SIMULATION AND ANALYSIS OF ATMOSPHERIC BOUNDARY LAYER PROCESSES UNDER STRONG
AND WEAK WIND STABLE CONDITIONS
By
ADITI
Thesis submitted
in the fulfillment of the requirements for the award of the degree of DOCTOR OF PHILOSOPHY
-To -1.t.e
Centre for Atmospheric Sciences Indian Institute of Technology Delhi
January 2007
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CERTIFICATE
This is to certify that the thesis entitled "SIMULATION AND ANALYSIS OF ATMOSPHERIC BOUNDARY LAYER PROCESSES UNDER STRONG AND WEAK WIND STABLE CONDITIONS" being submitted by Ms. Aditi to the Indian Institute of Technology Delhi for the award of the degree of DOCTOR OF PHILOSOPHY, is a record of the original bonafide research carried out by her. Ms. Aditi has worked under my guidance and supervision and has fulfilled the requirements for the submission of this thesis. The results presented in this thesis have not been submitted in part or full to any other University or Institute for award of any degree or diploma.
(Prof. MAITHILI SHARAN) Centre for Atmospheric Sciences Indian Institute of Technology New Delhi — 110016, INDIA
ACKNOWLEDGEMENTS
It is a matter of immense pleasure for me to express my heartfelt veneration to Prof Maithili Sharan, Head Centre for Atmospheric Sciences, Indian Institute of Technology Delhi, for his valuable guidance and whole-hearted supervision throughout the tenure of this work. I am extremely thankful to him for his painstaking efforts and immense care in going through the manuscript.
I wish to express my deepest gratitude to Dr. T. V. B. P. S. Ramakrishna, NEERI, Nagpur for his valuable support and help during my research work. I also gratefully acknowledge the help received from Gr. Capt. Dr. 0. P. Madan and Dr. S. G.
Gopalakrishnan. In addition, I take this opportunity to express my gratitude to Prof. Sethu Raman, NCSU, Raleigh (USA) for scientific discussions.
I am grateful to Prof. (Mrs.) P. Goyal for her invaluable suggestions throughout the course of this work. I also take this opportunity to express my gratitude to all other faculty members of the Centre for Atmospheric Sciences, for their whole hearted support, inspiration and encouragement. In addition, I also like to acknowledge the support I received from the staff of the Centre.
My sincere thanks to Manish, Jagabandhu and Pramod for their kind cooperation and nice company. At this happy moment, I fondly remember all my colleagues in the Centre especially, Sujata, Neeru, Rashmi, Bharti, Madhu, Mourani, Sunita, Sankalp, Sushi!, Swagata, Lalit, Senthil, Subrat, Palash, Anumeha, Hashmi, Ashish and many others.
All my thanks are due to my adorable daughter Asmi, who means world to me.
She has been very supportive by her own little ways. Loving cheers are also to my nephew Munmun.
To my parents, I owe so much that it is impossible for me to express in words. It would not have been possible for me to complete my work without their cooperation. My father was always there to encourage and motivate me to achieve my goal. My mother sacrificed all her social engagements to be with my baby, when I was busy in my research work. It was her constant encouragement, love and support, which inspired me to fulfill one of her dreams.
I would like to thank my husband Dr. Siddhartha Singh for his great help, care and patience to bring this thesis to completion.
My brothers were always there by my side for any help that I needed. I thank both of them from the deepest of my heart. And finally I thank my in-laws for their hearty cooperation.
Date : 25. 01.2007 (ADITI)
ABSTRACT
The study of atmospheric boundary layer (ABL) is significant because the ABL connects the land surface with the free atmosphere through the turbulent transport of momentum, heat and moisture. The study assumes further significance under weak wind stable conditions since weak and variable winds occur for a considerable period of time all over the globe and are associated with high pollution potential. A number of observational and numerical studies conducted so far provide a better understanding of the ABL in convective conditions whereas the knowledge of boundary layer is limited under stable conditions. The complexity grows with the weakening of winds and thus the structure of SBL is not fully understood under weak wind stable conditions. The contribution of radiation and turbulence, the two major processes that control the evolution of stable boundary layer over a horizontally homogeneous terrain is not clearly understood under these conditions. Under weak wind stable conditions turbulence is found to be intermittent even near the surface which complicates the determination of surface fluxes under these conditions. The objective of this work is to analyze and simulate the characteristics of surface/boundary layer characteristics under weak wind stable conditions.
