Association of Tropical Cirrus Formation With the Structure and Dynamics of the Upper Troposphere

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Tropical Cirrus in Megha-Tropiques Scenario
K. Parameswaran , K. Rajeev and C. Suresh Raju
Space Physics Laboratory, Vikram Sarabhai Space
Centre, Trivandrum 695 022
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CIRRUS
They are thin clouds, sometimes invisible or
sub-visible, forming at very high altitudes
consisting of ice crystals.
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In optical probing they appear as region of
enhanced Backscatter Ratio (R) where the
backscattered radiation under go significant
depolarization (?)
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Two mechanisms responsible for Cirrus formation
are the in situ nucleation of condensable vapours
and Out flow from Convective Anvils
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Cirrus from Convective Anvils
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Lidars are used to study the physical properties
of these cirrus and their temporal evolution.
But being positioned at a particular location
it cannot provide information of their spatial
coverage and structure. The temporal evolution
observed in lidar can also be due to spatial
in-homogeneities and movement of clouds because
of horizontal wind. This can be resolved to
some extent by using complimentary space borne
measurements. This can also be used to study the
cirrus IR radiative forcing. Here we examine
the physical properties of tropical cirrus
observed from lidar and possibilities of using
Megha-Tropiques data to compliment this.
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Based on their optical depth Cirrus are
classified in to three typesSub visual (SVC) for
?clt0.03 62Thin Cirrus
(TC) for 0.03lt ?c lt0.3 30Dense
Cirrus(DC) for ?c gt0.3 8Most favourable
altitude for Cirrus occurrence is 14 to 16 km

• Frequency of occurrence of clouds with different
  optical depths

• Ferquecy of occurrence of cloud mean
  altitude

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Tropical Cirrus generally occurs just below the
cold point of tropical tropopause.
18 January 1999
23 November 1998

• At times the thin Cirrus forms in the lower
  stratoshere just above the tropopause also.

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Figure shows the separation of cloud top (hct)
and cloud mean altitude (hm1) from tropopause.
Most of the cases the cloud occurs below the
tropopause
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On some days Cirrus is observed with multi-layer
structure
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On some days the cloud is strong and persists
throughout the night while on some other days it
is weak and intermittent

• Figure shows typical days on which the cloud is
  continuous

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Figure shows typical days on which the cloud is
weak and intermittent
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The cloud Depolarization also show significant
Spatial/Temporal variations
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Depolarization is large in thin cirrus than in
thick clouds!
Thin Cirrus Occur more frequently than thick
cirrus !
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The cloud optical depth shows significant
temporal variation over the night
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Dependence of Cloud strength and depth on cloud
altitude

• SVC
• TC
• DC

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Seasonal dependence of cirrus
Stacked bar diagram showing the
month-to-month variation of the percentage
occurrence of SVC, TC and DC
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The Ice Water Content in Cirrus is related to
Cloud optical depth as
Where a3.44E-3 m2g-1 and b2.43m2g-1

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In Tropics Cirrus clouds Occur at an altitude
where the temperature is 60?5 C
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Cirrus Optical Depth (in visible) Increases with
increase in cloud Temperature
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The IR Cirrus radiative forcing is Given by
?-Stephan Const. C Ratio of IR to visible
OD(2), D- the diffusivity factor(1.66)
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Very High Resolution Radiometer (VHRR) data from
satellites can be used to study the spatial
extent of clouds . The radiance measured in the
IR channel can be used to derive the brightness
temperature (Tb) which in turn can be used to
identify cloud type. This potential can be used
for Tropical cirrus Data from Geo-stationary /
Orbiting satellites will be very useful in this
context because they can provide data from same
location at close time intervals.
(a) 10.5-12.5 ?m
(b) 5.7-7.1 ?m
The un-navigated images of brightness temperature
from KALPANA-1 in TIR (TbTIR ) (a) and in WV
(TbWV ) (b) respectively at 1032 GMT, on 16
September 2003 .
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Brightness temperature from KALPANA-1 in TIR
(TbTIR in Kelvin) (a) and in WV (TbWV in Kelvin)
(b) respectively at 1032 GMT after pixel
navigation for 6 September 2003 .
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In TIR maximum BT will be form the surface or
low level clouds. It corresponds to clear sky or
low level clouds Presence of other clouds(mid,
upper, cirrus or a combination) reduces the
observed BT in TIR. This can be user to detect
cloudy regions but not precisely the
altitude There will not be any contribution to BT
in WV channel from surface (Clear sky is easily
masked) TbWV will be more weighted to mid
troposphere TbWV along with TbTIR can be used for
classifying different cloud types
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Cloud Classification based on Brightness
Temperature in TIR and WV
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Thin semi-transparent cirrus clouds observed on
16 September 2003 classified to be toped above
very deep convective, deep convective, mid-upper
and low clouds or clear sky at 1032 GMT. The
colour code used is also shown
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Formation and Persistence of Tropical Cirrus is
found to be strongly associated with the altitude
structure of Tropospheric Turbulence
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Contour plots of backscatter ratios for
co-polarized and cross-polarized components along
with the vertical wind velocity
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Plots of backscatter ratio (Rp and Rs) and
altitude profile of turbulent kinetic energy (?)
and eddy diffusion coefficient (Km) on 19 January
1999
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Altitude profiles of TKE dissipation rates, ?
(m2s-3), on 19, 20 and 29 January 1999 when the
cirrus was strong and persistent and on 3, 17 and
22 February 1999 when cirrus was weak and
intermittent and on 1, 12 and 13 February 1999
when cirrus is totally absent. The arrowhead on
left Y-axis indicates the tropopause level (htp).
Mean cloud top (hct) and cloud base (hcb) are
indicated by the two heads on the right Y-axis
for those nights in which cirrus cloud was
present
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Contour plots of ? and Km showing the day-to-day
variability of turbulent kinetic energy
dissipation and vertical eddy diffusion
coefficient in the altitude region 8 to 20 km
during the period 18 January 1999 to 5 March
1999. The frequency of occurrence of cirrus is
superposed on Km contour to illustrate the
correspondence. The lower panel shows the mean
cloud strength observed on the nights during the
above period
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In January when the altitude gradient of vertical
eddy diffusion coefficient is sharp the cirrus
cloud is strong and persistent and in February
when it is weak the cloud is weak and
intermittent if not absent
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The Cloud Strength is positively correlated with
the Turbulent Kinetic Energy dissipation rate
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Coordinated observations of Megh-Tropiques along
with other satellites( like METEOSAT, INSAT,
KALPANA etc.), SBL, Network of Lidars and ST
radar can be used for the study of following
aspects relating to Tropical Cirrus. .

• Type of Cirrus associated with the two suggested
  formation mechanisms for Tropical Cirrus (MT,
  KALPANA, Lidar)
• Horizontal extent of cirrus cover and their
  homogeneity (Network of lidar, SBL,MT, KALPANA)
• Radiative forcing of tropical cirrus in the IR
  window ScaRaB SC4- IR and cirrus optical
  depth from lidar
• Total cirrus IR forcing and Optical depth ScaRaB
  Total IR, cirrus optical depth from lidar
• Cloud microphysics IWC from MT-MADRAS, optical
  depth from Lidar
• Cloud albedo and optical depth ScaRaB Sc1
  radiance, Cirrus optical depth
• Association Cirrus (formation and persistence)
  with turbulence Lidar, MST/ST Radars, MT
  (SAPHIR) water vapour, GPS water vapour

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Thank you