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Ann. Geophys., 19, 873-882, 2001
www.ann-geophys.net/19/873/2001/
© European Geosciences Union 2001
MST radar and polarization lidar observations of tropical cirrus
Y. Bhavani Kumar, V. S. Siva Kumar, A. R. Jain, and P. B. Rao
National MST Radar Facility, Gadanki 517 112, India
Abstract. Significant gaps in our
understanding of global cirrus effects on the climate system involve the role
of frequently occurring tropical cirrus. Much of the cirrus in the atmosphere
is largely due to frequent cumulus and convective activity in the tropics. In
the Indian sub-tropical region, the deep convective activity is very prominent
from April to December, which is a favorable period for the formation of deep
cumulus clouds. The fibrous anvils of these clouds, laden with ice crystals,
are one of the source mechanisms for much of the cirrus in the atmosphere. In
the present study, several passages of tropical cirrus were investigated by
simultaneously operating MST radar and a co-located polarization lidar at the
National MST Radar Facility (NMRF), Gadanki (13.45° N, 79.18° E), India to
understand its structure, the background wind field and the microphysics at the
cloud boundaries. The lidar system used is capable of measuring the degree of
depolarization in the laser backscatter. It has identified several different
cirrus structures with a peak linear depolarization ratio (LDR) in the range of
0.1 to 0.32. Simultaneous observations of tropical cirrus by the VHF Doppler
radar indicated a clear enhancement of reflectivity detected in the vicinity of
the cloud boundaries, as revealed by the lidar and are strongly dependent on
observed cloud LDR. An inter-comparison of radar reflectivity observed for
vertical and oblique beams reveals that the radar-enhanced reflectivity at the
cloud boundaries is also accompanied by significant aspect sensitivity. These
observations indicate the presence of anisotropic turbulence at the cloud
boundaries. Radar velocity measurements show that boundaries of cirrus are
associated with enhanced horizontal winds, significant vertical shear in the
horizontal winds and reduced vertical velocity. Therefore, these measurements
indicate that a circulation at the cloud boundaries suggest an entrainment
taking place close to these levels. The analysis of simultaneous lidar and MST
Radar observations can thus yield valuable information on the structure and
dynamics of the cirrus, specifically near the boundaries of such clouds.
Key words. Atmospheric composition
and structure (cloud physics and chemistry; instruments and technique) -
Meteorology and atmospheric dynamics (tropical meteorology)
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