chapter 6: cloud interpretation Flashcards
Clouds constitute major factors in determining
the radiation budget of Earth,
Clouds constitute major factors in determining the radiation budget of Earth, and they play crucial roles in
modulating the climate
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
……………………………………. are the only means by which the global distribution of clouds.
space based instruments
Satellite remote sensing methods for determining these cloud parameters use
date from three sources:
- Visible and infrared imaging instruments (imagers)
- Microwave instruments (microwave radiometers)
- Sounding instruments (sounders)
Cloud estimation can be done using
window channels on imaging instruments
Cloud estimation can be done using window channels on imaging instruments.
These instruments have the advantage of
much higher spatial resolution than sounders.
The most often used technique for cloud characterization is
simple manual inspection of satellite photographs
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
nephanalyses
maps showing chiefly cloud type and amount at various atmospheric levels
cloud amount and height can be estimated using
objective techniques like threshold and pattern recognition techniques.
Threshold technique
The simplest, and most frequently used technique for objectively extracting cloud information from digital satellite images is the threshold technique.
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.
The fractional area covered with cloud is simply the
ratio of the number of cloudy pixels to the total number of pixels.
Thresholding is one of the means of
filtering clouds from data to be used in sea surface temperature (SST) retrievals
For some applications, it is important to know the
type of cloud (or surface)
Pattern recognition techniques attempt to
classify arrays of pixels in a manner analogous to the way a person might perform such a classification.
To determine whether a given area is clear, cloudy or partly cloudy using a
……………………………………. image, the simple pattern recognition algorithm proceeds as follows:
satellite
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.
Efforts to detect clouds using sounder data mostly consist of
removing cloud contamination
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.
If we assume that clouds reflect
little radiation at the wavelengths used by sounders
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
the equation is the basis to
determine clear‐column radiances for sounding retrievals and to determine the cloud properties.
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.
by applying Eq.1 to two scan spots and eliminating Lcld (lamda) yield the
clear‐column radiance.
Similarly, by applying Eq.1 to two scan spots and eliminatingܮௗ 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.
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.
……………………………………. weakly absorb micowave radiation
water vapor and cloud droplets
Both water vapor and cloud droplets weakly absorb microwave radiation and that scattering of microwave radiation is negligible for
non‐precipitating drops
For centimeter wavelengths, water vapor
absorption peaks at
22.235 GHz
For centimeter wavelengths, water vapor absorption peaks at 22.235 GHz and the absorption by water droplets increases with
frequency.
The absorption coefficient for each drop is proportional to
the amount of water (liquid or vapor) per unit volume.
by ………………………………………………. column integrated water vapor and liquid water content (mass per unit area) can be retrieved.
By exploiting two frequencies, one near 22.235 GHz and one in the atmospheric window near 31 GHz
