Transcript OldHydrologicCycle

Land-Atmosphere Interactions
Need to supplement material from textbook
The Hydrologic Cycle
Earth’s Water Distribution
Groundwater
Atmospheric Water
annual mean precipitatble water (mm)
• Mean ~ 25
mm (1 inch)
• Mean precip
rate is about
2.6 mm/day
• Residence
time ~ 9 days
• Very steady
Source http://www.cdc.noaa.gov/
Reanalysis for 1968-1996
• E ~ P ~ 2.6
mm/day
January
Precipitation
(mm/month)
July
•
•
•
•
Very wet over tropics
Seasonal shift (N/S)
Monsoon regions
Extremely dry
subtropical highs
• Midlatitudes get
more summer rain
• July rainfall looks
like a map of forest
cover
Atmospheric Water Balance
• P-E = Df = fin - fout
– Net water import
by atmosphere
• Water vapor is
imported into the
tropics and
midlatitudes
• Water vapor is
exported from
the subtropics
Sources of Atmospheric Water
• Water vapor is
concentrated in the
tropics (ClausiusClapeyron Eqn)
• Evaporation from the
sea surface depends
on Rnet,T, u, and RH
• The greatest water
source is in the
subtropics, with near
zero LE in the ITCZ
Seasonal Hydrology
• “Potential evap” tracks
temp and radiation
• Winter rain/summer
dry climates on the US
West Coast
• Summer rain climates
in tropics
Seasonal Hydrology (cont’d)
• Actual E is strongly
limited by water
availability in many
places (E ~ P rather
than PE)
• Some midlatitude
locations (e.g., Boston)
have little seasonality
in P, but strongly
seasonal E
Land-Ocean Transfers
fluxes in cm/yr (adjusted for area of land and ocean)
• Ocean transfers water to land in atmosphere
• Land returns this water in rivers
• Most precip over land (48/75=64%) is “recycled” water
Precipitation Measurement
Primary data on precipitation is a can with a stick
Precipitation Measurement
• These gauges can work
well without supervision
in remote areas
• What about snow?
• Wind shielding: Alter or
Nipher shields
• Gauge catch is abysmal
• These are the “ground
truth” by which radar
and satellite products
are judged!
Precipitation Climatologies
• L&W (1990) used
spherical
interpolation to
estimate 0.5º
precipitation from
about 20,000 gauge
stations
• GPCC merges gauges
with two kinds of
satellite imagery to
estimate precip on a
2.5 º grid
Precipitation Climatologies (cont’d)
• Two climatologies
agree that west is
drier than east
• Many details are
different
• Effects of resolution
• Where are the
gauges?
– Land vs ocean
– Valleys vs mountains
PRISM Climatology (SW Oregon)
• Start with gauge data
and a digital elevation
model
• Divide the region into
topographic “facets” by
slope and aspect
• Develop regression
relationships between
gauge catch at each
station and elevation,
for each prism “facet”
• Apply statistics to each
gauge to make a map of
precipitation
Orographic
Effects
• Rain gauges are where
the people are (flatlands
and valleys)
• Most precip falls where
the people aren’t!
• Precipitation rates in
the west are dominated
by orographic effects
PRISM Climatology
Annual precip
estimates
(PRISM)
Patterns of Climate and Vegetation
Classification of Land Vegetation
Land Use
(Percentage of Total Land Area)
Tropical and Subtropical Vegetation
• Rainfall and its
seasonal
distribution
determine the
distribution of
plant types
• Savannas and
grasslands are
adapted to
seasonal and
longer dry periods
• Landscape
patterns strongly
influence
radiation budgets
and climate
Tropical Forest
• Located in equatorial zone of mean rising motion and
heavy precipitation during much of the year
• Low albedo, very strong energy absorption
• Broadleaf evergreen trees with extensive
understory, as many as 300 tree species per km2
• The most productive ecosystems on Earth
• Some are very deeply rooted (> 10 m) and can
withstand periods of severe drought
Grasslands and Savannas
• Subtropical
subsiding air
• As much as 85%
of biomass is
belowground
• Highly adapted to
drought, fire, and
grazing
• May be very
productive in rare
wet periods
Deserts
• Little or no
precipitation
• Little or no
vegetation
• Very high albedo
• Negative energy
balance
• Subsiding air
Temperate and Boreal Vegetation
tundra
evergreen
needleleaf
forest
grasslands
bare ground
ice
• Moisture,
growing
season, and
human land
use play roles
crops
broadleaf
deciduous
forest
desert
broadleaf
evergreen
forest
• Latitude and
continentality
are both very
important
Broadleaf Deciduous Forest
• Very productive
forests located in
midlatitudes
• Abundant
precipitation, but
growing season limited
by long cold winters
• Leaf-area equals that
of tropical forests
during growing season
• Mostly evergreen,
needleleaf trees
with little
understory
• Short growing
season, susceptible
to drought and fire
• Low evaporative
demand, so surface
may be wet (bogs
and fens)
• Very low albedo
Boreal Forest
Permafrost
Tundra
• High latitudes: cold dry climates, but very little evaporative
demand, so surface may be very wet
• Underlain by permafrost in many places
• Low-growing, non-woody plants
• Very short growing season
• Supports migratory mammals
Surface Energy Budget
Energy Storage
Integrate through mass of atmospheric column
But for the ocean:
• Heat capacity of 102/42 ~ 2.5 m of ocean water is
equal to total atmospheric column
• Seasonal warming/cooling of ocean to ~ 70 m
… about 25 times the heat capacity of the air
• On longer time scales, when the ocean says “jump,”
the atmosphere says “how high?”
Energy Storage on Land
Vertical heat flux in soil or rock:
Formulate change in storage as a flux divergence:
If physical properties (thermal conductivity) is constant
with depth, can simplify to
Soil Temperature
Assume periodic forcing of period t
(e.g., diurnal or seasonal cycles, ice ages, whatever).
Response of T(z) is also periodic, but damped and delayed
with depth relative to surface forcing
“Penetration depth” (e-folding) of temperature oscillations
forced by surface periodicity depends on period of forcing and
physical properties of material
DT ~ 5 x 10-7 m2 s-1
t = 1 day
t = 1 yr
t = 10,000 yr
hT ~ 10 cm
hT ~ 1.5 m
hT ~ 150 m
Diurnal Variations of Soil Temperature
• Huge range near surface
– 25 K diurnal cycle at 0.5 cm
– Max T around 2 PM
• Damped and delayed with depth
– Only 6 K diurnal range at 10 cm’
– Max T about 6 PM
– Negligible diurnal cycle at 50 cm
• Similar phenomena on seasonal
time scales