) derived from NOAA14/16-AVHRR data during 1996-2003, and the model estimates of
the diurnal mean normalized aerosol radiative forcing (NADRF) which also compares fairly well with the earlier
observations reported over this region. The diurnal mean NADRF varies with latitude and Julian day. Its value at TOA
(for AOD at 550nm) is in the range -26.5 Wm-2AOD -1 to -29.5 Wm-2 AOD-1 while the corresponding value at surface is
in the range -77 to -95 Wm-2 AOD-1. In the Northern Hemisphere, ADRF is in the range -4 to -14 Wm-2 at TOA, -2 to -
42 Wm-2 at the surface and 8 to 28Wm-2 in the atmosphere. During the Asian dry season highest DMADRF is observed
in the northwest Bay of Bengal followed by the southeast Arabian Sea during the March-April period.
Conference Committee Involvement (1)
Lidar Remote Sensing for Environmental Monitoring XV
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