Bay Of Bengal Monsoon Experiment Research Articles

Empirical Estimation of Hourly Incoming Shortwave Radiation over the North Indian Ocean

Hourly estimation of incoming shortwave radiation formulae have been developed and given by many scientists. The empirical formulae have to be updated with the latest realistic data. Using Lumb procedure, author attempted to give new regression coefficients for incoming shortwave radiation with in situ observations made during Arabian Sea Monsoon Experiment (ARMEX) cruises over North Indian Ocean for different cloud amounts (from clear sky to overcast conditions). ARMEX field work program has been conducted in June-August 2002 and March-June, 2003 on board R/V “Sagar Kanya”. The shortwave radiation data has been collected using the Albedometer, which is covering entire cloud amounts. Empirical formulae have been developed using sine of solar altitude (sin (h)) with respect to different could amounts. Newly developed Empirical Formulae are better and more accurate as is evidenced by standard deviation. However newly developed empirical formulae are compared with other formulae given by other scientists. The empirical formulae have been compared with available observed data from Bay of Bengal Monsoon Experiment (BOBMEX) and NCEP re-analyzed data. With BOBMEX data is in agreement, however with NCEP showing lower values.

Observed Variability of Thermohaline Fields, Currents and Eddies in the Western Bay of Bengal During BOBMEX-99

Field measurements during the Bay of Bengal Monsoon Experiment (BOBMEX-99), data from a deep sea moored buoy, and satellite altimeter were used to describe variability in the hydrographic and meso-scale features in the Bay of Bengal (BoB) during the summer monsoon of 1999. The thermohaline fields showed two regions of upsloping of isopleths centered at 82°E and 84.75°E, ∼110 km and 450 km away from the coast, respectively, followed by downsloping. The upsloping/downsloping of isopleths and the alternating currents was part of cyclonic and anti-cyclonic circulation patterns in the western BoB. In this region, both wind and current were important in the dynamics of coastal upwelling. The observations showed a relationship between the propagating waves and eddy on variability of thermohaline fields. On an annual cycle, four Kelvin waves were observed in the BoB, but only the downwelling Kelvin wave formed during October entered the Arabian Sea. During the monsoon season, four eddies were formed in the western BoB, of which the anticyclonic eddy centered at 15°N, 84°E and the cyclonic eddy centered at 17.5°N, 84.5°E were prominent. The baroclinic instability caused by the opposing currents along the east coast and the wind stress curl favored the formation of eddies. Okhubo-Weiss and Isern-Fontanet parameter confirmed the presence of eddies in the BoB.

The Impacts of Indirect Soil Moisture Assimilation and Direct Surface Temperature and Humidity Assimilation on a Mesoscale Model Simulation of an Indian Monsoon Depression

This study investigates the impact of the Flux-Adjusting Surface Data Assimilation System (FASDAS) and the four-dimensional data assimilation (FDDA) using analysis nudging on the simulation of a monsoon depression that formed over India during the 1999 Bay of Bengal Monsoon Experiment (BOBMEX) field campaign. FASDAS allows for the indirect assimilation/adjustment of soil moisture and soil temperature together with continuous direct surface data assimilation of surface temperature and surface humidity. Two additional numerical experiments [control (CTRL) and FDDA] were conducted to assess the relative improvements to the simulation by FASDAS. To improve the initial analysis for the FDDA and the surface data assimilation (SDA) runs, the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) simulation utilized the humidity and temperature profiles from the NOAA Television Infrared Observation Satellite (TIROS) Operational Vertical Sounder (TOVS), surface winds from the Quick Scatterometer (QuikSCAT), and the conventional meteorological upper-air (radiosonde/rawinsonde, pilot balloon) and surface data. The results from the three simulations are compared with each other as well as with NCEP–NCAR reanalysis, the Tropical Rainfall Measuring Mission (TRMM), and the special buoy, ship, and radiosonde observations available during BOBMEX. As compared with the CTRL, the FASDAS and the FDDA runs resulted in (i) a relatively better-developed cyclonic circulation and (ii) a larger spatial area as well as increased rainfall amounts over the coastal regions after landfall. The FASDAS run showed a consistently improved model simulation performance in terms of reduced rms errors of surface humidity and surface temperature as compared with the CTRL and the FDDA runs.

