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Geog370Hdrlgy > Topic 2: Atmospheric Water > Flashcards

Topic 2: Atmospheric Water Flashcards

1
Q

Controls on Evaporation

A
  • Needs energy to break bonds (Scales with available radiative and sensible heat)
  • Need humidity gradient to drive moisture flux from high to low (Scales with change in q, i.e. qs-qa)
  • Need a way to remove moisture so the air above the surface does not get saturated (Scales with wind speed, v)
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2
Q

Shortwave radiation

A
  • Incoming solar radiaton

- Some sensible heat, breaks bonds

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3
Q

Longwave radiation

A
  • Absorbed at surface

- Re-radiated back

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4
Q

Net Radiation at the surface

A

Q*=Qs(in) -Qs(out) + Ql(in)-Ql(out)

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5
Q

Qe

A
  • Latent Heat
  • Energy transfer involved in phase changes
  • Consumed during evaporation
  • Released during condensation
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6
Q

Albedo (alpha) estimate

A

Q=Qs(in)(1-alpha) + Ql(in)-Ql(out)

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7
Q

Terrestrial Budget

A
  • Energy from the sun that is not scattered or absorbed by the atmosphere
  • Absorbed by Earth & Re-radiated as long wave radiation
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8
Q

Albedo

A

Reflective quality of a surface (clouds, cloud type, surface). Dictates amount of energy absorbed by Earth overall.

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9
Q

Qh

A
  • Sensible Heat
  • Energy (heat) flows from warm to cold
  • via conduction (molecular transfer)
  • via advection/convection (movement of the medium, e.g. water)
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10
Q

Advection/convection

A

Movement of the medium

e.g. water

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11
Q

Conduction

A

molecular transfer

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12
Q

Qg

A
  • Heat exchange w/ substrate (ground)

- Sometimes not included in surface energy balance

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13
Q

Surface Energy Balance

Net Energy Qn

A

Qn=Q*+Qh+Qe+Qg=0

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14
Q

Controls on Evaporation

A
  • Temperature
  • Energy
  • Wind (circulation)
  • How much water is on the ground available to be evaporated
  • Need a humidity (vapour pressure) gradient to drive moisture flux from high to low (Scales with change in q i.e. qs-qa)
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15
Q

Modelling evaporation/sublimation

A

From water/snow/ice): Evaporation Rate (E)= Qe (Latent Heat)/ Density of water *Lv(latent heat of evaporation
- Equation still holds for soils/sediments/vegetation but rates are limited by the available water (hence, potential evaporation)

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16
Q

When are Evaporation rates high?

A
  • Evap rates are high when there is lots of available energy (Q*,Qh)
  • When the atmosphere is dry
  • and when it is windy
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17
Q

Methods to measure both evaporation & transpiration

A
  • Evaporation pan (change in water height daily)
  • Lysimeter (Change in weight of a soil or snow sample)
  • Water Balance Equation
  • Energy Balance (theoretical calculation)
  • Hydrological model (e.g. Penman-Monteith)
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18
Q

Evaporation Pan

A
  • Method for measuring evapotranspiration
  • start daily with full pan
  • refill using graduated cylinder to record how much is filled back in during the day
  • Provides evap rate if water is available but isn’t representative of what is happening b/c standing water/soil water may not be available for evaporation at the time in reality
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19
Q

Lysimeter

A
  • Method to measure evapotranspiration
  • Add rain or actively add water
  • let it percolate
  • collect percolated water
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20
Q

Water Balance Equation

A

Evapotranspiration = Qin - Qout

  • basically water in - water out
  • works well for a small controlled basin
  • Can determine losses (animals, plants, groundwater)
  • May need to know about storage & precipitation
  • Evapotranspiration then = Qin - Qout +Precip + Change in Storage
  • Groundwater can create water balance with more out than in
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21
Q

Hydrological Model

A

The concept of potential evapotranspiration (Theoretical calculation)
- Penman-Monteith is common equation

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22
Q

Means of describing Humidity (with units)

A
  • Vapour pressure, ev (Pa or mbar)
  • Mixing ratio, wv (g water vapour/kg air)
  • Specific humidity, qv (g water vapour/kg air)
  • Absolute humidity rho v (g water vapour/m^3)
  • Relative humidity, RH (%)
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23
Q

Vapour Pressure

A
  • Partial pressure contribution of water to the total atmospheric pressure
  • Measured in Pa or mbar
  • Measures effect of water molecules in atmosphere
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24
Q

Relative Humidity

A
  • How close an air parcel is to saturation
  • Measured with a %
  • % = Actual (ev, vapour pressure)/ Potential (es, saturation)
  • Not the best method
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25
Q

Why is Relative Humidity not the best method for reporting/measuring humidity?

A
  • % is based on the capacity of the atmosphere to hold water vapour
  • But, cooler air has less capacity to hold water than warmer air (Potential for saturation is different)
  • Therefore, same amount of water vapour in the tropics would be less RH than that amount in the Arctic
  • % at one location has different volume than another location
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26
Q

At what temperature does vapour pressure double?

