Discuss the effects that a
hypothetical increase in clouds (high clouds, low clouds and both) could have
on the atmospheric temperature at the Earth’s surface. Discuss the effects of
increases in cloud amount and cloud properties.
20th October 2006
Understanding the atmosphere and that way in which it works is crucial in the discipline of atmospheric science. The IPCC (Intergovernmental Panel on Climate Change) is the largest collaboration of scientists (IPCC, 2001 [a]) working on the science behind climate change and their subsequent impacts. Their conclusions derive the greatest understanding so far on the role of anthropogenic influences on climate change. Their overall conclusion is that “the global average surface temperature has increased over the 20th century by about 0.6oC” (IPCC, 2001 [a]).
In an effort to understand the anthropogenic effects on climate, atmospheric scientists attempt to model the natural atmospheric processes. The increasing complexity of climate modeling can be seen in the diagram below; which illustrates how the inclusion of atmospheric science and anthropogenic emissions are only beginning to be understood.
The effects of the atmosphere act similar to that of choosing to shelter under a tree when it rains. Whilst the tree can be assumed to provide sufficient cover, the density of the leaves, the number of branches and the actual type of leaf will all determine your ability to stay dry. Such is the same with the atmosphere; as Earth is continually bathed in a stream of electromagnetic radiation, our atmosphere acts like the rain filtering trees. The atmosphere shields Earth through processes of absorption, scattering and reflection; keeping us protected from harmful UV radiation and more importantly, the surface 40oC warmer than without it (Barry, 1992). The effect of our atmosphere overall is to provide a blanket protecting us from the full effects of Incident Solar Radiation (ISR). However, it is also important to note that the atmosphere is not a single layer, but can be divided into layers according to its vertical profile of temperature, its gaseous composition, or its electrical properties (Ahrens, 2007).
All incoming radiation before reaching the surface must penetrate through these layers. What is of greatest importance with relation to this essay is that the presence of clouds, and their effects on climate occur within the lowest reaches of the atmosphere within the Troposphere (Ahrens, 2007) and at a maximum height of 18,000m within the Stratosphere (Ahrens, 2007 - Table 5.3).
Atmospheric composition, the layers and the complex interactions between different molecules within this layer are not the only natural forcing cycles affecting ISR. The Earth’s orbit varies in three ways over three different time periods, called the Milankovitch cycles (Barry, 1992):
1) The shape of the orbit – 92,000yrs
2) The tilt of the Earth’s axis on rotation – 42,000yrs
3) The time of year when the Earth is closest to the sun (perihelion) – 21,000yrs
These three variants are responsible for fluctuations in the amount of ISR that reaches a specific position on the Earth’s surface over long time periods. In simulating atmospheric radiation, one needs to determine the spectral irradiance (Jacobson, 2005). By calculating the ‘total spectral extinction coefficient’ this accounts for scattering by gases, absorption by gases, hydrometer particles, and absorption by hydrometer particles respectively (Jacobson, 2005). This equation can be seen below:
σλ
= σs,g,λ + σa,g,λ + σa,g,λ
+ σs,a,λ + σa,a,λ + σs,h,λ
+ σa,h,λ
Integrating the spectral extinction coefficient over an incremental distance (determined by the solar zenith angle) gives an optical depth. Optical depth quantifies scattering and absorbing that occurs between the top of the atmosphere and a given latitude.
Ruddiman (Earth’s Climate, 2001) relates
to this by saying, “Incoming solar radiation is stronger at lower latitudes,
where sunlight is concentrated more nearly overhead, than at higher latitudes,
where the sun’s rays strike Earth at a more indirect angle and cover a wider
area.” This variation is named the Solar Zenith Angle (and given the Greek
letter, Ө).
The solar zenith angle depends upon the latitude, season and time of day, it also determines energy available per unit area of the surface and albedo (Hartman, 1994).
Exactly how much ISR is available at the Earth’s surface is variable depending on your latitudal location and the effects of the atmosphere (Barry, 1992). This unequal variation in solar radiation throughout the Earth’s surface causes a pole-to-pole imbalance of absorption and reflection.
Before illustrating in greater detail the actual impact of clouds on determining the temperature on the Earth’ surface, it is important to be aware of the first law of thermodynamics (Hartman, 1994) defined as “The heat added to a system is equal to the change in internal energy minus the work extracted”. Therefore, the amount of energy that is input into the atmosphere from solar radiation, must equal the amount extracted (termed radiative equilibrium). If this wasn’t the case, and more energy was radiated from Earth then it would gradually cool, and vice versa.
