Interrelationship between cloud cover and sensible heat flux over land during MONTBLEX-1990

Micro-meteorological tower observations of MONTBLEX (Monsoon Trough Boundary Layer Experiment)- 1990, combined with routine surface observations at Jodhpur in the dry convective sector of Indian summer monsoon trough are used to examine the interrelationship between total cloud cover (TCC) and surface sensible heat flux (SHF) during the summer monsoon of 1990. A significant inverse relationship between TCC and SHF is found during various Intensive Observation Periods of the experiment. This relationship holds for the various methods of estimation of SHF. © Printed in India

A study of rainfall along the west coast of India in relation to low level jet and air-sea interactions over the Arabian Sea

Indian summer monsoon has a large inter-annual as well as intra-seasonal variability over temporal and spatial scales. Onset dates, monsoon activity within a monsoon season and quantity of monsoon rainfall are also found to vary from year to year. One important synoptic feature associated with the onset of monsoon is the existence of a strong cross equatorial low level jet (LLJ), with its core around 850 hPa over the Indian Ocean and South Asia. This LLJ generally supports the large-scale moisture and momentum transport from ocean to atmosphere and the consequent rainfall over the Indian mainland. In the present study, buoy data at a stationary position in the Arabian Sea (15.5°N, 61.5°E) have been used to understand the air-sea interface processes before, during and after the onset of monsoon 1995

Thermodynamic structure of the marine atmosphere over the region 80-87°E along 13°N during August (phase II) BOBMEX-99

Thermodynamic structure of the marine atmosphere in the region between 80 and 87°E along 13°N over the Bay of Bengal was studied using 13 high resolution radiosonde profiles from surface -400 hPa collected onboard ORV Sagar Kanya during the period 27th-30th August, during BOBMEX-99. Saturation point concept, mixing line analysis and conserved variable diagrams have been used to identify boundary layer characteristics such as air mass movement and stability of the atmosphere. The results showed relatively dry air near the ocean surface between 1000 and 950 hPa. This feature is confirmed by the conserved structure in this layer. Further, seldom showed any inversions in this region. The profiles showed persistent low cloud layers between 900 and 700 hPa. The conserved variable diagrams (c-q) showed the existence of double mixing line structures approximately at 950 and 700 hPa levels

Ocean-atmosphere interaction and synoptic weather conditions in association with the two contrasting phases of monsoon during BOBMEX-1999

Surface meteorological parameters acquired during the field phase experiment, BOBMEX-99, for the stationary periods (SP I and II) of the ship ORV Sagar Kanya over Bay of Bengal have been analysed. Active and weak monsoon conditions were observed during the first and the second phase of the experiment respectively over India as well as over the stationary ship location. The phase mean sea surface temperature (SST) is found to be the same in both the phases, however large differences have been observed in the phase mean values of wind speed, mean sea level pressure, latent heat and momentum fluxes at air-sea interface. Synoptic scale monsoon disturbances formed only during the period of strong north-south pressure gradient over the Bay region. Events of prominent fall in SST and in the upper 15 m ocean layer mean temperature and salinity values during typical rainfall events are cited. The impact of monsoon disturbances on ocean-atmosphere interface transfer processes has been investigated

Response of the Bay of Bengal to Gopalpur and Paradip super cyclones during 15-31 October 1999

Response of the Bay of Bengal to two tropical cyclones, i.e. Gopalpur and Paradip super cyclones, during 15-31 October 1999, is studied using a stationary mooring buoy for marine meteorological observations. The National Institute of Ocean Technology (NIOT, Chennai) has deployed this buoy at 13°N, 87°E, by fixing various meteorological instruments and sensors to acquire sea surface temperature (SST), air temperature (Ta), wind speed (Ws), wind direction (Wd) and ocean currents (Cs) using remote sensing technique through INSAT-1D satellite at an interval of 3 h. The results of the analysis of the above parameters have shown clearly a response (SST difference between before and after formation) of about 0.7°C for the Gopalpur cyclone and 0.9°C for the Paradip cyclone. Ta has shown rapid variations following the rapid movement of cloud decks across the buoy during the cyclone period. The observed changes in the wind speed and direction are in concurrence with analysed mean sea level pressure oscillations. Finally, this study recommends more buoy-based marine meteorological observations over this region and the neighbouring areas, where the tropical cyclones generally occur and subsequently hit the Coromandal coast