A

Every 11 degrees celsius vapour pressure will double

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27
Q

Dalton’s Law

A

Total pressure of a mixture of gases = sum of pressure constituents
- vapour pressure is the pressure exerted by water vapour that contributes to this Law

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28
Q

What is the standard atmospheric pressure exerted by air?

A 
101.325 kPa or 1013.25mb or 1 atm
(Not 101.335 KPa like it was on the class notes)
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29
Q

Actual Vapour Pressure

A

Pressure resulting from the water molecules

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30
Q

Saturation Vapour Pressure

A

Partial pressure of the water molecules when the air is saturated

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31
Q

Net evaporation

A

more water molecules enter the vapour phase than return via condensation

32
Q

Net Condensation

A

More water vapour molecules condense than vapourize

33
Q

What is happening at the air/water interface (saturation)?

A

A continual process of evaporation & condensation

  • Balanced & phase changes don’t halt entirely but exist together
  • Equilibrium at saturation
34
Q

What can affect the balance of evaporation & condensation at the saturation point?

A

When it’s colder there is:

  • Less KE, = less evaporation
  • shifts to condensing side of equilibrium
  • lower vapour pressure and mixing ratio
35
Q

Saturation vapour pressure

A

vapour pressure at equilibrium

36
Q

What are the ~3 ways to think of Evapotranspiration? (types)

A
  • Actual Evapotranspiration, ET
  • Potential Evapotranspiration, ET0
  • Crop Evapotranspiration
37
Q

Penman-Monteith Equation

A

Estimate of actual evapotranspiration
Needs:
- Temp, RH, 4 radiation components, pressure, aerodynamic resistance (wind), latent energy of vaporization, psychometric “constant” and surface parameters (terrain, vegetation, soil)

38
Q

Potential evaporation

A

A measure of what potentially could evaporate if there was a limitless supply of moisture
- Potentail ET = Max ET available

39
Q

Where does Delta stand for in the Penman-Monteith equation?

A
  • It is the curve (slope) of the saturation vapour pressure vs. temperature
  • change in vapour pressure vs change in Temp
40
Q

Thornewaite model

A
  • Used for Potential evapotranspiration
  • Needs:
    Length of day, number of days, average daily temperature, alpha (function of annual heat index), and heat index (function of 12 month mean Temp)
  • Length of day relates to solar radiation but doesn’t account for cloudy days
41
Q

Impact of Soil Moisture

A
  • Directly reduces available moisture at the surface
  • Decreases moisture in plants, increasing surface resistance
  • A function of soil type
  • Some soils hold water and don’t readily “give it up”
42
Q

Precipitation Processes (Forcing)

A
  • Orographic uplift
  • Frontal uplift (cold or warm fronts)
  • Forced convection: large-scale air mass convergence
  • Free (buoyant) convection
  • Condensation alone isn’t enough
43
Q

Orographic uplift

A

Topography creates obstacle for air (mountains)

  • precipitates on windward side
  • rain shadow on leeward side
44
Q

Adiabatic processes

A
  • Change in T based on change in volume
  • Don’t need to change energy b/c volume changes energy of an air parcel
  • adiabatic cooling as an air mass rises
  • No heat transfer with the environment (idealistic but a bad assumption)
  • PV=nRT
  • or P = ρ RT
  • Will eventually hit the Dewpoint
45
Q

Lapse Rate

A

The rate of change in temperature with altitude

46
Q

Dry adiabatic lapse rate (DAR)

A

DAR = -9.8 °C km^-1

47
Q

Moist Adiabatic Lapse Rate (MAR)

A

MAR ~ -6°C km^-1

- But not true for entire planet

48
Q

Why is the moist adiabatic rate different than the dry adiabatic rate?

A
  • Energy in the atmosphere from condensation releases latent heat
  • Moist rate is less because there is more water vapour
  • High altitude areas don’t have much water vapour, therefore, even if condensation is occurring and expelling energy, the water vapour isn’t enough
  • Tropics have lots of water vapour (lots of energy = lots of storms)
  • Assumes that latent heat of condensation isn’t actually leaving but stays in the cloud system (equation simplification)
49
Q

Frontal uplift

A

Driven by density contrasts

50
Q

Type 1 Frontal uplift process

A

Warm, wet air will rise over cold, dry air

- Buoyancy driven

51
Q

Type 2 Frontal uplift process

A

Cold air will wedge underneath warm air

52
Q

Forced Convection

A

Convergence on lower pressure centres = uplift

- Convergence at expected latitudes based on large earth cycles

53
Q

Free Convection

A

Also buoyancy driven

  • Small scale (~1-20km)
  • Differential heating based on different surface types resulting in different energy absorption
54
Q

How do polynyas affect cloud processes?

