What is Micrometeorology?

The study of the atmosphere at “small” scales.

Learning Objectives

• Some boring housekeeping stuff
• Define key units, and conventions
• Discuss the scales relevant to micrometeorology
• Talk about applications of the discipline

Fog in the Fraser Valley (Photo: A. Christen)

iClicker Test

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Scientific Notation

Table 1: Scientific notation can be used to units and make large/small numbers more readable.

Scientific notation	Prefix	Symbol
\(10^{12}\)	tera	T
\(10^{+9}\)	giga	G
\(10^{6}\)	mega	M
\(10^{3}\)	kilo	k
\(10^{2}\)	hecto	h
\(10^{-1}\)	deci	d
\(10^{-2}\)	cenit	c
\(10^{-3}\)	milli	m
\(10^{-6}\)	micro	\(\mu\)
\(10^{-9}\)	nano	n
\(10^{-12}\)	pico	p

The SI System

The SI (Système International d’Unités) is the official unit system in science. This system uses a small number of base units from which other derived units can be obtained.

Table 2: SI Base Units.

SI base unit	Symbol	Quantity
Meter	m	Length
Kilogram	Kg	Mass
Second	s	Time
Kelvin	K	Thermodynamic Temperature
Ampere	A	Electric current
Mole	mol	Amount of a substance
Candela	cd	Luminous intensity

Derived Units

Derived units can be formed and related to base units by the process of multiplication or division.

• This allows the representation of more complex phenomena (e.g., atmospheric pressure, radiative flux) in “relatively” simple terms.

Table 3: SI Derived Units.

SI unit	Symbol	Definition
Newton	N	kg m s-2
Pascal	Pa	N m-2
Joule	J	N m
Watt	W	J s-1

A Note: Kelvin vs. Celsius

• The unit for temperature is Kelvin (K, not \(^{\circ} K\)).
• Temperatures may be also indicated in degree Celsius ( \(^{\circ} C\) ); but differences should be in K.
  • 0 K represents absolute zero, lowest possible temperature.
  • 0 \(^{\circ} C\) is the freezing point of fresh water at 1 atm (mean atmospheric pressure at sea level).
• The conversion between Celsius and Kelvin is simple:

\[ T(K) = T(\deg C) + 273.15 \qquad(1)\]

Types of Data: Kelvin vs. Celsius

• Kelvin is on a ratio scale: a fixed, meaningful zero point and can only take positive values.
  • Allows relative comparisons: e.g., \(100 K = 2*50 K\)
• Celsius is on an interval scale: an arbitrary zero value and can take positive or negative values.
  • Prevents relative comparisons: e.g., \(100 ^{\circ} C \neq 2*50 ^{\circ} C\)
• Suggested Reading: Open Geomatics section 3.1 & 3.2 to understand the difference between types of data and measurement scales.

Programmatic Examples (iClicker)

Do you have any previous programming experience?

• A - No prior experience
• B - Some prior experience with R
• C - Some prior experience with Python
• D - Some prior experience with both R and Python
• E - No prior experience with R or Python, but some prior experience with another language

Programmatic Examples

These code blocks demonstrate a simple unit conversion in R and Python respectively. You can use syzygy to try them out.

R Code

# Converts between Celsius and Kelvin
TConvert <- function(Temp,uIn="C"){
    if (uIn == "C") {
       return(Temp+273.15)
    } else if (uIn == "K") {
       return(Temp-273.15)
    } else {
       stop("Invalid unit")
    }
}
# Celsius to Kelvin
TempC <- c(-10,0,10)
TConvert(TempC)

[1] 263.15 273.15 283.15

# Kelvin to Celsius
TempK <- c(200,300,400)
TConvert(TempK,uIn='K')

[1] -73.15  26.85 126.85

Python Code

# Converts between Celsius and Kelvin
import numpy as np
def TConvert(Temp,uIn="C"):
    if uIn == "C":
       return(Temp+273.15)
    elif uIn == "K":
       return(Temp-273.15)
    else:
       raise ValueError('Invalid unit')
# Celsius to Kelvin
TempC = np.array([-10,0,10])
TConvert(TempC)

array([263.15, 273.15, 283.15])

# Kelvin to Celsius
TempK = np.array([200,300,400])
TConvert(TempK,uIn='K')

array([-73.15,  26.85, 126.85])

Wind References

• In Meteorology: horizontal (2D) winds are named after the direction from which the air is moving.