On the plausible reasons for the formation of onset vortex in the presence of Arabian Sea mini warm pool

It has been established through a numerical model that the onset vortex (OV) was formed dramatically in the shear line on the northern flank of a low level jet (LLJ) at 850 hPa over the mini warm pool (MWP) in the East Central Arabian Sea with the aid of sea surface temperature (SST) anomalies using MONEX-79 data. This study has led to serious investigation of MWP over the ECAS, but little attention has been given to its counterpart, i.e. the atmospheric pattern at 850 hPa, the level at which OV generally forms and extends on either side during the course of development. The present study examines the SST distribution over the Arabian Sea and circulation at 850 hPa to identify the MWP and the LLJ positions for five consecutive days with onset day as its centre and for six consecutive years 2000-05. The study has revealed that OV had formed only in 2001 under the influence of MWP on the northern flank of LLJ. During other years it seldom formed due to (i) absence of MWP, (ii) lack of sufficient strength of LLJ, and (iii) absence of the location of shear line (over the northern flank of LLJ) over MWP. The air-sea flux transfer processes for the OV year 2001 and a non-OV year 2002 are studied and compared for better understanding of the above process in relation to the OV and non-OV weather conditions over the study area

Marine boundary layer characteristics during a cyclonic storm over the Bay of Bengal

During the period 12-16 June 1996 a tropical cyclonic storm formed over the southwest Bay of Bengal and moved in a north-northeasterly direction. The thermodynamic characteristics of this system are investigated by utilizing the surface and upper air observations collected onboard ORV Sagar Kanya over the Bay of Bengal region. The response of the cyclonic storm is clearly evident from the ship observations when the ship was within the distance of 600-800 km from the cyclonic storm. This study explores why (i) the whole atmosphere from surface to 500 hPa had become warm and moist during the cyclonic storm period as compared to before and after the formation of this system and (ii) the lower layer of the atmosphere had become stable during the formative stage of the cyclonic storm

Evolution of an atmospheric boundary layer at a tropical semi-arid station, Anand during boreal summer month of May - A case study

The evolution of an Atmospheric Boundary Layer (ABL) over a semi-arid land station, Anand, (22°35â²N, 72°55â²E, 45.1 m asl) in India, during the summer month of May, is examined using surface meteorological and radiosonde temperature and humidity data collected during LASPEX-97 for a 5-day period from 13-17 May 1997. These 5 days remained undisturbed, and clear sky weather conditions prevailed. However, the data obtained on these days are helpful in understanding the diurnal variation of the ABL over a land station. There are 5 observations per day at an interval of 3 h beginning with 0530 IST. The 0530 IST ascents are chosen to find out the initial ABL heights which exhibit the nocturnal cooling conditions. It is observed from the analysis of θv, θe, θes, q, and P profiles that the nocturnal boundary layer is stable with an inversion close to the ground. The top of an inversion layer is characterized by a θe minimum and a θes maximum. After dawn, the ABL grows to a height of 827 m at 0830 IST. Aloft, a residual layer up to 3200 m is observed. The daytime strong insolation causes formation of an unstable boundary layer close to the ground at 1130 IST with an elevated stable layer between 550 and 930 m. It is only by 1430 IST that the stable layer gets completely wiped out and a convective mixed layer develops up to a height of 3280 m. Lack of moisture inhibits formation of clouds. Hence the ABL at a semi-arid station like Anand is stable in the morning with residual layer aloft and develops into a dry convective boundary layer in the afternoon and evening. Growth of the convective boundary layer (CBL) is observed to be very rapid as it reaches a height up to 3280 m by the afternoon