G. S. Bhat

BOBMEX: The Bay of Bengal Monsoon Experiment

The first observational experiment under the Indian Climate Research Programme, called the Bay of Bengal Monsoon Experiment (BOBMEX), was carried out during July-August 1999. BOBMEX was aimed at measurements of important variables of the atmosphere, ocean, and their interface to gain deeper insight into some of the processes that govern the variability of organized convection over the bay. Simultaneous time series observations were carried out in the northern and southern Bay of Bengal from ships and moored buoys. About 80 scientists from 15 different institutions in India collaborated during BOBMEX to make observations in most-hostile conditions of the raging monsoon. In this paper, the objectives and the design of BOBMEX are described and some initial results presented. During the BOBMEX field phase there were several active spells of convection over the bay, separated by weak spells. Observation with high-resolution radiosondes, launched for the first time over the northern bay, showed that the magnitudes of the convective available potential energy (CA-PE) and the convective inhibition energy were comparable to those for the atmosphere over the west Pacific warm pool. CAPE decreased by 2-3 kJ kg(-1) following convection, and recovered in a time period of 1-2 days. The surface wind speed was generally higher than 8 m. s(-1). The thermohaline structure as well as its time evolution during the BOBMEX field phase were found to be different in the northern bay than in the southern bay. Over both the regions, the SST decreased during rain events and increased in cloud-free conditions. Over the season as a whole, the upper-layer salinity decreased for the north bay and increased for the south bay. The variation in SST during 1999 was found to be of smaller amplitude than in 1998. Further analysis of the surface fluxes and currents is expected to give insight into the nature of coupling.

Spring Warming of the Eastern Arabian Sea and Bay of Bengal from Buoy Data

Observations from moored buoys during spring of 1998-2000 suggest that the warming of the mixed layer (similar to20 m deep) of the north Indian Ocean warm pool is a response to net surface heat flux Q(net) (similar to100 W m(-2)) minus penetrative solar radiation Q(pen) (similar to45 W m(-2)). A residual cooling due to vertical mixing and advection is indirectly estimated to be about 25 W m(-2). The rate of warming due to typical values of Q(net) minus Q(pen) is not very sensitive to the depth of the mixed layer if it lies between 10 m and 30 m.

Experiments on a plume with off‐source heating: Implications for cloud fluid dynamics

We report here on a series of laboratory experiments on plumes, undertaken with the object of simulating the effect of the heat relase that occurs in clouds on condensation of water vapor. The experimental technique used for this purpose relies on ohmic heating generated in an electrically conducting plume fluid subjected to a suitable alternating voltage across specified axial stations in the plume flow [Bhat et al., 1989]. The present series of experiments achieves a value of the Richardson number that is toward the lower end of the range that characterizes cumulus clouds. It is found that the buoyancy enhancement due to heating disrupts the eddy structures in the flow and reduces the dilution owing to entrainment of ambient fluid that would otherwise have occurred in the central region of the plume. Heating also reduces the spread rate of the plume, but as it accelerates the flow as well, the overall specific mass flux in the plume does not show a very significant change at the heat input employed in the experiment. However, there is some indication that the entrainment rate (proportional to the streamwise derivative of the mass flux) is slightly higher immediately after heat injection and slightly lower farther downstream. The measurements support a previous proposal for a cloud scenario [Bhat and Narasimha, 1996] and demonstrate how fresh insights into certain aspects of the fluid dynamics of clouds may be derived from the experimental techniques employed here.

Near surface atmospheric characteristics over the North Bay of Bengal during the Indian Summer Monsoon

Observations were made from a ship at 17.5 °N & 89 °E in the North Bay of Bengal during July-August 1999. The Bay was convectively active during this period with almost one convective event every week. The surface wind speed varied from 3 to 15 m s−1. SST decreased by 0.5° to 1 °C following convection. The latent heat flux was about 25–35% lower over the Bay compared to that over the West Pacific at comparable wind speeds. High resolution radiosonde observations have been made for the first time in the North Bay. The values of atmospheric mixed layer height, equivalent potential temperature, convective available potential energy and convection inhibition energy over the North Bay are comparable to those observed over the western Pacific warm pool region. The recovery timescale of the atmospheric variables such as mixed layer height, equivalent potential temperature and convective available potential energy is one to two days.

Vertical structure of cumulonimbus towers and intense convective clouds over the South Asian region during the summer monsoon season

The vertical structure of radar reflectivity factor in active convective clouds that form during the South Asian monsoon season is reported using the 2A25 version 6 data product derived from the precipitation radar measurements on board the Tropical Rainfall Measuring Mission satellite. We define two types of convective cells, namely, cumulonimbus towers (CbTs) and intense convective cells (ICCs). CbT is defined referring to a reflectivity threshold of 20 dBZ at 12 km altitude and is at least 9 km thick. ICCs are constructed referring to reflectivity thresholds at 8 km and 3 km altitudes. Cloud properties reported here are based on 10 year climatology. It is observed that the frequency of occurrence of CbTs is highest over the foothills of Himalayas, plains of northern India and Bangladesh, and minimum over the Arabian Sea and equatorial Indian Ocean west of 90°E. The regional differences depend on the reference height selected, namely, small in the case of CbTs and prominent in 6–13 km height range for ICCs. Land cells are more intense than the oceanic ones for convective cells defined using the reflectivity threshold at 3 km, whereas land versus ocean contrasts are not observed in the case of CbTs. Compared to cumulonimbus clouds elsewhere in the tropics, the South Asian counterparts have higher reflectivity values above 11 km altitude.