The effect of a surface data assimilation technique and the traditional four-dimensional data assimilation on the simulation of a monsoon depression over India using a mesoscale model

The objective of this study is to investigate the impact of a surface data assimilation (SDA) technique, together with the traditional four-dimensional data assimilation (FDDA), on the simulation of a monsoon depression that formed over India during the field phase of the 1999 Bay of Bengal Monsoon Experiment (BOBMEX). The SDA uses the analyzed surface data to continuously assimilate the surface layer temperature as well as the water vapor mixing ratio in the mesoscale model. The depression for the greater part of this study was offshore and since successful application of the SDA would require surface information, a method of estimating surface temperature and surface humidity using NOAA-TOVS satellites was used. Three sets of numerical experiments were performed using a coupled mesoscale model. The first set, called CONTROL, uses the NCEP (National Center for Environmental Prediction) reanalysis for the initial and lateral boundary conditions in the MM5 simulation. The second and the third sets implemented the SDA of temperature and moisture together with the traditional FDDA scheme available in the MM5 model. The second set of MM5 simulation implemented the SDA scheme only over the land areas, and the third set extended the SDA technique over land as well as sea. Both the second and third sets of the MM5 simulation used the NOAA-TOVS and QuikSCAT satellite and conventional upper air and surface meteorological data to provide an improved analysis. The results of the three sets of MM5 simulations are compared with one another and with the analysis and the BOBMEX 1999 buoy, ship, and radiosonde observations. The predicted sea level pressure of both the model runs with assimilation resembles the analysis closely and also captures the large-scale structure of the monsoon depression well. The central sea level pressures of the depression for both the model runs with assimilation were 2–4 hPa lower than the CONTROL. The results of both the model runs with assimilation indicate a larger spatial area as well as increased rainfall amounts over the coastal regions after landfall compared with the CONTROL. The impact of FDDA and SDA, the latter over land, resulted in reduced errors of the following: 1.45 K in temperature, 0.39 m s−1 in wind speed, and 14° in wind direction compared with the BOBMEX buoy observation, and 1.43 m s−1 in wind speed, 43° in wind direction, and 0.75% in relative humidity compared with the CONTROL. The impact of SDA over land and sea compared with SDA over land only showed a further marginal reduction of errors: 0.23 K in air temperature (BOBMEX buoy) and 1.33 m s−1 in wind speed simulations.

Hydrographic observations and model simulation of the Bay of Bengal freshwater plume

Hydrographic observations were taken along two coastal sections and one open ocean section in the Bay of Bengal during the 1999 southwest monsoon, as a part of the Bay of Bengal Monsoon Experiment (BOBMEX). The coastal section in the northwestern Bay of Bengal, which was occupied twice, captured a freshwater plume in its two stages: first when the plume was restricted to the coastal region although separated from the coast, and then when the plume spread offshore. Below the freshwater layer there were indications of an undercurrent. The coastal section in the southern Bay of Bengal was marked by intense coastal upwelling in a 50 km wide band. In regions under the influence of the freshwater plume, the mixed layer was considerably thinner and occasionally led to the formation of a temperature inversion. The mixed layer and isothermal layer were of similar depth for most of the profiles within and outside the freshwater plume and temperature below the mixed layer decreased rapidly till the top of seasonal thermocline. There was no barrier layer even in regions well under the influence of the freshwater plume. The freshwater plume in the open Bay of Bengal does not advect to the south of 16 ∘ N during the southwest monsoon. A model of the Indian Ocean, forced by heat, momentum and freshwater fluxes for the year 1999, reproduces the freshwater plume in the Bay of Bengal reasonably well. Model currents as well as the surface circulation calculated as the sum of geostrophic and Ekman drift show a southeastward North Bay Monsoon Current (NBMC) across the Bay, which forms the southern arm of a cyclonic gyre. The NBMC separates the very low salinity waters of the northern Bay from the higher salinities in the south and thus plays an important role in the regulation of near surface stratification.

