Global climate and the hydrologic cycle
Key points
•The climate system in ultimately driven by the sun; through latent heat
exchanges, the global hydrologic cycle is an important component of
the climate system.
•Differential solar heating gives rise to an equator-to-pole temperature
gradient, resulting in a gradient in the height of atmospheric pressure
surfaces that drives the global atmospheric circulation.
•Patterns of precipitation and evapotranspiration reflect the effects of
latitude and regional aspects of atmospheric circulation linked to
orography, the distribution of continents and ocean, ocean currents and
other factors.
The hydrologic system is ultimately driven by
the sun, which radiates approximately as a
6000K blackbody (ε = 1). About 99.9% of
solar energy is emitted between wavelengths
of 0.15-4 μm (μm = 10-6 m). From Wien’s
law, the wavelength λ of maximum intensity is
at about 0.5 μm in the visible spectrum
λmax = 2.897x103 μm K/K
K is temperature in Kelvin. Net incoming solar
radiation at the top of the atmosphere Snet is
Snet = [(1-A).Sconπ.R2 ]/[4. π.R2]= 239 W m-2
Where A is planetary albedo (approximately
0.3), Scon is the solar constant (1367 W m-2)
and R is the earth’s radius (6371 km)
Stefan-Boltzmann equation for blackbody
emission E: E = ε.σ.T4, T in Kelvin, emissivity
ε = 1, σ = Stefan Boltzmann constant
The earth radiates back to space in longwave
radiation. Most of the radiation emission to
space is from the atmosphere and not the
surface. This is because of absorbtion and
emission by greenhouse gases like CO2 and
H20, i.e., the natural (and critical to life!)
greenhouse effect.
Global radiative balance and the greenhouse effect
The earth as a whole is close to a state of radiative balance, where net solar
radiation received at the top of the atmosphere is balanced by longwave
radiation emission to space:
Incoming = outgoing, or:
σ .Te4.4.π.R2 = (1-A).Sconπ.R2
where σ is the Stefan-Boltzman constant
(5.7x10-8 W m-2 K-4) and Te is the effective
radiation
emission
temperature
(or
effective temperature) of the earth of
approximately 255 K. The emission to
space peaks at about 10 μm.
From Wikipedia
As mentioned, because of absorbtion and emission by greenhouse gases, most of the
emission to space is from the atmosphere. Because of the counter-radiation back to the
surface from greenhouse gases (H20, CO2 , CH4 and others) the earth’s average surface
temperature is a habitable 288 K, 33K higher than the radiation emission temperature.
Average global energy flows (courtesy of Trenberth, Fasullo and Kiehl, BAMS, 2009).
Net solar radiation at the top of the atmosphere is the sum of the incoming solar and reflected
solar radiation, which balances the outgoing longwave radiation to space (radiative balance).
The above global mean view masks strong differences in all terms by related to latitude,
atmospheric and oceanic circulation and characteristics of the surface.
Top of atmosphere incoming solar radiation
http://visual.merriamwebster.com/earth/meteorology/seasons-year.php
Dingman 2002, Figure 3-6
The seasonal distribution of solar
radiation at the top of the atmosphere
(at left, in MJ m-2) is a function of earth
orbital geometry (top)
The global heat engine
Differential solar heating between low and high latitudes gives rise to a circulation of the atmosphere
and ocean that transports sensible heat, latent heat and geopotential poleward. As a result of these
transports, poleward of about 38 deg. in both hemispheres, longwave radiation emission to space
exceeds shortwave (solar) radiation gain. Without this transport, the polar regions would be colder and
the equatorial regions warmer than observed. Key point from the left-hand panel: radiative balance
holds only for the globe as a whole and not for individual latitudes [courtesy Kevin Trenberth, NCAR].
Northward energy transports
Zonal averages of the mean annual northward energy transport required by the net
radiation budget at the top of the atmosphere (RT). The total atmospheric transport
is AT and the ocean transport is OT. Units are petawatts (PW) [from Trenberth and
Caron, 2001]. Negative values in the southern hemisphere mean transport to the
south. Atmospheric transport dominates.
Average surface air temperature
http://planetariumweb.madison.k12.wi.us/smi-resources
Top of atmosphere net radiation budget
The global pattern of the annual mean net radiation budget at the top of the
atmosphere [from Trenberth et al., 2001, by permission of Springer-Verlag]. Note
that along with the basic latitudinal gradients, there are asymmetries that relate to
regional difference in albedo, ocean temperature and other factors.
Northward transport of total atmospheric energy
The zonal mean annual cycle of the northward atmospheric energy transport,
averaged for 1979-1998 (PW). Negative values in the Southern Hemisphere
mean transport to the south (towards the South Pole) [from Trenberth and
Stepaniak, 2003, by permission of AMS]. Note the strong seasonality in the
transport; the transport is strongest in the winter of each hemisphere.
