1-13 Impact of Clouds on the Radiation Balance

Introduction

In previous learning units we have discussed the concept of infrared, or longwave, radiation and its importance to the global energy balance. To this point, however, we have not given a quantitative description of this concept. In this unit, we will discuss the mathematical form of this concept and demonstrate how it is useful in understanding measurements taken from satellites.

Stefan-Boltzmann Equation

The amount of energy radiated from a body (such as the earth or a cloud) per unit area per unit time is given by the Stefan-Boltzmann equation given in Figure 1. The emissivity is a property of the radiating object, but its value is usually near 1. The Stefan-Boltzmann constant has a value of 5.6696 x 10^-8 Wm^-2deg^-4. The effective radiating temperature must be expressed using the Kelvin temperature scale (0 K being absolute zero and 273 $K = 0$ ^oC) for this formula to make any sense. The fact that the temperature is raised to the fourth power means that even a small change in temperature translates into a large change in radiated energy.

Review from Previous Units

Recall from the unit on atmospheric structure and circulation that the temperature of the atmosphere decreases with height. Clouds will have temperatures approximately equivalent to the surrounding air, so high clouds will be expected to have lower temperatures than low clouds. From what we have just discussed, therefore, we would expect high clouds to emit much less infrared radiation than low clouds, and low clouds will likely emit less infrared radiation than the underlying surface of the earth. Therefore, even though all clouds are somewhat uniform in reflecting solar (visible) radiation from their top sides, they differ significantly in the amount of energy they emit upward by infrared radiation.

Our discussion from the last unit included the topic of reflection of solar radiation from particles (dust, soot, volcanic materials, etc.) in the atmosphere. It was noted that volcanoes can cause temporary global cooling due to this effect. Three major volcanoes have erupted in the past 40 years that have allowed us to observe the impact of such events on global temperatures: Agung in the 1960s, El Chichon in Mexico in the mid 1980s, and Mt. Pinatubo in the Philippines in 1991. In each case, the global temperature dropped immediately and gradually recovered over a period of about three years. Global climate models have been used to estimate the effects of such volcanoes from estimated volumes of particulate material put into the atmosphere. These calculations have been quite accurate in estimating the effects on global climate. The University of North Dakota gives a listing of currently active volcanoes.

Outgoing Radiation Under Clear-Sky Conditions

Figure 2, produced by the Earth Radiation Budget Experiment (ERBE) program of the National Oceanic and Atmospheric Administration (NOAA) (Harrison et al, 1988), shows a map of outgoing longwave radiation, in Wm^-2, for the month of April 1985 under clear-sky conditions. Regions colored in red and purple define regions of high amounts of infrared radiation leaving the earth, and green and blue colors denote low IR values. From the Stefan-Boltzmann equation, we can also say that the radiating regions colored red and purple are warmer than those colored green and blue. As expected, the tropical and subtropical regions have the highest outgoing radiation (and temperature) and polar regions have lowest values. Very careful inspection, however, will reveal that some areas in the equatorial regions over land have substantially lower temperatures than adjacent subtropical areas to the north or south. Can you explain this?

Figure 3, also from Harrison et al, 1988, depicts the diurnal range, that is the day-to-night changes, in amount of radiated energy in Wm^-2 under cloud-free skies for April 1985. Note that the range of values is much lower than for the previous photograph. Regions having the largest diurnal variation are generally deserts in the subtropical zones. Having few clouds and low humidity (i.e., very little water vapor for greenhouse gas absorption) in the overlying atmosphere, these regions radiate to outer space directly from their surfaces, which range in temperature from over 60⁰C (333K) during the day to near 10⁰C (283 K) at night. You might use these values in the Stefan-Boltzmann equation to calculate the difference in outgoing radiation for these regions and compare your results with the values of about 60 Wm^-2 given in the photograph. Note that most ocean regions have very low changes in outgoing radiation (and, therefore, temperature) from day to night.

Effects of Clouds

Now if we consider the effect of clouds, we get a quite different picture. Figure 4 (Harrison et al, 1988) shows outgoing radiation, including effects of clouds, averaged over the entire month of April 1985. Comparing this with the clear-sky photograph shown above, you see that the tropical areas have a much lower outgoing longwave radiation. In fact, some areas over Indonesia, South America, and Africa on the Equator have temperatures comparable with polar regions. How can this be? A review of the temperature structure of the atmosphere and your observations of cloud patterns from satellite photographs from the third unit will help answer this question. Very strong surface heating in the tropical regions gives strong convection that creates very deep cloud layers, the tops of which are very high and therefore very cold.

Diurnal Variation

The fourth photograph of this set (Harrison et al, 1988), Figure 5, shows the diurnal variation for all days and include the effects of cloudiness. This shows the effect of clouds in reducing the diurnal variation. Note, for instance, that around the margins of the Sahara Desert in Northern Africa, the area of high diurnal range shrinks when clouds are present. Clouds tend to keep daytime temperatures lower and nighttime temperatures higher, thereby reducing the diurnal range in two ways.

