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G. S. Bhat

Abstract

A framework has been developed that brings together the important physical parameters and processes governing the vertical mass flux in deep convective clouds and their area cover. The main result is a simple relation for the cloud mass flux and area fraction in terms of the large-scale radiative cooling, environmental stratification, and the extent of lateral entrainment of the ambient air by the convective systems. It is shown that the contribution of the moist processes to the total vertical mass flux in deep clouds can become comparable to that of the large-scale radiative component, and thus the neglect of these subsynoptic-scale processes can severely underestimate the convective activity. Further, it is argued that the consideration of moist processes is not merely a question of the inclusion of a correction factor in the relationship, but the uncertainty that needs to be overcome before meaningful predictions of deep cloud area cover can be achieved.

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G. S. Bhat

Abstract

Atmospheric and oceanic data were collected in 1998 and 1999 over the tropical Indian Ocean during three cruises of the Indian research vessel Sagar Kanya, covering different geographic locations and seasons, including boreal winter, peak monsoon, and postmonsoon periods. The present study is mainly based on the measurements made during the three cruises. It is found that there are important differences in the near-surface characteristics during monsoon and other seasons. The largest variations in the net surface heat flux occurred during the monsoon period. The specific humidity difference between sea surface and air at 10-m height shows a strong seasonal dependence, with the lowest values observed during the monsoon period. An important finding from the ship observations is that, at a given SST, the surface air over the Indian Ocean is much warmer compared to that over other tropical oceans and the west Pacific warm pool in particular. These findings are supported by data obtained by a moored buoy in the north Indian Ocean and also from the Comprehensive Ocean–Atmosphere Data Set for the Indian Ocean.

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Shailendra Kumar and G. S. Bhat

Abstract

This study is based on the analysis of 10 years of data for radar reflectivity factor Z e as derived from the TRMM Precipitation Radar (PR) measurements. The vertical structure of active convective clouds at the PR pixel scale has been extracted by defining two types of convective cells. The first one is cumulonimbus tower (CbT), which contains Z e ≥ 20 dBZ at 12-km altitude and is at least 9 km deep. The other is intense convective cloud (ICC), which belongs to the top 5% of the population of the Z e distribution at a prescribed reference height. Here two reference heights (3 and 8 km) have been chosen. Regional differences in the vertical structure of convective cells have been explored by considering 16 locations distributed across the tropics and two locations in the subtropics. The choice of oceanic locations is based on the sea surface temperature; that of the land locations is based on propensity for intense convection. One of the main findings of the study is the close similarity in the average vertical profiles of CbTs and ICCs in the mid- and lower troposphere across the ocean basins whereas differences over land areas are larger and depend on the selected reference height. The foothills of the western Himalaya, southeastern South America, and the Indo-Gangetic Plain contain the most intense CbTs; equatorial Africa, the foothills of the western Himalaya, and equatorial South America contain the most intense ICCs. Close similarity among the oceanic profiles suggests that the development of vigorous convective cells over warm oceans is similar and that understanding gained in one region is extendable to other areas.

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Jayesh Phadtare and G. S. Bhat

Abstract

Synoptic-scale weather systems are often responsible for initiating mesoscale convective systems (MCSs). Here, we explore how synoptic forcing influences MCS characteristics, such as the maximum size, lifespan, cloud-top height, propagation speed, and triggering over the Indian region. We used 30-min interval infrared (IR) data of the Indian Kalpana-1 geostationary satellite. Cloud systems (CSs) in this data are identified and tracked using an object tracking algorithm. ERA-Interim 850-hPa vorticity is taken as a proxy for the synoptic forcing. The probability of CSs being larger, longer lived, and deeper is more in the presence of a synoptic-scale vorticity field; however, the influence of synoptic forcing is not evident on the westward propagation of CSs over land. There exists a linear relationship between maximum size, lifespan, and average cloud-top height of CSs regardless of the nature of synoptic forcing. Formation of CSs peaks around 1500 LST over land, which is independent of synoptic forcing. Over the north Bay of Bengal, CSs formation is predominantly nocturnal when synoptic forcing is strong, whereas, 0300 and 1200 LST are the preferred times when synoptic forcing is weak. Long-lived CSs are preferentially triggered in the western flank of the 850-hPa vorticity gradient field of a monsoon low pressure system. Once triggered, CSs propagate westward and ahead of the synoptic system and dissipate around midnight. Formation of new CSs on the next day occurs in the afternoon hours in the wake of previous day’s CSs and where vorticity gradient is also present. Formation and westward propagations of CSs on successive days move the synoptic envelope westward.