Evaluation of daily and diurnal signals of total precipitable water (TPW) over the Indian Ocean based on TMI retrieved 3‐day composite estimates and radiosonde data

AbstractTropical rainfall measuring mission (TRMM) TRMM microwave imager (TMI) retrieved total precipitable water (TPW) has been shown in previous studies to be useful for characterizing energy transport and rainfall variability over water bodies. In this study, 3‐day composite TPW values during the monsoon months of June–September in 1999 and 2003 were evaluated against radiosonde derived TPW over the coastal regions of Indian Ocean. The radiosonde measurements were from coastal locations, island stations and ship‐based observations made during the 1999 Bay of Bengal Monsoon Experiment (BOBMEX). Our study is one of the first attempts to compare TMI‐derived TPW with the BOBMEX data. Root mean square (RMS) error of 8.1 mm and a bias of −0.76 mm were obtained for TMI 3‐day composite TPW data with respect to the radiosonde data over the Indian coastal regions. TMI overestimated (underestimated) radiosonde measurements under dry (wet) atmospheric conditions. Daily TMI‐derived TPW was also validated with radiosonde measurements from the BOBMEX data for the period 21 July–28 August 1999. Diurnal signals of TPW over Bay of Bengal were found to be comparable to the in situ measurements during the same period. Copyright © 2006 Royal Meteorological Society

4D-variational assimilation over the Bay of Bengal using the Adjoint of a Variable Resolution Global Ocean Model

The adjoint technique has been widely used over the last two decades in applications involving the dynamics of the Atmosphere and Ocean. This present study applies this technique in the assimilation of oceanic parameters such as temperature and net-surface heat flux, in a variable resolution Ocean General Circulation Model (OGCM). The National Center for Environmental Prediction (NCEP) 1999 July–August global data (sea-surface temperature and derived surface heat flux) and vertical temperature profiles collected from ORV Sagar Kanya and INS Sagardhwani during the Bay of Bengal Monsoon Experiment (BOBMEX 1999) are used for assimilation over the Bay of Bengal. The net-surface heat flux and temperature obtained after assimilation show good agreement with their respective seasonal patterns over the whole Indian Ocean, though there are some differences in the numerical values of heat fluxes. The vertical profiles of temperature after assimilation over the Bay of Bengal show strong resemblance with the corresponding ship observations, emphasizing the importance of the adjoint technique in ocean data assimilation.

Intraseasonal Variability of the Summer Monsoon over the North Indian Ocean as Revealed by the BOBMEX and ARMEX Field Programs

During the summer monsoon season over India a range of intraseasonal modulations of the monsoon rains occur due to genesis of weather disturbances over the Bay of Bengal (BOB) and the east Arabian Sea. The amplitudes of the fluctuations in the surface state of the ocean (sea-surface temperature and salinity) and atmosphere are quite large due to these monsoonal modulations on the intraseasonal scale as shown by the data collected during the field programs under Bay of Bengal Monsoon Experiment (BOBMEX) and Arabian Sea Monsoon Experiments (ARMEX). The focus of BOBMEX was to understand the role of ocean-atmospheric processes in organizing convection over the BOB on intra-seasonal scale. ARMEX-I was aimed at understanding the coupled processes in the development of deep convection off the West Coast of India. ARMEX-II was focused on the formation of the mini-warm pool across the southeast Arabian Sea in April-May and its role in the abrupt onset of the monsoon along the Southwest Coast of India and its further progress along the West Coast of India. The paper attempts to integrate the results of the observational studies and brings out an important finding that atmospheric instability is prominently responsible for convective organization whereas the upper ocean parameters regulate the episodes of the intraseasonal oscillations.

Role of variation in vertical profiles of relative humidity on retrieval of humidity from AMSU‐B data

In this paper, we have compared the relative humidity (RH) retrieved from 183 GHz channel brightness temperature (BT) of AMSU‐B (Advanced Microwave Sounding Unit‐B) with those obtained from radiosondes in the Bay of Bengal during BOBMEX (Bay of Bengal Monsoon Experiment) and in the Arabian Sea during ARMEX (Arabian Sea Monsoon Experiment). The NCAR (National Center for Atmospheric Research) radiative transfer (RT) model has been used to obtain the relationship between layer‐averaged RH and BT. With the present RT model, it has been found that the linear dependence of natural logarithm of layer‐averaged RH on BT is valid, if the profile of humidity is invariant. However, the relative humidity profiles obtained during BOBMEX and ARMEX experiments showed a large variability in the vertical profiles of humidity in middle and upper troposphere. In such situation, the relation between layer‐averaged RH and BT was found to be linear.