Monin-Obukhov (M-0) similarity with the specification of similarity functions of momentum (4m) and heat OW is often used to determine the fluxes in the surface layer. Linear similarity functions proposed by Businger et al. (1971) and Dyer (1974) are commonly used to compute surface fluxes under stable conditions. However, these functions are not valid for large values of Richardson numbers found in a number of observational studies (Agarwal et al. 1995, Yadav et al. 1996), specially in weak wind stable conditions. A systematic mathematical analysis is carried out to analyze the extent of applicability of these linear similarity functions to compute surface fluxes in weak wind stable conditions. The linear
functional forms are found to be applicable as long as the value of bulk Richardson number (RIB) is smaller than Prty/f32 where Prt is the turbulent Prandtl number and 13 and y are constants appearing in the functions 4,„ and 4h. These linear forms underestimate the Obukhov length (L) resulting in large value of stability parameter (z/L) when RIB > 13,-(y/I32 occurring in weak wind stable conditions. Using the data of Electric Power Research Institute (EPRI), it is found that in 70 % of weak wind cases RIB > Pry/P2.
Surface layer characteristics are analyzed by computing surface layer parameters using similarity functions proposed by Beljaars and Holtslag (1991) in both strong and weak wind stable conditions. The observations from Cooperative Atmosphere Surface Exchange Study (CASES-99) experiment are utilized to validate the computed fluxes in both strong and weak wind stable conditions. Empirical formulations for the eddy diffusivities of momentum (K M) and heat (KH) and drag (CD) and heat (CH) exchange coefficients, as power law function of RIB are proposed under both strong and weak wind conditions. Proposed relations are validated with the observations taken from Indian Institute of Technology Delhi low wind diffusion experiment, the Land Surface Processes Experiment (LASPEX), the Hanford diffusion experiment, the Cabauw field experiment and the CASES-99 experiment. The fluxes computed using the proposed empirical relations are found to be in good agreement with those based on similarity theory as well as the turbulence measurements taken during CASES-99 experiment.
The linear similarity functions are modified by different investigators (Webb 1970, Clarke 1970, Hicks 1976, Beljaars and Holtslag 1991, Blumel 2000, Cheng and Brutsaert 2005) over the years to be applicable at large values of RIB. A performance analysis of these modified non-linear forms is carried out to compute surface fluxes under weak wind stable conditions. For this purpose, the data from three different field experiments such as EPRI, CASES-99 and LASPEX are utilized. Using EPRI data, it is found that all the functions
ii
perform equally well in strong wind conditions whereas in weak wind conditions, the fluxes computed from different similarity functions differ somewhat from each other.
The EPRI data set lacks turbulence measurements, thus observations from CASES-99 experiment are taken to check the applicability of different similarity functions. A good agreement is found in the observed and computed fluxes under strong wind conditions.
However, u• is under predicted by all the functions and the computed heat flux with different similarity functions are found to be closer to each other and relatively closer to observations under weak wind conditions.
Due to limited data in weak wind stable conditions in CASES-99 experiment, the observations from LASPEX experiment are utilized to obtain data under weak wind stable conditions. The justification of persistence of weak wind condition is given by means of regional climatology, upper air observations and surface observations. The winds are found to be weak for 67% of time during the experiment. The performance of all the functions is found to be comparable in weak wind stable conditions and the computed fluxes show deviations from the observations. A number of possible reasons are given for the deviations between the observed and computed fluxes at Anand. It is also concluded that the traditional M-0 theory is not able to simulate the observed fluxes well in weak wind stable conditions during the experiment.
A one-dimensional meteorological model originally developed by R.A. Pielke, modified with turbulent kinetic energy mixing length closure, a layer-by-layer emissivity- based radiation scheme is used to analyze the mean structure and evolution of the nocturnal boundary layer (NBL) under strong and weak wind conditions. The similarity functions proposed by Beljaars and Holtslag (1991) in the surface layer are incorporated in model. Four case studies — two in strong wind and one each in moderate and in weak wind conditions are undertaken. It is found that in all the four cases ranging from strong to weak geostrophic
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forcing, the model reproduced the observed mean profiles, their evolutions in the NBL, and the inertial oscillations reasonably well. The NBL developed into three layers: 1) Very close to the surface, radiative cooling dominated over turbulence cooling. In this layer, late in the night, cooling caused by radiation may be offset by turbulence warming, causing net reduction in cooling. 2) A layer above it, turbulent cooling was the dominant mechanism. 3) Near the top of the turbulent layer and above, clear-air radiative cooling was the dominant mechanism.
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CONTENTS
Abstract List of Figures List of Tables
Chapter 1 GENERAL INTRODUCTION
(i — iv) (viii — xvi)
xvii Page No.