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G. S. Bhat and A. Krothapalli

Abstract

The mean flow characteristics of a turbulent round jet and a plume are simulated in a high-resolution regional atmospheric model. The model used for the study is the Advanced Regional Prediction System (ARPS). It is observed that the predicted flow characteristics are very sensitive to the turbulence closure scheme used. Among the three closure options in ARPS, namely, the constant eddy viscosity scheme, the Smagorinsky diagnostic closure, and the 1.5-order total kinetic energy scheme, the constant eddy viscosity scheme predicts the jet characteristics in better agreement with experiments. For the plume, all three schemes are unsatisfactory. It is shown that a modification of the constant eddy viscosity scheme incorporating a length-scale variation as suggested by theory predicts plume characteristics in good agreement with experiments. The simulations are carried out at one fixed grid-box size and flow inlet conditions; extending the present simulation results to other cases is discussed.

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Arvind V. Gambheer and G. S. Bhat

Abstract

A detailed study of deep cloud systems (denoted by CSs) over the Indian region using INSAT-1B pixel data is presented. The life cycle characteristics of CSs are examined, including their preferred regions of formation and dissipation, frequency of occurrence, life duration, and speeds of propagation. A new automatic algorithm to track cloud systems has been developed that takes into account the mergers and splits in CSs. The algorithm is based on a combination of the maximum allowable displacement of a CS in 3 h and area overlap. The choice of the minimum size for CS is fixed at 4800 km2. The temperature threshold is varied from 201 to 261 K. It is observed that majority of CSs decay within a couple of hundred kilometers from where they form. There is a bimonthly modulation of the areal extent of more frequent convection. The number of CSs increases approximately linearly with threshold temperature up to 251 K. Tracking results are not very sensitive to the criterion chosen for identifying the successor in cases of multiple candidates, except for CSs that live longer than 36 h. Mean speeds of propagation of CSs range from 7 to 9 m s−1.

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G. S. Bhat, G. A. Vecchi, and S. Gadgil

Abstract

The Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) with the capability of measuring sea surface temperature (SST) in the presence of clouds, has been providing an unprecedented view of tropical basin-scale SST variability. In this paper, an assessment of the accuracy of the SST derived from TMI over the Bay of Bengal using in situ data collected from moored buoys and research ships, is presented. The authors find that TMI captures the evolution of the SST of the bay on seasonal time scales with reasonable accuracy. The mean difference between the SST from TMI and buoys is less than 0.1°C, and the rms difference is about 0.6°C. The time scales of the intraseasonal variation of the TMI SST are realistic. However, the amplitude of the SST variation on the intraseasonal scale is overestimated by a factor of about 1.3 when compared to buoy data. It is observed that the SST derived from TMI tends to be lower during periods with deep convection or winds stronger than 10 m s−1, or both. There is better agreement during weak conditions of convection/wind. This leads to a cold bias during convectively active periods when running average SST time series are constructed from SSTs retrieved from the TMI.

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Akshaya C. Nikumbh, Arindam Chakraborty, G. S. Bhat, and Dargan M. W. Frierson

