Simulation of marine boundary layer characteristics using a 1-D PBL model over the Bay of Bengal during BOBMEX-99

The characteristic features of the marine boundary layer (MBL) over the Bay of Bengal during the southwest monsoon and the factors influencing it are investigated. The Bay of Bengal and Monsoon Experiment (BOBMEX) carried out during July-August 1999 is the first observational experiment under the Indian Climate Research Programme (ICRP). A very high-resolution data in the vertical was obtained during this experiment, which was used to study the MBL characteristics off the east coast of India in the north and south Bay of Bengal. Spells of active and suppressed convection over the Bay were observed, of which, three representative convective episodes were considered for the study. For this purpose a one-dimensional multi-level PBL model with a TKE-ε closure scheme was used. The soundings, viz., the vertical profiles of temperature, humidity, zonal and meridional component of wind, obtained onboard ORV Sagar Kanya and from coastal stations along the east coast are used for the study. The temporal evolution of turbulent kinetic energy, marine boundary layer height (MBLH), sensible and latent heat fluxes and drag coefficient of momentum are simulated for different epochs of monsoon and monsoon depressions during BOBMEX-99.The model also generates the vertical profiles of potential temperature, specific humidity, zonal and meridional wind. These simulated values compared reasonably well with the observations available from BOBMEX

Conserved variable analysis of the marine boundary layer and air-sea exchange processes using BOBMEX-Pilot data sets

The present study is based on the observed features of the MBL (Marine Boundary Layer) during the Bay of Bengal and Monsoon Experiment (BOBMEX) - Pilot phase. Conserved Variable Analysis (CVA) of the conserved variables such as potential temperature, virtual potential temperature, equivalent potential temperature, saturation equivalent potential temperature and specific humidity were carried out at every point of upper air observation obtained on board ORV Sagar Kanya. The values are estimated up to a maximum of 4 km to cover the boundary layer. The Marine Boundary Layer Height is estimated from the conserved thermodynamic profiles. During the disturbed period when the convective activity is observed, the deeper boundary layers show double mixing line structures. An attempt is also made to study the oceanic heat budget using empirical models. The estimated short-wave radiation flux compared well with the observations

Numerical simulation of the marine boundary layer characteristics over the Bay of Bengal as revealed by BOBMEX-98 Pilot experiment

An attempt has been made to study the marine boundary layer characteristics over Bay of Bengal using BOBMEX (Bay of Bengal and Monsoon Experiment) pilot experiment data sets, which was conducted between 23rd October and 12th November 1998 on board ORV Sagar Kanya. A one-dimensional multilevel atmospheric boundary layer with TKE-ε closure scheme is employed to study the marine boundary layer characteristics. In this study two synoptic situations are chosen: one represents an active convection case and the other a suppressed convection. In the present article the marine boundary layer characteristics such as temporal evolution of turbulent kinetic energy, height of the boundary layer and the airsea exchange processes such as sensible and latent heat fluxes, drag coefficient for momentum are simulated during both active and suppressed convection. Marine boundary layer height is estimated from the vertical profiles of potential temperature using the stability criterion. The model simulations are compared with the available observations

A study on the structure of the convective atmosphere over the Bay of Bengal during BOBMEX-99

Convective activity is one of the major processes in the atmosphere influencing the local and large-scale weather in the tropics. The latent heat released by the cumulus cloud is known to drive monsoon circulation, which on the other hand supplies the moisture that maintains the cumulus clouds. An investigation is carried out on the convective structure of the atmosphere during active and suppressed periods of convection using data sets obtained from the Bay of Bengal and Monsoon Experiment (BOBMEX). The cumulus convection though being a small-scale phenomenon, still influences its embedding environment by interaction through various scales. This study shows the variation in the kinematic and convective parameters during the transition from suppressed to active periods of convection. Convergence in the lower levels and strong upward vertical velocity, significant during active convection are associated with the formation of monsoon depressions. The apparent heat source due to latent heat release and the vertical transport of the eddy heat by cumulus convection, and the apparent moisture sink due to net condensation and vertical divergence of the eddy transport of moisture, are estimated through residuals of the thermodynamic equation and examined in relation to monsoon activity during BOBMEX

