4/5/11!1!Phytoplankton!Nutrients!Zooplankton!h!!Light in the Ocean The Marine Environment!Side View Top View Terrestrial Marine4/5/11!2!Light!4/5/11!3!Light in water!• Solar radiation (light) must first reach the surface of the ocean by passing through the atmosphere!• It must then enter the water (rather than being reflected)!• It then is governed by optical properties of water!Light in the ocean!4/5/11!4!4/5/11!5!L(z;!) = L(0;!)exp "1µc(z' ' )dz''0z#$ % & ' ( ) +1µL*E(z' ;!) +1µc(z' )S(z' ;!)$ % & ' ( ) 0z#+ exp "1µc(z' ' )dz''z'z#$ % & ' ( ) dz'Rrs(!) "LuEd= constantbbw+ bbpaw+ aph+ adm+ bbw+ bbpRadiative Transfer Equation Beer-Lambert Law E=E0 • e-kz Light: Particles and Waves •" Light can be measured in terms of:!•" Energy (e.g. units of J m-2 s-1 or W m-2)!–" good for heat budgets and the like!•" Quanta (e.g. units of µmol quanta m-2 s-1)!–" useful for relating to photosynthesis or photochemical processes!4.120nm)()smquantamol()s nm(10 vacuumain light ofspeed)(sfrequencys)J10(6.63constant sPlanck'photonsofnumber)J(energy/1221-191-34-!!!=!==!!====="""#µ$%#$%mWchnnhcnh4/5/11!6!The atmosphere –"The atmosphere attenuates the amount of radiation impinging on earth’s surface. "!! !"Kirk 1994, Fig. 2.1, p. 27!Atmosphere4/5/11!7!Solar Radiation Incident on the Ocean •" Transmission through the atmosphere depends on:!•" Solar zenith angle (latitude, season, time of day)!•" Cloud cover!•" Atmospheric pressure (air mass)!•" Water vapor!•" Atmospheric turbidity !•" Column ozone (important for UV-B)!–" Ground albedo (how much light is reflected from the ground) also affects the incident irradiance.!•" Midsummer Solar Irradiance at 45°N (midday)!•" about 450 W m-2 (PAR, energy units)!•" 2000 µmol m-2 s-1 (PAR, quanta)!•" Midwinter Solar Irradiance at 45°N!•" about 200 W m-2; 800 µmol m-2 s-1 ((PAR only)!Note that “PAR” = Photosynthetically Available Radiation; it is the integral of the spectral irradiance over the visible wavelengths 400-700 nm.!Solar irradiance at the air-sea interface!•" Surface Reflection !–" 2 - 3% for solar zenith angles 0° - 45°!–" Increased greatly when angles exceed 70°; less so with surface roughness!Total internal reflection Focusing and defocusing air water Reflection and refraction •" Refraction!–" Refractive index of natural waters is about 1.34 (air is 1.0)!–" Light direction becomes more vertical !–" Focusing and defocusing effects of waves!4/5/11!8!What Determines the Optical Properties of Water?!Water color is determined mostly by absorption (not particles) Water clarity is determined mostly by scattering (the amount of “stuff” in the water)4/5/11!9!More Units… The ABCs of Light4/5/11!10!Absorption and Scattering Coefficients are Inherent Optical Properties (IOPs)!Absorbed (Heat) Incoming radiant power Transmitted power Scattered power Losses or attenuation from a beam are due to either absorption or scattering. c = a + b, Where c is the beam attenuation, a the absorption, coefficient and b the scattering coefficient, all w/ dimensions L-1. These terms are properties of the water and constituents; they do not vary with angular structure of the light field. But they do vary with the wavelength of light. Absorbed and re-emitted as fluorescence or Raman scattering Absorption, Scattering, and Attenuation Attenuation (c ) is the combination of absorption and scattering!Diffuse attenuation (k) is !4/5/11!11!Absorption in the ocean!0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 400 450 500 550 600 650 700 Absorption (m -1 ) Wavelength (nm) Water Detritus + dissolved colored matter Total Phytoplankton Blue water, green water… Scattering!•" Removal of photon from the incident light beam.!–" Forward scattering bf (m-1)!–" Backscattering bb (m-1)!a!bf!bb!4/5/11!12!Scattering in the ocean!Coccolithophore!Emiliania huxleyi!http://earthguide.ucsd.edu/earthguide/imagelibrary/emilianiahuxleyi.html!4/5/11!13!Absorption plus Scattering!Eddies off the coast of Chile Red tide Ocean Color is a+b!http://seawifs.gsfc.nasa.gov/SEAWIFS.html!4/5/11!14!L(z;!) = L(0;!)exp "1µc(z' ' )dz''0z#$ % & ' ( ) +1µL*E(z' ;!) +1µc(z' )S(z' ;!)$ % & ' ( ) 0z#+ exp "1µc(z' ' )dz''z'z#$ % & ' ( ) dz'Rrs(!) "LuEd= constantbbw+ bbpaw+ aph+ adm+ bbw+ bbpRadiative Transfer Equation Beer-Lambert Law E=E0 • e-kz Even More Units… –"Diffuse attenuation (K, m-1)!•" e.g., Kd (e.g. Ed (z+!z) = Ed (z) exp(- Kd. !z))!–"Reflectance (R)!•" e.g., Radiance reflectance (Lu/ Ed; sr-1)!The Fundamental problem in ocean optics is to relate the IOPs to the AOPs….4/5/11!15!Imagine…… ….An imaging optical sensor that meets the following specifications: •" A dynamic range of order 1010-1012 (~35-40 bits) •" An ability to resolve over 10,000 wavebands over the visible. •" A detection limit equivalent to photon flux at the 0.000001% light level (relative to surface noon) while still resolving colors, and three orders of magnitude more when shifted (automatically) to monochromatic mode centered at 498 nm •" A spectral response optimized for coastal, “green water” environments. •" Logarithmic ranging •" IR and UV blocking to high degree. •" Dynamic focusing. •" Ability to handle 104 radiance range in a single image. •" Spatial resolution of 60 arc-minutes (~250 meters at 700 km altitude) •" <100 msec response time •" On-sensor adaptive preprocessing, coupled to advanced image processing •" ~ 3 cm in size •" Other models available sensitive in UV, IR, polarization The product of 3 billion years of R&D!4/5/11!16!The Secchi Disk: •"First systematic usage reported in 1866, but observed and remarked upon much earlier. •"Early experiments carried out by Commander Cialdi, head of the Papal Navy, and Professor Secchi onboard the SS L’Immacolata Concezione (Cialdi, 1866). •"Used operationally for establishing aids to navigation over shallow water. Optical Observations at Sea Background reflectance just below surface Reflectance of disk Sea surface effects Eyeball Response Optical Properties of the Sea All of the things that one might think would interfere – the illumination conditions, sea-state, the nature of the disk, and human-to-human variability – actually have little effect since they are all contained inside the logarithm. The primary source of variability in the Secchi disk depth is the optical properties of the sea, specifically