Global atmospheric circulation
Dingman 2002, Figure 3-8
http://rst.gsfc.nasa.gov/Sect14/Sect14_1c.html
The global atmospheric circulation is shaped primarily by the Coriolis Force associated
with the earth’s rotation. large-scale orographic barriers such as the Rocky
Mountains and the Himalayas, and large-scale temperature contrasts between continents
and ocean. In middle and higher latitudes, most of the poleward atmospheric energy
transport is associated with “transient eddies”, better known as extratropical cyclones
and anticyclones. The atmospheric circulation has key controls on global, regional and
local patterns of precipitation and evapotranspiration.
Annual mean sea level pressure
Mean annual sea-level pressure calculated from the ECMWF 40-year reanalysis
(from Kallberg et al. 2005). Note the mean subtropical highs (anticyclones) and
mean subpolar lows in each hemisphere.
Global distribution of annual precipitation
http://oceanworld.tamu.edu/resources/oceanographybook/oceansandclimate.htm
Global Precipitation Climatology Program
The zonal mean pattern (averaging precipitation for each latitude across all
longitudes) is one of high precipitation in the equatorial regions associated with the
ITCZ, lower precipitation in the subtropics associated with the Hadley Cells, fairly high
precipitation in the middle latitudes linked to extratropical cyclones, and low precipitation
in the cold high latitudes. However, the spatial pattern is very complex, reflecting
things like orographic uplift, rain shadows, monsoonal circulations and ocean currents.
The figure at left is from the JRA-25 reanalysis.
Arctic precipitation
Mean precipitation north of 60°N for the four mid-season months, based on data from land
stations and the Arctic Ocean with bias adjustments, the NCEP/NCAR reanalysis and
satellite retrievals (over open ocean). The fields are based primarily on data for 1960-1989.
Contours are at every 10 mm up to 80 mm and at every 50 mm (dashed) for amounts of
100 mm and higher [from Serreze and Barry, 2005]. The Atlantic sector of the Arctic is
quite wet. Precipitation over other parts of the Arctic is quite low. Some land areas are
classified as polar desert. Precipitation peaks over the Atlantic sector during the cold
months and in summer over most land areas.
Global evapotranspiration
The global pattern of evapotranspiration, from Dingman 2002 (Figure 3-24). Evapotranspiration is in general largest over the equatorial regions and smallest in high
latitudes, reflecting the availability of energy to evaporate water. Evapotranspiration also
tends to be larger over ocean than land. However, values also reflect land precipitation
(over deserts, there is little water available to evaporate), wind speed and vertical
gradients in atmospheric humidity.
Soils are formed by the physical and chemical breakdown of rock.
The global distribution of soils is hence strongly determined by
patterns of temperature and precipitation
Biomes of the globe
http://www.colorado.edu/geography/courses/geog_1001_lab/
Vegetation across the globe can be divided into different biomes, and while the
distribution of biomes is strongly controlled by ranges in precipitation and
temperature, vegetation also affects climate through impacts on albedo and
evapotranspiration.
Storages and fluxes
Two schematics of the global hydrologic cycle showing key storages and fluxes,
the one of the left is courtesy of UCAR, and the one of the right is from Dingman 2002
(Figure 3-16). Storages are in 103 km3, and fluxes are in 103km3 yr-1. Note that the numbers
don’t match between the figures – it is hard to get estimates for some terms. Fluxes into
the atmosphere represent evapotranspiration, and fluxes out of the atmosphere represent
condensation; these represent strong energy transfers in the global climate system.
Northward transport of latent heat energy
The zonal mean annual cycle of northward latent heat energy transport
(water vapor), averaged for 1979-1998 (PW). Negative values in the
Southern Hemisphere mean transport to the south (towards the South Pole)
[from Trenberth and Stepaniak, 2003, by permission of AMS]. Transports
are strongest in the tropics .
Precipitable water is the equivalent liquid water depth of water vapor in the atmosphere. The
above figure is based on the NASA Atmospheric Infrared Sounder (AIRS) aboard the Terra
satellite. instrument. Over the tropics, precipitable water may exceed 70 mm. Over the
Antarctic continent it is only a few mm. The global average is about 25 mm. See previous
slide – compared to the global ocean, water storage in the atmosphere is very small.
http://photojournal.jpl.nasa.gov/catalog/PIA12096
A few basic considerations
Globally: P = ET
However, also globally,
•Oceans: E T > P
•Land: P > ET
Hence there is a net transfer of
water from the ocean to the land,
and this must appear as runoff, R
Considering all land masses together:
Residence time (S/I) from above numbers:
P
= ET + R
75 cm 48 cm 27 cm
100%
64%
36%
Global Atmosphere= 12,900/(71,000 + 1,000+
505,000) = 0.022 years = 8.2 days
Africa = 80% + 20%
N.A. = 55% + 45%
Antarctica = 17% + 83%
Global Ocean = 1,338,000,000/(458,000 +
44,700 + 2,200 +2,700 = 2,636 years
Atmospheric storage is small, with big fluxes into
or out of it. Ocean storage is very big compared
to the fluxes into or out of it.