Observational and Modelling Studies of the Marine Atmospheric Boundary Layer over the Tropical Indian Ocean during INDOEX

Table of Contents

1. Introduction

1.1 General Introduction

1.2 Atmospheric Boundary Layer (ABL)

1.2.1 Depth of the ABL over land and the ocean

1.2.2 General Structure of the ABL

1.2.3 Factors Influencing the Structure of the ABL

1.3 Diurnal Evolution of the ABL

1.3.1 Convective Boundary Layer (CBL)

1.3.2 Residual Layer (RL)

1.3.3 Stable Boundary Layer (SBL)

1.4 Stability of the ABL

1.5 Features of the ABL over Oceanic Surface

1.5.1 Sea Surface Temperature (SST)

1.5.2 Surface Waves

1.6 Oceanographic Field Experiments: An Overview

1.6.1 International Indian Ocean Expedition (IIOE)

1.6.2 Barbados Oceanographic and Meteorological Experiment (BOMEX)

1.6.3 Atlantic Trade Wind Experiment (ATEX)

1.6.4 GARP (Global Atmospheric Research Program) Atlantic Tropical Experiment (GATE)

1.6.5 Air Mass Transformation Experiment (AMTEX)

1.6.6 Joint Air-Sea Interaction (JASIN) Experiment

1.6.7 Maritime Remote Sensing (MARSEN) Experiment

1.6.8 Coastal Ocean Dynamics Experiment (CODE)

1.6.9 Genesis of Atlantic Lows Experiment (GALE)

1.6.10 Humidity Exchange over the Sea (HEXOS) Programme

1.6.11 Frontal Air Sea Interaction Experiment (FASINEX)

1.6.12 Tropical Ocean Global Atmosphere (TOGA) and TOGA Coupled Ocean Atmosphere Response Experiment (TOGA COARE)

1.6.13 Shelf Mixed Layer Experiment (SMILE)

1.6.14 Atlantic Stratus Transition Experiment (ASTEX)

1.6.15 Surface of the Ocean, Flux and Interaction with the Atmosphere (SOFIA)

1.6.16 Structure des Exchanges Mer-Atmosphère, Propriétés des Héterogénéités Océaniques: Recherche Expérimentale (SEMAPHORE)

