Effects of Clouds

Images figure 4 ERBE monthly mean LWRE for April 1985. figure 5 Diurnal range of LWRE from ERBE for April 1985. figure 6 Contour map of outgoing longwave radiation (OLR) for 10 Januarys. figure 7 Contour map of outgoing longwave radiation (OLR) for 10 Julys. figure 8 Standard deviation map of change in interannual OLR for 10 summers (June, July, August) and winters (December, January, February).

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. 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. 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 show 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. 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. PREVIOUS: Outgoing Radiation Under Clear-Sky Conditions NEXT: Conclusion