GHowSAW Cloud Type Recognition
The methodology used here has been adapted from "Estimating Cloud Type from Pyranometer Observations" by Caude E. Duchon and Mark S. O’Malley and published in the Journal of Applied Meteorology vol 38 1998. The authors present an inexpensive and automatable method to estimate cloud type at a given location during daylight hours using the time series of irradiance from a pyranometer. Since measurements of solar illuminance are quite easy to capture using an inexpensive photo-diode system (refer to the pages on radiation in GHowSA) I thought it might be interesting to try and replicate their results.
Pyranometer response to clouds
A pyranometer measures the hemispheric broadband solar radiation and these measurements naturally integrate the effects of clouds. The concept of using a pyranometer to estimate cloud type is illustrated in figure 1 in which comparatively fast-moving clouds cross the slow moving path of the solar beam. For cloud bases at 1 km the speed of the solar beam varies from 0.07 overhead to 2.4 m/s at a zenith angle of 80°, while at a cloud base of 10 km, the comparable speeds are 0.7 and 24.1 m/s. There will be times when the cloud speed is similar to the speed of the solar beam.
|Fig 1. The basis for using a pyranometer to estimate cloud type
is that the clouds cross the solar beam causing fluctuations in the pyranometer readings.
||Fig 2.Comparison of diffuse and global irradiance for (a) a clear
day, (b) a partly cloudy day, and (c) a mostly cloudy day.
With clear skies the irradiance signal is dominated by the solar beam. Diffuse irradiance is around 10-15% of the
total irradiance. With variable cloudiness, the signal fluctuates principally in response to the occurrence or nonoccurrence of clouds intersecting the path between the sun and the pyranometer.
When clouds are present along the beam path, the diffuse radiation as a fraction of the signal increases and becomes nearly indistinguishable from the total irradiance in overcast skies. This is shown in figure 2 for a clear day, in which the diffuse irradiance is about 15% of the global irradiance; a partly cloudy day, where the diffuse to global ratio is variable but greater than 15%; and a mostly overcast day, where the global and diffuse irradiances are essentially equivalent. In summary, the time series of irradiance captures the character of the cloudiness weighted toward the portion
of the sky where the sun is located and provides the basis for estimating cloud type.
Because the fluctuations due to clouds are in proportion to the clear-sky solar irradiance, which varies systematically during the course of a day it is necessary to linearize the estimated clear sky irradiance.
Figure 3 shows the decision criteria for each of the
seven cloud types. The determination of cloud-type
boundaries is based primarily on a comparison of human-
observed cloud types coincident with the measured
irradiance parameters and secondarily
on nominal values of the two parameters intuitively
expected for the different categories based on
the standard cloud-type descriptions. Thus the rectangular
boundaries should not be considered as precise
The lower-left-hand side of Fig. 5 contains stratus,
precipitation, and fog. Here the attenuation of solar radiation
is high, and the variability in the irradiance signal
is small. The large area of cumulus is bounded on
the left by a ratio of scaled observed to scaled clearsky
irradiances of 0.5 and on the bottom by a standard
deviation of scaled irradiance of about 120 W/m2,
which is 20 W/m2 higher than the upper-stratus boundary.
In addition, there must be at least one value of the
21-min time series that has an irradiance greater than
the clear-sky value, as is typical of cumulus clouds (see,
for example, Fig. 3). This criterion was used to separate
cumulus from cumulus and cirrus, the other large rectangle
in Fig. 5. The argument is that cirrus added to
cumulus lowers the general level of irradiance relative
to clear-sky irradiance to such a value that the above
criterion is not met. Nevertheless, the cumulus contribution
continues to yield large variance.
