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CALTECH ESE 148A - Summary of Radiatively Important Processes

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1 Summary of Radiatively Important Processes • In summary, to get the basic radiative budget of the Earth correct we need to understand: • H2O. Most important gas for radiative transfer. Distribution is variable. Evaporation and Condensation are fast compared to atmospheric motion that mixes dry and moist air. 6.3 µm band (bend) and pure rotational bands most important. It is also the most important absorber of solar radiation in the troposphere. • CO2. CO2 is going up at a fast clip and is expected to double in the next 100 years. 390 ppmv today. Its mixing ratio does not vary much with location or altitude (at least as far as its importance for radiation is concerned). Strong band at 15µm most important. Also a significant absorber of solar radiation (2.0 µm, 1.6 µm, etc.). • O3. Most ozone is present in the stratosphere; smog ozone produced near surface. Ozone has a strong band at 9.6 µm that is important for IR transfer. It also absorbes UV between 200 and 320 nm. The absorption of UV by ozone heats the stratosphere. • Aerosols. Atmospheric aerosol has been increasing. Aerosol influences cloud properties, can directly influence albedo (e.g. sulfates) and solar absorption (e.g. black carbon). • Surface Albedo. Quite variable and changing with land use changes. Varies with SZA, ice/snow. • Clouds. More next. The Earth Radiation Budget Experiment. (ERBE) Satellite observation of the radiance emitted to space both from thermal emission and from solar reflection have produced an important data set for understanding the role of clouds in Earth's energy budget. The 1980's ERBE program included three satellites in different orbits to sampling the upwelling radiance at different locations and different times of day. The first satellite was launched from the space shuttle in 1984. Much detail about the program can be found on the ERBE web page: asd-www.larc.nasa.gov/erbe/ASDerbe.html. A recent follow on is the CERES mission – http://ceres.larc.nasa.gov/ Figures below are Ramanathan et al, Science, 243, 57-63, 1989. These observations suggest that the shortwave forcing by clouds is ~-45 W m-2. The longwave forcing is ~30 W m-2 (i.e. clouds on net cool Earth's surface). The geographical distribution is, however, decidedly non-uniform. The cooling effect is large over the mid and high latitude oceans with values approaching -100 W m-2. In contrast, in the tropics, both the long and shortwave forcing are large but of nearly equal magnitude. The net forcing is much larger than the change in forcing due to doubling CO2. Small changes in the forcing can significantly influence climate. For example, if the stratus cloud deck associated with the mid and highlatitude storm tracks migrated southward during glaciation, the radiative impact of the cloud induced albedo increase could act as a strong and positive feedback mechanism. If the region of -50 to -75 W m-2 cooling (presently near 45 oN) shifted to 35 oN, for example, the hemispherically average cooling would be ~ 3Wm-2, larger than the radiative forcing produced by the observed changes in CO2 (280 to 180 ppm).23 The variation of surface energy balance with latitude and season. Figure 4.11 of Hartmann illustrates the variation of the surface forcing with latitude. The net radiation at the surface peaks in the tropics. In the tropics Rs is much higher over the ocean than the land, reflecting primarily the lower albedo. Differences between water vapor also contribute. In polar regions, the net radiative loss is balanced by downward flux of sensible heat. The polar temperature inversion keeps the surface warmer than it would otherwise be. 90% of the net radiative heating of the oceans is balanced by evaporation! This effect is strongest in the sub tropics because of the higher ocean surface temperature, the relatively dryer conditions aid evaporation, and the stronger trade winds stir the boundary layer. Overland, evaporation peaks near the equator because the subtropical lands are typically dry. Sensible heat, however, peaks in the subtropics - the windy deserts. In general the ratio SH/LH increases with latitude due to the dependence of the vapor pressure of water on temperature. At middle and high latitudes, the evaporative and sensible heat loss is substantially greater than the radiative input, and the deficit is closed by transport of heat by the oceans (ESE/GE 148b - winter term).4 Net radiative input to surface (Rs). LE: Latent cooling from evaporation SH: Sensible Heat Heat transported below surface (earth ocean) Figure 4.16 (Hartmann) shows seasonal variation in the surface flux at several locations in the US. The annual variation of net radiation follows that of insolation, with peak values in summer of ~ 200 W-2. The mechanisms for balancing this radiation depends on the local surface. Over land, it is only sensible and latent heat because storage is small. The apportionment between these depends on surface moisture, temperature, and humidity.5 Figure 4.17 Annual cycle of heat budget for the Gulf Stream at 38 oN 71 oW In the oceans, the energy required for evaporation can be derived from the thermal energy of the ocean. Evaporative loss is more often correlated with wind speed and/or the temperature and humidity contrast than with insolation. Much of the high-latitude evaporation over oceans occurs on the western boundary currents of the ocean basins where the gulf stream and Kuroshio currents carry warm water northward. When this current comes in contact with the dry and windy air coming off the contents, tremendous evaporation occurs and the oceans cool and become saltier. Evaporation rates in winter produce 400 W m-2 cooling (figure 4.17, Hartmann). Net Radiation Latent Heat Flux Net downward flux into ocean Sensible Heat The geographical distribution of energy transfer in and out of the oceans is illustrated in Figure 4.18,


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