Tropical Cirrus in Megha-Tropiques Scenario

K. Parameswaran , K. Rajeev and C. Suresh Raju

Space Physics Laboratory, Vikram Sarabhai Space Centre,

Trivandrum 695 022

CIRRUS

They are thin clouds, sometimes invisible or sub-visible, forming at very high altitudes consisting of ice crystals.

In optical probing they appear as region of enhanced Backscatter Ratio (R) where the backscattered radiation under go significant depolarization ()

Two mechanisms responsible for Cirrus formation are the in situ nucleation of condensable vapours and Out flow from Convective Anvils

Cirrus from Convective Anvils

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.

Based on their optical depth Cirrus are classified in to three typesSub visual (SVC) for c<0.03 62%Thin Cirrus (TC) for 0.03< c <0.3 30%Dense Cirrus(DC) for c >0.3 8%Most 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

• Tropical Cirrus generally occurs just below the cold point of tropical tropopause.

18 January 1999

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

23 November 1998

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

On some days Cirrus is observed with multi-layer structure

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

Figure shows typical days on which the cloud is weak and intermittent

The cloud Depolarization also show significant Spatial/Temporal variations

Thin Cirrus Occur more frequently than thick cirrus !

Depolarization is large in thin cirrus than in thick clouds!

The cloud optical depth shows significant temporal variation over the night

Dependence of Cloud strength and depth on cloud altitude

•SVC•TC•DC

Stacked bar diagram showing the month-to-month variation of the percentage occurrence of SVC, TC and DC

Seasonal dependence of cirrus

ecbctc r

bahhIWC )(

Where a=3.44E-3 m2g-1

and b=2.43m2g-1

The Ice Water Content in Cirrus is related to Cloud optical depth as

In Tropics Cirrus clouds Occur at an altitude where the temperature is 605 C

Cirrus Optical Depth (in visible) Increases with increase in cloud Temperature

4cc

4c

4s T)C/D(exp)TT(F

The IR Cirrus radiative forcing is Given by

-Stephan Const.C– Ratio of IR to visible OD(~2),D- the diffusivity factor(=1.66)

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 .

(a) 10.5-12.5 m (b) 5.7-7.1 m

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.

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 .

In TIR maximum BT will be form the surface or low level clouds. It corresponds to clear sky or low level cloudsPresence 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 altitudeThere will not be any contribution to BT in WV channel from surface (Clear sky is easily masked)TbWV will be more weighted to mid troposphereTbWV along with TbTIR can be used for classifying different cloud types

 

Cloud type DT= TbTIR- TbWV TbTIR TbWV

Clear sky 10<DT<50 >270 >240

Low cloud 10<DT<50 >270 >240

Mid-up Cloud (no cirrus)

DT< 19 240<Tb<270 >240

Deep Convection(no cirrus)

DT< 6 210<Tb<240 <240

Very Deep Convection (no cirrus)

DT< 6 Tb < 210 <240

Thin cirrus or detached cloud top

DT< 19 Tb<240 <240

Semi Transperent cirrus 19<DT<50 Tb>270 <240

Mid-up Cloud(with ST cirrus)

19<DT<50 240<Tb<270 <240

Deep Convection (with ST cirrus)

19<DT<50 210<Tb<240 <240

Very Deep Convection (with ST cirrus)

19<DT<50 Tb < 210 <240

Cloud Classification based on Brightness Temperature in TIR and WV

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

Formation and Persistence of Tropical Cirrus is found to be strongly associated with the altitude structure of Tropospheric Turbulence

Contour plots of backscatter ratios for co-polarized and cross-

polarized components along with the vertical wind velocity

Plots of backscatter ratio (Rp and Rs) and altitude profile of turbulent kinetic energy () and eddy diffusion coefficient (Km) on 19 January 1999

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

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

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

The Cloud Strength is positively correlated with the Turbulent Kinetic Energy dissipation rate

•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]

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. .