Lecture 6: Radiative
Properties in Natural Surfaces
Chapter 5, Petty
Thanks David Randall for sharing his notes.
Surface Reflectance and Ocean Temperature
http://visibleearth.nasa.gov/view_rec.php?id=2366
• MODIS's 36 spectral bands provide scientists the chance
to study many of Earth's terrestrial and oceanic
characteristics with a single instrument, for example, Sea
Surface Temperature (SST) and Land Surface
Reflectance (LSR). This image was made from data
collected during the month of May 2001. The LSR
portion of the image is made from data collected at three
wavelengths: 645 nm (red), 555 nm (green), and 469 nm
(blue). This combination is similar to what our eyes
would see. Combined with the land surface data are
MODIS's measurements of SST in May, using detectors
that capture thermal radiation at 4.0 µm, a design
innovation that improves measurements in moist areas,
such as the tropics, where persistent clouds often
interfere with satellite measurements of SST. Largescale temperature patterns are apparent, such as the
Gulf Stream off the east coast of the United States, and
the Kuroshio Circulation southeast of Japan
Land Cover
(www.bu.edu)
Table 5.1: Shortwave (solar)
reflectivity (in percent) of various surfaces.
Fresh, dry snow
Old, melting snow
Sand, desert
Dry vegetation
Deciduous Forest
Grass
Ocean surface (low sun)
Bare soil
Coniferous Forest
Ocean surface (high sun)
70–90
35–65
25–40
20–30
15–25
15–25
10–70
10–25
10–15
<10
• Absorptivity (a, unitless): Fraction of incident radiation that is absorbed.
• Reflectivity (r, unitless): Fraction of incident radiation that is reflected.
Since these depend on wavelength, refer to them as aλ and rλ.
Since they depend on angle of incident radiation, write them as fn(θ,Φ).
For opaque surface (no transmission): aλ(θ,Φ) + rλ(θ,Φ) = 1 [petty 5.1]
•
If the reflection is isotropic (independent of θ and Φ) then the reflected
monochromatic (spectral) irradiance can be written as:
Fλ,r = rλ Fλ,o [W m-2 nm-1] (Fλ,o = incident, Fλ,r = reflected) [petty 5.2]
And the absorbed irradiance is:
Fλ,a = Fλ,o - Fλ,r = Fλ,o - rλ Fλ,o = (1 – rλ) Fλ,o = aλ Fλ,o [W m-2 nm-1] [petty 5.3]
Petty Figure 5.2.
Shortwave
reflectance spectra
of various natural
surface types.
Where, in the visible, does
chlorophyll absorb?
Vegetation spectral reflectance
 Show a typical vegetation spectra –
Also show the spectral bands of TM in the VNIR and SWIR as well
as some of the basic physical process in each part of the spectrum
Spectral signature
The key will be that
different materials have
different spectral reflectances
 As an example, consider
the spectral reflectance
curves of three different
materials shown in the graph
 In the solar reflective part of the spectrum (350-2500 nm), the shape of the
spectral reflectance of a material of interest drives the band selection
Recall the spectral reflectance of vegetation
Select bands based on an absorbing or reflecting feature in the material
In the TIR it will be the emissivity that is studied
Spectral signature - Vegetation
Samples shown here are for a variety of vegetation types
 All samples are of the leaves only
 That is, no effects due to the branches and stems is included
NDVI –vegetation index
A common method of monitoring surface vegetation is through
the Normalized Difference Vegetation Index:
This has long been used to monitor the vegetation, and changes in vegetation
of the entire earth. NDVI for vegetation generally range from 0.3 to 0.8, with the
larger values representing 'greener' surfaces. Bare soils range from about 0.2 - 0.3.
Here is an example of NDVI over Europe, derived by
German Remote Sensing Data Center.
Graybody Approximation
What is the Lambertian Reflectivity?
The BiDirectional Reflection Function, BDRF – or BRDF
Global Albedo
ASTER: Death Valley
NASA http://visibleearth.nasa.gov/view_rec.php?id=1514
• Since different materials reflect and emit energy in different ways,
the Advanced Spaceborne Thermal Emission and Reflection
Radiometer (ASTER), with its multi-spectral infrared channels, can
provide detailed information about the composition of Earth’s
surface. In this 3-D perspective view looking north over Death
Valley, California, ASTER’s bands 13 (10.6µm), 12 (9.1µm), and 10
(8.3µm) are displayed in red, green and blue respectively. The data
have been computer enhanced to exaggerate the color variations
that highlight differences in types of surface materials.
• Salt deposits on the floor of Death Valley appear in shades of
yellow, green, purple, and pink, indicating the presence of
carbonate, sulfate, and chloride minerals. The Panamint Mountains
to the west and the Black Mountains to the east are made up of
sedimentary limestones, sandstones, shales and metamorphic
rocks. The bright red areas are dominated by the mineral quartz,
found in sandstones; green areas are limestones. In the lower
center of the image is Badwater, the lowest point in North America
Definition: Skin temperature is the radiometric temperature
retrieved from thermal emission via certain wavelengths
Upward longwave radiation
εσTskin4
Temperature Tskin
Surface
Atmosphere
Window
(8-12 µm)
3.1 Results: Global Scale
(Jin and Dickinson 2010)
(1-α)Sd +LWd-εσTskin4 -H-LE - G= 0
Rn
The skin temperature used in calculating heat fluxes and radiation:
G = f( Tskin- Tsoil)
H = CDHU(Taero-Ta)
LE =CDEU(qTskin*-qa)
Eq.
Eq.
Eq.
Snow Index - NDSI
• To detect the presence of snow, recent
satellite instruments include observations
at 0.66 and 1.6mm. The atmosphere is
transparent at both these wavelengths,
while snow is very reflective at 0.66 mm
and not reflective at 1.6mm . The
Normalized Difference Snow Index:
Why do we define NDSI this way?
Red regions are snow covered, white regions are cloudy.
Online Satellite Data Analysis
• http://gdata1.sci.gsfc.nasa.gov/daacbin/G3/gui.cgi?instance_id=neespi
• Plot land skin temperature for July 2008, USA
using MODIS Terra data
• Plot aerosol optical depth at 550nm for July
2008, USA, Terra MODIS
• Plot H20 for July 2000 to July 2008, USA Terra
• NDVI for July 2000-July 2008, 20-40N, 0-360
• Correlation between land skin temperature and
NDVI from July 2000 – January 2010 For USA