It is also the downward emission of thermal IR by clouds and water
4 1. Introduction -150 -100 -50 0 50 100 150 200 250 300 350 400 -90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 Flux (W m -2 ) Latitude Annual Average Radiative Flux Deficit Surplus Deficit Outgoing Longwave Radiation Absorbed Solar Radiation Net Radiation Fig. 1.1: Annual average radiation budget as a function of latitude. vapor in the lower atmosphere that explains why nighttime tem- peratures do not fall nearly as sharply on humid or overcast nights than they do on clear, dry nights. 1.1.3 The Global Heat Engine In broadest terms, the importance of radiation for weather and cli- mate can perhaps best be appreciated by examining Fig. 1.1. The top two curves represent the long-term zonally averaged distribution of absorbed solar radiation and outgoing longwave radiation, as seen at the top of the atmosphere. The bottom curve depicts the differ- ence between the input of energy from the sun and the loss of energy to space. In the tropical belt, more energy is received from the sun than is lost to space in the form of longwave radiation. If there were no compensating processes at work, the tropical belt would con- tinue to heat up to well above its current temperature, and the poles would plunge to even lower temperatures than exist there now. The net effect of this radiative imbalance is the creation of a meridional
Relevance for Climate and Weather 5 temperature gradient. Horizontal temperature gradients in a fluid such as our atmo- sphere inevitably create pressure gradients that in turn initiate cir- culations. These fluid motions serve to reduce the temperature gra- dient by transporting heat from warm to cold regions. In fact, they intensify until the net horizontal heat transport exactly offsets (on average) the imbalance in radiative heating and cooling. All circula- tions observed in the ocean and atmosphere — from ocean currents to the Hadley circulation to extratropical cyclones to hurricanes and tornadoes — can be viewed as mere cogs in a huge and complex machine serving this higher purpose. In fact, if you have previously taken a course in thermodynam- ics, you might recall that a heat engine is defined as a system that converts a temperature gradient into mechanical work. It does this by taking in heat energy at a high temperature and discharging the same amount of heat at a cooler temperature. If you take a second look at Fig. 1.1 you will agree that that is exactly what is occurring in the earth-atmosphere system: a net intake of heat energy in the warm tropics and a net discharge of heat from the cool polar re- gions. Problem 1.1: Referring to Fig. 1.1: (a) Estimate the latitude L C where the net radiation crosses over from positive to negative in the Northern Hemisphere. Through that point, use a straightedge to sketch a line that best fits the trend in net radiation toward the North Pole. Find the value Q np of the net radi- ation where your straight line intercepts the right axis. Use the two pieces of information to find the linear equation that approximately
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