Earth's Climate System Today
advertisement
Earth’s Climate System Today Heated by solar energy Tropics heated more than poles Imbalance in heating redistributed Solar heating and movement of heat by oceans and atmosphere determines distribution of: Temperature Precipitation Ice Vegetation Electromagnetic Spectrum Electromagnetic energy travels through space Energy heating Earth mostly shortwave radiation Visible light Some ultraviolet radiation Incoming Solar Radiation Radiation at top of Earth’s atmosphere = 1368 W m-2 If Earth flat disk with no atmosphere, average radiation = 1368 W m-2 Earth 3-dimensional rotating sphere, Area = 4r2 Average solar heating = 1368 4 = 342 W m-2 30% Solar Energy Reflected Energy reflected by clouds, dust, surface Ave. incoming radiation 0.7 x 342 = 240 W m-2 Energy Budget Earth’s temperature constant ~15C Energy loss must = incoming energy Earth is constantly receiving heat from Sun, therefore must lose equal amount of heat back to space Heat loss called back radiation Wavelengths in the infrared (long-wave radiation) Earth is a radiator of heat If T > 1K, radiator of heat Energy Budget Average Earth’s surface temperature ~15C Reasonable assumption Surface of earth radiates heat with an average temperature of 15C However, satellite data indicate Earth radiating heat average temperature ~-16C Why the discrepancy? What accounts for the 31C heating? Energy Budget Greenhouse gases absorb 95% of the long-wave, back radiation emitted from Earth’s surface Trapped radiation reradiated down to Earth’s surface Accounts for the 31C heating Satellites don’t detect radiation Muffling effect from greenhouse gases Heat radiated back to space from elevation of about 5 km (top of clouds) average 240 W m-2 Keeps Earth’s temperature in balance Energy Balance Greenhouse Gases Water vapor (H2O(v), 1 to 3%) Carbon dioxide (CO2, 0.037%; 365 ppmv) Methane (CH4, 0.00018%; 1.8 ppmv) Nitrous oxide (N2O, 0.00000315%; 315 ppbv) Clouds also trap outgoing radiation Variations in Heat Balance Incoming solar radiation Stronger at low latitudes Weaker at high latitudes Tropics receive more solar radiation per unit area than Poles Variations in Heat Balance What else affects variation in heat balance? Solar radiation arrives at a low angle Snow and ice reflect more radiation at high latitudes Albedo Percentage of incoming solar radiation that is reflected rather than absorbed Average Albedo Sun Angle Affects Albedo All of Earth’s surfaces absorb more solar radiation from an overhead sun Water reflects <5% radiation from an overhead Sun Sun Angle Affects Albedo Water reflects a high fraction of radiation from a low-lying Sun Earth average albedo = 10% Pole-to-Equator Heat Imbalance Incoming solar radiation per unit area higher in Tropics than Poles Sun angle higher in Poles than Tropics Albedo higher at Poles than Tropics Variations in cloud cover affect heat imbalance Seasonal Change in Solar Radiation & Albedo Tilt of Earth’s axis results in seasonal change in Solar radiation in each hemisphere Snow and ice cover (albedo) Seasonal Change in Solar Radiation Large seasonal change in solar radiation between the hemispheres Seasonal Change in Albedo Increases in N. hemisphere winter due mainly to snow cover and to lesser degree Arctic sea ice Increases in S. hemisphere winter due to sea ice Albedo-Temperature Feedback Water a Key to Earth’s Climate Water has high heat capacity Measure of ability to absorb heat Heat measured in calories 1 calorie = amount of heat required to raise temperature of one gram of water by 1C -3 -2 Heat Capacity (cal cm ) = Density (g cm ) x Specific Heat (cal g-1) Specific heat of water = 1 Ratio of heat capacity water:ice:air:land 60:5:2:1 Heat capacity of air linked to water vapor Differences in Heating Land & Oceans Low latitude ocean major storage tank of solar heat Sunlight direct, albedo low, heat capacity high Heats surface; winds mix heat Contrast with land Albedo high, heat capacity & conductance low Tropical/subtropical lands become hot, but don’t store heat Sensitivity of Land & Oceans to Solar Heating Change in mean seasonal surface temperature greatest over large landmasses and lowest over oceans Thermal Response Different Large land masses heat and cool quickly Extreme seasonal temperature reached 1 month after Solstice Upper ocean heats and cools slowly Extreme seasonal temperature reached 2-3 months after Solstice Redistribution of Heat Heat transfer in Earth’ atmosphere Sensible heat Heat that a person directly senses Sensible heat = T x specific heat Latent heat (hidden or concealed) Additional heat required to change the state of a substance Sensible and latent heat affected by convection Convection Sensible Heat Sensible heating greatest At low latitude Overhead Sun Over land Low heat conductance (air heats) Dry regions Low humidity Sensible heat lowest Over oceanic regions Latent Heat Heat is temporarily hidden or latent in water vapor Powerful process transferring heat long distances Transfer is two step process Initial evaporation of water and storage of heat in vapor Later release of stored heat during condensation and precipitation (typically far from site of evaporation) Latent Heat 0°C-100°C, 1 calorie of heat energy of water releases -1 