Topic 2: Atmospheric Water Flashcards
Controls on Evaporation
- 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)
Shortwave radiation
- Incoming solar radiaton
- Some sensible heat, breaks bonds
Longwave radiation
- Absorbed at surface
- Re-radiated back
Net Radiation at the surface
Q*=Qs(in) -Qs(out) + Ql(in)-Ql(out)
Qe
- Latent Heat
- Energy transfer involved in phase changes
- Consumed during evaporation
- Released during condensation
Albedo (alpha) estimate
Q=Qs(in)(1-alpha) + Ql(in)-Ql(out)
Terrestrial Budget
- Energy from the sun that is not scattered or absorbed by the atmosphere
- Absorbed by Earth & Re-radiated as long wave radiation
Albedo
Reflective quality of a surface (clouds, cloud type, surface). Dictates amount of energy absorbed by Earth overall.
Qh
- Sensible Heat
- Energy (heat) flows from warm to cold
- via conduction (molecular transfer)
- via advection/convection (movement of the medium, e.g. water)
Advection/convection
Movement of the medium
e.g. water
Conduction
molecular transfer
Qg
- Heat exchange w/ substrate (ground)
- Sometimes not included in surface energy balance
Surface Energy Balance
Net Energy Qn
Qn=Q*+Qh+Qe+Qg=0
Controls on Evaporation
- 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)
Modelling evaporation/sublimation
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)
When are Evaporation rates high?
- Evap rates are high when there is lots of available energy (Q*,Qh)
- When the atmosphere is dry
- and when it is windy
Methods to measure both evaporation & transpiration
- 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)
Evaporation Pan
- 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
Lysimeter
- Method to measure evapotranspiration
- Add rain or actively add water
- let it percolate
- collect percolated water
Water Balance Equation
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
Hydrological Model
The concept of potential evapotranspiration (Theoretical calculation)
- Penman-Monteith is common equation
Means of describing Humidity (with units)
- 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 (%)
Vapour Pressure
- Partial pressure contribution of water to the total atmospheric pressure
- Measured in Pa or mbar
- Measures effect of water molecules in atmosphere
Relative Humidity
- How close an air parcel is to saturation
- Measured with a %
- % = Actual (ev, vapour pressure)/ Potential (es, saturation)
- Not the best method
Why is Relative Humidity not the best method for reporting/measuring humidity?
- % 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
At what temperature does vapour pressure double?
Every 11 degrees celsius vapour pressure will double
Dalton’s Law
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
What is the standard atmospheric pressure exerted by air?
101.325 kPa or 1013.25mb or 1 atm (Not 101.335 KPa like it was on the class notes)
Actual Vapour Pressure
Pressure resulting from the water molecules
Saturation Vapour Pressure
Partial pressure of the water molecules when the air is saturated
Net evaporation
more water molecules enter the vapour phase than return via condensation
Net Condensation
More water vapour molecules condense than vapourize
What is happening at the air/water interface (saturation)?
A continual process of evaporation & condensation
- Balanced & phase changes don’t halt entirely but exist together
- Equilibrium at saturation
What can affect the balance of evaporation & condensation at the saturation point?
When it’s colder there is:
- Less KE, = less evaporation
- shifts to condensing side of equilibrium
- lower vapour pressure and mixing ratio
Saturation vapour pressure
vapour pressure at equilibrium
What are the ~3 ways to think of Evapotranspiration? (types)
- Actual Evapotranspiration, ET
- Potential Evapotranspiration, ET0
- Crop Evapotranspiration
Penman-Monteith Equation
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)
Potential evaporation
A measure of what potentially could evaporate if there was a limitless supply of moisture
- Potentail ET = Max ET available
Where does Delta stand for in the Penman-Monteith equation?
