AT 351
Homework 4
Due February 22, 2007
Name ________Answer Key________________
1. (5 points) What is the polarization of the wave shown below?
The polarization describes the orientation of the electric field. In this figure, the
electric field is pointing in the vertical direction so the polarization of this wave is
vertical.
2. (5 points).
a.
b.
c.
d.
e.
Tell if each of the following is an (a) passive or (b) active sensor:
CLOUDSAT
active
AMSR-E
passive
WSR-88D
active
GOES IR
passive
TRMM-PR
active
3. If a radar has a maximum unambiguous range of 300km, what is its PRF? (The speed
of light, c, is equal to 3x108 m/s)
The equation to solve for the unambiguous range is Rmax = c/2*PRF where c is the
speed of light and PRF is the pulse repetition frequency (units of cycles per second).
Therefore, rearrange the equation to solve for PRF: PRF = c/2* Rmax. Rmax is in km
and c is in m/s so convert the Rmax to m first before substituting the value into the
equation.
PRF = (3x108 m/s) / (2*300000m) = 500 cycles/s
4. Explain the presence of a bright band.
The bright band is a band of high reflectivity observed within the cloud,
surrounded above and below by lower reflectivities. It represents the region in the
cloud where snow is melting. As snow from the upper levels of the cloud falls
through the melting layer (the layer in the cloud with 0C), the snow acquires a coat of
water, giving it the appearance as a large raindrop. Because reflectivity is related to
the diameter of the hydrometeor to the sixth power, the water-coated snowflakes have
high reflectivity values and produce the bright band on radar.
5. Using the PPI radar reflectivity image below, answer the following questions:
a. Which letter indicates the location of the radar? D
b. Which letter indicates the region of ground clutter? C
c. Which letter indicates the region of highest reflectivity values? A
d. Which letter (A or B) indicates a region of lower rainfall rates? B
e. What type of precipitation system is this (supercell, snow, bow echo)?
Bow echo
B
A
C
D
6. On the reflectivity and velocity images below, indicate (on both) the most probable
location of the tornado?
7. What are the differences between the Clear Air Mode and the Precipitation Mode
(including which features can be identified easier depending on the mode)?
The Precipitation Mode is a faster scan, used when precipitation is present
or expected to be present and is useful for detecting smaller particles (such as
hydrometeors). The Clear Air Mode is a much slower scan used when
precipitation is not present. Reflectivities that appear on this mode are due to
scattering by differences in refractivity and scattering from insects and birds. The
reflectivity scale used for the Clear Air Mode is much lower than the Precipitation
Mode with typical maximum reflectivities usually less then 30dBZ as opposed to
reflectivities exceeding 70dBZ for the precipitation mode.
8. (10 points) Explain how emissivity () affects the brightness temperature measured
by a satellite. What are some factors that affect the emissivity of a surface?
Brightness temperature is the temperature of an equivalent blackbody
emitting radiation. A blackbody emits all the radiation it absorbs, but an
equivalent blackbody only emits a certain fraction of the absorbed radiation. The
amount of absorbed radiation that is emitted by the body is referred to as
emissivity. The greater the emissivity, the higher the fraction of absorbed
radiation being emitted, and the higher the brightness temperature will be. The
emissivity over land is affected by the amount of moisture in the soil, the
conductivity of the soil, the amount of vegetation cover, and other surface
characteristics. Over the ocean, the emissivity is most dependent on the sea
surface temperatures, as well as the wind speed over the ocean.
9. (10 points) Why is emissivity over land difficult to determine?
The emissivity over land is so difficult to determine because of the influence of
surface and soil characteristics, such as moisture, conductivity, vegetation, etc. These
surface characteristics are difficult to determine and frequently change so determining
emissivity over land is a challenge.
10. (10 points) On the Infrared image below, indicate the location of the highest clouds,
lowest clouds, and clear sky. At which point is it most likely precipitating?:
A
B
C
Infrared imagery displays the brightness temperature that is computed from the
amount of sensed infrared radiation. For high clouds, cloud top temperatures are colder
than lower cloud tops so the high clouds appear whitest on satellite imagery. Land
surfaces are warmer than clouds and therefore appear much darker on imagery.
Therefore, the highest clouds are located at C, lowest clouds at A, and clear sky at B.
11. (10 points) Keeping in mind that the ITCZ is a region of rising air, indicate on the
GOES visible image below the where this feature would likely be found.
Extra credit: (10 points) Using the websites provided in class (or by your own searching),
find a radar or satellite image of something you think is interesting, and describe what is
occurring in the image. Be sure to either print out the image, or provide a web address as
to where it can be found.