A study on air-sea exchange processes and conserved variable analysis over the Indian Ocean with INDOEX IFP-99 data

The exchange properties across the Marine Boundary Layer influence atmospheric convection and determine to a large extent the atmospheric circulation in the tropics. In view of these phenomena, an analysis of the surface parameters such as sea-surface temperature, air temperature, wind speed, moisture and cloud cover along the ORV Sagar Kanya and Ronald H. Brown cruise track was carried out. An attempt was made to examine the changes in these parameters while the ship was passing across the ITCZ region. The humidity was found to be considerably high over the ITCZ than non-ITCZ regions. The air-sea interaction processes were studied by computing the radiation fluxes (short-wave and longwave) and the turbulent heat fluxes (sensible heat and latent heat) using semi-empirical models. Over ITCZ region a net heat loss was found which can be attributed to large convergence, strong convective activity and dense cloud cover, leading to drastic reduction in short-wave radiation. Latitudinal and longitudinal variation of surface meteorological parameters and turbulent fluxes were studied to establish a link with the prevailing convective and clear weather regimes along the cruise tracks. A conserved variable analysis of the conserved variable such as potential temperature, virtual potential temperature, saturation equivalent potential temperature and specific humidity is carried out at every point of upper air observation from Ronald H. Brown. The values are estimated up to a maximum of 3 km to cover the boundary layer. In the ITCZ region due to the convective mixing deeper boundary layers are formed, where a double structure is observed. The Marine Boundary Layer Height is also estimated using thermodynamic profiles

A comparative analysis of the characteristics of the Marine Boundary Layer with GCM and 1-D PBL model simulations using INDOEX IFP-99 data

Glass-sonde observations consisting of wind, temperature and relative humidity at different pressure levels that were obtained on board ORV Sagar Kanya cruise #141 (INDOEX IFP-99), during winter monsoon of 1999 were used for the present study. An attempt has been made to compare the simulation of the evolution of the Marine Boundary Layer as obtained from the one-dimensional PBL model of IIT Delhi, having TKE-ε closure scheme with that obtained from the GCM of NCMRWF having first order closure scheme. Simulation of various boundary layer characteristics including surface and upper air has been studied. The model simulations are compared with the available observations. Both the models simulated the vertical profiles reasonably well compared with the observations

A study on marine boundary layer processes in the ITCZ and non-ITCZ regimes over Indian Ocean with INDOEX IFP-99 data

A one-dimensional numerical planetary boundary layer (PBL) model was applied to simulate the dynamical and thermodynamical characteristics of the tropical Indian Ocean under varying convective regimes. Using sounding as well as surface meteorological data obtained during the INDOEX field phase, the PBL was validated for three different regions within the INDOEX domain. The three regions identified were, a coastal location representing suppressed convection, an open ocean region with medium convection, and a region of intense convection in the vicinity of the Inter-Tropical Convergence Zone (ITCZ). The model was integrated using observed sounding as initial as well as lateral boundary conditions, for a period up to 48 h. The model simulated surface fields as well as vertical profiles were compared with observations for the three cases. In general the model performance was good. The one-dimensional model could not simulate the dynamical features associated with advection and winds satisfactorily. However, the convective regimes are well simulated. As such, the PBL processes near the ITCZ were better simulated compared to the coastal regions. Results suggest that such a model can be used as a tool to develop high resolution, time-varying profiles over data-sparse regions to enhance mesoscale analysis

A study on the characteristics of the marine boundary layer over Indian Ocean with ORV Sagar Kanya cruise # 120 during 1997

The CLASS system observations of wind speed, wind direction, temperature and moisture at different pressure levels taken during the pre-INDOEX (Indian Ocean Experiment) cruise # 120 were used for the study. This article presents a study of marine boundary layer (MBL) characteristics, such as turbulent kinetic energy, MBL height, eddy diffusivity, etc. over the Indian Ocean using a multilevel one-dimensional PBL model with one and a half order TKE-ε closure scheme. The model also generates the vertical profiles of zonal wind, meridional wind, potential temperature and specific humidity. The model simulations are compared with the observations and the statistical analysis, viz. correlation coefficient; the RMS error is found to be satisfactory