With the understanding of the first law of atmospheric thermodynamics, we can begin to look at the effect of clouds on this radiative equilibrium. The impact of clouds on total solar radiation can be illustrated by examining the following equation:
Q+q = (Q+q)0[b+(1-b)(1-c)]
(A) = (B) x (C)
The total solar radiation (A) must equal the global solar radiation for clear skies (B) multiplied by the effect of cloud type, thickness and cloudiness (a fraction of sky covered) (C) (Barry, 1992).
The different constituents of clouds, such as type, thickness and cloudiness are all influential factors in determining the impact of clouds on atmosphere radiation. Barry 1992 likens the direct impact of clouds on solar radiation as the drop in temperature experienced on a sunny day when a cloud may temporarily cut off direct radiation. How much radiation is reflected depends upon the amount of cloud cover and it’s thickness. “Clouds represent an important sink for radiative energy in the Earth – atmosphere system by absorption, reflection and re-radiation” (Barry, 1992).
One of the greatest variations in clouds, are their particular form. These different forms depend upon their height above the surface, which in turn determines their composition. For example, thin water (low) clouds have relatively weak solar absorption, but they effectively scatter solar radiation back toward space and so have high reflectivites (Hartman, 1994). Thick clouds can be assumed to be blackbodies, so that they absorb and reflect longwave radiation at the same temperature (Hartman, 1994).
Clouds are generally classified
into three categories, high, middle and low. These are briefly outlined below:
High – Generally form above 20,000ft, because the air is cold they are made almost exclusively of ice crystals. Most common form of high cloud is Cirrus.
Middle – Found between 6,500ft till 23,000ft. These clouds are composed of water droplets and when the temperature is low enough some ice crystals. For example Altocumulus.
Low – Below 6,500ft, are almost always composed of water droplets, however in cold weather may contain ice or snow. For example, Stratocumulus.
- Adapted from Ahrens 2007.
Clouds are very important for the sensitivity of climate, since they affect both solar (shortwave) and terrestrial (longwave) radiation (Hartman, 1994). The energy balance at the top of the atmosphere is a function of the ISR and Outgoing Longwave Radiation (OLR), as illustrated below from Hartman 1994:
RTOA
= So/4(1-αp) – F↑ (∞)
Energy balance at the top of the atmosphere = ISR - OLR
The following equation gives an approximate formula for the change in net radiation at the top of the atmosphere that is produced by the addition of clouds to a clear atmosphere (Hartman, 1994 eq.3.13):
ΔRTOA
= - So/4Δαp + Fclear↑
(∞) – σT24
This formula provides us with figure 5 below, illustrating that the insertion of a cloud layer into a clear atmosphere can either increase or decrease net radiative force.
This ability to change the radiative flux at the top of the atmosphere is also influential on surface temperature, as whatever radiation does penetrate the cloud layers can then warm the surface. Hartman 1994 states that “the variables that determine the fluxes of radiant energy in the atmosphere include the atmospheric gaseous composition, aerosol and cloud characteristics, surface albedo and insolation”.
Ahrens 2007, develops this idea further by comparing the difference between high and low clouds. “High clouds tend to promote a warming effect, they allow a good deal of sunlight to pass through (warms surface)… low clouds, tend to promote a net cooling effect. Composed mostly of water droplets, they reflect much of the sun’s incoming energy, and because their tops are relatively warm radiate away much of they energy the receive from Earth”.
Whilst the type of cloud is an important variable in affecting surface temperatures; the composition of the clouds is also a major influence on their radiative ability. Hartman 1994 summarises this “The nature of the interactions depends upon the total mass of water, the size and shape of droplets or particles, and their distribution in space”. Water vapour, a major constituent of low clouds is “the most important gas for the transfer of radiation in the atmosphere. It is also the principal absorber of solar radiation in the atmosphere” (Hartman, 1994).
Scattering is the process in the atmosphere by which the direction of a photon of radiation is changed by interaction with atmospheric gases and aerosols (Barry, 1992). Due to the influences of scattering by aerosols and water vapour, the actual density and prevalence of clouds becomes an important factor in influencing surface temperatures. “In very thick clouds most of the solar radiation is scattered before it can reach the particles deep in the cloud, and radiation scattered is unlikely to find it’s way back out to space” (Hartman, 1994).
Clouds become opaque to longwave radiation past a thickness of 20gm-2. If this liquid water path is achieved at an altitude of stable temperature, the cloud surface can be assumed to absorb and emit radiation like a blackbody. This assumption produces accurate results except for very thin clouds such as cirrus, which may be partially transparent to longwave terrestrial radiation (Hartman, 1994). This variation in the water carrying content of a cloud in turn affects it’s albedo (or reflectivity). “The albedo increases with total water content or depth of a cloud and also with the solar zenith angle” (Hartman, 1994). Albedo is the proportion of incident radiation that is reflected. Clouds have different albedo values, varying between 44-50% for Cirrostratus and 90% for Cumulonimbus (Barry 1992). The variation in albedo for clouds must therefore also be a significant influence on surface temperature, if a body has a reflectivity of 90% then it becomes a very influential blanket on solar radiation for the surface.
