ESSO - Indian National Centre for Ocean Information Services

(An Autonomous Body under the Ministry of Earth Sciences, Govt. of India)

Science Plan

Science Background
The large-scale rainfall over the Indian region during the summer monsoon is associated with a continental tropical convergence zone (CTCZ), which appears as a prominent zonal cloud band in satellite imagery (e.g. Fig 1.1). The major phases of the CTCZ are (i) the spring to summer transition which occurs in the onset phase, and (ii) the peak monsoon months of July and August, when the CTCZ fluctuates primarily in the core monsoon zone (Fig 1.2). An important feature of the intraseasonal variation of the CTCZ is the fluctuation between active and weak spells (e.g. Fig. 1.3). The onset phase involves northward propagation of the TCZ from the equatorial Indian Ocean onto the Indian subcontinent. The advance of the monsoon over the northern plains also involves westward propagation of synoptic-scale systems from the Bay of Bengal. There is considerable variation from year to year in the dates of onset and advance of the monsoon. The timing, duration and intensity of the active and weak spells/breaks in July-August also vary from year to year. Revival from breaks occurs with propagation of cloud systems emanating from the Bay of Bengal or the equatorial Indian Ocean onto the core monsoon zone.

FIG 1.1 : FIG 1.2 : FIG 1.3 :

Seasonal evolution and spatial distribution of monsoon rainfall are influenced by land-atmosphere-ocean-biosphere interactions. The Indian monsoon zone is one of the major 'hot spots' where soil-moisture variations have a significant impact on the precipitation on the synoptic timescale. Detailed studies of the atmosphere-land-hydrosphere-biosphere feedbacks are needed with special observational sites located in regions considered critical for understanding these processes and events, such as break-active-break monsoon transitions, in which they are expected to play an important role. The observations on the heat and water balance in selected watersheds/river basins are to be used for testing a variety of land-vegetation-hydrology models employed in atmospheric general circulation models.
In the large-scale cloud band characterizing the CTCZ, cloud systems of synoptic and meso-scale are generally embedded. Such systems, with a life cycle of a few days, produce heavy rainfall over the Indian region. Various facets of the meso-scale and synoptic scale systems, such as the preferred location of formation and movement of synoptic scale systems, their intensification, propagation, interaction with the land surface, duration, diurnal variation, etc., need to be understood. The microphysical and dynamical characteristics of cloud clusters associated with these systems have not been studied. The fractional area covered by convective and stratiform clouds, estimation of the cumulus mass flux, and the mesoscale mass fluxes are essential to ascertain the deficiencies of the representation of cloud dynamics in numerical models.
The atmospheric boundary layer (ABL) is the lowest layer of the atmosphere (typically less than 2 km deep) and forms the interface between the surface (be it land or ocean) and the rest of the atmosphere. The ABL plays an important role in the variability of convection over land and ocean. Properties of the ABL determine convective cloud properties and are required for the representation of cloud processes in global atmospheric models, i.e., cloud parameterization schemes. Over land, evapotranspiration is an important component of the hydrological cycle and the land-atmosphere-vegetation interactions are critically dependent on the ABL. There is a large variation in the surface characteristics across the monsoon zone. The largest amplitude of diurnal cycle in the troposphere occurs within ABL and more specifically in the surface layer. Soil temperature and moisture, and vegetation parameters evolve with the progress of the monsoon. We expect ABL properties to respond to these changes and also to vary between active and break monsoon conditions. The CTCZ region is often a low-wind regime even within a monsoon season. Formulations in fluxes under low-wind-speed regimes under unstable and stable conditions need to be established and validated. Thus, determining the surface fluxes under the entire range of wind speeds that prevail during the monsoon, including the low-wind regime, is an outstanding problem that needs to be undertaken during the CTCZ programme.
Recent studies have shown that aerosols can cause substantial alteration in the energy balance of the lower atmosphere and at the earth's surface, thus modulating the hydrological cycle. Observations of the space-time variation of aerosols particularly over regions which are considered to be critical for impact on the monsoon, aerosol life cycles in clouds and impact of aerosols on atmospheric radiation, are needed. The CTCZ programme offers a unique opportunity to address several challenging scientific issues related to the interactions of the monsoon with the ABL, ocean-atmosphere coupling and land-surface/cloud/aerosol processes.

Science Foci and Main Objective
The variability of the monsoon associated with the variability with the CTCZ is a highly complex phenomenon where local, regional and global scale processes/phenomena interact. Land-atmosphere interactions, hydrological feedbacks and aerosols influence the seasonal transitions and intraseasonal variation during the summer monsoon. Most of the cloud systems in the CTCZ (synoptic scale in particular) are generated over the warm oceans around the subcontinent and propagate onto the Indian region. Therefore, it is important to investigate the links of monsoon variability with the oceanic conditions and convection over the surrounding oceans. Accordingly, the science foci of CTCZ comprise of both important phenomena and process studies as listed below.

(a) Seasonal transition and intraseasonal variation of the monsoon. (b) Links of monsoon variability with the convection over the surrounding oceans.

Process studies
(i) Land-atmosphere interactions and hydrological feedbacks. (ii) Role of aerosols in monsoon variability. (iii) Scale interactions in convection ranging from the cumulus to the large scale.
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Proposal for CTCZ               Science Plan