Wind References

• In reality, wind is a vector consisting of three components (u, v, & w). They can be referenced to either:
  • Fixed cartesian coordinates (x, y, & z)
  • Mean flow: u is aligned with the mean flow, v (horizontal) and w (vertical) are orthogonal u

What is Micrometeorology

Measuring soil carbon loss after forest fires (<1m²)

Measuring wetland trace gas exchange (<1km²)

Clouds and circulation patterns in a mountainous terrain (<10km²)

How do feedbacks between ecosystems and the atmosphere influence microclimates and the global climate system? In this course we’ll focus on the interactions between the surface, vegetation and atmosphere, and the physical processes governing the transfer of heat, mass and momentum. Through this course you’ll also be exposed to the basic instrumentation and methods used in today’s monitoring and modeling of microscale climate and surface-atmosphere exchange. While this course examines the behavior of the atmosphere close to the surface, what we cover in this class implications for processes occurring at the scale of a single leaf to the entire planet!

Relevant Scales

Table 4: Spatial scales in meteorology.

Scale	Spatial Extent
Micro	10-2 to 103 m
Local	102 to 5 x 104 m
Meso	104 to 2 x 105 m
Macro	105 to 108 m

Oke and Christen (2014)

mm’s to hundreds to kms; <s to days

Relevant Scales

Micro-scale

Meso-scale

Macro-scale

In these terms GEOB 300 is mainly restricted to atmospheric features whose horizontal extent falls within the micro- and local scale categories.

Weather vs Climate?

What is the difference between weather & climate?

Micro-scale Weather vs Climate?

Micrometeorology focuses on short-term fluctuations of atmospheric variables (up to one hour) and the detailed physical processes (e.g. turbulence) that drive exchange at small scales.

Microclimatology deals with long-term integral effects (from hours to decades) namely, the average and variability of specific climates near the surface with respect to different geographic settings.

Boundary layers

In a fluid, a boundary layer refers to the region immediately above a surface where flow is significantly influenced by:

• Energy and mass exchange
• Surface friction.

Mean wind speed with height above a surface

We’re focused on what’s going on in the atmospheric boundary layer A boundary layer comes into picture when a fluid is interacting with a solid during its flow. 

When the fluid touches the surface, the layer making contact with the surface immediately comes to rest which is popularly known as no slip condition. Due to the property viscosity, this layer slows down the adjacent layer and so on. So the velocity profile varies parabolically and at some distance from the surface, the velocity is not affected by the viscous force. The thickness up to which the viscous forces are significant is known as the boundary layer thickness and this region is known as boundary layer. (Source: https://www.quora.com/What-is-a-boundary-layer)

The atmospheric boundary layer is formed as a consequence of the interactions between the atmosphere and the underlying surface (land or water) over time scales of a few hours to about 1 day.

Fluid Flow

Flow can be laminar (orderly) or turbulent (random).

• When flow interacts with a surface, it slows and generates turbulent motions.

What is a boundary layer?

Photo: Visualization of a boundary layer in a wind tunnel (M. A. Carper, Saint Anthony Falls Laboratory, University of Minnesota).

The Atmospheric Boundary Layer

The atmospheric boundary layer (ABL) refers to the lowest 300-3000 m of the troposphere. The ABL is controlled by the roughness, thermal mixing and the injections of moisture and pollutants from the underlying surface.

A humid and polluted ABL capped by clouds (Photo: A. Christen)

This layer is particularly characterized by well developed mixing (turbulence) generated by frictional drag as the Atmosphere moves across the rough and rigid surface of the Earth, and by the ‘bubbling-up’ of air parcels from the heated surface.

The boundary layer receives much of its heat and all of its water through this process of turbulence.

The ABL over Vancouver (Photo: A. Christen)

Lower parts of the fair weather cumulus clouds The height of the boundary layer (i.e. the depth of surface-related influence) is not constant with time, it depends upon the strength of the surface-generated mixing. By day, when the Earths surface is heated by the Sun, there is an upward transfer of heat into the cooler Atmosphere. This vigorous thermal mixing (convection) enables the boundary layer depth to extend to about 1 to 2 km. Conversely by night, when the Earths surface cools more rapidly than the Atmosphere, there is a downward transfer of heat. This tends to suppress mixing and the boundary layer depth may shrink to less than 100 m. (Sometimes called free air.) That portion of the earth’s atmosphere, above the planetary boundary layer, in which the effect of the earth’s surface friction on the air motion is negligible, and in which the air is usually treated (dynamically) as an ideal fluid.

Naturally this ideal picture can be considerably disrupted by large-scale weather systems whose wind and cloud patterns are not tied to surface features, or to the daily heating cycle.

The Atmospheric Boundary Layer

• The ABL reacts within several hours (< 1 day) to changes at the surface.
• We measure a diurnal cycle of temperature, humidity, wind and air pollutants in the ABL.
  • The free atmosphere above is less tied to the diurnal cycle; requires less frequent measurement

The ABL in the foreground, with the “free atmosphere” above.

