D. Bala Subrahamanyam

Variability of Mixed-Layer Heights over the Indian Ocean and Central Arabian Sea during INDOEX, IFP-99

The upper air data collected from the balloon-borne GLASS Sondes launched from the Oceanic Research Vessel (ORV) Sagar Kanya during the Intensive Field Phase of the Indian Ocean experiment (INDOEX, IFP-99;SK-141 Cruise) are utilized forstudying the variability in the mixed-layer heights observed over the western tropical Indian Ocean and central Arabian Sea. During the entire cruise, typical daytime convective mixed-layer heights (roughly corresponding to 1400 LT) obtained from θV and q profiles, were observed to be in the range 200–900 m. Shallowmixed -layer heights are observed, in general, over the Inter-Tropical Convergence Zone (ITCZ). Over the central Arabian Sea, vertical profiles of θV and q demonstrate a double mixed-layer structure of the marine atmospheric boundary layer (MABL), which gradually disappears close to the Indian coastline.

Characterization of the Vertical Structure of Coastal Atmospheric Boundary Layer over Thumba (.∘N, .∘E) during Different Seasons

Vertical profiles of meteorological parameters obtained from balloon-borne GPS Radiosonde for a period of more than two years are analyzed for characterization of the coastal atmospheric boundary layer (CABL) over Thumba (8.5∘N, 76.9∘E, India). The study reports seasonal variability in the thickness of three different sublayers of the CABL, namely, mixed layer, turbulent flow, and sea breeze flow. Among the three, the vertical thickness of sea breeze flow showed considerable dominance on the other two throughout the year. Mixed layer heights derived through gradients in virtual potential temperature (𝜃𝑣) showed large seasonal variability with a peak in the Summer and Post-Monsoon. On the other hand, the vertical thickness of turbulent flow remained steady all through the year. Results from the present study indicate that the magnitudes of mixed layer heights are often larger than the turbulent flow thickness.

Air–sea interface fluxes over the Indian Ocean during INDOEX, IFP-99

Abstract The Intensive Field Phase of Indian Ocean Experiment (INDOEX, IFP-99) was carried onboard Oceanic Research Vessel Sagar Kanya during January 20–March 12, 1999 over the latitudinal range 15°N–20°S and the longitudinal range 63°E–77°E. The present study deals with the spatial variation of air–sea fluxes over the Indian Ocean during the INDOEX, IFP-99 campaign. Drag coefficient ( C _D), and sensible heat ( C H ) and water vapor ( C E ) exchange coefficients are determined using an iterative scheme. The estimated values of these coefficients are utilized for the computation of air–sea fluxes using the bulk aerodynamic method. The variations of air–sea flux estimates are studied with respect to the variation of wind speed.

Parameterization of rain induced surface roughness and its validation study using a third generation wave model

The effect of raindrops striking water surface and their role in modifying the prevailing sea-surface roughness is investigated. The work presents a new theoretical formulation developed to study rain-induced stress on sea-surface based on dimensional analysis. Rain parameters include drop size, rain intensity and rain duration. The influences of these rain parameters on young and mature waves were studied separately under varying wind speeds, rain intensity and rain duration. Contrary to popular belief that rain only attenuates surface waves, this study also points out rain duration under certain condition can contribute to wave growth at high wind speeds. Strong winds in conjunction with high rain intensity enhance the horizontal stress component on the sea-surface, leading to wave growth. Previous studies based on laboratory experiments and dimensional analysis do not account for rain duration when attempting to parameterize sea-surface roughness. This study signifies the importance of rain duration as an important parameter modifying sea-surface roughness. Qualitative as well quantitative support for the developed formulation is established through critical validation with reports of several researchers and satellite measurements for an extreme cyclonic event in the Indian Ocean. Based on skill assessment, it is suggested that the present formulation is superior to prior studies. Numerical experiments and validation performed by incorporating in state-of-art WAM wave model show the importance of treating rain-induced surface roughness as an essential pre-requisite for ocean wave modeling studies.

