Transcript Lesson27
METO 621 Lesson 27 Albedo 200 – 400 nm Solar Backscatter Ultraviolet (SBUV) • The previous slide shows the albedo of the earth viewed from the nadir. Note that the y axis is on a log scale. • The albedo shows a very low minimum near 260 nm, but has risen to about 20% at 330 nm and above. • The large dip in the albedo matches the absorption cross section for ozone, shown in the next slide. • The SBUV method takes advantage of the relation between the rapid change in the albedo and the ozone cross section to derive an altitude profile for ozone between 30 and 60 km altitude. Solar Backscatter Ultraviolet (SBUV) Solar Backscatter Ultraviolet (SBUV) • SBUV is a satellite spectrometer designed to retrieve the altitude profile of ozone between 25 and 50 km. • Above about 25 km the absorption due to Rayleigh scattering is small, and we can ignore multiple scattering. • Let the solar zenith angle be q0 ., and the angle between the viewing direction from the spacecraft and nadir be q • We define the column amount of ozone at an altitude z to be X(z). Let the atmospheric pressure at z be P(z) • If we choose wavelengths below 300 nm, then all of the solar radiation is absorbed before it reaches the tropopause, hence there is no upward radiation from the ground. Solar Backscatter Ultraviolet (SBUV) At a given wave length the solar flux that reaches z is given by F Fs0 exp( 0 / 0 ) where 0 X ( z ) P( z ) where is the ozone absorption coefficien t and is the Rayleigh scattering cross section. 1 ( )dP S I ( ) d p ( 0 , ) F exp( ) exp( ) 4 0 0 1 ( ) F d p ( 0 , ) I ( ) exp( ) exp( )dP 4 cos( ) 0 0 S 1 dI/FS vs altitude, 245 to 305 nm Solar Backscatter Ultraviolet (SBUV) We now define a parameter Q( ) where Q ( ) * 4 cos( ) I ( ) exp (sec( 0 ) sec( ) dP S ( ) F d p( 0 , ) 0 • Q() is called the normalized radiance. Note that 0 and can be calculated knowing the orbital parameters. • The next slide shows the same figure as the previous slide, only for Q. The upper parts of each wavelength curve now lie on top of each other. dQ vs altitude, 245 to 295 nm Solar Backscatter Ultraviolet (SBUV) • The next figure plots dQ/dz for a single wavelength. • Also plotted is the dQ/dz that would be obtained if there were no ozone absorption i.e. Rayleigh scattering only. • The horizontal line is the altitude at which = 1.0 • The area B is almost equal to area C, hence the area within the curve (A+B) is equivalent to the area under the Rayleigh curve down to the horizontal line (A+C). • The area A+C can be simply related to the pressure at the horizontal line. But the optical depth for ozone absorption is much greater than that for Rayleigh scattering. • Hence Q at each wavelength can be related to the column density of ozone at an optical depth of one, versus the pressure altitude at that point. • For wavelength chosen, Q=.00122, pressure altitude=.00135 dQ versus altitude for 275 nm Contribution function and resolution • The shape of the curve shown in the previous slide is called the contribution function, and the half width in altitude is a measure of the resolution of the method. • However, it should be noted that the contribution at high altitudes follows the increase in pressure as the altitude becomes smaller. It is only when the optical depth due to ozone approaches one, that the contribution begins to fall off. • If we subtract the contribution functions for two consecutive wavelengths then we get the next figure. The resolution of this contribution function is much smaller. • It should be noted that there is a limit to the resolution that can be obtained – the scale height of ozone. dQ vs altitude, for 265 and 275 nm dQ vs altitude, for 265 and 275 nm ‘Difference’ for wavelengths, 245 to 285 nm Effect of scattering and reflection on the photolysis rates • As mentioned before most dissociation processes are limited to the ultraviolet. • At these wavelengths the Rayleigh cross section is high. • Hence scattering can become an important issue in calculating dissociation rates. • In addition we must also consider the effect of the radiation that is reflected by the Earth’s surface and clouds. • The next figure shows the effect of a change in the earth’s albedo on the dissociation rate at the ground. What is plotted is the enhancement factor i.e. the ratio of the dissociation at a given altitude to that at the top of the atmosphere (ф/ф∞). There is no enhancement at wavelengths below 330 nm, as the solar flux at these wavelengths does not reach the ground. Enhancement factors vs albedo Enhancement factors in the stratosphere • The following three figures show the enhancement factors for particular wavelengths for three cases (1) no scattering or albedo, (2) with scattering added, and (3) with both scattering and albedo added. • The figures are taken from Meier et. al, (1982) • The albedo chosen for the calculations is 0.5. There is a broad range observed, from 10% for the ground up to 100% for optically thick non absorbing clouds. Enhancement factor as a function of altitude for absorption only Absorption and multiple scattering With absorption, multiple scattering, and albedo