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Geography 201: Hydrology > Lecture 3-Precipitation 1 > Flashcards

Lecture 3-Precipitation 1 Flashcards

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1
Q

Why does precipitation matter?

A
  • controls the hydrology of a region (temporal and spatial distribution of precipitation)
  • driver of hydrological cycle (precip influences what happens in the rest of the catchment)
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2
Q

What aspects of precipitation are hydrologist interested?

A
  • how much
  • when
  • where does it occur
  • where does it go
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3
Q

Precipitations mechanisms - water vapour

A

Maximum amount of water vapour the atmosphere can hold varies with temperature

  • heating increases parcel of air - more space for water vapour
  • cooling decreases parcel of air - less space for water vapour
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4
Q

Dew point temperature

A

Limit between if the air can hold more water (below dew point) or is fully saturated (above dew point), dividing line between evaporation or condensation/precipitation

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5
Q

How does air become saturated?

A
  • get colder
  • add more water

in terms of energy - easier to reach saturation through cooling than adding water

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6
Q

Temperature dependence

A

temperature of surrounding air parcels will affect if the temperature can change, which then determines if we reach dew pt temp

  • if surrounding air is the same temp hard to change
  • if surrounding is colder, can rise and cool
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7
Q

Precipitation occurs when:

A
  • air cools to become saturated
  • condensation nuclei present (particles in air)
  • droplets become heavy enough to fall
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8
Q

Main cooling mechanism:

A

uplift

  • as air rises it cools
  • as it rises its ability to hold water redcues
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9
Q

Air cooling processes (3):

A
  • synoptic systems (frontal/cyclonic)
  • orographic uplift
  • convectional uplift
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10
Q

Global forcing

A
  • precip intimately related to global energy balance
  • how air is transported around globe, what temperature/energy it might have, so what it can hold and what it is going to deposit
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11
Q

Global forcing concept: energy gradient

A
  1. Sun’s energy is distributed onto surface, direct at tropics, diffuse at poles because of further travel/oblique angle. This creates an energy gradient, more at tropics, less at poles.
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12
Q

Global forcing concept: moving energy

A
  1. Energy likes to become balanced, so we move energy from where its high to where its low. System is set up to move energy from tropics to poles. Conveyor belt of energy as a general pattern
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13
Q

Global forcing concept: Hadley cell

A
  1. Air at the tropics gets heated and rises, starts to intract with cool air parcels, get south/north cools and sinks at the tropic of cancer/capricorn
    (low pressure)
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14
Q

Global forcing concept:

Katabatic airflow

A
  1. At the poles really cold air in polar front, warmer air from the mid-latitudes gets pushed up over this
    (high pressure)
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15
Q

Frontal/cyclonic processes (synoptic systems)

A
  • air moves from high to low pressure

- westerly in NZ

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16
Q

Types of front systems:

A
  • cold fronts
  • warm fronts
  • occluded front (combination of warm and cold fronts)
17
Q

Cold fronts

A
  • cold, dense air undercuts warm buoyant air
  • intensity depends on the temperature difference between air masses
  • vigorous convection: warm air forced to rise quickly and becomes unstable
  • quick/short/heavy precipitation and wind at the front (at the gradient between air masses)
18
Q

Cold front characteristics:

A

before: warm, atmosphere pressure decreasing, winds N-NW, showers
during: temperature cools suddenly, pressure levels off then increases, variable and gusty wind, heavy rain/snow/hail
after: cold and getting colder, pressure increasing steadily, winds S-SE, showers then clearing

19
Q

Other considerations with frontal systems for why it doesn’t always behave on the ground as theorised:

A
  • topography (friction)

- air splitting by landform

20
Q

Warm fronts

A

-warm air rises over colder denser air
-gradual change of weather
(gentle nature)
-less vigorous than cold fronts (showers instead of storm)
-longer duration but lower intensity than cold fronts

21
Q

Warm front characteristics:

A

before: temp cool, pressure decreasing, wind W-NW, showers, snow, sleet or drizzle
during: warms suddenly, pressure levels off, variable wind, light drizzle
after: warms then temp stablises, slight rise in pressure, wind S-SE, no precipitation

22
Q

Occluded front

A
  • cold front catches up to a warm front
  • cold air, short spell of warm air in the middle, and another front of cold air
  • less predictable than cold front and warm front alone
23
Q

Orographic phenomena

A
  • associated with airflow over mountain ranges
  • airflow fast and turbulent (unstable)
  • orographic rainfall
24
Q

Windward side

A

air rises, cools, condenses and precipitation occurs (more wet)
precipitation might occur on the side or at the top, depends the height of mountain and how much water the air can hold

25
Q

Leeward side

A

air descends, warms, expands water holding capacity (more dry)

26
Q

Orographic precipitation in NZ

A
  • up to 10m per yr on west slopes of Southern Alps
  • Foehn effect on East side
  • much lower precip on Canterbury plains
27
Q

Convectional processes

A
  • air parcels are heated and rise rapidly and cools quickly causing rapid condensation
  • generally limited to tropics, but can occur on really hot summer days
  • really intense rainfall later in the day
28
Q

Convection characteristics

A
  • hot ground
  • very heavy rainfall
  • thunderstorms
  • sometimes severe winds
  • usually intense, short duration, late in day
29
Q

Spatial patterns of precipitation

A

Depending on where we are in the world we get different amounts of water but also different processes causing precip so different intensities and durations on the ground

30
Q

Why does it rain?

A

Vapour pressure changes

31
Q

When does it rain?

A
  • when warm, wet air is uplifted cooled to dew pt temp

- caused by: frontal systems, convection, orographic lift

32
Q

Where does it rain?

A

-where we have large temperature variations, a lot of energy in the system

Geography 201: Hydrology (8 decks)

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