7.6 precipitation
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7.6 Precipitation. Climate vs. Weather . Climatological Rainfall. This chapter looks at the processes that control the “climatological” distribution of rainfall. Climate. The average of individual weather systems (mid latitude depressions, tropical convective cells) and patterns - PowerPoint PPT PresentationTRANSCRIPT
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7.6 Precipitation
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Climate vs. Weather
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Climatological Rainfall
• This chapter looks at the processes that control the “climatological” distribution of rainfall
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Climate
• The average of individual weather systems (mid latitude depressions, tropical convective cells) and patterns
• Weather cannot be predicted beyond a certain amount of time
• Climate takes averages to predict where and when systems and patterns will tend to occur again
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How do we predict weather?
• We can accurately predict 3-4 days in advance• Geostationary orbiting satellites gather visual
wavelength data– They use this to estimate cloud albedo, water
content, and in doing so the p(rain)
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What is rainfall climatology• Measuring, understanding, predicting rainfall
distribution across different regions of Earth • These predictions are made depending on air pressure,
humidity, topography, cloud type• The measurements are taken by remote sensing
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What causes precipitation
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What causes precipitation?
• Moisture and energy• Ocean gives unlimited moisture (talking global
average here)• Constraints are from energy
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Water cycle
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Atmospheric circulation
• Large scale movement of air • The means by which thermal energy is
distributed on Earth– Varies year to year but remains fairly constant
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Atmospheric circulation
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Atmospheric circulation
• These are the wind belts that girdle the planet• They are grouped into three cells: Hadley,
Ferrel, Polar – Most of the vertical motion occurs in Hadley
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Effects of warming on precipitation
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Effects of warming on large scale precipitation trends
• Globally averaged precipitation increases with the global mean surface temperature
• The change ranges from 1.5 to 3 % per degree C of warming that we see – Considerable regional variability
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Effects of warming on large scale precipitation trends
• Increase is more dramatic in wet latitudes
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Effects of warming on large scale precipitation trends
• Dry latitudes may see a decrease
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Effects of warming on large scale precipitation trends
• “wet get wetter” and “dry get drier” response is evident at large scales
• It is the result of a change in water vapor carried by circulations
• Also, wet regions import from dry regions
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Effects of warming on large scale precipitation trends
• At the marginal level, or local level, the precipitation response is less clear because of regional circulation shifts and model uncertainty
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Mitigation of effects• Especially in the dry regions• there will be a slowdown in atmospheric
circulation
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Overall understanding
• We can safely make claims about the effects of warming on ocean precipitation
• Responses over warming land are iffy because certain relationships are not well understood (soil moisture precipitation feedbacks)
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Radiative forcing and its effects on precipitation
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Radiative Forcing of the Hydrological Cycle
• The intensity of the hydrological cycle also depends on the radiative cooling of the troposphere
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Energy budget
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Radiative Forcing of the Hydrological Cycle
• Increases in GHG concentrations reduce the radiative cooling of the troposphere
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Radiative Forcing of the Hydrological Cycle
• When the radiative cooling of the troposphere is reduced, the rainfall rate is reduced
• The strength of the circulation is also reduced • So even through even though the mean
precipitation should go up by 1.5-3% per degree Celsius, the increase in GHG reduces it by about 0.5% per degree Celsius
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Effects of aerosol cloud interactions on precipitation
• Aerosols influence cloud microphysical structure (convective intensity)
• They mostly affect the atmospheric heating rate – For this reason they have mostly been studied
regarding their effects on the spatial-temporal distribution of precipitation, versus global averages
– Limited and ambiguous evidence
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convection-the atmosphere becomes unstable through heating (more than its surroundings)-significant evaporation, convective rain from convective clouds
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The effects of warming on extreme precipitation
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Warming’s effect on extreme precipitation
• Precipitation from individual storms will increase with available moisture content in the atmosphere near the surface
• The rate is 6-10% per degree C• But there are longer intervals between storms
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GCM predictions • Poor at simulating precipitation
extremes • Plus predictions on warming vary• Not generally regarded as reliable re:
extremes– Local temperature may not be a good
proxy for assessing the effects of warming
– They tend to covary with other meteorological factors• Humidity, atmospheric stability, wind
direction
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Solar radiation management
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Geoengineering
• Definition: broad set of methods to intentionally alter the climate system to alleviate the effects of climate change – Solar radiation management (SRM): counter the
warming associated with GHG by reducing the amount of sunlight that gets absorbed
– Carbon Dioxide Removal (ch. 6) • Reduce the amount of sunlight hitting the
earth, or make the planet more reflective (clouds, atmosphere)
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Geoengineering
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Geoengineering
• Relatively new field • Few studies look at it• Looking at SRM is limited by:– Gaps in understanding processes– Scarcity of studies
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Geoengineering
• How to reduce sunlight reaching earth?– Solid or refractive disks in space– Dust particles in space• Feasibility is not assessed
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SRM methods
• Increase stratospheric aerosol to produce a cooling effect, similar to an erupting volcano – Require replenishment
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SRM methods• Cloud brightening: boundary
layer clouds cool the planet – Small changes in albedo or
extent has big effects on radiation budget
– Systematically introduce cloud seeds to boundary layer (Cloud condensation nuclei)
– Could produce strong negative forcing
– Clouds with weak precipitation are best
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SRM methods• Surface albedo changes: urban areas, croplands,
grasslands, deserts, ocean surface• Whitening of urban areas might have effects such
as -0.17 W/square meter – High uncertainty – Limited studies – Side effects for photosynthetic activity?
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SRM methods
• Cirrus Thinning: these clouds enhance the greenhouse effect (high, thin clouds) by warming the surface– Reduce humidity in the upper troposphere
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Effects of SRM
• Simplest SRM studies can be performed in climate model through simulations
• SRM affects temperatures in the daytime only, versus GHG increases which raise temperatures regardless of the time
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Effects of SRM
• Some uniform decrease in sunlight reaching the surface will offset mean CO2 warming
• Could theoretically counteract anthropogenic climate change, cooling the Earth to preindustrial levels in 1-2 decades– Known from climate models– Data from eruptions
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Effects of SRM
• Mount Pinatubo 1991• 0.5 degrees C