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

• 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

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)

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

What causes precipitation

What causes precipitation?

• Moisture and energy• Ocean gives unlimited moisture (talking global

average here)• Constraints are from energy

Water cycle

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


• 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

Effects of warming on precipitation

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


• Increase is more dramatic in wet latitudes


• Dry latitudes may see a decrease


• “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


• At the marginal level, or local level, the precipitation response is less clear because of regional circulation shifts and model uncertainty

Mitigation of effects• Especially in the dry regions• there will be a slowdown in atmospheric

circulation

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)

Radiative forcing and its effects on precipitation

Radiative Forcing of the Hydrological Cycle

• The intensity of the hydrological cycle also depends on the radiative cooling of the troposphere

Energy budget


• Increases in GHG concentrations reduce the radiative cooling of the troposphere


• 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

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

convection-the atmosphere becomes unstable through heating (more than its surroundings)-significant evaporation, convective rain from convective clouds

The effects of warming on extreme precipitation

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

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

Solar radiation management

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)

Geoengineering

Geoengineering

• Relatively new field • Few studies look at it• Looking at SRM is limited by:– Gaps in understanding processes– Scarcity of studies

Geoengineering

• How to reduce sunlight reaching earth?– Solid or refractive disks in space– Dust particles in space• Feasibility is not assessed

SRM methods

• Increase stratospheric aerosol to produce a cooling effect, similar to an erupting volcano – Require replenishment

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

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?

SRM methods

• Cirrus Thinning: these clouds enhance the greenhouse effect (high, thin clouds) by warming the surface– Reduce humidity in the upper troposphere

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

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

Effects of SRM

• Mount Pinatubo 1991• 0.5 degrees C

7.6 Precipitation

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