HCondensation 2O(l) H 2O(g) requires 540 cal g -1 needed to increase540 1gH cal latent heat of vaporization 2Og by– 1°C 80 watertransformation, freezes – latentice heat of melting 80cal calgg-1-1heat heatreleased requiredwhen for phase water Latent Heat of Vaporization Important – evaporation occurs at any temperature between 0-100°C Latent heat is associated with any change of state Therefore, during evaporation heat is stored in water vapor in latent form for later release Water Vapor Content of Air Saturation vapor density Warm air holds 10X more water than cold Redistribution of Latent Heat Evaporation in warm equatorial region Stored energy carried vertically and horizontally Condensation and precipitation releases energy Water Vapor Feedback Unequal Heating of Tropics and Poles Latitudes <35° have excess incoming solar radiation over outgoing back radiation Excess heat stored in upper ocean drives general circulation of oceans and atmosphere Atmospheric Circulation Atmosphere has no distinct upper boundary Air becomes less dense with increasing altitude Air is compressible and subject to greater compression at lower elevations, density of air greater at surface Constant composition to 80 km What drives atmospheric circulation? Free Convection Atmospheric mixing related to buoyancy Localized parcel of air is heated more than nearby air Warm air is less dense than cold air Warm air is therefore more buoyant than cold air Warm air rises Forced Convection Occurs when a fluid breaks into disorganized swirling motions as it undergoes flow Fluid flow can be laminar or turbulent Laminar vs. Turbulent Flow Whether a fluid flow is laminar or turbulent depends on Velocity (rate of movement) Geometry (primarily depth) Viscosity Turbulent flow occurs during high velocity movement of non-viscous fluids in unconfined geometries Forced Convection in Atmosphere Horizontally moving air undergoes turbulence Air is forced to mix vertically through eddy motions because of High velocity Depth of atmosphere Low viscosity Atmospheric Circulation Force of gravity maintains a stable atmosphere Most of the mass of air near surface As a result of atmospheric pressure Dense air at surface Air flows from high pressure to low pressure Flow is turbulent Turbulent flow produces vertical mixing Mixing by Sensible Heat Convection driven by sensible heat Air parcels rise if they become heated and less dense than surrounding air As air parcels rises, Air expands Air cools Air becomes less dense Air parcels stop rising Heat transferred vertically, since air forced from high to low pressure, heat also moves horizontally Adibatic Process Rising and sinking air change temperature with no gain or loss of heat Consider sinking parcel of air As it sinks, it contracts Contraction takes work Work takes (mechanical) energy Temperature of air rises Conservation of energy 1st law of thermodynamics Thermodynamics of Air First law of thermodynamics Heat added + work done = rise in Temp But, adibatic process (no heat added) Heat added + work done = rise in Temp Second term is not zero Work of compression results in a rise in temperature of air parcel Mixing by Latent Heat Water vapor is less dense than mixture of gases composing the atmosphere Evaporation adds water vapor to atmosphere and lowers its density Moist air rises, expands and cools until dew point reached When air becomes fully saturated Condensation begins Air releases latent heat Air heats and becomes less dense causing it to rise further Eventually water vapor lost, air parcel stops release of latent heat and stops rising Which Process More Important? Atmospheric circulation driven by adiabatic processes (sensible heat) redistributes about 30% heat Atmospheric circulation driven by latent heat redistributes about 70% heat Greater amount of heat stored in water Larger distances moist air parcels move Dry Adibatic Lapse Rate Rising and sinking dry air parcel cools and heats at a constant rate -1 Dry adibatic lapse rate = 10°C km Work required to lift an air parcel Mix of gases Acceleration of gravity Regardless of latitude, season, altitude, etc. a dry parcel of air will heat or cool at 10°C km-1 Dew Point Lapse Rate Consider a rising parcel of air with constant humidity Dew point decreases as parcel expands Drop in pressure, drop in dew point Lapse of dew point as parcel rises Dew point lapse rate 2°C km-1 Over 1 km, air cools by 10°C Air temperature rapidly approaches dew point as parcel rises As air temperature approaches dew point, cloud forms Wet Adibatic Lapse Rate As wet air rises, it cools, dew point reached and condensation begins Latent heat released Decreasing rate of cooling Wet adibatic lapse rate 4°C km-1 minimum (rapid condensation) 9°C km-1 maximum (slow condensation) Differences in temperature For same amount of cooling, warm air looses more water than cold air Summary Once saturation reached latent heat released as long as parcel continues to rise The saturated process assumes condensation products fall out of parcel, so the parcel maintains 100% humidity Upon decent, the parcel warms, relatively humidity falls below 100% After decent the parcel is warmer because latent heat was added during ascent Dry adibatic process reversible Wet adibatic process non-reversible
