ATMOSPHERIC BOUNDARY LAYER : An Overview

By SHARAD PACHPUTE PI: Prof. PMV Subbarao Department of Mechanical Engineering, IIT Delhi

Task 3: Atmospheric boundary layer and orographic dragRequirements: One student & one Post Doc Investigators PMV Subbarao, Dept. of Mechanical Engineering (Lead): Shantanu Roy, Dept. of Chemical Engineering G Jayaraman, Centre for Atmospheric Sciences OP Sharma, Centre for Atmospheric Sciences Subodh Kumar, Dept. of Computer Science and Engineering

Task elements (i)Formulation of the ABL equations with suitable closure assumptions and discrete analogues

Timeline 0 18 months

(ii) Gravity wave drag, boundary layer height, fluxes etc.

18 24 months 24 30 months

(iii) Code description

OUTLINE OF PRESENTATION1. What is ABL 2. Structure of ABL 3. Velocity Profile within ABL 4. Boundary Layer Depth 5. Similarity Theory : Flow equations for Velocity, Turbulence 6. CFD Modeling of ABL- Modification of turbulence model applied to ABL 7. Case Study 8. Cloud Topped ABL: Types of cloud, Modeling and types of cloud BL 9. Rainfall Boundary Layer 10. References : PhD thesis, Books and Journals, Research papers and list ofPresentations

1.What is the atmospheric boundary Layer (ABL/PBL) ?Stull (1988) The part of the atmosphere that is directly influenced by the presence of the earth s surface, and responds to surface forcing with a timescale of about an hour or less . Kaimal and Finnigan (1994) lowest 1-2 km of the atmosphere, the region most directly influenced by the exchange of momentum, heat, and water vapor at the earth s surface. Giiorgiio Crastto (2007) The Atmospheric Boundary Layer (ABL) is the turbulent atmospheric layer adjacent to the Earth s surface that directly feels the effect of the diurnal cycle at the surface. ABL is layer which is directly influenced by surface friction or the layer in which eddy viscosity plays important role.

Day time ABL : Sun heats the surface and turbulence is dominantly driven by buoyancy Large convective eddies provide vigorous mixing over, typically 1-2 Km depth Convective mixed boundary layer Night time ABL: Thinning at night due to radiative surface cooling Buoyancy suppresses the turbulence intensity (sink) Forced convection is source of turbulence Shallower BL, 100-200m in deep Stable Boundary Layer (SBL) Stably stratified ABL It prevails at night time as well as daytime in winter in mid latitudes as well as in polar regions

2.Atmospheric Boundary Layer Structure

Fig.2 Structure of ABL

Noon Diurnal BL

Sunset Nocturnal BL (SBL)

Sunrise

Noon Diurnal BL

Daytime: convective mixed layer + clouds (sometimes) Nocturnal : stable boundary layer + residual layerPhD Thesis: Investigation of stable and unstable boundary layer phenomena using observations and a numerical weather prediction (2009 )http://www.grin.com/en/doc/271723/investigation-of-stable-and-unstable-boundary-layer-phenomena-using-observations

2.1 The Mixed Boundary Layer (Day time BL) Characterized more by buoyant instability, i.e. free convection than by forced convection (wind shear) Diurnal variations in meteorological variables smaller than in surface layer. Ekman turning can be significant

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Convection and Advection Convection - the circulatory motion that occurs in a fluid at a non-uniform temperature owing to the variation of its density and the action of gravity, usually referring to vertical motions. Hot cells are transported within cooler surroundings

Advection horizontal transport in the atmosphere, including heat and mass!1/4/2012 Lecture 01 - Introduction 8

Free Convection Rising or sinking air caused by density differences Buoyant stability (s)

T2 T1 sw z 2 z1 Stable if S>0 Unstable if S