近地面微氣象學

授課老師 : 游政谷

(Micrometeorology near the Ground)

Micrometeorology(1)

課程簡介 近地面微氣象學是在探討發生在大氣邊界層 (或可稱為行星邊界層 )的各種大氣現象 , 在大氣邊界層中 , 大氣與地球表面具有顯著的動量 ,質量及熱量交換 , 而其觀測特徵如溫度 ,速度及密度具有高度的變異性 , 且這些現象主要牽涉到小尺度的過程 .雖然大氣邊界層只佔大氣層的極小部分 , 然而其伴隨的小尺度物理過程對於人類活動及生物生存有重要影響 . 本課程內容將針對下列主題作有系統的闡釋 : 1. 微氣象學簡介 2. 近地面層的能量收支 3. 土壤溫度與熱量傳送 4. 大氣邊界層的溫度與濕度 5. 大氣邊界層的氣流特性 .

授課內容

1. 近地面微氣象簡介 (2週 )2. 近地面層的能量收支 (3週 )3. 土壤溫度與熱量傳送 (3週 )4. 大氣邊界層的溫度與濕度 (4週 )5. 大氣邊界層的氣流特性 (4 週 )

參考書名 :1. Introduction to Micrometeorology. Second Edition. By S.

Pal Arya, 2001.2. An Introduction to Boundary Layer Meteorology. By Rola

nd B. Stull, 1988.

成績計算方式 :• 作業成績 20%• 課堂參與及討論 20%• 期中考試 30%• 期末考試 30%

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 mainfested 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: 氣流遇障礙物 , 在障礙物後方形成 Turbulence 之實驗室照片

溪流遇障礙物 ( 石頭 ) 所產生的擾流與漣漪 (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

香港近地面層空氣污染嚴重 (Fig1.9)

花粉除了可透過昆蟲傳送 , 也可透過近地面氣流作較長距離傳送 (Fig1.10)

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

Cloud base

dust

暖濕的邊界層空氣從對流胞前緣進入系統 , 提供對流所需的能量 (Fig1.12)

海洋邊界層的暖濕空氣提供颱風內部對流所需 (Fig1.13)