1.6.17 Central Equatorial Pacific Experiment (CEPEX)

1.7 Features of the Tropical Indian Ocean

1.7.1 Historical Background

1.7.2 Tropical Indian Ocean Region

1.8 Indian Ocean Experiment (INDOEX)

1.8.1 Objectives of the INDOEX Campaign

1.8.2 Cruise Track of INDOEX, IFP-99 Campaign

1.9 Objectives and Scope of the Present Study

2. Instrumentation and Database

2.1 General Introduction

2.2 Ship-Borne Instrumentations onboard ORV Sagar Kanya

2.3 Tower-based Instrumentation at KCO

2.4 Upper Air Observations (GLASS Sonde Launches)

2.4.1 GLASS Facility Description

2.4.2 GLASS Radiosonde Deployment

2.4.3 Radiosonde Sensor Specifications

2.5 Database Used in the Present Study

3. Air-Sea Interaction Processes over the Tropical Indian Ocean

3.1 General Introduction

3.2 Surface Layer Meteorological Observations

3.2.1 INDOEX, IFP-99: Cruise Track

3.2.2 Spatio-Temporal Variation of Meteorological Parameters along the cruise track

3.3 Method of Analysis

3.3.1 Estimation of friction velocity (u*) and scaling Temperature (θ*)

3.3.2 Estimation of Integrated Stability Functions (ψm and ψh)

3.3.3 Estimation of Roughness Length (z0)

3.3.4 Determination of Exchange Coefficients (CD, CH and CE) and the surface fluxes (τ, HS and HL)

3.4 Wind Speed Dependence of Air-Sea Exchange parameters

3.4.1 Variation of Surface Roughness length (z0, z0t and z0q)

3.4.2 Variation of Air-Sea Exchange Coefficients

3.4.3 Variation of Neutral Drag Coefficient (CDN)

3.4.4 Wind Speed Dependence of Air-Sea Fluxes

3.5 Spatio-Temporal variation of Air-Sea Exchange parameters Along the cruise track

3.5.1 Cruise Leg-1 (Meridional track-AB)

3.5.2 Cruise Leg-2 (Zonal track-BC)

3.5.3 Cruise Leg-3 (Meridional track-CD)

3.5.4 Cruise Leg-4 (Zonal track-DE)

3.6 Fluxes in relation to NCMRWF Wind Analysis

3.7 Comparison with Pre-INDOEX, INDOEX - FFP98 and TOGA-COARE Fluxes

3.8 Summary and Conclusions

3.9 Discussion

4. Vertical Structure of the MABL over the Tropical Indian Ocean

4.1 General Introduction

4.2 Data Processing and Analysis

4.2.1 Moisture Variables

4.2.2 Classification of Data

4.3 Mean Vertical Structure of the MABL over the Indian Ocean during INDOEX, IFP-99

4.4 Degree of Convection prevailing over the Tropical Indian Ocean along the cruise track

4.4.1 Variation in q, θE and θES along the cruise track at seven standard altitude levels

4.4.2 Comparison of θE, δθ and PSPD profiles over the ITCZ and the non-ITCZ regions

4.5 Mixed Layer Height Variability along the cruise track of INDOEX, IFP-99 campaign

4.5.1 Mixed Layer Height Variability during forward track

4.5.2 Mixed Layer Height Variability during return track

4.5.3 Spatial Variation in Mixed Layer Heights and LCL along the cruise legs

4.6 Double Mixed Layer Structure over the Central Arabian Sea

4.7 Summary and Conclusions

5. ABL Characteristics over Kaashidhoo through Observations and Simulations

5.1 General Introduction

5.1.1 Kaashidhoo: Site Description

5.2 Surface Layer Meteorological Observations over Kaashidhoo during INDOEX, IFP-99

5.3 GLASS Sonde Observations over Kaashidhoo during INDOEX, IFP-99 campaign

5.3.1 Degree of Convection Prevailing over Kaashidhoo

5.3.2 Mixed Layer Height variability over Kaashidhoo

5.4 Mesoscale Modelling over Kaashidhoo

5.4.1 Mesoscale Models

5.4.2 OSU 1-D PBL (CAPS) Model

5.4.3 ARPS Model

5.4.4 Model Run Configuration

5.5 Sensitivity of Model to Input Parameters

5.5.1 OSU Model Sensitivity

5.5.2 ARPS Model Sensitivity

5.6 Simulation of ‘u’, ‘v’, ‘θ’ and ‘q’ over Kaashidhoo

5.7 Concluding Remarks

6. Summary and Conclusions

Research Objectives and Themes

This thesis investigates the structural characteristics and associated dynamics of the Marine Atmospheric Boundary Layer (MABL) over the western tropical Indian Ocean. By utilizing meteorological data collected during the Indian Ocean Experiment (INDOEX), the research aims to improve the understanding of air-sea interaction processes, the vertical structure of the MABL in relation to convective regimes, and the diurnal variability of the ABL over the Kaashidhoo island using both observational data and mesoscale model simulations.

• Physics of air-sea interaction processes and the role of wind speed in estimating exchange parameters.
• Dynamics of the vertical MABL structure in relation to convection near the Inter Tropical Convergence Zone (ITCZ).
• Observational analysis of the ABL structure over the Kaashidhoo island.
• Mesoscale model simulations (OSU and ARPS) of thermodynamic profiles and their validation against observations.

Excerpt from the Book

Summary of Chapters

Chapter 1. Introduction: This chapter introduces the concept of the Atmospheric Boundary Layer (ABL) and the Marine Atmospheric Boundary Layer (MABL), highlighting their importance for weather prediction and reviewing major international oceanographic field experiments conducted prior to the INDOEX campaign.

Chapter 2. Instrumentation and Database: This chapter details the meteorological sensors mounted on the ORV Sagar Kanya and the Kaashidhoo Climate Observatory (KCO), including specifics on the GLASS Sonde launches and the data acquisition systems used for the study.

Chapter 3. Air-Sea Interaction Processes over the Tropical Indian Ocean: This chapter addresses air-sea interaction through the development of a revised bulk aerodynamic algorithm, examining the wind speed dependence of exchange parameters and the spatio-temporal variations of fluxes across the cruise track.

Chapter 4. Vertical Structure of the MABL over the Tropical Indian Ocean: This chapter analyzes the vertical structure of the MABL, focusing on convection in the ITCZ and non-ITCZ regions, mixed layer height variability, and the occurrence of an anomalous double mixed layer structure over the Central Arabian Sea.

Chapter 5. ABL Characteristics over Kaashidhoo through Observations and Simulations: This chapter describes the ABL features over the island of Kaashidhoo using surface and upper air observations and evaluates the performance of two mesoscale models, the OSU 1-D PBL and ARPS, in simulating the lower atmosphere circulation.

Chapter 6. Summary and Conclusions: This chapter synthesizes the major findings from the study, confirms the significance of the results for improved PBL and Global Circulation Modelling, and discusses the scope for future research.

Keywords

Marine Atmospheric Boundary Layer, MABL, INDOEX, Tropical Indian Ocean, Air-Sea Interaction, Convection, ITCZ, Kaashidhoo, Mesoscale Modelling, OSU 1-D PBL Model, ARPS Model, Bulk Aerodynamic Algorithm, Mixed Layer Height, Thermodynamics, Wind Speed Dependence.

Frequently Asked Questions

What is the primary objective of this thesis?

The primary objective is to improve the scientific understanding of the structural characteristics and dynamics of the Marine Atmospheric Boundary Layer (MABL) over the data-sparse western tropical Indian Ocean using unique data from the INDOEX, IFP-99 campaign.

What are the core research themes?

The core themes include the underlying physics of air-sea interaction processes, the vertical structure of the MABL in relation to varying convective regimes (ITCZ vs. non-ITCZ), and the application of mesoscale models to study the ABL over the isolated island of Kaashidhoo.

Which scientific methods are employed?

The research employs a combination of observational analysis using surface tower-based sensors and balloon-borne GLASS Soundings, alongside one-dimensional vertical column numerical modelling using the Oregon State University (OSU) 1-D PBL model and the Advanced Regional Prediction System (ARPS).

What is covered in the main body of the work?

The main body focuses on the derivation of a revised bulk aerodynamic algorithm, statistical analysis of exchange parameters, vertical profiling of thermodynamic variables, and the simulation of ABL diurnal evolution to test model performance under differing convective conditions.

How is the data organized for analysis?

Data is organized into two main categories: surface layer observations from ship-based booms and towers, and upper air observations obtained from GPS/Loran Atmospheric Sounding System (GLASS) Sondes.

Which keywords best characterize this research?

The research is best characterized by terms such as MABL, INDOEX, air-sea interaction, convective regimes, Kaashidhoo, and mesoscale modelling.

What is the significance of the double mixed layer structure?

The double mixed layer observed over the Central Arabian Sea is identified as an atypical structure likely sustained by anticyclonic circulation, providing new insights into MABL dynamics that are currently lacking in standard climate models.

Why are the mesoscale model simulations important?

These simulations are critical for bridging the gap between observed data and weather forecasting capabilities, particularly for understanding how thermodynamic profiles evolve diurnally over small islands where routine observations are sparse.