Land-Atmosphere InteractionsLand-Atmosphere Interactions

Need to supplement material from textbook

The Hydrologic CycleThe Hydrologic Cycle

Earth’s Water DistributionEarth’s Water Distribution

GroundwaterGroundwater

Atmospheric WaterAtmospheric Waterannual mean precipitatble water (mm)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

• E ~ P ~ 2.6 mm/daySource http://www.cdc.noaa.gov/

Reanalysis for 1968-1996

Precipitation Precipitation (mm/month)(mm/month)

January

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 BalanceAtmospheric Water Balance

• P-E = f = 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 Sources of Atmospheric WaterWater

• Water vapor is concentrated in the tropics (Clausius-Clapeyron 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 HydrologySeasonal 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)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 TransfersLand-Ocean Transfers fluxes in cm/yr (adjusted for area of land and fluxes in cm/yr (adjusted for area of land and

ocean)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 MeasurementPrecipitation Measurement

Primary data on precipitation is a can with a stick

Precipitation MeasurementPrecipitation 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 ClimatologiesPrecipitation 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 Precipitation Climatologies (cont’d)(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)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 Orographic EffectsEffects

• 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 ClimatologyPRISM Climatology

Annual precip estimates (PRISM)

Patterns of Climate and Patterns of Climate and VegetationVegetation

Classification of Land Classification of Land VegetationVegetation

Land Use Land Use (Percentage of Total Land Area)(Percentage of Total Land Area)

Tropical and Subtropical Tropical and Subtropical VegetationVegetation

• 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 ForestTropical 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 SavannasGrasslands 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

DesertsDeserts• Little or no

precipitation

• Little or no vegetation

• Very high albedo

• Negative energy balance

• Subsiding air

Temperate and Boreal Temperate and Boreal VegetationVegetation

• Moisture, growing season, and human land use play roles

• Latitude and continentality are both very important

desert

evergreenneedleleafforest

broadleafdeciduousforest

tundra bare ground ice

grasslands

crops

broadleafevergreenforest

Broadleaf Deciduous ForestBroadleaf 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

Boreal ForestBoreal Forest

• 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

PermafrostPermafrost

TundraTundra

• 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 BudgetSurface Energy Budget

Energy StorageEnergy Storage

• 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?”

Integrate through mass of atmospheric column

But for the ocean:

Energy Storage on LandEnergy 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 TemperatureSoil Temperature

Assume periodic forcing of period (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

= 1 day hT ~ 10 cm

= 1 yr hT ~ 1.5 m

= 10,000 yr hT ~ 150 m

Diurnal Variations of Soil Diurnal Variations of Soil TemperatureTemperature

• 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