Observations indicate that clouds increase surface albedo from a surface average of 15% to around 30% which results in a reduction of solar radiation reaching the surface of 50Wm-2. This cooling is partially offset by the greenhouse effect of clouds which positively contributes 30Wm-2. Therefore if clouds were removed without changing any other atmospheric variable, the earth would gain 20Wm-2 and begin to warm (Hartman, 1994). Hartman’s conclusion from 1994 differs from the conclusion drawn by Ahrens in Meteorology Today. Ahrens believes that models of cloud interactions with surface temperature show that as “surface air warms, there will be more convection, more convective type clouds and an increase in Cirrus clouds. This situation would tend to provide a positive feedback on the climate system”. The WMO define the feedback mechanism as “When climatic variables change, it alters another in a way that changes the climate. A positive feedback reinforces the impact of the initial stimulus. The reverse occurs when the response tends to dampen the impact of the initial stimulus”. The WMO illustrate a negative feedback by illustrating when a warming leads to more water vapour in the atmosphere, which produces more clouds. These reflect more sunlight into space which cools the surface.
In simple atmospheric modeling, cloud types are assumed to be uniform, which is called the plane parallel cloud assumption (Hartman, 1994).
The surface energy balance is expressed as ISR + reflected from atmosphere, or:
σTS4
= So/4(1-αp) + σTA4
From the above equation we can see how clouds affect both the surface energy balance and albedo within this calculation.
Intermediate complexity models of atmospheric radiation are termed a one-dimensional radiative-convective model (McGuffie, 2005). These models allow for many layers in the atmosphere and seek to compute atmospheric and surface temperatures (McGuffie, 2005). The RC model is a vertical column which contains many layers of atmosphere, there would be 18 layers of atmosphere which when computed would allow the complex interactions between associated layers to be modeled.
McGuffie 2005 goes on to explore the sensitivity of clouds in determining surface temperatures. The absorption of solar energy at the Earth’s surface is derived by “using separate albedos for the cloudless part of the system and the cloud covered part”. McGuffie 2005, draws a very important conclusion; “It is not yet possible to generalise globally averaged results to yield information on the local effect of variations in cloudiness, since the effect or changes in cloudiness on surface net heating depends upon the local values of the cloud amounts, heights and albedo, the albedo of the surface, the average solar zenith angle and the local vertical distribution of temperature and radiatively active constituents” (McGuffie, 2005).
The most complex modeling incorporates interactions between the atmosphere, sea ice, land and the oceans. They are three dimensional models of general circulation and are known as GCM’s which is either a Global Climate Model or General Circulation Model (McGuffie, 2005). A further derivation, AGCM or OGCM can be applied when modeling the Atmosphere or Ocean, respectively (McGuffie, 2005). An AGCM divides the atmosphere into a series of ‘boxes’, which is the easiest way to visualise the complexity of the multi-layered atmosphere.
The variabilities affecting clouds, such as their: type, density, height and albedo combine to influence radiative transition and therefore resulting surface temperatures. The importance of understanding the atmospheric processes involved in clouds impacting on the surface temperature; calculating exactly the feedback’s experienced by an increased amount of cloud amount is difficult because “probably the greatest uncertainty in future projections of climate arises from clouds and their interactions with radiation” (IPCC, 2001 [b]). Exactly how the climate will respond to changes in cloudiness will probably depend on the type of clouds that form, their physical properties, such as liquid water (or ice) content and droplet size distribution (Ahrens, 2007). It is important to understand that modeling of the atmosphere must take into consideration a significant number of variables, clouds of which form only a small part of either influential or indeed resulting natural climate forcing factors. The observed or hypothetical surface temperature will be a result of a combination of all cloud variabilities and many other natural cyclical processes such as seasonal variations in ISR and the latitudal differences associated with the solar zenith angle.
2,705 words.
Bibliography
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Barry, R.G. 1992 6th ed. Atmosphere, Weather and Climate – Routledge
Hartman, D.L. 1994. Global Physical Climatology – Academic Press
IPCC 2001. Working Group 1. The Scientific Basis.
[a] Summary for Policymakers (SPM).
[b] Technical Summary D.1 Climate Processes and Feedbacks
Jacobson, Mark Z. 2005. Fundamentals of atmospheric modeling – Cambridge University Press
McGuffie, K. 2005. 3rd ed. A climate modeling primer – Wiley Publishing
Ruddiman, W.F. 2001. Earth’s climate: past and future – Freeman
World
Meteorological Organisation. 2003 – Climate into the 21st
Century –