From this you can see that clouds insert a large amount of local variability in the amount of energy the earth radiates to outer space. It also is important to remember that these photographs are averages over many days; if we were to look at a snapshot of a particular day, we would see much more variability from place to place and time to time. For a glimpse at current global cloudiness go to the University of Wisconsin-Madison satellite composite.

Seasonal Variation

Figure 6 shows a 310-day composite of the outgoing longwave radiation for 10 Januarys (Bess et al, 1989). A notable feature of this plot is that, while the South American and African minima in outgoing longwave radiation are confined to the continental borders, the longitudinally extended minimum in outgoing longwave radiation over Indonesia is much larger and spans a large area of ocean. This particular region of enhanced amount of deep cloudiness will be discussed later when we discuss the Southern Oscillation and El Nino effects.

Figure 7 (Bess et al, 1989) for a composite of 10 Julys shows a general northward seasonal shift, reflecting summer in the Northern Hemisphere and winter in the Southern Hemisphere, and marked reduction of the South American and African cloudiness patterns. The Indonesian pattern has shifted northward and westward to encompass the Indian Monsoon phenomenon. The South American pattern also has evolved into what is known as the Mexican Monsoon. The regions of highest outgoing radiation are again the subtropical high-pressure zones which now have drifted somewhat northward with the movement of the season into North Africa, and the Mediterranean and Middle East Regions.

Interannual Variability

The final photograph of this set (Figure 8) shows the standard deviation of the change in annual outgoing longwave radiation for ten summer (June, July, and August) periods and ten winter (December, January, and February) periods. The standard deviation reveals regions of highest variability from one winter (or summer) season to the next. This shows that June, July and August do not experience large changes from one year to the next but, rather, tend to be reasonably constant. On the other hand, in the Northern Hemisphere winter, a region along the equator has a very high variability: that is, it can be extremely warm one year and quite cool the next. This shows that there is something quite peculiar occurring in this region. We will come back to study this phenomenon in more detail when we consider El Nino.

A recent (Oct. 2000) NASA report suggests clouds in a warmer climate will be thinner and contribute less to global cooling than previously thought.

Jet Contrails Impact on the Radiation Budget

Contrails (condensation trails) from jet aircraft represent man-made clouds in the lower stratosphere that can have the same effect on the radiation budget as natural clouds. Stratospheric clouds generally reflect solar radiation during the day but contribute to trapping of long-wave radiation both day and night. The net effect is that they lead to slightly cooler daytime temperature and warmer nighttime temperatures, in other words they reduce the diurnal temperature range (DTR - difference between the daytime high temperature and the nighttime low temperature). Travis et al (2002) seized on the opportunity presented by the grounding of all US aircraft from 11-14 September 2001 to see whether the DTR would increase without contrails. They found the DTR was 1 degree C above the 30-year average for this three year period, adding evidence that jet aircraft do have an impact on the radiation budget over the US.

Have Volcanoes Caused Ice Ages?

The eruption of Mt. Pinatubo spewed enough debris into the atmosphere to lead to a global cooling of a few tenths of a degree that lasted for a couple of years before gradually diminishing. Since this was a medium-sized volcano by comparison to some eruptions known from geological history, what would happen to climate if a major eruption occurred? Richard Kerr (Science 272, 817; 10 May 1996) summarizes some recent data suggesting that huge volcanoes don't necessarily lead to major long-term cooling.

Gregory Zelinsky et al (1996) report recent analysis of the Greenland ice core that lends new insight on this issue. The volcano Toba that erupted in the Indonesian island of Sumatra 71,000 years ago put about 100 times as much sulfuric acid into the atmosphere (1 to 10 billion tons) as did Mt. Pinatubo. Although conclusive evidence has not been presented, it is now estimated that such an event might cause a 3 - 5 degree C cooling for a few years but would not likely plunge the planet into a prolonged cold period. More acid likely means larger droplets which would fall out quicker and allow the surface temperature to recover relatively quickly.

Global Dimming

Global dimming is a term that has been used to describe the reduction in apparent solar radiation received at the Earth's surface in the last half century. The magnitude of the radiation reduction varies around the globe but is more evident in the Northern Hemisphere. Some regions have experienced as much as 5% reduction in recent decades. This trend would counteract a part of the global warming due to greenhouse gases, but the trend seems to have reversed in recent years. This factor could lead to an acceleration of the observed rate of global warming. This dimming evidently is due to particulates in the atmosphere (atmospheric aerosol) which reflect sunlight back to space. Recent successful efforts to reduce atmospheric concentrations of particulates have reduced this dimming since 1990.

Conclusion

One major point that can be concluded from this survey of patterns of outgoing longwave radiation is that clouds play a very significant role in the variability of our weather and climate. Unfortunately clouds are very difficult to describe mathematically in weather and climate models. For this reason, progress in both weather prediction and simulation of future climates is limited by our ability to characterize the occurrence and effects of clouds. I can't help but be reminded of this in one of my favorite popular songs from several years ago by Judy Collins (Figure 9) entitled "Both Sides Now", which has a line that goes 'I've looked at clouds from both sides now, from up and down and still somehow it's clouds illusions I recall, I really don't know clouds at all.

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1-13: Impact of Clouds on the Radiation Balance