Continental Tropical Convergence Zone (CTCZ) Programme under the Indian Climate Research Programme (ICRP)

Main Focus : Understanding monsoon variability on timescales ranging from sub-seasonal to inter annual and decadal, and its impact on critical national resources Observational Modeling Data Analysis Inter-Agency & Multi-Institutional

ICRP Experiments Bay of Bengal Monsoon Experiment (BOBMEX): Arabian Sea Monsoon Experiment (ARMEX): Jun-Aug 2002, Mar - June 2003, April-May 2005 BOBMEX – Air-sea Coupling Rapid SST Recovery Northward Propagation North-South gradients (Bhat et al, BAMS, 2001) ARMEX – , Intense Rainfall Events on WC 2.Warm pool build up & Collapse 3.Monsoon Onset over Kerala Mausam Jan (Special Issue) July 26, 2005 Mumbai ~940 mm 1998

Indian Oceanographic Research Vessel ~110 m long 31 Scientists + 65 Crew ORV Sagar Kanya

Maximum Precipitation Region of highest clouding in the entire region Cloud systems - Cut off solar radiation Wind speed generally increases SST decreases How SST is maintained above the threshold level (28 o C) Top fresh water layer – shallow mixed layer (solar penetration, Debasis, et al. GRL, 2002) OLR, Rainfall (GPCP) (shaded) (cont.) June-Sept.: North Bay SST- maintenance

(Source: NIOT Buoy data) ARMEX : 1. Intense Rainfall Events on West Coast 2. Warm pool dynamics Year : 1998

Differences in the Ocean Mixed Layer North Bay and Arabian Sea (AS) Density Tempsalinity (ARMEX & BOBMEX)

BAY weak winds High RH – lower LHF & NLW AS Winds stronger Lower RH – High LHF

Continental Tropical Convergence Zone (CTCZ) Programme under the Indian Climate Research Programme (ICRP) Pilot:2008 Main expt from 2009

Fig 1a: INSAT picture of the CTCZ on an acttive monsoon day:7 August 2007

Fig 1b: Winds at 1.5 km above sea-level on 7 August 07

Figure 2 : Average June-September rainfall; the approximate limits of the monsoon zone indicated by red dashed lines

The major objective the CTCZ programme of the Indian Climate Research Programme is to understand the mechanisms leading to space-time variation of the CTCZ and the embedded monsoon disturbances during the summer monsoon.

A multiscale approach with study of the major interacting scales viz. planetary, regional, synoptic and mesoscale is essential. For adequate observations on all important time-scales from diurnal to intraseasonal and interannual, a multi-year programme is required.

Science foci and objectives: A. Phenomena a: Links of monsoon variability with the convection over the surrounding oceans

to understand the underlying mechanisms of the three basic elements of the variation of convection over the ocean on synoptic and larger scales viz. genesis of synoptic scale cloud systems over the warm oceans around the subcontinent fluctuations in the intensity of the TCZ propagations of the synoptic scale systems and of the TCZ emanating from the oceanic regions (iv) the variation of convection over the critical regions of the (i) Bay of Bengal and eastern Arabian Sea and (ii) equatorial Indian Ocean: EEIO, WEIO, on intraseasonal and interannual scales

Fig. 3 Variation of the daily rainfall over central India during June-September 1972,75 Active spells, weak spells and monsoon breaks

b: Important phases of the monsoon Unravelling the factors that determine the life-span of active and weak spells of the CTCZ and Understanding the role of land hydrological feedbacks and aerosols in the transitions between active and weak spells and hence the life-span of these spells.

B: Process studies Under the CTCZ programme, detailed studies of such processes specific to the CTCZ such as the atmosphere- hydrosphere biosphere feedbacks (in which land surface processes, and aerosols will be important) are planned. The focus will be on features of the variation/ events (such as active spells and breaks, propagation of systems) in which these processes play an important role. Special observational sites will be located in regions which are considered critical for understanding such processes and events.

Fig.4. Regions over which variation of soil moisture has an impact on rainfall (derived from 12 atmospheric general circulation models participating in GLACE : the Global Land Atmosphere Coupling Experiment) Hot Spots

(ii): Aerosols : role in the variability of the monsoon Elucidation of the space-time variation of aerosols particularly over regions which are considered to be critical for impact on the monsoon, of aerosol sources and sinks, aerosol life cycles in clouds, impact of aerosols on atmospheric radiation, is one of the objectives of the CTCZ

(iii) Interaction between different scales of convection In the observational programme, a major goal will be the elucidation of the nature of the cloud systems in the CTCZ. The microphysics of warm and cold clouds, the characteristics of the meso and synoptic scale cloud systems in the CTCZ will be studied. Special observations aimed at elucidating the nature and variation of cloud condensation nuclei and aerosols (natural and anthropogenic) over the land and some oceanic regions, and cloud microphysics and aerosol- cloud-precipitation interactions are planned.