A

Polynyas open arctic air to moisture from the ocean

  • inject moisture into cold air
  • Can cause low-lying clouds
55
Q

Manufacturing Precipitation

A
  • Condesation droplets float suspended by slight updrafts
  • many per cm^3
  • Droplets grow very slowly through condensation processes
  • Typical cloud droplets are much smaller (100x) than a typical raindrop
  • Need ~1 million droplets for one raindrop
56
Q

Collision & Coalescence

A

Small droplets collide and coalesce with each other former larger droplets that begin to fall and capture even more droplets in its wake

57
Q

Bergeron Process

A
  • Nucleation and growth of snowflakes or ice crystals in cold clouds (-15 to -40)
  • Ice grows at the expense of liquid raindrops in the cloud
  • Smaller particles are more susceptible as they fall
  • Pressure difference between liquid and ice
58
Q

How do crystals grow in the Bergeron Process?

A
  • Riming (accretion): supercooled water droplets freeze on impact
  • Aggregation: Like coalescence
59
Q

Ice particle changes in cloud processes (Bergeron)

A
  • As ice crystals fall and collided with super cooled drops, they get bigger by accretion
  • Ice crystals colliding with each other form aggregates
60
Q

What are the Problems with Measuring Rain with a Standard Rain Gauge ?

A
  • Doesn’t give intensity (only volume)
  • Must be read regularly in person
  • Potential evaporation between readings
61
Q

Measuring Rain w/ a Recording Gauge

A
  • Tipping bucket & weighing rain gauges record precipitation rate at shorter time intervals
  • Provides intensity data (rain rate)
  • Remote, don’t need to check personally (Potentially for years)
62
Q

What are the Problems with Measuring Rain with a Recording Gauge?

A
  • Tipping mechanism can’t keep up with intense rain

- Only measures liquid, BUT some setups can convert snow to liquid to get precipitation

63
Q

Measuring Rain with Optical Methods

A
  • Laser estimates as precipitation falls through laser point
  • Can give snow rate
  • Can also give size distribution of particles for liquid to snow
64
Q

What are some potential errors involved with measuring precipitation

A
  • Wind results in under catch (can minimize with a shield)
  • Sublimation can affect the amount of precipitation that actually reaches the ground
  • Gauge must be level (knocked over by wind, animals, etc.)
65
Q

Measuring Precipitation:

WMO recommended gauge densities

A
  • Small mountain region 140-300km2/gauge (higher variability)
  • Temperate and tropical mountain regions 300-1000km2/gauge
  • Flat areas 5000-20000km2/gauge
  • More gauges are usually needed but cost & labour is expensive
66
Q

What can a network of precipitation gauges be used for?

A

Modeling & interpolating a precipitation surface with isohyetal contours
- More gauges = more detail of model

67
Q

Weather Radar

A
  • Electromagnetic signal sent by radar
  • Some is scattered back by precipitation
  • Can be used to detect clouds, rain etc.
  • Can give distance to precipitation and how much there is
  • Needs certain set-ups/ wavelength for different types of layers
  • Canada has poor coverage, US had great
  • Can put on planes/satellites
  • Gives Cross-section of cloud
  • Can help determine precipitation lost to sublimation that can’t be measured with a traditional gauge
68
Q

Spatial Distribution of Precipitation: Thiessen Polygons

A
  • works better over flat terrain
  • Connect gauges with a line and draw perpendicular line bisecting
  • Gives polygon where every point within is closest to only one station
69
Q

Spatial Distribution of Precipitation: Methods

A 
- All have benefits & limitations and give a different value for areal precipitation
Interpolate between stations:
- Theissen
- Isohyetal
- Inverse Distance Weighting (IDW) (GIS)
- Kriging (GIS)
70
Q

IDW

A

Inverse Distance Weighting

- Interpolation weights points based on which ones are closer having more influence on interpolated point

71
Q

Kriging

A

Uses spatial autocorrelation

- Correlates a variable with itself over space

72
Q

Areal precipitation

A

Precipitation (mm)* Area (km^2) = mm*km^2

- Weighted average which differs from average of just adding numbers and dividing by number of measurements

73
Q

Isohyets

A
  • Like contour lines, they connect areas of equal precipitation
  • Take the mid value between the lines for average precipitation depth
74
Q

Global Winds & Precipitation

A
  • Precipitation patterns are closely tied to winds from atmospheric circulation
  • Precipitation is produced by rising air
75
Q

Where is precipitation most abundant?

A
  • Where the atmosphere is unstable (thunderstorms)
  • Where surface winds converge, low pressure areas (ITCZ)
  • Where prevailing winds intercept a mountain slope
76
Q

Major patterns of global wind & precipitation

A
  • Precipitation follows temperature to some degree, hence a general decrease with latitude
  • The tropics are wet, subtropics are dry
  • Continental interiors are dry
  • West-East asymmetry with latitude (winds)
  • Seasonality including monsoons
77
Q

Why do the west coast of all continents between 30-50 degrees have rainy winters?

A

The large subtropical high and sub polar low follow the sun and so does the belt of onshore winds & rising air which brings dry summers and wet winters

Geog370Hdrlgy (9 decks)

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