Therefore we measure a diurnal cycle of temperature, humidity, wind and air pollutants in the PBL. We often forget that this daily routine does not affect the whole atmosphere! The influence of the surface is effectively limited to the lowest 10 km of the Atmosphere in a layer called the troposphere. Over time periods of about one day this influence is restricted to a very much shallower zone known as the atmospheric (planetary) boundary layer, often referred to simply as the ‘boundary layer’.

The Free Atmosphere (iClicker)

The free atmosphere refers to the region of the atmosphere farther away from the Earth’s surface that is less directly influenced by short time-scale processes at the surface, and is therefore not the primary focus of this course.

• A - True
• B - False

Applications

• Global climate change
• Clean energy

• Food and water resources
• Air pollution and industry

Climate Change

Rising CO2 concentrations

We know CO2 concentrations are rising And of course we know that this has important implications for the planet and is the leading cause of climate change (including rising temperatures, changes in precipitation patterns, more drought and heat waves, hurricanes will become stronger and more intense, rising sea levels (1-4 feet), loss of artic sea ice).

Climate Change

Sources

Land use changes: 12% 4.8 GtCO2/yr

Industry and agriculture: 88% 34.4 GtCO2/yr

Sinks

Atmosphere: 46% 17.2 GtCO2/yr

Oceans: 24% 8.8 GtCO2/yr

Terrestrial Ecosystems: 30% 11.0 GtCO2/yr

However, thankfully not all the CO2 that we emit stays in the atmosphere – Of what is emitted, about 46% remains in the atmosphere, 24% is absorbed by our oceans and 30% by the land. So the exchange of CO2 into and out of ecosystems is really key in regulating the global carbon cycle. Understanding land atmosphere interactions is key to understanding global carbon dynamics and climate change. -Average for 2007-2016

The breathing of the biosphere

This is one of my favorite clips that really highlights the importance and magnitude of land-atmosphere interactions how this influences GHG concentrations in the atmosphere This nicely illustrates how dynamic the exchange of GHG into and out of ecosystems is and how this really influences the global carbon cycle.

Global climate change

Knowledge on micrometeorology and climatology help:

• Determine greenhouse-gas emissions and uptake between.
• Study climate-surface feedbacks (e.g. thawing permafrost).
• Determine the melt rate and mass balance of ice-sheets and glaciers.

Important concepts from the class: Energy and mass balance, radiation, flux-gradient relations, eddy covariance, plant-atmosphere interactions

Food security

Food security is a major challenge of our century.

• Microclimates play a key role in understanding agriculture

Photo: J. Verfaillie

Food and Water Resources

Food requires water for growth, in many places the availability of water is the limiting factor.

Food and Water Resources

Warmer temperatures mean even when the rains come, evaporation can wick much of the moisture away.

Photo: A. Christen

In a world scorched by high temperatures, Canada’s Western provinces – home to more than 80 per cent of Canada’s farmable lands – have been the country’s hot spot this week. On Friday, the forecast highs in Medicine Hat, Alta. and Medina, Saudi Arabia, were both in the low 40s.

Warmer temperatures mean even when the rains come, evaporation can wick much of the moisture away.

(Source: https://www.theglobeandmail.com/canada/alberta/article-drought-in-western-canada-is-becoming-an-agricultural-nightmare-for/)

Food and Water Resources

Knowledge on micrometeorology and climatology help:

• Develop strategies to use water efficiency (soil moisture, evapotranspiration).
• Quantify and model local and regional rates of evapotranspiration.
• To predict frost and protect crops properly from frost.
• To manage wind erosion and sheltering.
• To efficiently plan agricultural operations (seeding, spraying, burning).

Course relevant materials: The water cycle at land-atmosphere interfaces, Measuring and modeling evapotranspiration, Soil thermal properties and heat transfer

Clean Energy

Energy is a central need of growing populations and economies. How can we secure energy without harming our climate system?

• Clean energy production, along with efficient distribution and use of energy, are key tasks of moving into a more sustainable 21st century.

Clean Energy

Knowledge on micrometeorology and climatology helps:

• Maximize use of renewable energies (wind, solar).
• Optimize thermal climate and energetics of buildings.
• Calculate wind loading of buildings and structures.

Wind turbine on Grouse Mountain, Vancouver (Photo: A. Christen)

Related to concepts we’ll touch on radiation and turbulence

Coming Up Next?

A deep dive into energy and mass balances.

• Next week we’ll start taking an in-depth look at the processes governing ecosystem scale fluxes of energy and mass.