微氣象學微氣象學 (( 全英文全英文 ))

授課老師授課老師 : : 游政谷游政谷

Instructor: Cheng-Ku YuInstructor: Cheng-Ku Yu

((MicrometeorologyMicrometeorology))

Micrometeorology(1)

What is “Micrometeorology”?What is “Micrometeorology”?The scope of micrometeorology is limited to only those

phenomena occurring in the atmospheric boundary layer (or called planetary boundary layer).

Significant exchange of momentum, heat, or mass takes place between the surface and the atmospheric air

These characteristics are primarily related to the small-scale processes. Although the atmospheric boundary layer comprises only a tiny fraction of the atmosphere, its associated small-scale physical processes are important for the human activities and even survival of life on earth

Course Outline

(1) Introduction (PBL Definition and Structure, (1) Introduction (PBL Definition and Structure, Turbulence)Turbulence)

(2) Energy Budget near the Surface (2) Energy Budget near the Surface

(3) Soil Temperature and Heat Transfer

(4) Air temperature and Humidity in the (4) Air temperature and Humidity in the Atmospheric Boundary LayerAtmospheric Boundary Layer

(5) Wind Distribution in the Atmospheric Boundary (5) Wind Distribution in the Atmospheric Boundary LayerLayer

Text bookIntroduction to Micrometeorology. Second Edition. By S.

Pal Arya, 2001.

Reference bookAn Introduction to Boundary Layer Meteorology. By Roland

B. Stull, 1988.

Grading:• Homework 20%• Discussions 20%• Paper review 25%• Final exam 35%

Chapter 1 Introduction

• The scope of micrometeorology is limited to those phenomena which originate in and are dominated by the shallow layer of frictional influence adjoining the earth’s surface, commonly known as the atmospheric boundary later (ABL) or the planetary boundary layer (PBL).

• The atmospheric boundary layer is formed as a consequence of strong interactions between the atmosphere and the underlying surface (land or water).

Boundary-layer definition: the boundary layer as that part of the troposphere that is directly influenced by the presence of the earth’s surface, and responds to surface forcings with a timescale of about an hour or less.

Within PBL, significant exchange of momentum, heat or mass takes place between the surface and the atmospheric air.

• PBL height (several tens of meters to several kilometers)

Depend on:

1. rate of heating or cooling of the surface 2. strength of winds 3. the roughness and topographical characteristics of the surface 4. large-scale vertical motions ( 數個 cm/s) 5. horizontal advections of heat and moisture

daytime: PBL height on the order of 1 km (0.2-5 km) nighttime: PBL height on the order of 100 m (20-500 m)

• A strong diurnal variation in meteorological variables can be found within the PBL

Fig1.1 to be illustrated in the class

• The atmospheric surface layer: Comprises the lowest one-tenth or so of the PBL and in which the earth’s rotational or coriolis effects can be ignored.

• The sharpest variations in meteorological variables with height occur within the surface layer and, consequently, the most significant exchange of momentum, heat and mass also occur in this layer.

Fig1.2 to be illustrated in the class

Turbulence: refers to the apparently chaotic nature of many flows, which is manifested in the form of irregular, almost random fluctuations in velocity and temperature around their mean values in time and space.

• The motions in the PBL are almost always turbulent

• In the FA, turbulence usually occurs in clouds except CAT (Clear Air Turbulence)

Fig1.3 to be illustrated in the class

Importance: responsible for the efficient mixing and exchange of mass, heat, and momentum throughout the PBL. Without turbulence, such exchanges would have been at the molecular scale in magnitudes 10-3~10-6 times the turbulent transfers that commonly occur.

• Much of the boundary layer turbulence is generated by forcings from the ground

Examples of forcing:

1. Solar heating of the ground during sunny days causes thermals of warmer air to rise

Fig1.4 to be illustrated in the class

2. Frictional drag on the air flowing over the ground causes wind shears to develop, which frequently become turbulent.

Fig1.5 to be illustrated in the class

3. Obstacles like trees and buildings deflect the flow, causing turbulent wakes adjacent to, and downwind of the obstacle.

Fig1.6 to be illustrated in the class

front wake

Fig1.7: When fluid encounters barrier, turbulences may form in the wake of the barrier

A B

Initial

A B

Few minutes

A B

Final

(30 min later)

Stream meets obstacles, producing turbulences and wavelet (Fig1.8)

Significance of the boundary layer (BL)

• People spend most of their lives in the BL

• Daily weather forecasts of dew, frost, and maximum and minimum temperatures are really BL forecasts

• Pollution is trapped in the BL (Fig1.9)

• Fog occurs within the BL

• The primary energy source for the whole atmosphere is solar radiation, which for the most part is absorbed at the ground and transmitted to the rest of the atmosphere by BL processes

• Crops are grown in the BL. Pollen is distributed by boundary layer circulations (Fig1.10)

• Cloud nuclei are stirred into the air from the surface by BL processes (Fig1.11)

• Virtually all water wapor that reaches the FA is first transported through the BL by turbulent and advective processes

• Thunderstorm and hurricane evolution are tied to the inflow of moist BL air (Fig1.12 and Fig1.13)

• About 50% of the atmosphere’s kinetic energy is dissipated in the BL

• Wind stress on the sea surface is the primary energy source for ocean currents

Severe air pollutions in Hongkong (Fig1.9)

In addition to insects, pollen can be transported to a much longer distance by near-surface airflow (Fig1.10)

Microburst associated with a small-scale vortex touching the ground (Fig1.11)

Cloud base

dust

Warm and moist BL air feeds thunderstorms and provides convective energy required for storm development (Fig1.12)

Warm and moist BL air over the ocean is critical for the development of tropical cyclones (Fig1.13)