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1

Clouds constitute major factors in determining

the radiation budget of Earth,

2

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

modulating the climate

3

Cloud properties which are of interest to the meteorologist are:

  • 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 

4

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

space based instruments

5

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

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

6

Cloud estimation can be done using

window channels on imaging instruments

7

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

much higher spatial resolution than sounders.

8

The most often used technique for cloud characterization is 

simple manual inspection of satellite photographs

9

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

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

10

nephanalyses

maps showing chiefly cloud type and amount at various atmospheric levels

11

cloud amount and height can be estimated using

objective techniques like threshold and pattern recognition techniques.

12

Threshold technique

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

13

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

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.

14

The fractional area covered with cloud is simply the

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

15

Thresholding is one of the means of 

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

16

For some applications, it is important to know the

type of cloud (or surface)

17

Pattern recognition techniques attempt to

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

18

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

satellite

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:

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.

 

20

Efforts to detect clouds using sounder data mostly consist of

removing cloud contamination

21

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

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

22

If we assume that clouds reflect

little radiation at the wavelengths used by sounders

23

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

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

24

the equation is the basis to 

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

25

cloud amount N.

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.

26

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

clear‐column radiance.

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 .................

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

28

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

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

29

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

 water vapor and cloud droplets

30

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

non‐precipitating drops