chapter 6: cloud interpretation

Question 1

Clouds constitute major factors in determining

Answer 1

the radiation budget of Earth,

Question 2

Clouds constitute major factors in determining the radiation budget of Earth, and they play crucial roles in

Answer 2

modulating the climate

Question 3

Cloud properties which are of interest to the meteorologist are:

Answer 3

• cloud top temperature
• cloud top height (or pressure)
• cloud amount (the fractional area coered by clouds)
• phase (ice or water)
• liquid water contenet
• drop size distribution

Question 4

……………………………………. are the only means by which the global distribution of clouds.

Answer 4

space based instruments

Question 5

Satellite remote sensing methods for determining these cloud parameters use
date from three sources:

Answer 5

1. Visible and infrared imaging instruments (imagers)
2. Microwave instruments (microwave radiometers)
3. Sounding instruments (sounders)

Question 6

Cloud estimation can be done using

Answer 6

window channels on imaging instruments

Question 7

Cloud estimation can be done using window channels on imaging instruments.
These instruments have the advantage of

Answer 7

much higher spatial resolution than sounders.

Question 8

The most often used technique for cloud characterization is

Answer 8

simple manual inspection of satellite photographs

Question 9

The most often used technique for cloud characterization is simple manual inspection of satellite photographs
These analyses are used

Answer 9

daily for weather analysis and forecasting and in the construction of nephanalyses

Question 10

nephanalyses

Answer 10

maps showing chiefly cloud type and amount at various atmospheric levels

Question 11

cloud amount and height can be estimated using

Answer 11

objective techniques like threshold and pattern recognition techniques.

Question 12

Threshold technique

Answer 12

The simplest, and most frequently used technique for objectively extracting cloud information from digital satellite images is the threshold technique.

Question 13

The simplest, and most frequently used technique for objectively extracting cloud information from digital satellite images is the threshold technique.
In this, a

Answer 13

visible brightness or infrared temperature threshold is set such that if a pixel is brighter or colder than the threshold, the pixel is assumed to be cloud covered.

Question 14

The fractional area covered with cloud is simply the

Answer 14

ratio of the number of cloudy pixels to the total number of pixels.

Question 15

Thresholding is one of the means of

Answer 15

filtering clouds from data to be used in sea surface temperature (SST) retrievals

Question 16

For some applications, it is important to know the

Answer 16

type of cloud (or surface)

Question 17

Pattern recognition techniques attempt to

Answer 17

classify arrays of pixels in a manner analogous to the way a person might perform such a classification.

Question 18

To determine whether a given area is clear, cloudy or partly cloudy using a
……………………………………. image, the simple pattern recognition algorithm proceeds as follows:

Answer 18

satellite

Question 19

To determine whether a given area is clear, cloudy or partly cloudy using a
satellite image, the simple pattern recognition algorithm proceeds as follows:

Answer 19

Suppose that the mean and standard deviation (SD) of the pixel brightness in the area were calculated.

• • Uniform scenes, either clear or cloudy, would have a low SD.
• • Partly cloudy scenes must have a high SD.
• • Cloudy scenes would have high mean brightness, whereas clear (ocean) scenes would have low mean brightness.
• • Partly cloudy scenes would have intermediate values of mean brightness.

The algorithm could first check the standard deviation. If it were sufficiently high, the scene would be classified as partly cloudy

• For low standard deviation scenes, the mean brightness would discriminate between clear and cloudy cases.

A training data set is necessary to determine the discrimination values used in pattern recognition techniques.
If we have sets of scenes of clear, cloudy, or partly cloud; then the mean brightness and standard deviation foreach classification would be used to select threshold values.

Question 20

Efforts to detect clouds using sounder data mostly consist of

Answer 20

removing cloud contamination

Question 21

Efforts to detect clouds using sounder data mostly consist ofremoving cloud contaminationso that

Answer 21

accurate, clear‐column radiances can be calculated and also to retrieve the properties of the clouds like amount and cloud‐top pressure.

Question 22

If we assume that clouds reflect

Answer 22

little radiation at the wavelengths used by sounders

Question 23

If we assume that clouds reflect little radiation at the wavelengths used by sounders, then

Answer 23

the radiation sensed by a radiometer can be written as the sum of three terms:

• A. the radiation from the clear area
• B. the radiation emitted by the cloud and
• C. the radiation transmitted through the cloud

Question 24

the equation is the basis to

Answer 24

determine clear‐column radiances for sounding retrievals and to determine the cloud properties.

Question 25

cloud amount N.

Answer 25

If we measure the radiation at two adjacent scan spots, the difference between the radiation from these scan spots is assumed to be the cloud amount N.

Question 26

by applying Eq.1 to two scan spots and eliminating L_cld (lamda) yield the

Answer 26

clear‐column radiance.

Question 27

Similarly, by applying Eq.1 to two scan spots and eliminatingܮ௖௟ௗ ߣ yield the clear‐column radiance. If the above calculation is repeated for a sufficient number of ................................ the calculated ......................................................................... and are .................

Answer 27

scan spot pairs, the calculated radiances are averaged (along with radiances from any spots identified as clear) and are returned as clear‐column radiances.

Question 28

In the microwave portion of the spectrum, the ......................................... can be measured which is ...

Answer 28

liquid water content of clouds can be measured, which is not measurable in other portions of the spectrum.

Question 29

........................................... weakly absorb micowave radiation

Answer 29

water vapor and cloud droplets

Question 30

Both water vapor and cloud droplets weakly absorb microwave radiation and that scattering of microwave radiation is negligible for

Answer 30

non‐precipitating drops

Question 31

For centimeter wavelengths, water vapor absorption peaks at

Answer 31

22.235 GHz

Question 32

For centimeter wavelengths, water vapor absorption peaks at 22.235 GHz and the absorption by water droplets increases with

Answer 32

frequency.

Question 33

The absorption coefficient for each drop is proportional to

Answer 33

the amount of water (liquid or vapor) per unit volume.

Question 34

by ....................................................... column integrated water vapor and liquid water content (mass per unit area) can be retrieved.

Answer 34

By exploiting two frequencies, one near 22.235 GHz and one in the atmospheric window near 31 GHz