Net Radiation In The Middle East

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Net radiation at first seems like a complicated topic, but it is no more complicated than your bank account. The Earth receives solar radiation from the sun. About 30% is reflected back to space; the rest is absorbed. Absorbed solar radiation heats the Earth, just as deposits increase your bank balance. The Earth also emits infrared radiation to space. This emitted radiation cools the Earth, just as writing a check decreases your bank balance. Net radiation is simply the difference between the absorbed solar radiation and the emitted infrared radiation. Where the net radiation is positive, the Earth tends to warm; where it is negative, the Earth tends to cool. The bank analogy is that if you write checks for less than you deposit, your bank balance will increase; if you write checks for more than you deposit, your bank balance will decrease.

Using satellites, we can measure the amount of solar radiation which the Earth absorbs and the amount of infrared radiation which the Earth emits. Subtracting the latter from the former yields the net radiation. The top figure shows the net radiation for the four-year period June 1985 through May 1989 as measured by three satellites: the Earth Radiation Budget Satellite, NOAA 9, and NOAA 10 (from the Earth Radiation Budget Experiment; Barkstrom, 1984).

The Middle figure shows the zonal average net radiation. Not surprisingly, the tropics and subtropics are warming radiatively (positive net radiation) due to the intense solar radiation. The mid-latitudes and poles are cooling radiatively. The net radiation varies substantially in the north-south direction, but not in the east-west direction--except for North Africa and the Middle East.

The bottom figure shows the variation of net radiation along the Tropics of Cancer and Capricorn. The Sahara and the Arabian Peninsula show large, negative values of net radiation, which indicates that these hot regions are always cooling radiatively. They emit more infrared radiation to space than the sunlight that they absorb. If we estimate (conservatively) that the average net radiation in this region is -10 W/m (watts per square meter), we can calculate the rate at which the atmosphere is cooling: about 30C (54F) per year. Needless to say, the Sahara is not cooling at this rate. (If it were, we would soon have a Saharan ice cap!)

To balance this radiative loss, the atmosphere must import energy from outside the Middle East. This is primarily accomplished by importing high-energy air at upper levels in the atmosphere and exporting low-energy air at low levels. To conserve mass, there must be sinking motion over the entire area on average. Since clouds and precipitation form in rising air, this radiative imbalance in effect suppresses precipitation. What effect does this have on the Middle East? Click Next Page to find out.

Annual Net Radiation

Mean annual net radiation (W/m) from the ERBE data set.

Compare with precipitation | Close-up of the Middle East

Zonal Net Radiation

Zonally averaged annual net radiation (W/m) from the ERBE data set.

Compare with precipitation

Tropics Net Radiation

Mean annual net radiation along the Tropics of Cancer and Capricorn. Zero is shown as a dashed line.

Compare with precipitation

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