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 E ~ P ~ 2.6 mm/day Source Reanalysis for

Precipitation (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 Balance P-E =  f = f in - f out –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 (Clausius- Clapeyron Eqn) Evaporation from the sea surface depends on R net,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 km 2 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 Moisture, growing season, and human land use play roles Latitude and continentality are both very important desert evergreen needleleaf forest broadleaf deciduous forest tundrabare groundice grasslands crops broadleaf evergreen forest

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

Boreal 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

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 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 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  (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 D T ~ 5 x m 2 s -1  = 1 day h T ~ 10 cm  = 1 yr h T ~ 1.5 m  = 10,000 yrh T ~ 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