- It is the curve (slope) of the saturation vapour pressure vs. temperature
- change in vapour pressure vs change in Temp
Thornewaite model
- 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
Impact of Soil Moisture
- 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”
Precipitation Processes (Forcing)
- Orographic uplift
- Frontal uplift (cold or warm fronts)
- Forced convection: large-scale air mass convergence
- Free (buoyant) convection
- Condensation alone isn’t enough
Orographic uplift
Topography creates obstacle for air (mountains)
- precipitates on windward side
- rain shadow on leeward side
Adiabatic processes
- 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
Lapse Rate
The rate of change in temperature with altitude
Dry adiabatic lapse rate (DAR)
DAR = -9.8 °C km^-1
Moist Adiabatic Lapse Rate (MAR)
MAR ~ -6°C km^-1
- But not true for entire planet
Why is the moist adiabatic rate different than the dry adiabatic rate?
- 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)
Frontal uplift
Driven by density contrasts
Type 1 Frontal uplift process
Warm, wet air will rise over cold, dry air
- Buoyancy driven
Type 2 Frontal uplift process
Cold air will wedge underneath warm air
Forced Convection
Convergence on lower pressure centres = uplift
- Convergence at expected latitudes based on large earth cycles
Free Convection
Also buoyancy driven
- Small scale (~1-20km)
- Differential heating based on different surface types resulting in different energy absorption
How do polynyas affect cloud processes?
Polynyas open arctic air to moisture from the ocean
- inject moisture into cold air
- Can cause low-lying clouds
Manufacturing Precipitation
- 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
Collision & Coalescence
Small droplets collide and coalesce with each other former larger droplets that begin to fall and capture even more droplets in its wake
Bergeron Process
- 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
How do crystals grow in the Bergeron Process?
- Riming (accretion): supercooled water droplets freeze on impact
- Aggregation: Like coalescence
Ice particle changes in cloud processes (Bergeron)
- As ice crystals fall and collided with super cooled drops, they get bigger by accretion
- Ice crystals colliding with each other form aggregates
What are the Problems with Measuring Rain with a Standard Rain Gauge ?
- Doesn’t give intensity (only volume)
- Must be read regularly in person
- Potential evaporation between readings
Measuring Rain w/ a Recording Gauge
- 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)
What are the Problems with Measuring Rain with a Recording Gauge?
- Tipping mechanism can’t keep up with intense rain
- Only measures liquid, BUT some setups can convert snow to liquid to get precipitation
Measuring Rain with Optical Methods
- Laser estimates as precipitation falls through laser point
- Can give snow rate
- Can also give size distribution of particles for liquid to snow
What are some potential errors involved with measuring precipitation
- 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.)
Measuring Precipitation:
WMO recommended gauge densities
- 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
What can a network of precipitation gauges be used for?
Modeling & interpolating a precipitation surface with isohyetal contours
- More gauges = more detail of model
Weather Radar
- 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
Spatial Distribution of Precipitation: Thiessen Polygons
- 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
Spatial Distribution of Precipitation: Methods
- All have benefits & limitations and give a different value for areal precipitation Interpolate between stations: - Theissen - Isohyetal - Inverse Distance Weighting (IDW) (GIS) - Kriging (GIS)
IDW
Inverse Distance Weighting
- Interpolation weights points based on which ones are closer having more influence on interpolated point
Kriging
Uses spatial autocorrelation
- Correlates a variable with itself over space
Areal precipitation
Precipitation (mm)* Area (km^2) = mm*km^2
- Weighted average which differs from average of just adding numbers and dividing by number of measurements
Isohyets
- Like contour lines, they connect areas of equal precipitation
- Take the mid value between the lines for average precipitation depth
Global Winds & Precipitation
- Precipitation patterns are closely tied to winds from atmospheric circulation
- Precipitation is produced by rising air
Where is precipitation most abundant?
- Where the atmosphere is unstable (thunderstorms)
- Where surface winds converge, low pressure areas (ITCZ)
- Where prevailing winds intercept a mountain slope
Major patterns of global wind & precipitation
- 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
Why do the west coast of all continents between 30-50 degrees have rainy winters?
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