A comparative study of air-sea exchange coefficients and turbulent fluxes over Indian sub-continent and Korean peninsula

In this article, we describe the variation of air-sea exchange coefficients and air-sea interface fluxes over the East Asian marginal seas surrounding the Korean peninsula and compare them with the similar estimates reported for the tropical Indian Ocean. Surface layer meteorological observations for a period of about five years obtained from five oceanic buoys in the adjoining seas of Korean peninsula form the database for this study. Depending on the stability of the atmosphere, buoy data is classified into three categories - unstable, neutral and stable data. For unstable conditions, sensible and latent heat flux show good correlation with the wind speed, whereas it is not so for the neutral and stable condition. Irrespective of the stability of the atmosphere, momentum flux always shows a steady dependence on the varying wind speed. Sensible and latent heat fluxes also show good correlation with the difference between sea surface temperature and air temperature. Unlike the linear regression between the exchange coefficients and wind speeds reported for the Indian Ocean, we suggest second order and exponential fits for these exchange coefficients, which give better representation of their wind speed dependence. The results presented in this article form very useful input to the coupled ocean atmospheric models and the oceanic wave models, hence significant.

Nudging of vertical profiles of meteorological parameters in one-dimensional atmospheric model: A step towards improvements in numerical simulations

In this article, we describe a simple yet effective method for insertion of observational datasets in a mesoscale atmospheric model used in one-dimensional configuration through Nudging. To demonstrate the effectiveness of this technique, vertical profiles of meteorological parameters obtained from GLASS Sonde launches from a tiny island of Kaashidhoo in the Republic of Maldives are injected in a mesoscale atmospheric model — Advanced Regional Prediction System (ARPS), and model simulated parameters are compared with the available observational datasets. Analysis of one-time nudging in the model simulations over Kaashidhoo show that incorporation of this technique reasonably improves the model simulations within a time domain of +6 to +12 Hrs, while its impact on +18 Hrs simulations and beyond becomes literally null.

PARAMETERIZATION OF WAVE ATTENUATION IN MUDDY BEDS AND IMPLICATION ON COASTAL STRUCTURES

The complex interaction between surface waves and muddy sea bottom is pivotal for wave evolution studies in coastal regions. Considerable absorption of wave energy resulting in decreased wave height had been observed in numerous geographical locations which include countries like China, India, South Korea and Louisiana (Gulf of Mexico), USA. Strong dissipative effects of cohesive sedimentary environments on waves are well known, but little understood. The spectral action balance equation used in numerical wave modeling expresses evolution of wave energy as a function of frequency and direction, balanced by various source and sink mechanisms. In this research paper, we propose an improved parameterization of bottom induced wave attenuation which is an inherent limitation in the existing state-of-the-art third generation wave models. We explain through theoretical formulations wave propagation and attenuation in heterogeneous environments, where significant wave damping over entire wavelength is observed in bottom strata having mixture of sand with mud (hereafter referred to as slurry). In the context of engineering relevance, such a mixture is termed “slurry” commonly referred in dredging technology. Hence, in this work we propose and incorporate a parameterization form of wave attenuation in the action balance equation which accounts for dissipative mechanism for slurry dominated bottoms. The attenuation coefficient is thereby determined and a comparison is made with the theoretical formulation due to slurry and sand for a one-dimensional case. The study also addresses implication of such wave attenuation on coastal structures.

Atmospheric Boundary-Layer Processes and Atmospheric Modeling

1Space Physics Laboratory, Vikram Sarabhai Space Centre, Department of Space, Government of India, Indian Space Research Organisation, Thiruvananthapuram 695022, India 2Environment Canada, Meteorological Research Division, Cloud Physics and Severe Weather Research Section, Toronto, ON, Canada M3H 5T4 3Public Authority for Civil Aviation, Oman National Meteorological Services, Muscat, Oman 4Centre for Oceans, Rivers, Atmosphere and Land Sciences, Indian Institute of Technology Kharagpur, Kharagpur 721302, India