Simulation of Marine Boundary Layer Characteristics Using a 1-D Pbl Model over the Bay of Bengal During Bobmex-99

Simulation of Marine Boundary Layer Characteristics using a 1-D PBL Model over the Bay of Bengal during BOBMEX-99

N. V. Sam, U. C. Mohanty* and A. N. V. Satyanarayana

Centre for Atmospheric Sciences

Indian Institute of Technology, Delhi,

New Delhi, India

* Corresponding Author:

Prof. U. C. Mohanty, Centre for Atmospheric Sciences, IIT Delhi, New Delhi ÐŽV110 016

email: mohanty@cas.iitd.ernet.in

Abstract

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 multiÐŽVlevel PBL model with a TKE-Ñ"Ð¥ 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.

Keywords: Sensible and Latent Heat Fluxes, Marine Boundary Layer, and Turbulent

Kinetic Energy

1. Introduction

One of the primary objectives of the Bay of Bengal and Monsoon Experiment (BOBMEX) is to study the convective systems over the Bay of Bengal. The mechanism of its genesis and propagation as a synoptic scale system over the Bay of Bengal is also an important part of the experiment. BOBMEX, an observational programme under the Indian Climate Research Programme (ICRP) was aimed at measuring important variables of the atmosphere, ocean and from their interface, to have a greater depth of insight into some of the processes that govern organized convective activity over the Bay of Bengal and its variability (Bhat et al 2001). Understanding the nature of the feedbacks between the atmospheric convection and surface conditions of the bay is important for understanding the variability of convection over the bay. TOGA COARE (Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment), which was aimed to describe the coupling of west Pacific warm pool to the atmosphere (Webster and Lukas 1992), gave an insight into atmosphere-ocean coupling on timescales that is intra-seasonal (Godfrey et al. 1998; Shinoda et al. 1998). The movement of the Tropical Convergence Zone (TCZ) over the oceans during the active and weak phases of monsoon may depend on the feedbacks due to the mid-tropospheric warming by clouds in the same region. This movement is supposed to be characterized by the intensified circulation by other TCZs in the vicinity (Sikka and Gadgil, 1980). It is well established that Convective Available Potential Energy (CAPE) is high before the growth of a deep convection (Williams and Reno, 1993) and as it starts precipitating, winds and downdrafts lower the energy of the air near the surface, while deep cloud activity makes the upper troposphere warmer, making the atmosphere stable and hence substantially reducing CAPE (Emmanuel, 1994).

The surface fluxes of sensible and latent heat provide the energy for driving the atmospheric disturbances and the Marine Boundary Layer (MBL), plays an essential role in regulating the transport of energy and moisture upward into the atmosphere from the surface. The energy supplied to the atmosphere is mostly trapped in the MBL, except in regions of deep convection. Therefore, it is important to understand the various characteristics of the MBL namely the MBL height, the thermal stratification of the atmosphere, etc., during the growth of a deep convective activity. Studies carried out in the past over the Pacific Ocean show that MBL plays a key role in the growth and sustenance of the tropical disturbances. In regions of deep convection, there is an upward transport of moisture rich air in the MBL into the cumulonimbus clouds and downward transport of drier air to the surface. This interaction between convection and the boundary layer processes can lead to intra-seasonal variability of convection (Neelin et al, 1987; Yano and Emmanuel, 1991). It is also well known fact that majority of the disturbances that are responsible for the monsoon rainfall over the Indian region are generated over the warm ocean around the subcontinent, the maximum numbers observed over the Bay of Bengal. The frequency of the genesis of these disturbances and the role of MBL in its growth and modifications needs to be understood to link it with the variability of the monsoon.

The objectives of this paper are to analyse the MBL characteristics, while accounting for viz., the vertical stability of the atmosphere and the structure of the atmospheric boundary layer and the surface fluxes over the Bay of Bengal during the final phase of BOBMEX (1999), during three different synoptic situations. Some of the important questions that have been addressed are: what is the structure of the boundary layer over the Bay of Bengal during the southwest monsoon period? What are the local factors that influence the variability in the boundary layer height? The analysis further provides insights into the temporal evolution of the turbulent kinetic energy (TKE) that is the contribution due to shear and buoyancy with respect to high wind speed and convectively active state of the atmosphere respectively. One of the secondary objectives of the study is to validate the model