ABSTRACT

The subseasonal and synoptic-scale variability of the Indian summer monsoon rainfall is controlled primarily by monsoon intraseasonal oscillation (MISO) and low pressure systems (LPSs), respectively. The positive and negative phases of MISO lead to alternate epochs of above-normal (active) and below-normal (break) spells of rainfall. LPSs are embedded within the different phases of MISO and are known to produce heavy precipitation events over central India. Whether the interaction with the MISO phases modulates the precipitation response of LPSs, and thereby the characteristics of extreme rainfall events (EREs), remains unaddressed in the available literature. In this study, we analyze the LPSs that produce EREs of various spatial extents (small, medium, and large) over central India from 1979 to 2012. We also compare them with the LPSs that pass through central India and do not produce any ERE (LPS-noex). We find that thermodynamic characteristics of LPSs that trigger different spatial extents of EREs are similar. However, they show differences in their dynamic characteristics. The ERE-producing LPSs are slower, moister, and more intense than LPS-noex. The LPSs that lead to medium and large EREs tend to occur during the positive phase of MISO when an active monsoon trough is present over central India. On the other hand, LPS-noex and the LPSs that trigger small EREs occur mainly during the neutral or negative phases of the MISO. The large-scale dynamic forcing, intensification of LPSs, and diabatic generation of low-level potential vorticity due to the presence of active monsoon trough help in the organization of convection and lead to medium and large EREs. On the other hand, the LPSs that form during the negative or neutral phases of MISO do not intensify much during their lifetime and trigger scattered convection, leading to EREs of small size.

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Akshaya C Nikumbh, Arindam Chakraborty, G.S. Bhat, and Dargan M. W. Frierson

Abstract

The sub-seasonal and synoptic-scale variability of the Indian summer monsoon rainfall are controlled primarily by monsoon intra-seasonal oscillations (MISO) and low pressure systems (LPS), respectively. The positive and negative phases of MISO lead to alternate epochs of above-normal (active) and below-normal (break) spells of rainfall. LPSs are embedded within the different phases of MISO and are known to produce heavy precipitation events over central India. Whether the interaction with the MISO phases modulates the precipitation response of LPSs, and thereby the characteristics of extreme rainfall events (EREs) remains unaddressed in the available literature. In this study, we analyze the LPSs that produce EREs of various spatial extents viz., Small, Medium, and Large over central India from 1979 to 2012. We also compare them with the LPSs that pass through central India and do not give any ERE (LPS-noex). We find that thermodynamic characteristics of LPSs that trigger different spatial extents of EREs are similar. However, they show differences in their dynamic characteristics. The ERE producing LPSs are slower, moister and more intense than LPS-noex. The LPSs that lead to Medium and Large EREs tend to occur during the positive phase of MISO when an active monsoon trough is present over central India. On the other hand, LPS-noex and the LPSs that trigger Small EREs occur mainly during the neutral or negative phases of the MISO. The large-scale dynamic forcing, intensification of LPSs, and diabatic generation of low-level potential vorticity due to the presence of active monsoon trough help in the organization of convection and lead to Medium and Large EREs. On the other hand, the LPSs that form during the negative or neutral phases of MISO do not intensify much during their lifetime and trigger scattered convection, leading to EREs of small size.

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R. Harikumar, T. M. Balakrishnan Nair, G. S. Bhat, Shailesh Nayak, Venkat Shesu Reddem, and S. S. C. Shenoi

Abstract

A network of ship-mounted real-time Automatic Weather Stations integrated with Indian geosynchronous satellites [Indian National Satellites (INSATs)] 3A and 3C, named Indian National Centre for Ocean Information Services Real-Time Automatic Weather Stations (I-RAWS), is established. The purpose of I-RAWS is to measure the surface meteorological–ocean parameters and transmit the data in real time in order to validate and refine the forcing parameters (obtained from different meteorological agencies) of the Indian Ocean Forecasting System (INDOFOS). Preliminary validation and intercomparison of analyzed products obtained from the National Centre for Medium Range Weather Forecasting and the European Centre for Medium-Range Weather Forecasts using the data collected from I-RAWS were carried out. This I-RAWS was mounted on board oceanographic research vessel Sagar Nidhi during a cruise across three oceanic regimes, namely, the tropical Indian Ocean, the extratropical Indian Ocean, and the Southern Ocean. The results obtained from such a validation and intercomparison, and its implications with special reference to the usage of atmospheric model data for forcing ocean model, are discussed in detail. It is noticed that the performance of analysis products from both atmospheric models is similar and good; however, European Centre for Medium-Range Weather Forecasts air temperature over the extratropical Indian Ocean and wind speed in the Southern Ocean are marginally better.

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