I Introduction 1
1.1 Atmospheric Boundary Layer 2
1.1.1 Structure of ABL 3
1.2 Surface Layer 5
1.2.1 Monin - Obukhov Similarity 6
1.2.2 The Richardson Number 10
1.3 Governing, Equations in the Boundary Layer 11
1.4 Characteristics Features of the SBL 18
1.4.1 Surface Inversion 19
1.4.2 Low Level (Nocturnal) Jet 20
1.4.3 Gravity (Buoyancy) Waves 21
1.5 Status of the SBL Studies 22
1.5.1 Observational Studies in the SBL 22
1.5.2 Modeling Studies in the SBL 25
1.5.3 Weak Wind SBL 31
1.6 Organization of Thesis 36
Chapter 2 ANALYSIS OF APPLICABILITY OF LINEAR SIMILARITY FUNCTIONS TO COMPUTE SURFACE FLUXES IN WEAK WIND STABLE CONDITIONS
2.1 Introduction 42
2.2 Observational Data 43
2.2.1 Analysis of Data for Stable Conditions 44
2.2.2 Classification of Wind 45
2.3 Methodology 46
2.3.1 Surface Layer 46
2.3.2 The Bulk Richardson Number 48
2.4 Analytical Expression for Stability Parameter z/L using Linear 49 Similarity Functions
2.5 Estimation of z/L in terms of RIB using Beljaars and Holtslag 52 Similarity Functions
2.6 Results and Discussions 54
2.7 Conclusions 56
Chapter 3 STUDY OF SURFACE LAYER CHARACTERISTICS IN THE STABLE BOUNDARY LAYER UNDER STRONG AND WEAK WIND CONDITIONS
3.1 Introduction 68
3.2 Computation of Surface Layer Parameters 69
3.2.1 Surface Fluxes 69
3.2.2 Eddy Diffusivities of Momentum and Heat 70 3.2.3 Drag and Heat Exchange Coefficients 71
3.3 Data 71
3.4 Determination of Surface Layer Parameters 72
3.5 Results and Discussions 73
3.5.1 Variation of RiB 73
3.5.2 The Inverse of Monin-Obukhov Length (l/L) 73 3.5.3 Variation of Surface Fluxes 73
3.5.4 Validation of Fluxes 74
3.5.5 Eddy Diffusivity Coefficients of Momentum and Heat 75 3.5.6 Drag and Heat Exchange Coefficients 76 3.5.7 Validation of Empirical Relations for KM and KH & CD 77
and CH with Different Experiments
3.6 Conclusions 79
Chapter 4 PERFORMANCE OF DIFFERENT SIMILARITY FUNCTIONS TO COMPUTE SURFACE FLUXES UNDER WEAK WIND STABLE CONDITIONS
4.1 Introduction 95
4.2 Methodology 97
4.2.1 Computation of Surface Fluxes 97 4.2.2 Different Functional Forms of Similarity Functions 98
4n, and 4h
4.2.3 Analytical Method for Estimation of z/L Proposed by 104 B lum el
4.3 Results and Discussions 106
vi
4.3.1 EPRI Experiment 108
4.3.2 CASES-99 Experiment 109
4.3.3 LASPEX 111
4.3.3.1 Justification of Weak Wind Conditions at 112 Anand
4.3.3.2 Screening of Data for Strong and Weak 115 Wind Stable Conditions
4.3.3.3 Validation of Fluxes 116
4.4 Conclusions 118
Chapter 5 MEAN STRUCTURE OF THE NOCTURNAL BOUNDARY LAYER UNDER STRONG AND WEAK WIND CONDITIONS : EPRI CASE STUDY
5.1 Introduction 142
5.2 Observational Data 144
5.3 Numerical Model 145
5.3.1 Model Equations 145
5.3.2 Radiation Parameterization 146 5.3.3 Determination of the Inversion Layer Depth 148 5.3.4 Initial and Boundary Conditions 148
5.4 Results and Discussions 150
5.4.1 Mean Structure of the NBL 150 5.4.2 Evolution of the Lower NBL 154 5.4.3 Integrated Cooling Budget within the Inversion 158
Layer
5.4.4 Shear and Buoyancy Budget 159 5.5 Discussion on Assumptions of the Model and Availability of 160
Data
5.6 Conclusions 161
Chapter 6 CONCLUSIONS AND SCOPE FOR FUTURE RESEARCH
6.1 Conclusions 185
6.2 Surface Layer 186
6.3 Boundary Layer 188
REFERENCES 192
BIO-DATA 219
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