Warm pool thermodynamics from the Arabian Sea Monsoon Experiment (ARMEX)

Before the onset of the south Asian summer monsoon, sea surface temperature (SST) of the north Indian Ocean warms to 30–32°C. Climatological mean mixed layer depth in spring (March–May) is 10–20 m, and net surface heat flux (Q net ) is 80–100 W m−2 into the ocean. Previous work suggests that observed spring SST warming is small mainly because of (1) penetrative flux of solar radiation through the base of the mixed layer (Q pen ) and (2) advective cooling by upper ocean currents. We estimate the role of these two processes in SST evolution from a two-week Arabian Sea Monsoon Experiment process experiment in April–May 2005 in the southeastern Arabian Sea. The upper ocean is stratified by salinity and temperature, and mixed layer depth is shallow (6 to 12 m). Current speed at 2 m depth is high even under light winds. Currents within the mixed layer are quite distinct from those at 25 m. On subseasonal scales, SST warming is followed by rapid cooling, although the ocean gains heat at the surface: Q net is about 105 W m−2 in the warming phase and 25 W m−2 in the cooling phase; penetrative loss Q pen is 80 W m−2 and 70 W m−2. In the warming phase, SST rises mainly because of heat absorbed within the mixed layer, i.e., Q net minus Q pen ; Q pen reduces the rate of SST warming by a factor of 3. In the second phase, SST cools rapidly because (1) Q pen is larger than Q net and (2) advective cooling is ∼85 W m−2. A calculation using time-averaged heat fluxes and mixed layer depth suggests that diurnal variability of fluxes and upper ocean stratification tends to warm SST on subseasonal timescale. Buoy and satellite data suggest that a typical premonsoon intraseasonal cooling event occurs under clear skies when the ocean is gaining heat through the surface. In this respect, premonsoon SST cooling in the north Indian Ocean is different from that due to the Madden-Julian oscillation or monsoon intraseasonal oscillation.

An experimental study of a jet with local buoyancy enhancement

We report here an experimental study of the behaviour of a fully developed axisymmetric turbulent jet whose buoyancy is enhanced by volumetric heating over the region between two streamwise stations. The buoyancy enhancement is achieved by ohmic heating of an electrically conducting liquid jet, and the measurements are made using a laser Doppler velocimeter. It is found that, with heating, the axial component of mean velocity can increase appreciably relative to the unheated jet; however the turbulent intensity (normalized by the jet centreline velocity) decreases. The shape of the normalized mean velocity distribution across the jet is not significantly affected by the heating, but that of the fluctuating velocity is. The decay of the centreline velocity is considerably slowed down, or even reversed, due to the heating; similarly the spread rate is arrested at larger values of the Richardson number. As a result of the enhanced buoyancy the mass flux in the jet at first increases more rapidly than in the unheated jet but further downstream remains nearly constant over a distance of the order of the length of the heat injection region.

A new method of producing local enhancement of buoyancy in liquid flows

We describe here a novel method of generating large volumetric heating in a liquid. The method uses the principle of ohmic heating of the liquid, rendered electrically conducting by suitable additives if necessary. Electrolysis is prevented by the use of high frequency alternating voltage and chemically treated electrodes. The technique is demonstrated by producing substantial heating in an initially neutral jet of water. Simple flow visualisation studies, made by adding dye to the jet, show marked changes in the growth and development of the jet with heat addition.

Dynamic eduction of coherent structures in turbulent jet flow imagery by wavelet techniques: part I

The use of two-dimensional wavelet techniques enables us to discern, at appropriate wavelet scales, certain features of a coherent structure from a single flow visualization image. By analysing a large number of planar laser-induced fluorescence images of chiefly the diametral section of a turbulent jet, with some supplementary axial sections, we show that it is possible to describe the passage of whole coherent structures through a specified section of the flow, and thereby acquire useful information on their life cycle. In particular it is found that a typical cycle is characterized by a lobed ring-like structure about 8% of the time, a dye-filled core a similar fraction of the time, with a mixed regime in between. Based on comparisons with dye-concentration data and DNS results, it is suggested that the lobed ring seen in the wavelet transform of the raw image may represent a vortex ring exhibiting the Widnall instability.

ASIRI : an ocean–atmosphere initiative for Bay of Bengal

AbstractAir–Sea Interactions in the Northern Indian Ocean (ASIRI) is an international research effort (2013–17) aimed at understanding and quantifying coupled atmosphere–ocean dynamics of the Bay of Bengal (BoB) with relevance to Indian Ocean monsoons. Working collaboratively, more than 20 research institutions are acquiring field observations coupled with operational and high-resolution models to address scientific issues that have stymied the monsoon predictability. ASIRI combines new and mature observational technologies to resolve submesoscale to regional-scale currents and hydrophysical fields. These data reveal BoB’s sharp frontal features, submesoscale variability, low-salinity lenses and filaments, and shallow mixed layers, with relatively weak turbulent mixing. Observed physical features include energetic high-frequency internal waves in the southern BoB, energetic mesoscale and submesoscale features including an intrathermocline eddy in the central BoB, and a high-resolution view of the exchange...