Cirrus occupies an area with the ratio of irradiances
varying from 0.8 to 1.05. A ratio greater than unity is
again due to scattering of the solar beam, this time by
patchy cirrus clouds. The standard deviation is low because
of the thinness of the clouds and limited attenuation. The
clear-sky area is defined by an irradiance ratio extending
from 0.88 to 1.05 and a standard deviation of scaled irradiance
less than 30 W/m2(Originally 10 W/m2 but increased to accommodate the observer standard deviation of clear sky reading). The area outside specific
cloud types represents clouds of indeterminate types.
|FIG. 3. Decision criteria for estimating cloud type based on the standard deviation of scaled observed irradiance and the ratio of scaled observed irradiance to scaled clear-sky irradiance
The seven original cloud types shown as coloured regions in the above figure plus and additional type of Stratus-Hazy are shown on the left-hand
side of the table below. On the right-hand side are the associated human observations and the lower and upper bound limits of irradiation ration and
standard deviation for each cloud type.
||No cloud reported
||Any high clouds of 0.1 to 1.0 coverage
||Any low and/or midlevel cumuloform clouds with 0.1 to 0.8 coverage
||Cirrus and cumulus
||Any high clouds of 0.1 to 1.0 coverage and any low or midlevel cumuloform clouds of 0.1 to 1.0 coverage
||Stratus - Hazy
||Any low or midlevel light clouds of 0.9 to 1.0 coverage
||Stratus - Overcast
||Any low or midlevel heavy clouds of 0.9 to 1.0 coverage
|Precipitation reported and/or sky obscured by fog
||Indeterminate or other
||Multiple levels containing different cloud types
Obtaining the parameters
The basis for our classification scheme is the premise that certain types of clouds have statistical properties
that can be used to identify their occurrence.
The first step in the scheme is to determine the clear-sky irradiance for the date and time under investigation using the Bird and Hulstrom model. This value is then used to scale the measured irradiation to a constant value
of 1400 W/m2. This scale is greater than the solar constant and should not be exceeded on a cloudless day at any latitude.
Scaled Irradiation Value = Measured Irradiance Value X 1400 / Model Clear-Sky Irradiance Value
A clear-sky irradiation model is required each day since the irradiation varies on a daily basis.
Next the mean and standard deviation of the scaled values over the preceding 20 minutes are then determined as follows.
|Fig 4. Measured global irradiance and modeled clear-sky irradiance. The data are 1-min averages.
||Fig. 5. The solid line is the 21-min running mean and the short
dashed line is the 21-min running standard deviation of the observed
irradiances in Fig. 4 after they are scaled. The long dashed line is
the scaled modeled clear-sky irradiance.
Figure 4 shows the unscaled clear-sky model and measured irradiance. The character of the observed irradiance
in Fig. 4 reveals that beginning at about 1630 UTC there is sufficient convection to initiate cumulus clouds,
which last through much of the remainder of the day. Observed irradiance greater than clear-sky irradiance is
a consequence of reflection from the sides of clouds.
Application of scaling and smoothing to Fig. 4 results
in the time series of the 21-min running means of 1- min-scaled measured irradiances and the corresponding
standard deviations shown in Fig. 5 by the solid and short dashed lines, respectively. The rapid rise in standard
deviation coincides with the development of cumulus clouds; simultaneously, the mean scaled irradiance decreases. The horizontal long dashed line is the scaled modeled clear-sky irradiance. The ratio of the running mean scaled irradiance to the scaled clear-sky irradiance and the running standard deviation of scaled observed irradiance in Fig. 5 are the two parameters used in the cloud-type decision criteria.
Figure 6 above shows a sample of my own data where between 09h30 and 11h45 the Irradiance ratio was about 0.4 with a Standard deviation of about 100 W/m2 indicating stratus clouds bordering towards cumulus or indeterminate.
The cloud regimes presented in the table and figure 5 are approximate and can be refined using regular comparison of observed cloud types and the
measured mean and standard deviaation of the measured irradiation. This obviously requires consistency in the visual identification of cloud types
and therefore for inexperienced users such as myself the use of photographs or images of the various cloud types
cloud types or
Clouds at TheAirlinePilots.com)
- Bird and Hulstrom A Simplified Clear Sky model for Direct and Diffuse Insolation on Horizontal Surfaces by R.E. Bird and R.L Hulstrom, SERI Technical Report SERI/TR-642-761, Feb 1991. Solar Energy Research Institute, Golden, CO.
- Solrad.zip MS EXCEL™ VBA version of the NOAA calculator by Greg Pelletier of the Washngton Department of Ecology