Some salient features of the atmosphere observed over the north Bay of Bengal during BOBMEX

This paper describes the near surface characteristics and vertical variations based on the observations made at 17.5‡N and 89‡E from ORV Sagar Kanya in the north Bay of Bengal during the Bay of Bengal Monsoon Experiment (BOBMEX) carried out in July–August 1999. BOBMEX captured both the active and weak phases of convection. SST remained above the convection threshold throughout the BOBMEX. While the response of the SST to atmospheric forcing was clearly observed, the response of the atmosphere to SST changes was not clear. SST decreased during periods of large scale precipitation, and increased during a weak phase of convection. It is shown that the latent heat flux at comparable wind speeds was about 25–50% lower over the Bay during BOBMEX compared to that over the Indian Ocean during other seasons and tropical west Pacific. On the other hand, the largest variations in the surface daily net heat flux are observed over the Bay during BOBMEX. SST predicted using observed surface fluxes showed that 1-D heat balance model works sometime but not always, and horizontal advection is important. The high resolution Vaisala radiosondes launched during BOBMEX could clearly bring out the changes in the vertical structure of the atmosphere between active and weak phases of convection. Convective Available Potential Energy of the surface air decreased by 2–3 kJ kg-1 following convection, and recovered in a time period of one or two days. The mid tropospheric relative humidity and water vapor content, and wind direction show the major changes between the active and weak phases of convection.

Thermohaline features of the subsurface cyclonic Eddy the south central Bay of Bengal during August 1999

Analysis of the spatial data collected along two sections of temperature and salinity from Chennai to 13‡N and 87‡E and back to Chennai onboard INS Sagardhwani during the Bay of Bengal Monsoon Experiment (BOBMEX) from 10th to 20th August 1999 revealed the presence of a prominent cyclonic eddy centered around 280 km away from the coast. Analysis of the dissipation rate of the cyclonic eddy from transect one to transect two and from the analysis of the TOPEX data, it may be inferred that the cyclonic eddy is possibly due to the presence of westward propagating Rossby waves in the Bay of Bengal.

Near‐surface variations and surface fluxes over the northern Bay of Bengal during the 1999 Indian summer monsoon

Detailed observations of the atmosphere and the ocean were carried out in the Bay of Bengal during the peak monsoon months of July and August 1999 under the Bay of Bengal Monsoon Experiment (BOBMEX). The emphasis in BOBMEX was on time series observations of the ocean and the atmosphere from fixed locations in the bay. Indian research vessels INS Sagardhwani and ORV Sagar Kanya were deployed at 13°N, 87°E and 17.5°N, 89°E in the southern and northern Bay of Bengal, respectively. This paper describes the near‐surface characteristics during phase I (27 July to 6 August) and phase II (13–24 August) of BOBMEX observed at the northern location. During phase I, average surface pressure was 999 hPa, average winds were 10 m s−1, and three monsoon systems developed. Sea surface temperature (SST) remained around 28.5°C. Phase I also saw the arrival of low‐saline water plume, and the surface salinity decreased from 33 practical salinity units (psu) to less than 29 psu in a span of just 4 days. During phase II of BOBMEX, average surface pressure was 1006 hPa, average wind speed was lower at 7.5 m s−1, and no monsoon system developed. There was a weak phase of convection characterized by low winds and clear‐sky conditions, during which period SST increased from around 28°C to more than 29.5°C in 5 days. Convective available potential energy of the surface air decreased by 2–3 kJ kg−1 following convection and recovered in a time period of 1 or 2 days. SST predicted by using observed surface fluxes showed that horizontal advection is important in the northern bay and that a one‐dimensional heat balance model works sometimes, but not always. It is also shown that near‐surface characteristics over the northern Bay of Bengal are different from those observed over the west Pacific warm pool.

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 (CAPE) and the convective inhibition energy were comparable to those for the atmosphere over the west Pacific warm pool. CAPE decreased by 2–3 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.

Bay of Bengal Monsoon Experiment (BOBMEX) — A component of the Indian Climate Research Programme (ICRP)

The Indian Climate Research Programme (ICRP) focuses on the study of climate variability and its impact on agriculture. To address the role of the Bay of Bengal in monsoon variability, a process study was organised during July–August 1999, deploying research ships, buoys, INSAT, coastal radar and conventional observational systems to collect information about the coupled ocean-atmosphere system over the warm waters of the Bay of Bengal. The paper gives the background of the ICRP and the organisation and implementation of the Bay of Bengal Monsoon Experiment (BOBMEX) in its field phase.